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COOKBOOK Book One

AI 1200

A t o m i c A b s o r p t i o n Sp e c t r o m e t e r

Updated: Jan 2002


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AI 1200 Cookbook Table of Contents

AI 1200 COOKBOOK

Table of Contents

BOOK ONE- FAAS

Chapter 1: Theory of AASIntroduction 2Flame Atomic Absorption Spectrometry (FAAS) 3Graphite Furnace Atomic Absorption Spectrometry (GFAAS) 4Vapor Hydride Generation Atomic Absorption Spectrometry (VG AAS) 5

Chapter 2: AAS InstrumentationFundamentals 9

Light Source

AtomizerOpticsDetector

Optics 10LensesMirrorsMonochromatorDiffraction GratingSlit Width

Atomizer 15FlameGraphite Furnace

Detector 18

Chapter 3: Background CorrectionFundamentals 22

The Frequency of MeasurementThe Interval between MeasurementsThe Function used to Calculate Net AbsorptionSpectral and Structured BackgroundsThe Effect on the Linear Working Range

Deuterium (D2) Background Correction 24Smith-Hieftje (S-H) Background Correction 27Zeeman Background Correction 28Comparison of Background Correction Methods 29

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Chapter 4: Comparison of Analytical Techniques

Things to Consider 34Applications 34Expected Concentration Ranges 34Elements 34

Atomization Efficiency 34Interferences 35

SpectralBackgroundMatrix

Detection Limits 35Sensitivity 35Precision 35Linear Working Range 35Minimum Sample Volume 36Sample Throughput 36Sample Usage 36Total Dissolved Solids 36

Method Development 36Ease of Use 36Automation/Unattended Operation 36Costs 37

Initial InvestmentRunning Costs

Chapter 5: Standard and Sample Preparation

Apparatus 41Water 41

Standard and Blank Solutions 41Sample Solutions 42Storage of Solutions 42Calibrations 43Matrix Effects 43Chemical Interferences 43

Incomplete dissociation of analyte compoundsIonization

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Chapter 6: FAASAnalytical Data Sheets

Introduction 47THE ELEMENTS

Aluminum, Al 49Antimony, Sb 50

Arsenic, As 51Barium, Ba 52Beryllium, Be 53Bismuth, Bi 54Boron, B 55Cadmium, Cd 56Calcium, Ca (air/acetylene) 57Calcium, Ca (nitrous oxide/acetylene) 58Cesium, Cs 59Chromium, Cr (air/acetylene) 60Chromium, Cr (nitrous oxide/acetylene) 61Cobalt, Co 62Copper, Cu 63

Dysprosium, Dy 64Erbium, Er 65Europium, Eu 66Gadolinium, Gd 67Gallium, Ga 68Germanium, Ge 69Gold, Au 70Hafnium, Hf 71Holmium, Ho 72Indium, In 73Iridium, Ir 74Iron, Fe 75

Lanthanum, La 76Lead, Pb 77Lithium, Li 78Lutetium, Lu 79Magnesium, Mg 80Manganese, Mn 81Mercury, Hg 82Molybdenum, Mo 83Neodymium, Nd 84Nickel, Ni 85Niobium, Nb 86Osmium, Os 87Palladium, Pd 88

Phosphorous, P 89Platinum, Pt 90Potassium, K 91Praseodymium, Pr 92Rhenium, Re 93Rhodium, Rh 94Rubidium, Rb 95Ruthenium, Ru 96Samarium, Sm 97

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Scandium, Sc 98Selenium, Se 99Silicon, Si 100Silver, Ag 101Sodium, Na 102Strontium, Sr 103

Tantalum, Ta 104Tellurium, Te 105Thallium, Tl 106Tin, Sn 107Titanium, Ti 108

Tungsten, W 109Uranium, U 110Vanadium, V 111Ytterbium, Yb 112Yttrium, Y 113Zinc, Zn 114Zirconium, Zr 115

Chapter 7: Practical Applications - FAASMarine 120Water 121Biological 122Food 123Agricultural 124Petroleum 127Miscellaneous 129

References 132

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AI 1200 Cookbook - Book One 1 - Theory of AAS

Chapter 1:

Theory of AAS

Introduction

Flame Atomic AbsorptionSpectrometry (FAAS)

Graphite Furnace Atomic AbsorptionSpectrometry (GFAAS)

Vapor Hydride Generation AtomicAbsorption Spectrometry (VG AAS)

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Introduction

The essential elements of the theory behind the analytical technique of atomicabsorption spectroscopy (AAS) are compacted into the following paragraphs.

Atomic absorption spectroscopy (AAS) relies on the fact that the light

absorption of free atoms [1-7]. All atoms can absorb light, but only at discretewavelengths corresponding to the energy requirements of the particular atom. Inother words, each element absorbs light at some specific and unique wavelength anddoes not absorb light at all on other wavelengths. For example, in a sample withmultiple elements (say, copper, lead, iron, and nickel), only copper will absorb lightthat is at the characteristic wavelength for copper. Furthermore, the amount of lightabsorbed depends on the number of absorbing atoms that are present in the lightpath. All these factors enable AAS to be used as a tool for quantitative analysis.

In practice, measuring the amount of light absorbed by several knownstandards allows a calibration curve to be constructed. Then, the unknownconcentration of a sample can easily be determined based on the amount of light it

absorbs.

The amount of light energy absorbed at this wavelength depends on theconcentration of the atoms in the medium (as dictated by Lamberts law and Beerslaw). Lamberts law states that the portion of light absorbed by a transparentmedium is independent of the intensity of the incident light and each successive unitlayer of the medium absorbs an equal fraction of the light passing through it. Beerslaw states that the light absorbed is proportional to the number of absorbing atomsin the medium. Mathematically, when light of intensity Iopasses through a mediumof length x with atom concentration of C, the intensity I of the light beam emergingfrom the medium is given by:

I = Ioe-kCx

where k is a proportionality constant (the absorption coefficient). Theabsorption of the medium, A, is defined to be:

A = lg (Io/I) = kCx

This equation states that the absorbance, A, of the medium is linearlyproportional to the concentration of the absorbing atoms. The absorption coefficient(or absorptivity), k, can be determined by constructing a calibration curve (i.e.plotting the observed absorbance versus the known sample concentration). Theslope of the calibration curve is kx, and x is easily measurable or already known.Unknown sample concentrations may be determined from the calibration curve based

on their measured absorbances.

Any way that you look at, every AAS experiment can be broken down to thefollowing procedure:

A sample has an unknown amount of a known element (e.g. the sample isknown to contain lead, but not how much lead). The sample must be madeinto a homogeneous, liquid solution (if it is not already).

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A blank solution must be prepared. This blank must contain none of theelement of interest.

A series of standard solutions must be prepared. These standards haveknown (but varied) concentrations of the element of interest. Thesestandards are used to prepare a calibration curve.

Analyze the blank solution to determine the "blank" absorbance value. This is

the absorbance value for a sample with a zero concentration of the element ofinterest.

Individually analyze all of the standard solutions. Construct a calibration graph. For the blank and each standard solution, plot

its absorbance value against its concentration. Analyze the unknown sample. The concentration of the unknown sample,

based on its measured absorbance value, can be determined from thecalibration curve.

There are three main techniques of atomic absorption spectrometry: Flameatomic absorption spectrometry (FAAS), graphite furnace atomic absorptionspectrometry (GFAAS), and vapor hydride generation atomic absorptionspectrometry (VG AAS). Each has its own distinct advantages and disadvantages.

Each has specific applications for which they are the superior AAS technique. Forexample, FAAS is suitable for analyses where the sample is above trace quantities,and where high sample throughput, ease of use, and low initial investment arerequired. GFAAS is ideal for samples that are in the parts per billion (ppb) range andwhere the sample volume is limited. VG AAS is useful for determining elements thatform volatile hydrides at sub-trace levels. A brief outline of each technique isprovided below.

Flame Atomic Absorption Spectrometry (FAAS)

The atomization process by which the atom population is generated is ofprimary importance in AAS because analysis depends entirely on the fact that free,

uncombined atoms will absorb light of a particular wavelength. The key to successfuloperation of an atomic absorption spectrometer lies in generating a supply of free,uncombined atoms in the ground state and exposing this atom population to light atthe characteristic absorption wavelength. The source of energy for free atomproduction is heat, most commonly in the form of an air-acetylene or nitrous oxide-acetylene flame (Flame AAS). With this type of atomizer, the sample solution isintroduced in the form of a spray of fine droplets. This is accomplished by apneumatic nebulizer in most case. The spray of droplets is carried by a gas (usuallythe oxidant for the flame) through the spray chamber and burner head into theflame. The heat of the flame is sufficient to dry (desolvate) each of the sampledroplets and (usually) to decompose chemical components from the resulting driedparticles into their constituent atoms. Thus a population of ground state atoms iscreated in the flame and atomic absorption measurements can be made.

Flame systems for AAS give excellent results, and they are simple,inexpensive, convenient and extremely useful. They permit rapid analyticalmeasurements through a very simple sample introduction technique. The majorlimitation of flame AAS is that the burner-nebulizer system is a relatively inefficientsampling device. Only a small fraction of the sample that is taken up reaches theflame. Additionally, once atomized, the sample passes quickly through the lightpath. An improved sampling device would atomize the entire sample and retain it inthe light path for an extended period of time to enhance the sensitivity of the

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technique. Electrothermal atomization using a graphite furnace provides thesefeatures.

Graphite Furnace Atomic Absorption Spectrometry (GFAAS)

Graphite Furnace Atomic Absorption Spectrometry (GFAAS) has gained a

reputation in the field of analytical chemistry as a routine technique for thedetermination of very low levels of trace metals in a variety of sample matrices.With GFAAS, the flame has been replaced by an electrothermally heated graphitetube. The sample is injected directly into the tube as a small liquid volume (5 to 100L), which is then heated in a programmed series of steps to remove the solvent andmajor matrix components. Free analyte atoms in the gaseous state are eventuallyproduced inside the graphite furnace by rapidly heating with a strong electric currentto temperatures between 1500 and 3000C. All of the analyte is atomized, and theatoms are retained within the tube (and the light path, which passes through thetube) for an extended period (typically 0.2 to 0.5 second). The performance of thistechnique relied on the stability of the temperature. The recently developedtransversely heated integrated contact graphite furnace ensures the temperatureover the entire length of the tube is very uniform. As a result, sensitivity anddetection limits are significantly improved while matrix interferences and memoryeffects are reduced. After the measurement, the analyte vapor is then purged fromthe graphite furnace by helium or argon gas. The magnitude of the absorbance isrecorded as a function of time by the readout system.

The mechanism of atomization in a graphite furnace depends significantly onthe chemical nature of the analyte element, the availability of active sites, thegaseous species within the graphite furnace, the atomization temperature, and thegraphite furnace itself. After drying and pyrolysis the analyte atoms may be presentin reduced, oxidized or complex form. Upon heating the graphite tube to theatomization temperature, free analyte atoms are generated by:

(1) vaporization of the reduced form from the surface,

(2) by dissociation of the oxide form into gaseous free analyte atoms as thegraphite tube heats up, and

(3) vaporization as oxides (or any other molecular species) and dissociationinto free analyte atoms in the gaseous phase.

Examples of these mechanisms are given below - where M is an analyte, C iscarbon and O is oxygen and subscripts g and s refer to gaseous and solid phases:

M (s) M (g)MO (s)+ C(s) M (g)+ CO (g)MO (s) MO (g)MO (g) M (g)+ O (g)CO (g)+ O (g) CO2 (g).

Analysis times for GFAAS are longer than those for FAAS, and fewer elementscan be determined with this technique. Nonetheless, GFAASs enhanced sensitivity,ability to analyze very small sample sizes, and ability to directly analyze certaintypes of solid samples significantly expand the capability of atomic absorptionspectrometry.

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Vapor Hydride Generation

Atomic Absorption Spectrometry (VG AAS)

For the determination of As, Bi, Ge, Pb, Sb, Se, Sn, Te and Hg with AAS,vapor/hydride generation (VG) techniques have been proven to provide very highsensitivities and reduced interferences. With VG AAS, analytes are first reduced totheir corresponding volatile hydrides (or metallic form for Hg) by sodium borohydridein an acidic medium. The vapors are then transported by a carrier gas into theatomizer for atomization and AA measurement.

Aurora Instruments AI 1200 uses an open ended, temperature controlledelectrothermally heated quart tube for continuous flow vapor/hydride generationdeterminations. The heating unit can be installed and removed easily.

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AI 1200 Cookbook - Book One 2 - Instrumentation

Chapter 2:

AAS Instrumentation

Fundamentals

Light Source Atomizer

Optics

Detector

Optics

Lenses

Mirrors

Monochromator Diffraction Grating

Slit Width

Atomizer

Flame

Graphite Furnace

Detector

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Fundamentals

There are four components that are essential to every AAS instrument: alight source, an atomizer, an optics system, and a detector.

Light SourceUsually a hollow cathode lamp (HCL) is used as the light source in AAS. A

less common light source is the electrodeless discharge lamp (EDL). An HCLproduces an intense, narrow line emission of light at a wavelength that is specificto the element that the HCL cathode is coated with. Most elements emit atmultiple wavelengths, but all emissions are intense, sharp lines (called resonancelines). For example, a copper HCL emits at 324.75 nm, 327.40 nm, 222.6 nm,249.2 nm, and 244.2 nm. Generally, one line is more intense than the othersand is therefore the most sensitive and useful line for AAS analysis.

HCLs are coated with the element of interest to produce light of theresonance wavelength(s) that is/are specific to element. When this light ispassed through a medium that contains atoms of the same element, the light willbe partially absorbed.

Atomizer

The purpose of an atomizer is to create a population of free atoms that issuitable for absorption of light. The atomizer must have an energy source inorder to do this. Usually the energy comes from heat, and the most commonsource of the heat is a flame (either an air/acetylene or a nitrous oxide/acetyleneflame). An AAS instrument with a flame atomizer is called a flame atomicabsorption spectrometer (FAAS). With a flame atomizer, the sample isintroduced into the flame as an aerosol (a mist of tiny droplets). The flameburner head [8] is designed to be long, thin, and aligned with the light path.Such a design causes the aerosol atoms to be atomized in the flame while theyare inthe light path so that they can absorb the light.

A very important part of the atomizer system is the nebulizer. Thenebulizer is responsible for nebulizing a liquid sample into an aerosol. Thesensitivity of a FAAS instrument depends heavily on how efficiently the nebulizercan convert a sample to an aerosol.

Optics

The optics system of an AAS instrument is responsible for getting the lightfrom the light source to the detector. Along the way, the light must be passedthrough the atomized sample and through a monochromator. A monochromatoris used to isolate specific wavelengths from the bulk light that it receives. Forexample, it may be necessary to isolate the analytical wavelength of interestfrom light that was emitted from the fill gas of the HCL, or from stray room lightthat entered the spectrometer.

Detector

The detector part of an AAS instrument measures how much light istransmitted through the spectrometer. Most commonly a photomultiplier tube(PMT) is employed for this purpose.

While the above four components are the essential ones of an AASinstrument, there are still others that play important roles.

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There inevitably are electronic devices that convert the signals from thedetector into something that is useful for the human researcher. Olderinstruments used to employ signal meters and plotters that would chart thestrength of the absorbance signal on a moving strip of paper. Instruments oftoday have replaced the meter and chart recorder with computer software thathas many more capabilities. Modern software provides real-time plots of

absorbance versus time, constructs calibration curves, and calculates statisticssuch as RSD values.

On the starting end of the sample analysis spectrum, computer softwarecan keep track of samples that are running, that you will run, and that you didrun. It can also be used to setup and run the instrument without operatorintervention at all.

Optics

The ideal optics system will have the following characteristics:

Have 100% efficient light throughput. In other words, if there is noatomized sample in the light path, then 100% of the light from the sourcewill reach the detector.

Allow zero stray light. Provide a high signal to noise ratio (S/N). Provide absolute selectivity and resolution for the wavelength being

measured. Provide constant dispersion, regardless of wavelength. Have no optical aberrations. Provide unique performance over a wide wavelength range.

Unfortunately, there is no such thing as an ideal optics system. The best thatyou can hope for is to have a system that is optimized for your needs.

There are two ways to control the path of light within an optics system: withmirrors or with lenses. Most optics systems make use of both.

LensesGood quality lenses are made from silica glass and have good light

transmission over a broad wavelength range (~190 - 900 nm). Transmissionlosses occur at both interfaces of the lenses (i.e. at the lens surface where thelight enters and at the lens surface where the light exits). The losses aretypically between 10-14% for each lens in the optical path.

A feature of lenses that must be kept in mind is that the refractive indexof the lens is dependent on the wavelength of the light being refracted. Thismeans that the focal length of the lens will be different for every wavelength.Rather than moving a lens to keep the focal point in the same position fordifferent wavelengths, optics systems will keep their lenses fixed and tolerate therelatively minor losses associated with the changing focal lengths.

Most lenses used are designed for wavelengths in the UV region. This isbecause most analytical wavelengths are in this range and the median refractiveindex is about 250 nm.

For an air/acetylene flame, where the path length is around 10 cm, thelosses due to focal length differences are negligible. For a nitrousoxide/acetylene flame, however, the path length is only 5 cm and the losses canstart to become noticeable. In GF AAS, where the sample atomization occurs

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only at a small point in the center of the graphite tube, the focal length from thelight source is absolutely critical. As the light source wavelength is increased, thefocal point will move away from its original position at the center of the graphitetube. Because of this, optics systems for GF AAS systems usually employ onlymirrors, and not lenses.

Mirrors

Mirrors perform much better than lenses, both in terms of reflectingefficiency and focal length change. A mirror has a very thin (e.g. 2 m) topcoating of aluminum that reflects more than 90% of the light that strikes it (forthe range 190 - 900 nm). Also, when light reflects off a mirror there is nochange in the focal length. That is, the focal length of a mirror depends only onits shape and is independent of wavelength.

Plane mirrors are used to fold light and curved mirrors (also calledcollimating mirrors) are used to focus light. For example, a plane mirror isneeded to fold light around a 90corner, and a curved mirror is needed to focusthat light onto an entrance slit. While the use of mirrors does solve the problemof focal length differences, it raises the challenge of designing and manufacturing

focusing mirrors that are free from other optical aberrations, such asastigmatism.

Because of the thinness of the mirror coating, mirrors areextremely fragile and must be handled with care. Finger prints, and even softtissues, can irreversibly damage a mirrors coating. Reactive liquids and gasescan even cause harm. To increase their longevity, most optics mirrors arefurther coated with a transparent silica or magnesium fluoride film for protection.Even still, one should avoid any kind of direct contact with mirror surfaces.

MonochromatorThere are several different designs of monochromators available. No

matter which design is used in an AAS instrument, however, some fundamental

principles remain the same. Light enters the monochromator through anentrance slit. The light is folded and focused in the monochromator by use ofmirrors. The light is dispersed into its component wavelengths by some sort ofdiffracting element. The diffracted light then leaves the monochromator throughan exit slit.

The most common monochromator design is the Czerny-Turner design,which is used in the AI 1200. A schematic of this type of monochromator isshown in Figure 2.1.

The Czerny-Turner monochromator uses two separate mirrors to collimateand focus light. Mirror #1 receives the light that was focused through theentrance slit. The light that reflects off this mirror is collimated into parallelbeams and then strikes the diffraction grating, which diffracts the light into a

spectrum of wavelengths. This spectrum is dispersed at a variety of angles,depending on the wavelength of each component of the spectrum. The light thenstrikes Mirror #2, which focuses the light through the exit slit and into thedetector. Some monochromators make use of only one mirror, but there is adefinite advantage to using two mirrors. The two mirrors will each be smallerthan a single mirror, so they are easier to manufacture. This means that there isless chance for surface aberrations and therefore allows optimum lightthroughput and resolution.

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Figure 2-1 Schematic diagram of a Czerny-Turner monochromator

The detector is dumb it doesnt know whether the light it receives is ananalytical signal or not. The detector merely counts the number of photons thatit receives (the intensity of the light) and sends a signal to an amplifier thensends back to computer. So, it is the job of the monochromator to isolate aspecific, narrow resonance line from the rest of the spectrum before the light isallowed to reach the detector. The monochromator ensures that onlytheanalytical wavelength of light reaches the detector. This wavelength selectivity isachieved by rotation of the diffraction grating with respect to the incident light.

Turning the grating moves the spectrum across the exit slit, and thereforechanges the wavelength of light that passes through the slit. Another means ofcontrolling the light that passes through the exit slit is by changing the width ofthe slit. A narrower slit allows two closely spaced wavelengths to be resolved,but it also decreases the light throughput of the optics system as a whole. Ifthere are no interfering wavelengths close to the analytical wavelength, then awider slit can be used to increase the light throughput, say, improve the signalnoise ratio.

Other, less common used monochromator designs are the Ebert-Fastieand the Littrow designs.

The Ebert-Fastie monochromator design makes use of a single, largemirror to focus light. See Figure 2.2 for a schematic. One area of the mirror

collimates incoming light onto the diffraction grating and then another area of themirror focuses the light dispersed from the grating onto the exit slit. This singlemirror does both of the jobs of the double mirrors in the Czerny-Turner design. Adisadvantage of the Ebert-Fastie design is that the single mirror must be large.This increases the probability of surface imperfections during the manufacture ofthe mirror, which in turn impairs the light throughput and resolution of themonochromator. An Ebert-Fastie monochromator is, however, less expensivethan a Czerny-Turner.

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Figure 2-2 Schematic diagram of an Ebert-Fastie monochromator

The Littrow monochromator design is similar to the Ebert-Fastie design inthat it also employs only one mirror. See Figure 2.3 for a schematic. Thedifference lies in that the Littrow monochromator uses the same area of thesingle mirror for collimating the light onto the grating as for focusing thedispersed light onto the exit slit. This fact increases the chances of opticalaberrations (even more so than the Ebert-Fastie design).

Figure 2-3 Schematic diagram of a Littrow monochromator

Diffraction Grating

There are two types of diffraction gratings that are commonly used todayin AAS instruments: ruled gratings and holographic gratings. Ruled gratingshave been around longer than holographic gratings, which were only introducedin the late 1960s. A grating, in general, is a closely spaced series of grooves in aflat, reflecting surface. The grooves must be perfectly parallel and uniformly

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spaced from each other. The closer the grooves are spaced, the better theresolving capability of the grating. Gratings are produced with groove densitiesfrom 500 to 6000 grooves/mm.

The blaze wavelength of a diffraction grating is the wavelength of lightthat will be most efficiently diffracted by the grating. Generally, gratings can beused to diffract light that is 2/3 to 3/2 of the blaze wavelength. For example,

consider a grating blazed at 400 nm. The grating can be used for wavelengthsfrom 270 nm to 600 nm, but will diffract most efficiently wavelengths of 400 nm.

Ruled grating are physically etched, groove-by-groove, by a machine.Basically, a mirror is mounted on a grooving machine, a diamond bit etches astraight groove, the mirror is moved a short distance and then the bit etchesanother straight groove parallel to the previous one.

Holographic gratings, on the other hand, are manufactured with light, nota physical machine. A piece of glass is coated with a light-sensitive material,which is then exposed to two parallel beams of coherent light that produce aninterference pattern on the coated glass. The bright areas of the interferencepattern (where the light beams add constructively) form the grooves in thedeveloped photoresist. A thin layer of aluminum is then applied onto theetched glass piece to form a mirrored, grooved surface a diffraction grating.

The advantage of the holographic technique over the machine etching techniqueis that the holographic technique produces no systematic errors, since thegrooves are the result of a perfect optical phenomenon. The machinedtechnique, on the other hand, is only as good as the quality of the etchingmachine itself (which, for many purposes, is excellent, but will never be trulyperfect).

Gratings that are used in monochromators are always copies of mastergratings. A master grating is the original grating that was manufactured (eitherholographically or physically). Subsequent gratings can be made from themaster by a process that essentially makes a molded copy of the master.

There are advantages and disadvantages to both types of gratings. Theappropriate one to use in a monochromator depends on several factors. Ruled

gratings produce significant more stray light than holographic gratings, and thisis especially true when groove density increases. For this reason, the maximumgroove density of rules gratings is around 3600 grooves/mm. Holographicgratings can have up to 6000 grooves/mm. So, based on this factor, aholographic grating is better to use if a higher groove density is required toachieve a higher resolution and maintain a high signal to noise ratio.

Ruled gratings do exhibit significantly better efficiency than holographicgratings. So, based on this factor, a ruled grating is more appropriate to use iflight throughput is critical (for example, if the light source is very weak). If thelight source is intense (as is the case in AAS instruments), then using aholographic grating is more advantageous than using a ruled grating.

Slit Width

The slit width affects how much light enters and exits the monochromator,and so is very important for light throughput. A wide slit width will allow morelight to reach the detector and will improve the signal strength. But, if non-analytical lines also reach the detector, then this will increase the noise anddecrease the signal to noise ratio. Conversely, a narrow slit width will block outall non-analytical wavelengths, but may reduce the light throughput so much tomake the signal to noise ratio unsatisfactory. So, the best slit width is the onethat allows the most light to reach the detector and blocks out most of the noise.

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In other words, finding the optimum slit width is a compromise betweenmaintaining high light throughput and maintaining a high signal to noise ratio.

The maximum allowable slit width is generally determined by how closelyspaced the analytical line of interest and its nearest neighbor in the spectrumare. The slit width must be narrow enough to block out any non-analytical lines,since they would increase the noise that the detector would see. Every spectrum

is different, so the optimum slit width must be determined for each analysis. Forelements whose analytical wavelengths are high (for example, rubidium at 780nm), the spectra are usually not as dense as for elements with shorter analyticalwavelengths (for example, zinc at 214 nm). Therefore, a larger slit width isusually permissible for elements like rubidium, whereas elements like zinc usuallynecessitate narrower slit widths.

Atomizer

The atomizer is arguably the most important part of an AAS instrument,since this is where the sample is converted into atoms that can absorb light.Having an efficient atomizer is essential. The absorbance signal is completelydependent on how many atoms there are to absorb the source light. So, a goodatomizer will display good sensitivity to the sample being analyzed, and a pooratomizer will display poor sensitivity.

The ideal atomizer has the following characteristics:

Atomizes 100% of the sample delivered to it. No ionization occurs. No sample is left in the molecular, complexed form.

Of course, no real atomizer is ideal, and the degree to which any sampleis atomized depends to a large extent on the element being analyzed.

Atomization is achieved by heating the sample to an extent where freeground state atoms are formed. In FAAS, atomization is done with a flame. In

GFAAS, atomization is done with an electrically heated graphite tube furnace. InVG-AAS, atomization is done with an electrothermally heated furnace or a flame.

FlameThe atomizer in a FAAS instrument uses a nebulizer to convert a liquid

sample into an aerosol. The sample is introduced into the nebulizer by aspirationthough capillary tubing. The aspiration occurs pneumatically from the flow of fueland oxidant gases through the nebulizer chamber. After the sample has traveledthrough the capillary tubing, it strikes a glass impact bead. This impact bead isdesigned so that when a stream of liquid strikes it, the liquid breaks apart into amist of drops (an aerosol). This aerosol invariably contains drops of many sizes.The larger drops fall out, but the smaller drops remain suspended and arethoroughly mixed with the fuel and oxidant gases as the mixture is carried intothe spray chamber. As the sample mist gets mixed with the gases, it movesalong through the spray chamber and up towards the burner head. The sampleentering the burner head is a uniform mixture of fuel gas, oxidant gas, and tinysample droplets. Once the mixture enters the flame, the process of atomizationby heat begins. The heat from the flame is usually sufficient to desolvate thesample droplets. Then, the solid particles that were formed (e.g. salts) arebroken down, melted, or volatilized into gases. Finally, the molecules arethermally dissociated into atoms that are capable of absorbing their characteristicwavelength of light. Of course, the heat from the flame may be excessive and

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may ionize some atoms (remove an outer electron from the atom), thusdecreasing the absorption signal. As well, the heat from the flame may beinsufficient and not atomize enough of the sample molecules. This alsodecreases the absorption signal. The process of thermally atomizing the samplein the flame is a complex equilibrium, and the actual chemistry inside the flameat any stage (especially the atomization stage) is not clear. There are often

numerous side reactions that occur simultaneously.The degree to which a sample may be ionized in a flame depends on the

element, since each element has different energy requirements for ionization. Toreduce ionization of atoms, easily ionizable elements (EIEs), such as the Group Ielements (Li, Na, K, Cs) can be added to the original sample solution. If the EIEsare much more easily ionized than the sample atoms, then they will create alarge population of electrons in the flame and shift the atomization/ionizationequilibrium of the sample atoms towards the atomization side.

There are two types of flames that are commonly used for FAAS:air/acetylene and nitrous oxide/acetylene. The air/acetylene flame (air being theoxidant, acetylene being the reductant, or fuel) burns at around 2300 C. Thenitrous oxide/acetylene flame burns much hotter at around 3000 C. So, theflame temperature is a factor when determining which type of flame is best

suited for atomization of a given element. Because of its cooler burningtemperature, the air/acetylene flame works well for elements that are relativelyeasily atomized, such as copper, iron, nickel, and gold. The nitrousoxide/acetylene flame, with its higher burning temperature, is needed to atomizeelements that require more energy to atomize, such as aluminum, silicon,titanium, and tungsten.

Another important factor in optimizing the atomization of an element in aflame is the stoichiometry of the oxidant and reductant gases. A lean flame isfuel poor, and is therefore an oxidizing flame. A rich flame has excess fuel, andis therefore a reducing flame. For each type of flame, certain elements areatomized best in reducing flames, and certain elements are atomized best inoxidizing flames. There is extensive data on the absorbance characteristics of all

the elements in flames. For example, in a reducing flame there are excesscarbon and hydrogen atoms present in the flame (from the acetylene molecules).These excess atoms help break down the strong oxide bonds that form with someelements, such as chromium. Other elements that are best atomized in areducing flame are tin and molybdenum. On the other end of the spectrum,elements like silver, cadmium, gold, and nickel are best atomized in an oxidizingflame. Some elements, like iron and gallium, are best atomized in astoichiometric flame (i.e. neither rich nor lean). Furthermore, some elements aresatisfactorily atomized over a wide range of flame gas mixtures. Copper, forexample, is atomized in both rich and lean flames. For this reason, copper isoften used to test or validate the sensitivity of an AAS instrument.

The major disadvantage of the flame atomization technique is that it isvery inefficient at converting the original sample to atoms. Overall, the atomizer

system of a FAAS instrument can convert less than 0.1% of the original sampleto absorbing atoms. The nebulizer component usually transports only less than10% of the aspirated sample into the aerosol, and the other 90% is lost as wastein the spray chamber and nebulizer chamber. Furthermore, the sample that doesmake it into the flame (in aerosol form) is already greatly diluted from its mixingwith the flame gases. And once the sample gets atomized in the flame, theatoms residence times in the light path are extremely short. Atoms travelthrough the light path at great speeds (at least 1 cm/10 ms) as they exit the slitin the burner head and travel up the flame.

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GraphiteFurnaceThe graphite furnace (GF) atomizer solves the two major problems of the

flame atomizer: poor atomization efficiency and short residence times. Agraphite furnace is a tube that is connected to two low voltage electrodes. Whena current is forced through the tube, the tube heats up. The amount of heating

caused by the current flow can be accurately controlled, and the atomization ofsamples in a graphite furnace is usually performed over several heating steps.The graphite furnace is mounted in the electrodes so that one open end of thetube faces the light source and the other open end faces the entrance to theoptics. This allows the light to pass freely through the graphite furnace along theaxis of the tube. The tube is aligned so that the light path travels down thegraphite tube axis and right through the center of the tube.

With a GF atomizer, a very small amount of liquid sample (between 5 and100 L) is placed inside the center of the graphite tube. A heating program toatomize the sample consists of at least three principal steps:

1 . D r y i n g S t e p

The graphite tube is quickly heated to a temperature just below the

boiling point of the solvent. Then the temperature is slowly ramped pastthe boiling point. This step gently evaporates the solvent (withoutcausing splattering or sample ejection) and leaves the dried sampleinside the tube.

2 . A sh i n g S t e p ( o r C h a r r i n g )

This step removes any dry or semi-dry matrix that is left over fromthe drying step. Matrix modifiers can be added to the sample before theheating program to stabilize the sample during the ashing step. Modifiergases, such as oxygen or hydrogen, can be added to the graphitefurnace workhead during the ashing step to help remove the matrix.Ashing temperatures depend on the element being analyzed in its

matrix. For example, cadmium ashes at 300 C, arsenic at 1400 C, ironat 600 C, and lead at 480 C.

3 . A t om i z a t io n S t e p

At this stage, the sample is a dry solid at the bottom of the graphitetube. The sample is atomized by rapidly increasing the temperature ofthe tube. The AI 1200 can heat at a rate upto 3800 K/s. The requiredatomization temperature depends on the element being analyzed. Forexample, cadmium atomizes at 1250 C, arsenic at 2250 C, nickel at2250 C, and lead at 1400 C.

There are distinct advantages to the graphite furnace atomizer when itsperformance is compared to the flame atomizer. The graphite furnace atomizes

100% of the sample (compared to less than 0.1% for the flame). Also, theresidence time of the atomized sample in the light path is much longer in thegraphite furnace than in the flame. The residence time in the graphite tube canrange from 0.2 to 0.5 second, whereas in the flame it is only milliseconds. Bothof these factors increase the sensitivity of the graphite furnace atomizer, anddetection limits with GFAAS are typically one to two orders of magnitude betterthan with FAAS.

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Detector

The detector is the part of the AAS instrument that receives the lightoutput from the monochromator. The detector quantifies how much light itreceives and creates an electrical current. That current is then amplified andconverted into a digital signal that is recorded by a data acquisition system. By

far the most common detector used in AAS instruments is the photomultipliertube (PMT).

Essentially, a PMT is a photon counter. Light from the monochromatorenters the PMT through a quartz window. Photons (the quantum packets thatmake up light) strike the photocathode of the PMT. This converts the photon to aphotoelectron via the photoelectric effect. However, the production of a singleelectron from a single photon wont generate a very strong signal, so anamplification 5 or 6 orders of magnitude is required. This amplification isachieved through the use of a series of 8 to 12 dynode plates. A voltage isapplied between the photocathode and first dynode (on the order of 100 V). Thisvoltage difference causes the photoelectron to accelerate from the photocathodeto the dynode. When it strikes the dynode, several more electrons are produced.These electrons are in turn accelerated towards the second dynode, since there isalso a voltage difference across the first and second dynodes. Each of theelectrons striking the second dynode creates several more electrons. Forexample, if the electron collision into the first dynode created 5 electrons, thenthe collision of those 5 electrons into the second dynode will create 25 electrons.This chain reaction of electron production continues along the series of dynodes.If there are 12 dynodes in the chain, then the original single photon will produce244,140,625 electrons from the final twelfth dynode. These final electrons arethen collected by the PMT anode. The current that can result from the collectiondue to a single photon by the PMT can be as high as 100 mA! Clearly, PMTs areextremely sensitive detectors. Furthermore, the large amplification of the signalthat is achieved by the PMT is achieved with very little increase in noise.

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AI 1200 Cookbook - Book One 3 - Background Correction Methods

Chapter 3:

Background CorrectionMethods

Fundamentals The Frequency of Measurement

The Interval betweenMeasurements

The Function used to CalculateNet Absorption

Spectral and StructureBackgrounds

The Effect on Linear WorkingRange

Deuterium (D2) BackgroundCorrection

Smith-Hieftje (S-H) BackgroundCorrection

Zeeman Background Correction

Comparison of BackgroundCorrection Methods

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Fundamentals

Background correction is a necessary part of any good AAS instrument. Basically,there are two types of backgrounds that need to be corrected for: non-specific radiationand non-specific absorption.

Non-specific radiation was dealt with in Chapter 2. Non-specific radiation is theextra light (non-analytical wavelengths) that can pass through the optics of an AAS

instrument, enter the monochromator, and have the chance of reaching the detector. Ifthis non-specific radiation reaches the detector, it will result in a falsely signal. Non-specific radiation can come from many sources, including the HCL fill gas, sunlight, roomlight, and the light emitted by the flame itself. As was discussed in Chapter 2(Monochromator section), it is the job of the monochromator to effectively filter all enteringlight and allow only a specific, variable wavelength to exit and reach the detector. So, themonochromator ensures that the only light that reaches the detector is the wavelength oflight that is being absorbed by the sample being analyzed.

The other type of background that must be corrected for is non-specific absorption(and will be discussed in this chapter). This correction cannot be accomplished by themonochromator, since it involves the same analytical wavelength that is selected by themonochromator. Non-specific absorption (also called background absorption) has a

broadband effect and occurs when the source light is prevented from reaching the detectorby means other than absorption by analyte atoms, such as scattering and blocking of lightby other species in the light path. Molecular species and solid particles present in the flameare the major causes of non-specific absorption. When the heat of the flame is notsufficient to fully break down all molecular species (for example, matrix compounds), therecan be sufficient remaining molecules to absorb, block, or scatter the source light. Thesemolecules can be thought of as causing the same interference as putting ones hand in thelight path: the light gets blocked, less light reaches the detector, and a falsely highabsorbance signal results (because the signal analysis software thinks that less light isreaching the detector because more light is being absorbed by the analyte atoms).

In FAAS, the background absorption is relatively minor (usually less than 0.05absorbance units). In GFAAS, on the other hand, background absorption is severe and canreach levels of 2.0 absorbance units. Therefore, effective background correction methods

become essential for accurate GF analyses.

There are three background correction methods currently being used by AASinstrument manufacturers: Deuterium (D2), Smith-Hieftje (S-H), and Zeeman. For allthree methods, there are several common factors that determine the effectiveness of themethod:

The frequency at which the peaks are measured; The interval between total absorbance and background absorbance measurements; The mathematical function used to calculate net atomic absorption; The ability to correct for spectral or structured background; The effect of the method on the linear working range.

The Frequency of Measurement

How rapid and transient the absorption peaks are dictates how fast absorptionmeasurements must be made [9, 10].

In GF analyses, signal durations are typically 0.2 to 0.5 seconds. The temporallyuniform, isothermal atomization of the AI 1200 provides peak durations that are in the lowend of this range. The non-specific absorbance normally exhibits similar peak durations.Furthermore, the change in the absorbance values of these peaks can often be as high as10 absorbance units per second. Therefore, a very high sampling frequency is required toaccurately measure the very rapid and narrow peaks and to determine peak shapes.

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For a GFAAS system, a 10 Hz sampling frequency can produce significant errors inthe measurement of peak height and/or peak area. While the performance can beimproved if a 30 Hz frequency is used, the errors still remain significant. An increase to a60 Hz sampling frequency will provide acceptable results in a GFAAS system. The AI 1200utilizes a sampling frequency as high as 1000Hz in single beam mode and 120 Hz in doublemode, giving excellent peak definition and negligible errors in peak area and peak heightmeasurements.

The Interval between MeasurementsThe Net Absorbance Signal (NAS) is the analytical signal of interest. NAS is the

absorption of the light that is attributed only to the analyte atoms. It is obtained bysubtracting the Background Absorbance Signal (BAS) from the Total Absorbance Signal(TAS).

Ideally, if BAS and TAS were measured simultaneously, then there would be noerror in the NAS calculation. In reality, however, only one measurement can be made at atime. The best thing to do, then, is to make the time interval between successivemeasurements as short as possible, to approachsimultaneous measurements. The shorterthe time interval between a BAS measurement and a TAS measurement, the less errorthere will be in the calculation of NAS. This factor is especially important for GFAAS, wherethe background signal can change quite rapidly compared to the analyte signal.

The AI1200, which uses the D2 background correction method, uses a D2/HCLmodulation frequency of 1 KHz, resulting in a time interval between successivemeasurements of less than 0.5 ms. With this system, the HCL and D2 lamps are alternatelypulsed so that only one light source is passing through the sample at a time. When theHCL is pulsed, the D2 lamp is turned off and only the HCL light passes through the sample.The signal measured is the TAS. When the D2 lamp is pulsed, the HCL is turned off andonly the D2 light passes through the sample. The signal measured is the BAS.

The Function used to Calculate Net Absorption

If a sufficiently fast switching frequency between the BAS and TAS measurements isemployed (such as in the AI 1200), then a simple subtraction of the BAS from the TAS canbe performed to obtain the NAS.

If relatively long time periods are elapsed between the BAS and TAS measurements,then interpolation techniques will be needed to approximate the NAS values. Furthermore,there are increased chances of significant changes in the background signals occurring overthe elapsed time periods. Therefore, these interpolation techniques are much moresusceptible to errors and inaccurate NAS values.

Spectral and Structured Background

Less than 1% of the samples encountered in the real world exhibit spectral orstructured background interferences that cannot be overcome by optimizing the atomizer(for GFAAS, in particular, by optimizing the furnace heating program) and/or using anappropriate chemical modifier.

The Effect on the Linear Working Range

Due to complex splitting patterns, the TAS response of an instrument equipped withZeeman background correction can be non-linear. As a result, when the BAS is subtractedfrom the TAS, a roll over point on a calibration curve can occur. That is, two differentconcentration values could correspond to a single NAS value. In such a case, the lineardynamic range has been reduced by the effects of the background correction method.

In comparison, the D2 background correction method does not produce a roll overpoint in calibration curves and so has no negative effects on the linear dynamic range.

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Deuterium (D2) Background Correction

The light source used in the Deuterium (D2) background correction method is theD2 lamp. The D2 lamp is a continuum source, rather than a sharp line emitter. That is, itemits a spectrum of radiation covering 180 nm to 425 nm. Even though this means thatthe use of the D2 method is limited to the UV range, it is not much of a detriment since themost significant background absorptions occur at low wavelengths anyway.

See figure 3.1 for a graphical explanation of how D2 background correction works.The key thing about D2 background correction is the assumption that the absorption ofradiation by the analyte atoms alone has a negligible effect on the D2 spectrum. Themonochromator exit slit is relatively wide (at least 0.2 nm), so a spectrum of D2 light ofthat width is allowed through. Compare this to the tiny width of spectrum that the analyteabsorption occurs in (approximately 0.002 nm), and its easy to see how absorptionoccurring in such small region of a wide spectrum will have a negligible effect on thatspectrums overall intensity. In other words, only a small fraction of the D2 spectrum isattenuated by the analyte absorption, and the rest is completely unaffected. Conversely,the atomic absorption due the analyte atoms has a significant effect on the intensity of theHCL radiation, since the HCL line is very sharp and narrow to begin with.

But the absorption that occurs in real-life experiments is not due to analyte atoms

alone. It is always a combination of analyte absorption and background absorption.Background absorption is broadband and so has an equal effect on the intensity of both theHCL line and the D2 band. So, when the individual effects of the analyte absorption andthe background absorption are added together, the result is that the HCL line is attenuatedcumulatively by both and the D2 band is only attenuated by the broadband backgroundabsorption. This fact forms the foundation of the D2 background correction method. Whenthe D2 lamp is pulsed, the absorption of the D2 band is measured (the BAS). When theHCL lamp is pulsed, the absorption of the HCL line is measured (the TAS). The differencebetween the two (TAS - BAS) is the absorption due to the analyte atoms alone (the NAS),and this is the figure of analytical significance.

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Figure 3-1 How D2 background correction works(continued on next page)

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Figure 3-1 (continued)How D2 background correction works

In an AAS instrument with D2 background correction, the optics will have to beconstructed to accommodate the additional D2 lamp, and they must also be designed toensure that the D2 light and the HCL light follow exactly the same path through the opticssystem. In order to achieve accurate and reliable background correction, it is imperative

that the two light sources follow exactly the same path. If they did not, then thebackground signal measured with the D2 lamp will have no relevance to the totalabsorption signal measured by the HCL lamp. Imagine if the D2 light path went throughthe top of a flame or graphite tube, and the HCL light path went through the bottom offlame or graphite tube. In such a case, it cannot be assumed that the background in onearea is the same as the background in another. Unless both the background and totalabsorption signals are measured along exactly the same path, then no net analyteabsorption signal can be determined. Today, instrument optics are easily manufacturedwith enough precision and with adequate means of optimization to virtually eliminate allproblems due to poor optical path alignment.

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Smith-Hieftje (S-H) Background Correction

The Smith-Hieftje background correction method is based on the phenomenon ofself absorption (or self reversal) that occurs when HCLs are operated at very high currents[11]. When an HCL is operated at a very high current, the atoms that are sputtered insidethe HCL can sometimes absorb the radiation that is emitted by other atoms inside the HCL.

This causes the emission line to be broadened and the intensity of the emission line at theresonance wavelength to be decreased. The resulting emission profile looks like a broad,two-headed peak. The two heads sit on either side of the resonance wavelength, and thevalley between the peaks is right at the resonance wavelength (the atomic absorptionwavelength). Figure 3.2 shows emission profiles for an HCL line at different currents. Atlow current, the line is sharp and narrow. As the current is increased, the line gets moreintense but it also broadens. At very high currents, self absorption occurs within the HCL,and the two-headed peak straddling the resonance wavelength is observed.

Figure 3-2 The principle behind the Smith-Hieftje background correction technique

When the S-H method is used, the HCL is alternately pulsed between normaloperating currents (where no significant self absorption occurs) and high intensity currents.The duration of the HI pulses is very short compared to the normal current pulses. Duringthe period of the normal pulses, the analyte absorbance signal is measured. During theperiod of the HI pulse, the background absorbance signal is measured.

With the S-H method, the profile of the emission from a HCL is semi-broadband (inthat the emission has been broadened to an extent) and the intensity of the analyticalwavelength (the valley between the shoulders) has been reduced. So, during the HI pulse,the wavelengths above and below the analytical wavelength (the shoulder wavelengths)are used to measure the background absorbance. The attenuation of the already-

diminished analytical wavelength by analyte atoms is negligible compared to theattenuation of the shoulder wavelengths by the non-specific absorbing species. So, thesignal measured during the HI pulse is the background signal (BAS)

Of course, during the normal pulses, the HCL emits the same sharp line as for theD2 background correction method. So, during the normal pulse of the HCL, the totalabsorption signal (TAS) is measured.

A key advantage of S-H background correction method, in terms of accuracy, is thatonly one lamp is needed. Since a single lamp acts as both the continuum source and thesharp line emitter, there will never be any problems with the two optical paths not beingexactly the same.

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However, there is also a disadvantage of the method related to accuracy. Since theshoulder wavelengths of the HI pulse are used to measure the background (and not theanalytical wavelength itself), there is some degree of approximation going on when thatbackground measurement is subtracted from the total absorbance signal (for which theanalytical wavelength is used). The assumption is that the background doesnt changemuch in the short spectral width between the two shoulder wavelengths. In practice, thisassumption is adequate.

There are several other disadvantages to the S-H background correction method.While the requirement of a special HCL (designed for S-H background correction) cantreally be called a disadvantage in itself (since all background correction methods requiresome kind of additional equipment), the fact that special HCLs are not available for allelements limits the methods usefulness. Also, it is required that the high and low currentsof the HCL be stabilized before any measurements can be made. This results in a typicalsensitivity loss of 50%. Calibration curves can sometimes exhibit roll over points at highersample concentrations, so this decreases the linear dynamic range of analysis. Also, the S-H method cannot correct for any structured or spectral interferences. Finally, the methodis limited to relatively low modulation frequencies (normal 10 Hz), so is incompatible withthe rapid transient signals common to GFAAS experiments.

Zeeman Background CorrectionThis background correction method is based on the splitting of spectral lines that

occurs in magnetic fields [12]. In the presence of a magnetic field, an emission line from

an HCL will be split into pi () and sigma () lines. The original resonance line becomesthe pi line and remains at the resonance wavelength. There are two sigma lines and theyare shifted from the pi line (one above and one below) by thousandths of a nanometer.The exact amount of the sigma lines shift depends on the strength of the magnetic field.An important part of this background correction method is that the sigma lines areperpendicular to the applied magnetic field, and the pi line is parallel. So, when a polarizeris used to filter the light beam, the pi line can be excluded.

This method is effective because the species that are in most cases to blame fornon-specific absorption (molecules and solid particles) are not affected by the magnetic

field. So, when the magnetic field is applied, an accurate background absorption signal(BAS) can be measured since the non-specific absorbing species remain unaffected and theanalytical resonance wavelength can be blocked by polarization. Of course, when themagnetic field is not applied, the measurement made will simply be of the total absorptionsignal (TAS). Again, the net absorption signal (the NAS, the absorption due to the analyteatoms alone) is determined by the difference between the TAS and the BAS.

A key advantage to the Zeeman background correction method is that both the TASand BAS are measured along the exact same optical path, so the technique is very accuratein that sense. As well, Zeeman-equipped instruments generally operate at a highfrequency of measurement (usually 120 Hz), so they are able to deal with the rapidtransient peaks that occur in GFAAS analyses. The Zeeman method is applicable to anywavelength of light, so it works for all elements. This is in contrast to both the D2 and S-H

methods. The D2 method is limited to those elements with analytical wavelengths that fallin the range 180 - 425 nm, and the S-H is limited by the availability of a HCL for theelement of interest. Furthermore, the Zeeman method is the only one that can correct forspectral and structured background interferences.

While that last advantage listed may seem impressive, it is in reality of noconsequence. Less than 1% of samples that are run in labs today have problems withspectral or structured background interferences that cannot be overcome with optimizationof the atomizer or by the addition of suitable chemical modifiers.

There are other drawbacks of the Zeeman background correction method thatshould be mentioned. There is typically a sensitivity loss of 20% associated with theZeeman method (the exact value is determined by the magnetic susceptibility ratio of the

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element being determined). Also, it frequently results in roll over of calibration curves athigher concentrations, so the linear dynamic range is significantly reduced. Anotherimportant point relates to the cost of the Zeeman instrumentation, which requires arelatively large initial investment. This is a result of all the additional equipment that isrequired on an AAS instrument equipped with Zeeman background correction (a stableelectromagnet, a power supply, and a polarizer).

Comparison of Background Correction Methods

An important part of the decision to purchase an AAS instrument is the decision ofwhich background correction method to include. In general, this decision is governed bythe applications that the instrument is expected to be used for. Of course, there arespecific applications that each background correction method is the best suited for. But itis quite rare that an instrument will be dedicated for a single, specific type of analysisthroughout its entire lifetime. With that in mind, perhaps it is wisest to choose abackground correction method that is versatile and can meet the background correctionneeds of the majorityof samples encountered.

Table 3.1 below is a summary of the comparisons between the three backgroundcorrection methods discussed here.

Table 3.1:Summary Comparison of Background Correction Methods

D2 S-H Zeeman

Sensitivity Good Poor, 50% loss Moderate, 20% loss

Type of Instrument All FAAS GFAAS

Wavelength Range 180-425 nm 180-900 nm 180-900 nm

Number of Elements Limited to elements inwavelength range

Limited to lampavailability

All

Longevity of Lamp Normal Decreased Normal

Calibration Linearity Not affected Roll over occurs Roll over occurs

Linear Dynamic Range Not affected Decreased Decreased

Accuracy Two optical paths

must be optimized

Good (single

optical path)

Good (single optical

path)Correct for Spectral orStructured Interferences?

No No Yes

Frequency of Measurement 120 Hz 10 Hz 60-120 Hz

Interval betweenMeasurements

0.5 ms 2-4 ms 4.5 ms

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D2 seems to be the overall best background correction method of all the three,since it offers the broadest range of advantages and does not suffer in the really key areasof sensitivity and calibration linearity.

There seems to be an impression in the analytical chemistry community thatZeeman background correction is the superior method available. However, once thefactors have all been weighed, it should become apparent that this impression is flawed.As was mentioned before, the fact that Zeeman method is the only one that can correct for

spectral and structured background interferences is of no consequence in over 99% of thesamples that will be encountered in a laboratory. Even if it is determined that this featureis necessary and Zeeman background correction is required, there will be other importantfactors that will suffer. Initially, the cost of the Zeeman instrumentation is very highcompared to D2 and S-H. Also, Zeeman has significantly poorer sensitivity, calibrationlinearity, and linear dynamic range than the D2 method offers. Another misconception isthat D2 background correction is inadequate for samples with rapidly changing backgroundsignals. The frequency of measurement and interval between measurementscharacteristics show that D2 is on par or better than Zeeman is in these areas.

Based on all these considerations, the decision between the D2 and Zeemanbackground correction methods can be boiled down to the following:

Choose D2if you need: Choose Zeemanif:

maximum sensitivity calibration linearity maximum linear dynamic range rapid measurement capability low cost

you expect spectral andstructured backgroundinterferences, and this overridesall other factors

cost of equipment is no concern

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AI 1200 Cookbook - Book One 4 - Comparison of AAS Techniques

Chapter 4:

Comparison of AASTechniques

Things to Consider

Applications

Expected Concentration Ranges

Elements Atomization Efficiency

Interferences

Spectral

Background

Matrix

Detection Limits

Sensitivity Precision

Linear Working Range

Minimum Sample Volume

Sample Throughput

Sample Usage

Total Dissolved Solids

Method Development Ease of Use

Automation/Unattended Operation

Costs

Initial Investment

Running Costs

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Things to Consider

When deciding whether to purchase a flame AAS or a graphite furnace AAS, one shouldfirst make an assessment of ones analytical needs:

Which elements will I be analyzing? What are the sample matrices? What are the solvents? How much sample volume will I have? Do I need high throughput? Will the concentrations be high or at trace levels? What detection limits do I need? How important are precision and sensitivity? How skilled are the intended operators? Can operators afford to spend time on method development and optimization? How much money am I willing to spend?

Applications

The applications that you will be running (e.g. environmental, marine, petroleum,mining) will play a part in determining the AAS technique that is right for you. However,many of the techniques overlap when they are compared on the basis of applications.Therefore, your particular application will be a relatively minor factor in the decision. Thataside, there are some applications that one technique is much better suited for than theother (this is usually based on the element(s) being analyzed).

Its also important when deciding on a AAS instrument to think of the future. Willyour anticipated long-term needs be satisfied by the instrument that is appropriate topurchase for your needs today?

Expected Concentration Ranges

Do you expect most of the samples that you will analyze to have high or lowconcentrations? Since GFAAS is much more sensitive than FAAS (typically, GFAAS hasdetection limits 100 to 1000 times better), it makes more sense to run the concentratedsamples on a FAAS, and use a GFAAS to run the trace analyses. Diluting samples down toconcentration levels that a GFAAS can handle is not only time consuming and laborintensive, it also introduces the possibilities of errors and contamination.

Elements

The FAAS technique can analyze a total of 61 elements with its air/acetylene andnitrous oxide/acetylene flames. The GFAAS technique can analyze a total of 41 elements.Also, any element that GFAAS can analyze can also be analyzed with FAAS.

Atomization Efficiency

The atomization efficiency is arguably the most important performancecharacteristic of any AAS technique. The air/acetylene flame burns at around 2300 C, andthe nitrous oxide/acetylene flame burns significantly hotter at around 3000 C. For someelements, the cooler air flame is not hot enough to atomize the sample, and the nitrousoxide flame is required instead. However, the excess energy of the hotter nitrous oxideflame can ionize other elements. So, its often a trade-off with air flame atomizers. Someelements have low atomization energies, and so are better suited to the cooler flame

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(copper, iron, nickel, and gold). Others have high atomization energies, and so a nitrousoxide flame is needed (aluminum, silicon, titanium, and tungsten).

And for some elements, no flame is hot enough to efficiently atomize the sample.In these cases, the GFAAS technique is needed, because the furnace can reachtemperatures up to 3000 C.

Another consideration is the nebulization efficiency of the FAAS nebulizer. Only less10% of original sample introduced into the spray chamber gets converted in an aerosol and

makes it into the flame for atomization. In this sense, the FAAS technique is verywasteful. The GFAAS technique, on the other hand, is very efficient. All of the sampledeposited in the graphite furnace remains inside the furnace and therefore has a goodchance of being atomized. As well, atomized sample have relatively long residence timesinside the furnace tube (0.2 0.5 s)

Interferences

Spectral

Spectral interferences do occur, but are rare, in both FAAS and GFAAS techniques.

Background

Background interferences are much more common, and both techniques arevulnerable. Background correction techniques are usually employed to compensate for theinterference.

In GFAAS, background interference typically arises from vaporized and atomizedmatrix components.

Matrix

Matrix interferences can be serious for both FAAS and GFAAS analyses. For GFAAS,the effects can be countered by adding modifiers to the samples. For FAAS, the ionizationbuffers are usually employed to control matrix effects related to ionization.

Detection Limits

The detection limits of the GFAAS technique (ppb range) are generally 100-1000times better than the FAAS technique (ppm range).

Sensitivity

This factor is closely linked to detection limits. GFAAS is much more sensitive thanFAAS, since the graphite furnace atomizer has much longer residence times and muchhigher atomization efficiency than the flame atomizer.

Precision

The flame atomizer has better short term precision (0.5%) than the graphite

furnace atomizer (2-5%). Long term precision is usually better with the flame atomizertoo, since the graphite furnace invariably degrades and gets worn out as it is used.

LinearWorkingRange

The linear working range of the AAS technique is extremely important. It is theconcentration range over which standard samples can be measured and used to generate alinear calibration curve. Having a narrow working range means that the calibration curvesusefulness is limited because not as many unknown samples will fall within the linearcalibration range.

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In general, on an element-by-element basis, the linear working ranges of the FAAStechnique are better than those of the GFAAS technique. In this respect, FAAS is moreversatile and useful than GFAAS.

Minimum Sample Volume

The minimum sample volume required to measure an absorption signal of a solution

can be very important sometimes, such as when there is almost no sample solution tobegin with.

The minimum volume required by the GFAAS technique is much smaller than for theFAAS technique. GFAAS requires as low as 20 L of sample for a good measurement,whereas FAAS requires at least 0.5 mL of solution.

SampleThroughput

The sample throughput is the rate at which individual samples can be analyzed. Itis often expressed in the format minutes per sample. FAAS has a higher samplethroughput rate than GFAAS. The reason for this is that the graphite furnace atomizationsequence has multiple steps, each with hold times, and a cool-down time is required at the

end of each analysis.Typical sample throughput values are 10 seconds per sample for FAAS analyses and

4 minutes per sample for GFAAS analyses.

SampleUsage

Sample usage refers to how much sample is consumed by the instrument while itperforms an analysis. FAAS uses a lot of sample solution for each analysis, and much of itis wasted and never even makes it into the atomizer for an absorption measurement.GFAAS, on the other hand, uses very little sample and wastes none. The entire volume ofsample deposited in a graphite furnace remains in the graphite furnace during theatomization steps.

TotalDissolvedSolids

The graphite furnace atomizer can handle relatively large amounts of dissolvedsolids (> 20%), whereas the flame atomizer can only tolerate about 3% maximum (this isthe case for liquid introduction atomizers in general).

MethodDevelopmentThe FAAS is easier to develop methods for than the GFAAS instrument. This is

mostly because of the strict constraints set on the optical alignment of the furnace atomizerand the requirement of accurate and consistent sample injections. As well, the operation ofthe FAAS doesnt require as much experience as the operation of the GFAAS. Nonetheless,for both instruments, a skilled operator is required to develop a good method for a specific

application. Fortunately, most instruments come equipped with extensive libraries of pre-made, common methods that are ready to use.

Easeofuse

The FAAS instrument is very easy to use, and an expert operator is not required forroutine analyses. The GFAAS instrument is also relatively simple and straight-forward touse, but a skilled operator is still required because accurate and consistent sampleinjections are essential for good results, even for the most routine analyses.

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Automation/Unattendedoperation

GFAAS lends itself well to automation. The use of inert gases does not pose anydangers, so unattended operation is also feasible. Overnight, automated operation ofGFAAS is commonly employed to achieve a higher sample throughputs. FAAS is equallysuited to automation, but unattended operation is unadvisable due to the use ofcombustible gases. An operator should always be present to ensure the safe operation of

the instrument and to be able to deal with gas leaks or other emergencies as they arise.

Costs

Initial Investment

There is no way to specifically say how much an instrument will cost, since there aremany factors involved. The cost depends on the configuration, accessories, options,background correction method, and, of course, the vendor. Roughly speaking, FAASinstruments can cost from $20,000 to $35,000. GFAAS instruments are more expensive,and usually cost about 2 times as much ($40,000 to $70,000).

Running Costs

Both techniques are comparable in terms of the costs of running the instruments ona day to day basis, although FAAS is generally a cheaper instrument to run. There is aconstant expense for the fuel and oxidant gases in FAAS (although, air is free), and theinert gas used in GFAAS (usually argon) isnt cheap. In addition, a periodic expense of theGFAAS technique is the replacement cost of the graphite furnace atomization tube, whichhas a limited life and needs to be replaced from time to time to ensure efficientatomization.

Another consideration is whether your specific applications with GFAAS will requireclean room conditions. Maintaining clean room conditions is no small task and addsconsiderably to the operating costs of the instrument.

The following table is a summary of the comparisons between FAAS and GFAAS.

Table 4.1:A Summary Comparison of Flame AAS and Graphite Furnace AASFAAS GFAAS

Detection Limit sub ppm sub ppb

Linear Working Range 103 102

Precision 0.5% 2 - 5%

Sensitivity good excellent

Spectral Interferences virtually none occasionally

Chemical/Matrix Interferences many many

Ionization occasionally minimal

Minimum Sample Volume 0.5 mL 20 L

Sample Throughput ~10 s/sample ~4 min/sample

Sample Usage high low

Number of Elements 61 41Maximum Dissolved Solids ~ 3% > 20%

Ease of routine use easy requires expertise

Method Development/Optimization easy difficult

Easily Automated? moderately yes

Combustible Gases? yes no

Initial Investment low high

Running Costs low moderate

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AI 1200 Cookbook - Book One 5 - Standards and Sample Preparation

Chapter 5:

Standards and SamplePreparation

Apparatus

Water

Standard and BlankSolutions

Sample Solutions

Storage of Solutions Calibrations

Matrix Effects

Chemical Interferences

Incomplete Dissociationof Analyte Compounds

Ionization

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Apparatus

All apparatus to be used during the process of an analysis, from the sample andstandard preparation to the introduction into the instrument, must be scrupulously clean.Even glassware that had been cleaned previously and stored should be cleaned againimmediately before use.

As well, the instrument itself should be periodically cleaned to ensure that no

contamination errors or memory effects are introduced. In FAAS instruments, the sampleaspiration tubing is easily and inexpensively replaced. The nebulizer chamber should alsobe cleaned and washed regularly. Residue buildup can be a problem, especially when flowspoilers or baffles are employed. Corrosion resistant nebulizer should be used whencorrosive acids or organics are being analyzed (e.g. use a Teflon constructed spraychamber and glass impact bead).

Volumetric flasks should be Class A (accurate to 0.01%), since Class B glasswarewill introduce systematic measurement errors into your analysis.

Analytical balances should be high-accuracy and high-precision. Less than 1% erroris needed when weighing out samples to ensure minimal error in prepared solutions.

Pipettes and micropipettes should always be calibrated to ensure accuracy ofdelivery volumes.

Water

All water used for AAS analyses should be deionized. Distilled water alone is likelyto have significant amounts of dissolved metals and gases, and can introduce significanterror into analyses. Using distilled water that has also been deionized is acceptable.

For some GF applications, particularly when trace levels are being determined,simple deionized water is not good enough. In these cases, an ultra-pure water treatmentis needed (eg. the Milliporesystem).

In general, one can expect most acids to contain trace amounts of metallicimpurities. While the levels are trace, they can still introduce error in some applications(particularly in trace-analyses and GF analyses). Fortunately, specially purified acids arecommercially available to alleviate this source of error.

Standard and Blank Solutions

Bulk standard solutions are available from many chemical suppliers, usually in 1000mg/L (1000 ppm) concentrations. They have a shelf life of about one year. These bulkstandards come with certificates of analysis, and are traceable to NIST (National Instituteof Standards and Technology) standards. These bulk standards should be of the highestpurity available. Atomic Absorption grade is acceptable for many applications, but Ultra-pure is preferred (and essential for very sensitive, trace-level determinations).

Bulk standard solutions can also be prepared by the operator, in the laboratory.This method is useful to prepare standards from pure metals, metallic salts, and metaloxides. When weighing out the raw materials, the materials must be treated to ensure that

they are in a pure, standard form. Metals should be washed with acetone or ether toremove any oil layers. Oxide coatings can be removed by scrubbing them off the metalssurface (with an emery cloth, for example). Metal oxides, salts, and other compoundsshould be dried for several hours at an elevated temperature to drive off excess water (110C or higher will suffice). The material should be dried to a constant mass.

In the interest of minimizing errors, it is best to perform serial dilutions whenperforming large dilutions of the bulk standards. For example, imagine that you needed toprepare a 0.1 ppm standard from the 1000 ppm bulk. This is a dilution factor 10,000.Rather than performing such a dilution in one step, it is advisable to break it into twosteps: perform a 100 times dilution, and then another 100 times dilution. The end result

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is the same, but the chance for error is much less, since a small volume measurementerror has a relatively greater effect on a small volume than on a larger volume. However,the error in volume measurements that do occur in the two step method are doubled, sincethe dilution is performed twice. However, the relative effect of the volume measurementerror is of far greater concern. In this example, the two step dilution uses volumes of thebulk standard that are 100 times greater than volumes used with the single dilutionmethod. Therefore, the two step dilution method, in theory, has 50 times less error.

Ideal standard solutions are solutions that are identical in chemical and physicalcomposition to the sample solutions, except that they have known concentrations of theanalytical component. Ideal blank solutions are solutions that are identical in chemical andphysical composition to the sample solution, except that they dont have any of thecomponent that is of analytical interest. Proper standards and blanks should be preparedby the same procedure that the sample is prepared by. For example, if the sample ismicrowave digested by nitric acid, then the standards and blanks should also be microwavedigested by nitric acid (expect that the standards will have a known amount of theanalytical component added, and the blanks will have none added).

Matching the physical properties of sample with standards and blanks is importantbecause the amount of solution that is aspirated by the nebulizer and into the flamedepends, to an extent, on the physical properties of the solution. The viscosity, density,surface tension, and solvent vapor pressure will all affect the aspiration rate.

Sample Solutions

There are many different kinds of sample solutions. Sometimes the sample isobtained from a third party, and is already in ready-to-analyze form. Often, however,digestion and dissolution steps are required. In either case, it is important that samplesolutions be homogenous and free of solids. This is especially important for the GFAASanalyses, since the sample volumes analyzed are much smaller than for FAAS. If 50 L ofa heterogeneous sample is deposited in the graphite furnace and analyzed, it is very likelythat its signal will be different from the next 50 L sample analyzed, producing inconsistentresults. In other words, it is very difficult to get a representative sample aliquot from aheterogeneous sample solution.

Storage of Solutions

It is recommended that all working standard solutions to be used in an analysis beprepared (from the bulk standards) fresh on the same day as the analysis. All solutionsshould be labeled and dated to keep track of them. This is to prevent any errors due to thestandard solution ions absorbing into the container walls, and also molecules leaching fromthe container walls into the sample solution. The same applies to sample solutions. Bulksolutions stored on the shelf for long periods should have high concentrations (at least1000 ppm). There is significant degradation solutions with low concentrations (less than10

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