IntroductionGraphite furnace atomic absorption spectrometry (GFAAS) hasbecome a popular analytical technique in recent years, mainlybecause of its enhanced sensitivity that allows measurements inthe picogram (10-12 g) range. However, it has also acquired areputation of being a rather difficult technique to use. Papersdescribing intractable interferences and excessive background levelshave publicised some of the 'real life' problems encountered inGFAAS.

As a direct result of the high sensitivity, another practical problemthat must be faced is the control of external contamination.Contaminants can be introduced to standards and samples alike andcome from a variety of sources, including reagents (such as matrixmodifiers), solvents (used in dissolution procedures), the laboratoryatmosphere and contaminated glassware. Taken together, theseproblems can give the impression that GFAAS is a technique thatrequires many complex and poorly understood steps to be made beforean analytical method can be successfully developed and proved.

Although the difficulties in GFAAS must not be underestimated,the situation is, in fact, not as fraught with problems as itsometimes appears.

The key to success is that any analytical method developed forGFAAS must be made as simple as possible, with the minimumaddition of reagents and a minimum number of sample handling steps.

This paper attempts to lay down such a procedure, based onThermo's accumulated knowledge gained from a considerableperiod of involvement in advanced graphite furnace technology andanalytical methods development.

Analytical Method Development in GraphiteFurnace Atomic Absorption Spectrometry.

TechnicalNote: 40699

Modern Graphite FurnaceAtomizersThermo Electron Corporation offers theAA Series range of atomic absorptionspectrometers that use Windows-basedsoftware to offer high performance and allthe flexibility required by modernanalytical laboratories.

The GF95 and GF95Z Graphite Furnacesare major accessories that have beendeveloped to complement the AA Seriesspectrometers. They are similar designs,with the GF95Z providing a backgroundcorrection system based on the Zeemanopto-magnetic effect, while the GF95 relieson the continuum source Quadlinebackground correction system included in allthe AA Series spectrometers. The design ofthese furnaces is discussed in detail in (1),but at their heart is a graphite cuvette thatis 28 mm long with an internal diameterof 5 mm.

Figure 1. GF95Z Graphite Furnace System

A range of different types of graphitecuvettes is available, so that the optimumtype can be selected for the analysis. Someexamples are shown in Figure 2.

Figure 2. Thermo Graphite cuvettes.

An analyst new to GFAAS analysesmay not fully appreciate why there are

different types of graphite cuvettesavailable. The normal cuvette is madefrom a form of graphite known aselectrographite. This cuvette is suitable forthe determination of volatile elements suchas lead and cadmium in simple matrices,such as clean waters. Electrographite is arelatively porous material and allowssamples to soak into the graphite lattice.Many elements, including iron, vanadiumand molybdenum, then react with thegraphite during the furnace program to formstable carbides and so cannot be measuredwith an electrographite cuvette. Similarly,matrix components such as alkali metalhalide salts can penetrate the carbonlattice, to be subsequently released in thehigh temperature atomization phase, wherethey will often cause chemical, vapor phaseinterference effects.

For these elements and types ofsample, it is necessary to use a modifiedelectrographite cuvette, which is coatedwith a layer of pyrolytic graphite, generallyabout 12 - 20 µm thick. Pyrolytic graphite isa much denser form of carbon, with verylow porosity, so that the sample cannotpass through it to soak into the graphitelattice. Pyrolytically coatedelectrographite cuvettes are required forthe carbide forming, medium-volatile andrefractory elements, and for samples withcomplex, high salt matrices.

Extended Lifetime Cuvettes (ELC's)are a unique Thermo development, and havea pyrolytic coating that is up to 10 timesthicker than the standard coating. This givesthem more stable performance and muchlonger useful lifetimes than either of theelectrographite-based cuvettes. Thesecuvettes are especially recommended forhigh throughput analyses with complexsamples, and when measuring the mostrefractory elements.

Omega Platform cuvettes have anintegral L'Vov platform built into thecuvettes, which makes them particularlysuitable for determining the more volatileelements in heavy matrices where vaporphase interferences cause severe problems.Platform atomization and the Omegacuvettes are discussed in more detail in (2).

An inert sheathing gas is used toprotect the cuvette and graphite enclosurefrom oxidation. Argon is normallyrecommended for the purpose, but it ispossible to use high purity nitrogen if argonis not available. Some elements, such asvanadium or aluminum, must be measured

using argon because they form stablenitrides with nitrogen.

An independent flow of the inert gas ispassed through the cuvette during thefurnace cycle, to help carry away vapor andsmoke produced by the decomposition ofthe sample. It is also possible to introducean alternate gas in place of the argonduring some or all of the furnace programphases. Oxygen or air can be introducedduring the ashing (pyrolysing) phase of theheating program to assist thedecomposition of samples containing a highcarbon content - for example biologicalmaterials or foodstuffs, and can prevent thebuild-up of a carbonaceous residue insidethe cuvette. It is also possible to connectother gases, such as argon/methane forin-situ pyrolytically coating of the cuvetteinterior, to the alternate gas channel.

The final important accessory is theautosampler, which is nowadays consideredan essential part of a GFAAS analyses.Modern graphite furnace autosamplers,such as Thermo's FS95, provide manyfeatures and facilities to assist the analyst;some are listed in Table 1.

Figure 3. The FS95 Furnace Autosampler

FS95 FURNACE AUTOSAMPLER FEATURES

Automatic matrix modification -wet and dry mixing options availableAutomatic standard preparation -fixed or variable volume preparation may be usedAutomatic sample dilution -either a fixed dilution ratio for all samples, orintelligent dilution of over-range samples onlyAutomatic re-concentration of samples, usingmultiple injectionsAutomatic standards addition preparationAutomatic QC spike addition

Table 1. Some Features of the FS95 FurnaceAutosampler

Sample PreparationContamination problems are common inGFAAS analyses, but most laboratories donot have easy access to Class 100 cleanrooms in which to prepare their samples.However, a careful, systematic approach tosample handling can successfully controlcontamination even in a normal workinglaboratory. Glassware, such as volumetricflasks, should be filled with 10 % v/v nitricacid and stored until required for use. Whenneeded, the flasks are simply rinsed a fewtimes with doubly deionized water. Ingeneral, it is better to mark each flask witha particular analyte concentration and usethis one flask to contain that concentrationof analyte only.

Glass volumetric pipettes are oftenmajor contributors to contamination. Evenwhen precautions are taken to acid-soakthem and rinse them with pure water, theyare often left to dry in the open laboratory,dried in contaminated ovens, or rinsed withcontaminated acetone. It is preferable toperform volumetric transfers where possibleby using automatic pipettes with disposabletips. The tips can be cleaned relativelyeasily, by dispensing several volumes of10 % v/v nitric acid followed by severalvolumes of deionized water before taking upthe sample. When using such a pipette forpreparing accurate standards and sampledilutions, it is important that the analystshould validate the pipette's accuracy toensure that it does not introduce significantvolumetric errors. Glassware normally usedfor flame AAS analysis should not be usedfor furnace determinations without thoroughcleaning, and where possible it isrecommended that a specific set ofglassware should be reserved for GFAASsolution preparation.

All solutions used for GFAAS workshould be prepared freshly each workingday, and should contain a minimum of10-2M nitric acid to stabilize the tracemetals in solution.

Method DevelopmentConsiderationsGraphite Furnace AAS is a well-establishedanalytical technique, and there is a largebody of applications literature available. Itis likely that information already exists formost of the common types of analyses, sothat “information retrieval” is important andshould be the first step in developing newmethod. The journals of interest to analysts

working in GFAAS are the Journal ofAnalytical Atomic Spectrometry (JAAS), TheAnalyst, Spectro Chimica Acta, AnalyticalChemistry and Anal. Chim. Acta. The journalJAAS from The Royal Society of Chemistry,U.K. contains frequent reviews of all therecent publications in a particularapplication area, which form an excellentstarting point for the research.

Consideration should be given to anyspecific sample collection and transportprocedures that may be required to ensurethat stable, representative samples arrive inthe laboratory for analysis. Solid sampleswill typically require some form of digestionprocedure to bring them into solutions sothat they can be analyzed, while liquids mayrequire dilution or concentration to bring theexpected analyte concentrations into themeasurable range. These samplepreparation procedures can dramaticallyaffect the performance of the analyticalmethod, but are outside the scope of thisshort article.

Preliminary ProceduresThe starting point when developing a newGFAAS method is to obtain the set of basicinstrumental data for the analytes that areto be determined. This information isavailable from a variety of sources, and isconveniently collated in the On-Line'Cookbook' that is provided with theSOLAAR Data Station software suppliedwith all AA Series spectrometers. TheCookbook includes suggested values of theinstrument parameters such as wavelength,bandpass, lamp current and whetherbackground correction is required or not. Italso includes information on commoninterferences and suggests some methodsfor overcoming them. An example of theCookbook page is shown in Figure 4.

Figure 4. Example of a SOLAAR Cookbook page.

When a new Method is created, theAA Series spectrometers will load defaultinstrument and furnace parameters from theCookbook for each element that is selectedfrom the software. It is important to

appreciate that these defaults have beenoptimized for the measurement of theselected element in clean aqueousstandards. When developing a method foranalyzing real-world samples, the defaultsform a useful starting point, but willinvariably require optimization to obtain thebest possible results for any particular typeof sample.

Setting up the instrumentBefore an analysis begins, both the hollowcathode and deuterium lamps should beswitched on for about 15 minutes in orderto allow them to stabilize properly.

It is important to check that the furnacehead unit is correctly aligned with respectto the optical beam of the spectrometer.Failure to do so can result in problems suchas emission breakthrough, especiallynoticeable when the analyte resonance linelies above 300 nm. Figure 5 shows theeffects of emission breakthrough on theblank signal with a badly mis-alignedfurnace head, and the signal when thealignment is correct.

Figure 5. Effects of emission breakthrough on thebaseline signal at the barium wavelength of553.6nm.

The furnace head is permanentlymounted in the right hand samplecompartment of the Dual Atomizer M Seriesinstruments, and is correctly aligned duringthe installation of the instrument. Themounting of the GFS97 Combined Furnaceand Autosampler accessory for the singlecompartment S Series instruments has beendesigned so that the accessory can beremoved and replaced without disturbingthe alignment. Nevertheless, it is goodpractice on all instruments to check thefurnace head alignment periodically, andparticularly after the furnace head has beendisassembled for routine cleaning.

Capillary tip is toohigh - sample is notdeposited oncuvette floor

Capillary tip is toolow - sample runsback up the outsideof the capillary tip

Capillary tip iscorrectly positioned -sample is depositedcorrectly on to thefloor of the cuvette.

The furnace autosampler has to bealigned with the furnace head, so that thesample can be correctly deposited into thecuvette. Mechanical adjustments areprovided on the autosampler body for thispurpose, and, as with the furnace headitself, these have been designed to maintaintheir alignment even when the autosampleris removed from the instrument andreplaced. The FS95 provides replaceablecapillary tips, which perform the actuallysample deposition, and these can becomecontaminated, as well as bent or distorted,by careless handling. It is important tocheck the capillary tip alignment whenevera tip is replaced, and it is good practice toexamine the tip and confirm that thealignment remains accurate each time theinstrument is used. A small dental mirrorcan be used to examine the position of thecapillary tip inside the cuvette, as shown infigure 6, but this requires a certain amountof skill and experience.

Figure 6. Using a dental mirror to examine theinterior of the cuvette.

Thermo's AA Series instruments offer abetter solution, in the form of the GraphiteFurnace TeleVision (GFTV) accessory. This isa small CCD camera mounted within theoptical system of the spectrometer so that itcan produce a live image of the interior ofthe cuvette on the Data Station VDU display.

The height adjustment of the capillarytip (i.e. the distance between the tip andthe floor of the cuvette during the injectionsequence) is critical, and is dependent onthe viscosity characteristics of the sample.Some typical GFTV images are shown infigure 7, showing the effects of incorrectcapillary tip height adjustments, but aposition 1 - 2 mm from the cuvette floor is

suitable for simple aqueous solutions, andis a good starting point for morecomplex samples.

Figure 7. GFTV images, showing the effects ofautosampler capillary tip positioning.

All AA Series spectrometers areequipped with the Quadline continuumsource background correction system. Forthis to perform to specification, it isimportant that both the hollow cathodelamp and deuterium background correctorlamp beams are correctly aligned. AA Seriesinstruments provide automatic alignment ofthe hollow cathode lamp, but the deuteriumarc lamp requires manual alignmentwhenever it is replaced. Alignment proceduresare included in the User Manuals. Theyhave also been published in some journalswith a more detailed discussion of theeffects of misalignment (3,4).

Calibration rangeThe next step is to calculate theconcentration of the analytical standards(taking due account of the injected volumeto be employed) that would be required togive a calibration curve in the optimumrange of 0.1 to 0.8 A (measured in the peakheight mode). These concentrations can bederived from published characteristic massvalues (defined as the mass of analyte, inpicograms, required to generate a signal of0.0044 A) or from the furnace check valuesin the Cookbook, shown as the concentrationof analyte which will give an absorbancereading of 0.1 A when a 20 µL volume ofsolution is injected.

As an example, the characteristic massfor aluminum is 3.6 pg, so that a sampleinjection containing 3.6 pg will give a signalwith a peak height of 0.0044 A. It can easily

be calculated by simple ratios that a massof about 700 pg of aluminum should give asignal with a peak height of about 0.8 A,which would be ideal for the top point onthe calibration graph. It is normal to use 10or 20 µL injection volumes, so that the topstandard concentration should be equivalentto 700 pg in, say, 10 µL i.e. 70 µg/L or 70 ppb.The lowest standard should give a signal ofabout 0.1 A, and in this case, a concentrationof 10.0 µg/L would be suitable. GFAAScalibrations are not usually perfectly linear,and a minimum of three standards is usuallyused to define the curve accurately. Themid-point standard concentration shouldtherefore be selected so that the data pointfalls mid-way between the blank and thetop standard, and a concentration of 35 or40 µg/L would be suitable. The publishedCharacteristic Mass figures are measuredusing aqueous solutions, with the defaultinstrument parameters. Interferences andchanges away from the default parameterscan cause significant reduction of the signaland, therefore, the practical concentrationrange may well be higher in the presence ofinterferences.

Furnace Heating programThe Furnace Heating Program is critical tothe success of a method developmentprogram. It is conventionally divided intofour distinct phases, and, as with all otherparameters, the SOLAAR software provideselement dependent default parameters thatwill serve as a starting point forprogram optimization.

Figure 8. The default furnace program for lead.

The DRY phaseIf the furnace autosampler has been alignedcorrectly, it should be possible to inject adiscrete volume of sample into the cuvette,so that it remains as a droplet on thecuvette floor when the autosampler capillarytip is withdrawn.

The first task is to dry this sample pool,so that only the solid material remains. It isimportant to ensure that the sample iscompletely dried before moving on to thenext, higher temperature phase, but it alsoimportant that the cuvette temperature is

not increased to the point at which theliquid boils. The ideal behaviour is that thesolvent gently evaporates, leaving the solidmaterial in the same place on the cuvettefloor. If the sample does boil, or undergoesany type of vigorous disturbance, it willsplash and become dispersed over theentire inner surface of the cuvette. At best,this will degrade the measurementprecision, and in extreme cases may preventany meaningful signal from appearing.

The DRY phase temperature is usuallyset to a few degrees above the boiling pointof the main solvent in the sample - 110 ºCwould be a typical for an aqueous sample.The time required to dry the sample dependson the volume of the sample to be used andthe nature of the sample, and 30 secondswould be a typical value. Both the time andthe temperature of this phase are bestoptimized by observation, either using theGFTV image, or using a dental mirror todirectly view the sample in the cuvette.

The DRY phase largely controls theprecision and reproducibility of the finaloptimized method, and careful optimizationof this phase is necessary to obtain theexcellent relative standard deviation valuesof 5 % or better that GFAAS is capableof delivering.

The ASH phaseWhen the sample has been successfullyinjected into the cuvette and dried, the solidmaterial that was dissolved in it will remainon the floor of the cuvette, ready for thenext stage in the process.

If the sample was a clean aqueousstandard, this solid material will consistentirely of the analyte element, in the formof the salt of the major anion present,which is usually nitrate. If this is the case,then a discrete ASH phase will not berequired. However, more usually the samplecontains other metal salts, and possiblyorganic materials such as proteins - theseare collectively known as the samplematrix components.

The purpose of the ASH phase is toremove as much of the sample matrixmaterial as possible before the atomicabsorption measurement takes place. Thiswill minimize the occurrence of chemicalinterferences and background absorbanceeffects derived from the matrix, and so willenable the analyte signal to be measuredcleanly and easily.

The ASH phase is normally performedat a fairly high temperature, selected to

volatilize the major matrix components.However, the temperature must not beraised to the point where the analyte itselfis volatilized, as then the analyte will beremoved from the cuvette, and so will notbe available for later measurement. TheCookbook provides suggested maximum ashtemperatures for each element. These arederived from measurements in simplenitrate solutions of the element, and maynot be applicable if the sample matrixcontains a different major anion, or issignificantly different in any other way.

The time required to complete the ASHphase will typically be in the range 10 - 60seconds, and will, of course, be dependenton the nature of the sample and thetemperature selected. In general, ASHphases that are too long do not cause anyproblems other than wasted time, and inthe initial investigations, it is usually best toset a reasonably long period, typically 30 -45 seconds. As the program is refined, itmay be possible to reduce the ASHphase time.

Running the full program cycle, so thatan analytical signal is generated, thensystematically varying the ash phasetemperature while observing changes in thesignal is the best way to optimize the ASHphase temperature. If the ash phasetemperature exceeds the maximum ashtemperature that applies to the analyte inthe particular type of sample underinvestigation, the signal will sharplydecrease above the critical temperature. If amatrix component is responsible for aparticular type of chemical interference,then the signal will increase noticeably astemperature is increased so that thecomponent is removed. This type ofexperiment is time-consuming to perform,and so the SOLAAR software provides anAsh Atomize function to perform themeasurements and gather the dataautomatically. This will be described below.

The ASH phase is the most criticalphase in the furnace program, and requirescareful optimization. As it affects anychemical interferences that may be present,it directly influences the overall accuracy ofthe analysis.

The ATOMIZE phaseWhen the ASH phase has been successfullycompleted, the only sample materialremaining in the cuvette will be the analyteitself, now usually in the form of an oxide,and any refractory matrix components, such

as calcium or magnesium oxides, that havenot been removed in the ASH phase.

The cuvette is now heated sufficientlyto atomize the analyte, and the spectrometerrecords the atomic absorbance signal. Thisis the ATOMIZE phase. For each analyteelement, there is a minimum atomizetemperature that must be used, to ensurethat all the analyte is atomised, and, likethe maximum ash temperatures, these areprovided in the Cookbook, and loadedautomatically as part of the defaultparameter set for the analyte element.

It is important that the cuvette isheated as fast as possible at the start of theATOMIZE phase, so that the analyte isatomized as quickly as possible. Thisgenerates a dense atom cloud, andmaximizes the analytical sensitivity. TheThermo furnaces provide a TemperatureControl feature, that uses an opticaltemperature feedback system to ensure thatnot only is the atomize temperature accuratelycontrolled, but also that the temperaturerisetime is always the maximum that ispossible. The Temperature Control system isdiscussed in more detail in (2), and is almostinvariably selected for the atomization phase.

The ATOMIZE phase time is selected sothat all the analyte is atomized, and theatomic absorbance signal returns to thebaseline. Because of the fast temperaturerisetime provided by the use of theTemperature Control feature, this period canbe as short as 2 seconds, and is unlikely tobe longer than 5-6 seconds. Excessivelylong ATOMIZE phases may degrade themeasurement precision when peak areameasurements are used, and will certainlyshorten the cuvette lifetime unnecessarily.

Optimization of the ATOMIZE phasetemperature is usually performed in thesame way as optimization of the ASH phasetemperature, by performing an experimentin which the temperature is systematicallyvaried and examining the effects of this onthe analytical signal. In general, theCookbook atomization temperatures will befound to be suitable for many types ofsamples, but when the sample containslarge amounts of involatile matrixcomponents that cannot be removed in theASH phase, it may be necessary to select atemperature that will minimize the amountof matrix volatilized with the analyte atoms,to minimize both background absorptionsignals and the possibility of vapo phaseinterferences.

The CLEAN phaseThe final phase in the furnace program isintended to remove any residual matrixmaterial from the cuvette, so that it is readyto measure the next sample. The temperatureand time required for the CLEAN phase areobviously dependent on the nature of thesample, but as a rule of thumb, the temperatureshould be at least 2500 ºC, or 100 ºC abovethe ATOMIZE phase temperature, whicheveris greater. However, cuvette temperaturesabove 2800 ºC will result in more rapidwear of the cuvette, and will reduce thecuvette lifetime. They should therefore beavoided if possible.

It is sometimes difficult to confirm thatthe CLEAN phase is properly optimised, asthe consequences of an incorrectly set upCLEAN phase are associated with memoryeffects and longer term drift that will occuras the residues build up in the cuvette.

Practical Steps to MethodOptimisation

Preliminary experimentsIt is usually necessary to perform a fewpreliminary experiments to confirm that theinstrument is set up correctly, and toestablish the feasibility of the proposedanalysis. The type of cuvette to be usedshould be selected, based either on the typesuggested in the Cookbook, or from previouswork and consideration of the nature of theanalyte and sample matrix. Using theaqueous calibration standards (including anyadditional reagents used to stabilize themor matrix match them) it is then important to:• Check the blank levels • Check the precision of the measurements

by analyzing one of the standards severaltimes (at least 6 to 10 measurements arerequired for a meaningful value)

• Run all the standards to check forcalibration linearity and

• Calculate the measured characteristicmass for the analyte and compare it withthe Cookbook value.

The default parameters are normallysuitable for these preliminary experiments.Blank signals should normally be less than0.05 A, and if possible should be <0.01 A.If this is not the case, then steps should betaken to identify the source of thecontamination. Typical sources are thereagents used in preparing the standards(i.e. any acids or matrix modifiers),glassware and any other apparatus used tocarry out the preparation steps. Even the

sample cups used in the furnace autosamplercan cause problems (particularly for lowlevel aluminum, for example). The laboratoryatmosphere is also a source of contaminationand the use of clean boxes or rooms may berequired. Once the source is identified thencorrective action can be taken to overcomethe problem.

Measurement precisions of better than3 to 5 % relative standard deviation shouldbe obtained from the standard solutionswith the FS95 autosampler, when the peakheight signals are in absorbance range 0.1to 0.6 A. If the precision values are poorerthan this (higher RSD values), there are avariety of possible causes that must beinvestigated. The sample injection into thecuvette should be observed carefully, usingeither a dental mirror or the GFTV accessory,and checked for any misalignment, i.e. liquiddeposited on the lip of the injection holerather than inside the cuvette. A commoncause of poor injections is contamination onthe outer surface of the capillary tip. This isoften the result of handling the tip, and cancause the sample to run back up the outsidethe tip instead of being deposited properlyon to the cuvette floor. Wiping the outsideof the capillary tip with a tissue moistenedwith alcohol will usually solve the problem.If the injection is satisfactory, but theprecision remains poor, it is likely that theDRY phase is not set up correctly. The GFTVimage can show if the sample is boiling,indicating that the drying temperatureshould be reduced, or that the sample is notdrying completely, so that either or both ofthe phase temperature and time should beincreased. It is possible, although difficult,to observe the sample drying with adental mirror.

It is important to appreciate that atomicabsorption calibrations are inherently curved,and that the SOLAAR software providessophisticated calibration algorithms tocorrect the effects of this curvature. Thesoftware will identify calibrations where thecurvature is excessive, beyond the correctionability of the algorithm, and will both flagthe data and display a suitable error message.If the FS95 is being used to prepare theworking standards by automatic dilution ofa master standard, then the calibrationlinearity should normally be typical for theanalyte element. Problems here arenormally associated with contamination ofthe diluent solution used, and measuring asample of the diluent alone should confirmthis. It is not possible to correct calibration

data for the effects of a contaminateddiluent, and the only effective remedy is totrace the source of the contamination andremove it. If the calibration standards havebeen prepared manually, it is possible thatconcentration errors and/or contaminationmay be present in one standard solution. Itis often possible to identify the incorrectstandard by visual inspection of the calibrationplot displayed by the software, and facilitiesare provided to allow bad calibration pointsto be removed from the calibration.Nevertheless, it is preferable to identify andresolve the source of the problem.

When the calibration measurementsare complete, the SOLAAR software willdisplay the calibration plot and variousfigures of merit, including the curvature andcorrelation coefficients. It will also calculatethe characteristic concentration (theconcentration equivalent to a signal 0.0044units above the blank measurement). Anexample is shown in figure 9.

Figure 9. A calibration graph display in SOLAARsoftware

This characteristic concentration will bereported in the concentration units that havebeen specified for the calibration standards.To compare the result with the cookbookvalue, the actual mass of the analyteequivalent to the reported characteristicconcentration should be calculated, bytaking account of the volume of solutioninjected into the cuvette. This characteristicmass should typically be close to theCookbook value for the element if peakheight measurements are used. Variationsover the range of 0.5 x to 2 x the Cookbookvalue can be expected, and are due toindividual difference between differentinstruments, hollow cathode lamps andcuvettes, but variations outside this rangemay require further investigation.

Sample measurementsWhen satisfactory measurements of thecalibration standards have been obtained,investigations using the real samplesolutions can begin. If the composition ofthe samples is similar to that of thecalibration standards, then it is likely thatuseful results will be obtained from thesamples immediately, but if there aresignificant differences, it may be necessaryto repeat the optimization of the injectionand the DRY phase.

A sample should be chosen that gives asignal that is somewhere near the middle ofthe calibration graph. If all the samples givevery low signals, then it may be necessaryto spike a sample with the analyte elementso that a measurable signal is producedthat can be optimized - it is not practicableto attempt to optimize very small signals, asthe effects of the changes are obscured bythe noise on the signal. If the signals fromthe sample are very high, it may be necessaryto review the sample preparation procedure,and perhaps increase the sample dilution.

Background correctionSamples analyzed by GFAAS normallyexhibit significant background (non-specific)absorption signals when the wavelength ofthe analyte element is in the ultra-violetregion of the spectrum, below 400 nm.Such background signals can be correctedby the automatic background correctionsystems provided by the spectrometers.

The S Series and the M5 spectrometersare equipped with the Thermo Quadlinecontinuum source background correctionsystem, while the M6 and MQZspectrometers also provide backgroundcorrection based on the Zeeman opto-magnetic effect. The Zeeman backgroundcorrection system is capable of correctingfor certain types of structured backgroundthat cannot be corrected accurately by theQuadline system, but will also degrade boththe sensitivity and the noise levels of themeasurements, resulting in poorer detectionlimits than those obtained from theQuadline system. Background correction isdiscussed in more detail in (6).

Selection of the optimum backgroundcorrection system to use, on spectrometerssuch as the M6 and MQZ that offer achoice, is sometimes difficult. It is suggestedthat the Zeeman system should be used forthe initial investigations, as it is more likelyto give the correct results. Towards the end

of the method development process, anexperiment to compare the performance ofthe two systems should be carried out, assignificantly better sensitivity and detectionlimits can often be obtained from theQuadline system, if it is capable of correctingaccurately for the background signals.

The analytical signals derived from thesample solutions should be examined, andthe size of the background absorbancesignals noted. Both the Quadline andZeeman background correction systems arecapable of correcting for background signalsup to 2 A in peak height, but is preferablethat the background signals should be lessthan this if possible. Large backgroundsignals often go along with significantchemical interferences, as they indicate thata large amount of matrix material is beingvolatilized with the analyte atoms, whichcan result in gas phase reactions takingplace that reduce the analyte atompopulation. There are various techniquesavailable for reducing the backgroundsignals, including the Ash Atomizeexperiment and the use of matrixmodification both described below.

The Ash Atomize experimentAn Ash Atomize experiment permits theeffects of changes in the critical ASH andATOMIZE phase parameters to beinvestigated in a systematic manner, and asthe SOLAAR software provides facilities forperforming the experiment automatically, itshould be considered as an essential part ofany GFAAS method development work.

It is necessary to perform thepreliminary work described above, so thatreasonably stable measurements, withsignals magnitudes well within the rangecovered by the calibration can be obtainedfrom a typical sample solution.

The Ash Atomize experiment itself isset up from the Ash Atomize dialogue boxshown in figure 10.

Figure 10. The Ash Atomize dialogue box.

Either or both of the Ash and Atomizeparts of the experiments can be investigatedin a single run. It is recommended that theATOMIZE phase temperature should beoptimized first, as the optimum is usuallyfairly close to the default value. The starttemperature, end temperature andtemperature increment for the relevantphase can be specified, and the instrumentwill then proceed to make the measurements,and display the results as an Ash Atomizeplot, shown in figure 11.

Figure 11. An Ash/Atomise plot for lead in naturalwaters

It is possible to display the results onan Ash Atomize plot in various ways,showing any combination of the Total,Background and Corrected absorbancesignals, with the data presented in peakheight or peak area measurements, and it iseven possible to add a line showing theeffect of the temperature changes on theprecision of the measurements.

Interpretation of the data presented onthe Ash Atomize plot can be difficult for aninexperienced analyst, and so the softwareattempts to determine the optimum settings.As with all such automatic processes, theseoptima should be considered as a guideonly, and are not a substitute for thoughtfulconsideration by an experienced analyst.The optimum ATOMIZE phase temperaturewill be the temperature that gives themaximum Corrected absorbance signal,consistent with the minimum Backgroundabsorbance signal and the best measurementprecision. It may be necessary to review thepeak shape of the analytical signals toconfirm that clean signals without distortionsor other artefacts are obtained. Finally, thelowest temperature that meets all thesecriteria should be selected as the optimum,and the Furnace Program should be updatedwith this value.

athan the original sample concentrationto allow the differences to easily bemeasured.

The experiment can then be repeated,this time investigating the ASH phase. Thecriteria for selecting the optimum temperatureare similar to those used for the ATOMIZEphase temperature, but it is possible thatthe optimum found may be quite differentfrom the default value. If this is the case, itmay be necessary to re-optimize the ATOMIZEphase temperature using the new value ofthe ASH phase temperature.

Finally, it is often useful to run thewhole experiment again, varying both theASH and the ATOMIZE phase temperaturesover a narrow range around the previouslydetermined optima to refine the choicesmade and obtain a fully optimizedfurnace program.

At this point, it is worthwhile to re-runthe calibration standards again usingoptimized program, to confirm that thecalibration is still satisfactory. It is quitepossible, for example, that the furnaceprogram changes have resulted in a slightlydifferent value for the analyte characteristicmass - this should not cause concern, aslong as the change is moderate.

InterferencesAt this point in the method developmentprocess, it will be possible to obtain a goodcalibration curve and good, reproduciblesignals from the sample solutions. It istherefore possible to perform a full samplerun and obtain an analyte concentrationvalue for the sample solution. The next stepis to confirm that this is likely to be thecorrect concentration value.

There are two well-establishedtechniques for confirming the correctness ofan analytical result. These are:• The analysis of one or more reference

materials• The analysis of one or more spiked

samples

Reference materials (often referred toas Certified Reference Materials or CRM's)are stable, carefully characterized materialrepresentative of the type of sample beinganalyzed. They contain known concentrationsof the analyte elements that have usuallybeen determined by more than oneanalytical technique. They may be producedby external organizations, or may be in-house materials that have been preparedand reserved for this purpose.

It is relatively simple to use a CRM tocheck for interferences that might bepresent in the proposed method. One or

more CRM's should be selected that are assimilar as possible to the actual samplesthat require analysis, and contain theanalytes at approximately the same levels.The CRM's are then prepared and analyzedin the same way as the samples, and thefinal concentration results obtained arecompared to the certified values providedwith the CRM. Most certified values areprovided with a range of uncertainty limitsabout the mean certified concentration, andif the measured result falls within theselimits, then the proposed method can beconsidered to be free of significantinterferences. If uncertainty limits are notprovided, measured concentrations within arange of 90 - 110 % of the certified valueare usually considered to be acceptable.

Spiked samples can be used whensuitable a suitable CRM is not available. Asample is selected that is representative ofthe type of sample being analyzed, and thathas a relatively low concentration of analyte.A portion of this sample is taken, and theanalyte concentration is increased (spiked)by adding a known amount of the analyteelement. The way in which the analytespike is added is dependent on the type ofsample and the sample preparationprocedure. If the sample requires extensivepreparation procedures, it is preferable toadd the spike at the beginning of theseprocedures. The size of the spike should bechosen so that the concentration of thespiked sample remains within the calibrationrange of the method, and is also sufficientlylarger than the original sample concentrationto allow the differences to easily bemeasured.

The analyte concentrations in both theoriginal and the spiked samples can then bemeasured, and the recovery of the spike canbe calculated by subtraction. The measuredspike concentration can then be comparedwith the known added value, and the actualrecovery can be calculated. The actualrecovery of the spike is usually presented asa ratio of the original concentration added.Spike recoveries in the range of 90 - 110 %are usually considered to be excellent, andfor many analyses, a wider range may beacceptable.

The FS95 autosampler is capableautomatically adding a spike to a samplesolution, and the SOLAAR software willcalculate the spike recovery. This provides arelatively quick and simple check forinterferences that affect the actualmeasurement, but will not identify any

problems that may occur in the samplepreparation steps.

Analyte recoveries from either CRM'sor spiked samples that are greater than 110 %or so of the expected values are usuallyindicative of a contamination problemsomewhere in the process. While there area few known GFAAS interference effectsthat result in concentration enhancement(typically those caused by structuredbackground absorbance signals), these arerare, and potential sources of contaminationshould be investigated first.

Low recoveries are indicative of thepresence of interferences. Various strategiesare available to enable these to be identifiedand overcome, which are discussed below,but it is important to recognize that sometypes of interference cannot be completelysuppressed. In these cases, an alternativecalibration strategy, such as standardadditions calibration, may be appropriate, orchanges to the sample preparationprocedure, for example, to remove theinterfering species before the analysis, maybe required.

Interference controlInterferences that can occur in GFAASmeasurements fall into one of threepossible classes. These are:• Spectral interferences, particularly

those caused by background absorbancesignals that display fine structure in theirwavelength dependence

• Physical interferences, where thephysical characteristics of the sample aresignificantly different from thecharacteristics of the standards used

• Chemical interferences, whereresidual sample matrix components reactwith the atomized analyte in theATOMIZE phase, and reduce theconcentration of free atoms availablefor measurement.

Spectral interferencesTrue spectral interferences, where a matrixcomponent has an absorption line thatoverlaps the analyte absorption line, arevery rare in atomic absorption spectrometry,and can usually be ignored. Most instancesof spectral interference are actually causedby matrix components that exhibit broadband,non-specific absorption with fine structureon the same wavelength scale as theatomic absorption line. Continuum sourcebackground correction systems, such as theQuadline system provided by the AA Seriesspectrometers, are unable to correct

accurately for such this type of non-specificabsorption, and correction systems basedon the Zeeman effect are required.

It is sometimes possible to identifystructured background interferences byexamination of the Quadline backgroundsignals displayed by the SOLAAR software.Negative background signals that gosignificantly below the zero absorbancebaseline, especially if they then suddenlyjump to large positive values, are indicativeof problems in this area. Alternatively, theanalysis can be repeated at a differentwavelength if one is available - if the sameor similar concentration results for the samesample are obtained at both wavelengths,then it is unlikely that a spectral interferenceof any kind is present, and if the backgroundsignal shows distortion at one wavelength,but is normal at another, then it is morelikely that the second wavelength will givethe correct result.

If both Zeeman and Quadline backgroundcorrection systems are available, as theyare on the M6 and MQZ spectrometers,then it is an easier matter to identify thistype of interference. All that is required isto measure the same sample using eachform of background correction. If theconcentration results are the same, then itis unlikely that this type of interference ispresent, and Quadline correction will usuallybe the best choice. If the concentrationresults are different, and especially if theQuadline background signal shows unusualdistortion and artefacts that are not presenton the Zeeman background signal, then it islikely that structured background interferenceis present, and the Zeeman result is morelikely to be correct.

Physical interferencesPhysical interferences occur when thephysical properties of the samples are verydifferent from the properties of the standardsused to generate the calibration. Typicalexamples are found in clinical analyses,where the viscosity of body fluids such asblood serum, is markedly different to that ofthe aqueous standard solutions. Thesedifferences affect both the way in which thesample is deposited in the cuvette, and theway in which it is dried.

Low viscosity samples, such as simpleaqueous samples, tend to be deposited in tothe cuvette in the form of a 'pool', wherethe liquid wets the graphite surface andspreads out over the cuvette floor until itreaches the internal ridges in the cuvette

that prevent it from moving out of thecentral hot zone. This is a generallydesirable behaviour, as it ensures thatmaximum surface area of the sample liquidis in contact with the heated cuvette, sothat the solvent evaporation (drying) andsample decomposition (ashing) occur smoothly.

More viscous samples, and those thatdo not easily wet the graphite surface, havea tendency to be deposited in the form of a'drop'. This type of behaviour is lessdesirable, as thermal contact between thesample and the heated cuvette is less good,which can lead to local over-heating duringthe DRY phase, and non-reproducible dryingand ashing behaviour. Problems of this sortcan often be clearly identified from the liveGFTV images.

The FS95 provides facilities to varyboth the speed at which the sample is takenup from the sample cup, and the speed atwhich it is injected into the cuvette, whichcan help to alleviate the effects of this typeof interference. Adding surfactants to thesamples and standards modifies the sampleviscosity and wetting behaviour and canalso help to overcome this type ofinterference. This technique is so successfulthat most published clinical analyses nowinvolve the addition of Triton X-100, a readilyavailable non-ionic surfactant, to the sample,typically at concentrations between 0.1 and0.5 % m/v.

As with any procedure that involvesaddition of reagents to samples, it isimportant to check the contamination levelsof the surfactant before using it. It is alsonormal to add the surfactant to the liquidused to rinse the autosampler betweenmeasurements, to prevent cross-contamination and carryover betweensamples. Surfactants can exacerbate thetendency of some types of sample to foamas they are dried, which can lead toirreproducible results and the accumulationof carbon residues in the cuvette. Theaddition of an anti-foam agent, such as asilicone emulsion, has been suggested as ameans to overcome the foaming problem,and does work successfully in certain cases.

A different type of physical interferencecan occur when the samples to be analyzedare in a non-aqueous solvent, rather thanwater or dilute acid. Many such solventshave very low viscosities and are highlymobile. They spread rapidly along thecuvette floor as they are injected, and insome cases will flow right over the internalcuvette ridges and onwards out of the

cuvette altogether. This causes severeproblems in the measurement, and canresult in significant contamination of thefurnace head itself These highly mobilesolvents are also usually highly volatile, andthis provides a means to overcome theproblem. The GF95 and GF95Z furnace incombination with the FS95 autosamplerallow the cuvette to be pre-heated beforethe sample is injected. If the pre-heattemperature is chosen correctly, it is possibleto evaporate the sample as fast as it isdeposited on to the hot surface, and socompletely prevent the sample fromspreading. This technique has the addedadvantage of concentrating the solid analyteand matrix components in the centre of thecuvette, resulting in excellent sensitivity.

Chemical interferencesChemical interferences occur when matrixcomponents remain after the ASH phase,and either prevent the analyte from beingfully atomized in the ATOMIZE phase (theso-called 'solid phase' interferences), orco-volatilize with the analyte and react withthe free atoms as they are formed ('vaporphase' interferences). Both types ofchemical interference reduce the free atomconcentration in the cuvette during themeasurement phase, so that the result islower than it would be in the absence ofthe interferent.

Chemical interferences are unfortunatelycommon in GFAAS, and there are a fewbasic ground rules that will help the analystto predict possible problems. Refractoryanalyte elements, such as aluminum, whichhave high atomization temperature alsousually have high maximum ash temperatures,so that the interfering matrix componentscan be ashed away at high temperatureswithout loss of analyte and, therefore, thedetermination of these elements is usuallyfree of vapor phase interferences. Vaporphase interferences are more common forthe volatile elements, particularly whenthey are present in sample matrices thatcontain appreciable quantities of alkalimetal salts such as sodium chloride. Thetwin techniques of matrix modificationand platform atomization have beendeveloped to overcome many of theseeffects, and these are described below.

Solid phase interferences are moredifficult to predict and to overcome. Theyare likely to occur when the sample containsa large amount of a matrix component thatwill decompose to a refractory oxide, such

as magnesium or calcium. The analytebecomes trapped in the crystal lattice of theoxide, and is only released slowly, if at all,in the ATOMIZE phase. Methods forovercoming this type of interference usuallyinvolve some form of matrix matching, inwhich an excess of the interferingcomponent is added to both sample andstandards, together with extensivemodifications of the furnace program in anattempt to break down the crystal latticethat is trapping the analyte.

Solid phase interferences can alsooccur when the matrix contains a largeexcess of a metal that is less volatile thanthe analyte. The analyte can then form analloy or solid solution with the matrixcomponent, and it is necessary to increasethe atomization temperature to the point atwhich the involatile matrix component, notjust the analyte itself, is volatilized. Thistype of behaviour is sometimes exploiteddeliberately, in the form of matrixmodification, to control the behaviour ofvolatile analytes.

Matrix ModificationThe term 'matrix modification' describes theprocess of adding a reagent to the sampleto modify the thermal behaviour of thematrix or, in some cases, the analyte duringthe furnace program. Specific reagents canbe used to:• Stabilize the analyte during the ashing

stage to permit a higher ashingtemperature to be used

• Convert a matrix into a more volatile formwhich can be removed at lowertemperature during the ashing stage

• Delay the atomization of the analyte toestablish isothermal conditions inside thegraphite cuvette, which, especially whenused with platform atomization, canovercome many vapor phaseinterferences.

In some situations more than onematrix modifier may be necessary to achievethe desired effect, and the whole topic ofmatrix modification has acquired thereputation of being a poorly understood'black art'. This, of course, is not true, and abasic understanding of simple chemistry canusually be used to predict the effects of aparticular modifier.

The purpose of a matrix modifier is tomodify the thermal behaviour of the analyteand sample matrix. This implies that the useof a modifier will require changes to thefurnace program. The default furnace ASH

phase and ATOMIZE phase temperaturesprovided in the Cookbook are almost allmeasured without using modifiers, and sowill require optimization if a modifier is tobe used. The automated Ash Atomizefunction described above is the best way toperform this optimization, and a full AshAtomize plot from the sample with themodifier addition should always be measured.

Many types of modifiers have beenproposed and examined as the techniquehas developed, and the uses of the mostuseful and common specific modifiers arediscussed below.

1. Nitric acid and ammonium nitrate (0.1 to 5.0 % v/v)

Both nitric acid and ammonium nitrate havebeen used to remove halide salts from thegraphite cuvette during the ASH phase ofthe furnace program. Ammonium nitratereacts with halide salts to form volatileammonium halides that volatilize easily attemperatures of about 350 ºC. Nitric acidreacts to form volatile hydrogen halide gas,which is usually volatilized in the DRYphase. The anions remain as the nitratesalts that decompose readily to the oxidesas they are heated, and the alkali metaloxides then volatilize. Sodium chloride, forexample, will not volatilize until it reaches atemperature of around 1200 ºC. In thepresence of ammonium nitrate, however, anexchange reaction takes place and theammonium chloride volatilizes at 350 ºC,followed by the sodium oxide at about 700 ºC.The ASH phase temperature required toremove the sodium chloride is thereforereduced from 1200 ºC, well above themaximum ash temperature for volatileelements such as lead, to 700 ºC, wherelead is not lost.

Nitric acid is probably the preferredmodifier since it is simple to handle and canbe obtained readily in a purified state. It isalso normally added to samples andstandards to lower the pH and stabilize theanalyte metals in solution. For matrixmodification purposes, nitric acid should beadded until it is just in excess of the halideconcentration present in the sample.Standards should be treated similarly. Highconcentrations of nitric acid should beoptimized carefully, and only the minimumconcentration required to achieve the desiredeffect for a particular sample type should beused. Although the acid can be automaticallyadded to the sample by the FS95 immediatelybefore the sample is injected into the cuvette,

it is more usual to add it during the samplepreparation and dilution steps.

2. Nickel(10 - 50 µg for each injection)

Nickel has been used for many years tostabilize elements such as antimony,arsenic, bismuth, selenium and tellurium inthe graphite furnace. Nickel salts are readilyreduced to nickel metal by the carbonpresent in the cuvette, and the analyteelements react with the metal to formthermally stable compounds, such as nickelarsenide. Many arsenic compounds, such asthe halides and oxides are volatile, andevaporate at temperatures below 500 ºC.Nickel arsenide, on the other hand, is astable compound with a well defined boilingpoint in the region of 1200 ºC. By addingnickel to the sample, therefore, themaximum ash temperature can be raised to1100 ºC or so, which will allow manytroublesome matrix components to beremoved without loss of the arsenic analyte.It is, of course, necessary to increase theATOMIZE phase temperature as well, toensure that arsenic is fully atomized so thatthe maximum sensitivity is obtained.

Nickel matrix modifier is usually addedautomatically by the FS95 to each discretesample aliquot as it is injected into thecuvette. Typically, a modifier solution isprepared from nickel nitrate that containsapproximately 2 % m/v of nickel nitratehexahydrate salt in deionized water, and a5 µL portion of this is added to each injection.As the nickel is added in large excess,neither the actual concentration nor thevolume added are critical.

Other transition metal salts can beused in the same way as nickel, and copperhas been shown to be an effective modifierfor the arsenic group elements. However,these modifiers do have one major drawback.Because they are added in relatively largequantities, the furnace inevitable becomescontaminated with the modifier metal, andthis can cause severe problems if it is laterrequired to perform trace analyses of thesame metal.

3. Ammonium phosphate(50 - 100 µg for each injection)

Ammonium phosphate is used to stabilizeanalytes such as lead, tin and cadmium.The phosphates of these metals arereasonably temperature stable, whichpermits ASH phase temperatures up to 700- 800 ºC to be used without loss of the

analyte. Also, the basic component of themolecule helps to volatilize halides in thesample matrix, in the same way asammonium nitrate.

Ammonium phosphate is available inthe monobasic form ([NH4]H2PO4) or thedibasic form ([NH4]2HPO4). Both are equallyuseful as modifiers for the determination ofelements such as cadmium, lead and tin.The choice between the mono- and di-basicforms depends largely on the contaminationlevels present in the commerciallysupplied materials.

Unfortunately, high concentrations ofphosphate increase the background signallevel, and at some wavelengths exhibitstructured background absorption thatrequires a Zeeman effect backgroundcorrection system, so care should beexercised when assessing the advantagesof this modifier.

4. Palladium(2 - 50 µg for each injection)

Palladium is one of the newer matrixmodifiers to be employed in GFAASanalyses. It works in the same way asnickel, in that the analyte metals aretrapped as solid solutions or alloys in thepalladium metal. It is effective because theboiling point of palladium is high, so thatthe analytes can be stabilized to hightemperatures. Several studies have shownthat palladium is most effective when it isreduced to the metal as soon as possible inthe furnace cycle. It has therefore becomecommon practice to add a mild reducingagent, such as ascorbic acid or hydroxylaminehydrochloride to the palladium solution.These mixed solutions are unstable, andwill gradually deposit metallic palladium inthe sample containers and autosamplercapillary tips. It is possible to use aninternal furnace gas that contains hydrogen- typically 5 % hydrogen in argon, that willalso effectively reduce the palladium. Thistechnique does not require the addition ofreducing agents to the palladium solutions,and so is sometimes preferred.

Palladium is often used in conjunctionwith magnesium nitrate, and this mixturehas been proposed as a 'universal' modifier,suitable for use in all situations. Given thevery wide variety of samples that can beanalyzed by GFAAS, it seems most unlikelythat a genuinely 'universal' modifier exists,and it is generally preferable to investigateand develop the best methodology for eachtype of sample individually.

5. Magnesium nitrate(50 - 500 µg for each injection)

Magnesium nitrate is often used inconjunction with palladium, but has alsobeen recommended for use alone. Thenitrate anion can provide a source of oxygento assist in the decomposition of organicmatrix components, but the more importantmode of stabilization involves the magnesiumoxide that remains. Magnesium oxide has aloose, open crystal lattice, and willeffectively trap other metals within thelattice. These will not be released until thetemperature reaches the point when thecrystal lattice itself breaks down.

While magnesium nitrate does allowthe use of higher ASH phase temperatures,the main benefit is to delay the atomizationof the analyte, and so it is used extensivelywith platform atomization, where both theatomization mechanism and modifier actionwork together to slow the atomization ofthe analyte so that the gas phase temperatureinside the cuvette has stabilized, and vaporphase interferences are minimized.

Platform AtomizationWith a conventional type of cuvette, thecuvette walls are heated by the electriccurrent from the power supply. The gasinside the cuvette is heated by conductionof heat from the hot cuvette walls, so thatthe temperature of the gas lags behind thatof the walls. When an analyte is volatilizedfrom the cuvette wall, it is atomized into agas phase that is relatively cooler than thesurface from which it has been volatilized. Ifthe sample also contains residual matrixcomponents that have not been removed inthe ASH phase, then this lower temperaturecauses the free atoms to re-combine withthe matrix salts to form molecular species,which cannot be measured by the atomicabsorption spectrometer. Analyte atomicconcentration is reduced and hence theinterference manifests itself as a signaldepression when compared to a simpleaqueous standard. Such an effect is knownas chemical vapor-phase interference andaffects mainly the determination of volatileelements in halide-rich matrices.

Platform atomization attempts toreduce or eliminate this type of interferenceby delaying the atomization of the analyteuntil the gas phase inside the cuvette hasreached the same temperature as thecuvette walls. Platform cuvettes contain asmall piece of graphite located inside the

cuvette, on to which the sample is placed.The platform is designed so that it makesvery poor thermal contact with the cuvette,and so is heated mainly by radiation fromthe cuvette wall and conduction from thegas phase. The temperature of the platformtherefore lags behind that of the cuvettewall. When the platform does eventuallyreach the temperature necessary to atomizethe analyte, the gas phase temperature hasstabilised. The analyte is therefore atomizedinto a gas phase that is at the sametemperature as the atomization surface,which reduces analyte molecular formationand, therefore, reduces vapor phaseinterferences.

It is usually necessary to use a matrixmodifier such as magnesium nitrate tofurther delay the atomization of the analyteto obtain the most effective interferencecontrol.

Platform atomization is discussed inmore detail in (3). Figure 12 shows thecross-section of the latest OMEGA platformcuvette that employs ELC technology toproduce long lifetimes and good performancewith near refractory elements such aschromium.

Figure 12. Cross-section of an OMEGA platformcuvette.

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ConclusionsThe development of satisfactory GFAAS methods can be made amuch simpler task by a well thought out, systematic approach.Using a logical, step-by-step procedure, it is possible to tackleproblem areas one at a time, and thus ensure that the finaldeveloped method represents the optimum for that sample. If thesesimple guidelines are followed, a reliable working method will beobtained and poor quality data avoided.

References1. Design Features of the GF95 Graphite Furnace Accessory. Thermopublication PS407012. Design Considerations for a new Platform Cuvette for GraphiteFurnace Atomic Absorption Spectrometry. Thermo publicationPS407043. A A Brown and P J Whiteside, Intl. Labmate, 1984, 9 Issue 3,June.4. A A Brown, Anal. Chim. Acta, 1985, 175, p319.5. Design Considerations for High Performance BackgroundCorrection Systems in atomic Absorption Spectrometry. Thermopublication PS40690