Journal of Automatic Chemistry, Vol. 10, No. 3 (July-September 1988), pp. 130--134

Acquisition and analysis of GFAAS data

M. J. Adams,* G. J. Ewen and C. A. ShandThe Macaulay Land Use Research Institute, Craigiebuckler, Aberdeen AB9 2QJ,UK

Since its inception as an analytical technique some 30 years agoatomic absorption spectrometry has become a jqrmly establishedmethodfor the analysis of trace metals. Graphite furnace atomicabsorption spectrometry provides the analyst with the capability ofanalysis ofsolutions containing g l-1 levels of the analyte, but,because of the transient nature of the signals, a sophisticatedapproach to the data aquisition and handling ofdata is required.Most modern commercial graphite furnace atomic absorptionspectrometers have built in microprocessorsfor this purpose but theyoften have limited capability for extensible user programs andlimited data storage facilities. In this communication we describethe use ofan Apple lie microcomputerfor the acquisition ofdatafrom a Pye Unicam SP9 graphite furnace atomic absorptionspectrometer. Details of the interface which utilizes an in-housedesigned AD converter, and an overview of the Pascal andassemblerprograms employed are given. The system allows the userto record, store and dump the graphical display of the furnacesignalsfor all analyses performed. Files containing details ofpeakheight, and area are formatted on an eight-column spreadsheet.Details ofsample type, concentrations ofstandards, dilutions andreplication are enteredfrom the keyboard. The calibration graph isconstructed using a moving quadratic fit routine and theconcentrations of the analyte in unknown solutions calculated. Inaddition to this, greater processing power and integration of thedata into other analytical schemes can be achieved by exporting thedata to other software packages and computers. Details of datatransfer between the Apple lie and an AmstradPC 1512 are given.Some examples of the use of the system in the development of ananalytical methodfor silver in plant material are given.

Introduction

Atomic absorption spectrometry (AAS) is an establishedanalytical technique, which is routinely employed inmany analytical laboratories for the determination oftrace quantities of metals in solutions [1]. Since itsdevelopment by Walsh over 30 years ago the growth inthe application of AAS has become part of the trendtowards increasing use of instrumentation in chemicalanalysis. The first commercial AAS instruments wereintroduced in the early 1960s and generally employedflames as cells for the production of analyte atoms. Theflame is still the most common atom cell for routineanalyses and, in general, analytical sensitivities rangefrom g 1-1 to mg -. Ifgreater sensitivity is required, orthe sample size is inadequate for nebulization into a

Present address: Dr M. J. Adams, School of Applied Sciences,The Polytechnic, Wulfruma Street, Wolverhampton WV1 1SB,UK.(C) 1988 The Macaulay Land Use Research Institute.

conventional flame system, then non-flame techniquescan be employed. Of these, the most widely used isgraphite furnace atomic absorption spectrometry(GFAAS). An increase in sensitivity of between one andthree orders of magnitude can be realized for mostelements using GFAAS compared with flame AASprocedures. GFAAS has some disadvantages comparedwith flame AAS. The duration of the analytical signalfrom a flame system can be extended to any convenientperiod, provided there is sufficient sample, and the signalcan be integrated over this time to achieve a highmeasurement precision. The signal produced by elec-trothermal excitation in GFAAS, however, is transitoryand requires a more sophisticated approach to thetechniques of data capture and recording. Continuedimprovements and refinements of instrumentation, inparticular the application of microelectronic technology,have removed or reduced many of the problems asso-ciated with early GFAAS techniques. Most moderncommercial instruments used for GFAAS have somebuilt-in microprocessor-controlled unit for recording theanalytical signal and many have sophisticated graphicdisplay output facilities and video screen to show thesignal trace, the shape of which can be a useful aid inoptimizing and monitoring the analytical procedure.

The built-in microprocessors associated with most com-mercial analytical instruments are programmed to attendto routine tasks and have only limited capability forextensible user programs and, usually, they provideminimal data storage facilities. Where greater dataprocessing facilities are required and there is a need tostore data and provide an integrated data managementscheme within a laboratory environment, then interfacingthe instrument to a personal microcomputer can presentthe user with a more flexible and extensible analyticalsystem. A wide variety ofinexpensive personal computersare available and most are capable of being interfaced toanalytical instrumentation. One such machine, whichhas found extensive application in many laboratories, isthe 8-bit Apple II microcomputer.

In this communication we describe the use of an AppleIIe microcomputer for the acquisition and recording ofanalytical data directly from a Pye Unicam SP9 VideoFurnace AAS system (Pye Unicam Ltd, Cambridge,UK). The details of the computer-spectrometer interfaceare provided and the use of the system is illustrated withtypical results obtained from an optimization study ofthedetermination of silver in plant material. An overview ofthe Pascal and assembler programs employed in the datalogging and manipulation procedures is provided. Thetransfer of data acquired by the Apple IIe from thespectrometer to a second microcomputer operating a

commercial spreadsheet and graphics programs is alsodescribed.

130

M. J. Adams et al. Acquisition and analysis of GFAAS data

SP9

Spectrometer

Re corder Output(0-I V)

RecorderTrigger

COM

PU 9095VideoProgrammer

TTL(games)I/O

12-bit

AD

Serial(RS232C)I/O

Apple lie

PC1512

Figure 1. The general components of the computerized GFAAS system and the interfacing links.

Instrumentation

The commercial atomic absorption spectrometeremployed is a Pye Unicam SP9 system complete with aPU9095 video furnace programmer, a PU9090 datagraphics system and SP9 furnace autosampler. The SP9is a single-beam spectrometer in which backgroundcorrection for non-specific absorption is achieved with theaid of a deuterium-filled hollow cathode continuumsource which is pulsed alternately with the analyte’shollow cathode lamp. The intensity of the analyticalsignal output from the photomultiplier is converted toabsorbance using a logarithmic amplifier and this signalis output to the data graphics system. In conjunction withthe video furnace programmer this data processingsystem can display ’cookbook’ details of analyticalconditions for the elements determinable by AAS. Inaddition, the graphics facilities of the video programmerpermit the display of the transient peaks observed inGFAAS studies and the calibration plots for the analy-tical standards employed. Autosamplers are a desirableaddition to any GFAAS system both to improve measure-ment precision and enable automation of the analyticalprocedure to be achieved. The SP9 furnace autosampleremployed here accepts up to 38 sample cups and twowash cups.

The microcomputer employed to interface to the SP9system was an Apple IIe. This microcomputer hasproved to be a popular choice for laboratory applicationslargely because of the ease with which interface andexpansion circuits can be accommodated within themicrocomputer. The Apple IIe machine uses a 8-bitmicroprocessor (type 6502) as the central processing unitand can directly access up to 64K of memory, which canbe expanded to 128K using page-switching techniques.As standard, the computer was equipped with dualfloppy disk drive units, a video monitor and an EpsonFX-80 printer. Interface circuit cards were installed inthe computer using its backplane bus structure; several

edge connector slots are available for these boards. All thecomputer software was written under the Pascal UCSDOperating System.

Interfacing

The general interfacing scheme is illustrated in figure 1.The .spectrometer was connected to the microcomputervia an analogue-to-digital (AD) converter within thecomputer and its so-called games socket which served as aTTL trigger detector. The AD converter was designedand constructed in-house and has been described in detailelsewhere [2 and 3]. It comprises a precision 12-bitCMOS AD574 integrated circuit (Analog Devices) whichcan complete a 12-bit data conversion in about 25 ts.This device and all other components, including the X10signal amplifier, were mounted on a single circuit boardfor direct connection to the computer using one of itsinput/output expansion slots. The AD converter wasoperated in a unipolar mode, 0-10V FSD. The analogueanalytical signal was taken directly from the spec-trometer. Normally, this signal is supplied as a voltage inthe range 0-10 mV, corresponding to 0-1 AbsorbanceUnits, for use with a standard analogue chart recorder.To eliminate potential problems due to electrical pick-upon the spectrometer-computer cable the absorbance andscale expansion circuit board within the spectrometerwas modified to provide a 0-1V analogue output. Thiswas achieved by bypassing an internal signal attenuatorcircuit.

In employing analogue signals for digital data acquisitionprograms, special care must be taken to control theinitiation of the data logging procedure and the rate atwhich data is accumulated. In the application discussedhere the cycle start pulse was obtained from the recordersocket of the video furnace programmer. This.signal isconventionally used to activate a high-speed chartrecorder, which can be employed to monitor the transient

131

M. J. Adams et al. Acquisition and analysis of GFAAS data

analytical trace. This signal (0/5V TTL) was led to thegames socket of the Apple computer. The rate of dataacquisition was under program control and selected toachieve about 450 readings in the maximum 9s analyticalmeasurement period.

The microcomputer interfacing and data acquisitionsoftware was written in Pascal with the AD conversionroutine being prepared as assembly code and called fromthe main program as an external function. Once initiated,this program strobes the computer keyboard and theTTL (games) socket to test if a quit key has been pressedby the operator or if an analytical program has beenstarted from the video programmer. Ifan analytical cycleis initiated then digitization and recording begins. Thespectrometer system is programmed to provide a 7sautozeroing period prior to firing the graphite furnaceand during this period 350 digital readings are recordedand averaged by the Apple computer to provide a meanbackground reading which is subtracted from each pointof the resultant analytical AAS trace. The AAS data isrecorded until the trigger signal is deactivated asprogrammed at the video furnace programmer. Follow-ing each analytical cycle the Apple computer scales theAAS data and presents the complete trace on its graphicsdisplay unit along with the maximum signal recorded(peak absorbance) and the signal area (absorbances). Thetrace data is stored in a disk file after each analysis tominimize data loss should there be a program crash orany loss of electrical power to the computing system. Thewhole procedure is repeated until the quit key is pressedby the user. When the data acquisition program is exitedthe analytical data in the disk file is re-formatted toprovide a data file containing the peak height and areadata and a graphics file containing the original data forthe profile of the trace. These files can be used forsubsequent calibration and manipulation programswithin the Apple computer or exported to other computersystems. A second computer, an Amstrad PC1512, wasemployed for off-line processing and management of theanalytical data. The Apple microcomputer was connec-ted to the PC via a serial, RS232C, interface link. Datatransferred to the PC could be studied using a wide rangeof commercial software packages and integrated withother analytical and management data in this secondmicrocomputer.

Data processing and analysis

The principal data processing software for manipulatingand analysing the pre-recorded GFAAS data is anin-house program, DataCalc. DataCalc loads the analy-tical data into computer memory and displays it a screen’page’ at a time, on the video monitor. The format of thedisplay is similar to many of the common spreadsheetprograms, viz. the analytical data occupies discrete rowsand columns in the screen. The user can move a cursorfrom cell to cell between columns in a record and betweenrecords or rows. The data can be edited, listed on theprinter and saved back on floppy disk for storage andfuture processing operations. Data processing commandsinclude the determination of the analytical calibrationcurve and fitting the sample absorbance data to this

132

DataCalc

Hove to cell

<ESC Move to record

<CR Edit current cell

<L> Print spreadsheet contents

<P> using peak height-<F> Fit Data<C Cslibration <A> using peak <Q> Quit

<S> Display graph of trace of current record

<P> Print last graphical image (trace calibration)

<Q> Quit DataCale

Transmit spreadsheet

DataXmit Transmit graphical trace element

Quit DataXmit

Figure 2. The command structure of the Pascal DataCalcspreadsheet program used to analyse the GFAAS data.

curve. The command structure of DataCalc is illustratedin figure 2.

As well as the date and filename, displayed on the top rowof the screen, up to 16 records (rows) of analytical data,formatted in eight columns are displayed at any time.These data columns provide the record (sample) number,class, replicate code, dilution factor, analytical signalpeak height and area and the computed, or user-entered,concentrations. The record class defines the nature of thesample as being a standard, unknown, blank or null. Anyrecord marked by the user as being null is ignored by thecomputer in subsequent calculations. Analyses byGFAAS are rarely performed singly. Sample duplicationis usually encountered and to indicate this replication, theprogram permits an integer replicate code to be used toindicate similar solutions, the absorbance or concentra-tion values of which will be automatically averaged. Thepeak height and area fields contain integer valuescorresponding to the analytical instrument’s signal out-put, as digitized by the computer (these values can beentered by the user ifthe program is used in isolation fromthe spectrometer). The concentration column containseither values for standards keyed-in by the user or valuescomputed following calibration.

The major function ofthe DataCalc processing package isto compute an analytical calibration curve from thestandards’ data and calculate previously unknown sam-ple concentrations by interpolating their measured signalabsorbance values on the standard curve. The set ofstandards’ records is sorted into an order of increasingconcentration, using a simple bubble-sort routine, andthe curve-fitting coefficients are calculated. The proce-dure adopted to compute the calibration graph is similarto that employed by the spectrometer manufacturers anddetailed by Whiteside et al. [4]. The average absorbancevalue determined from all designated blank solutions issubtracted from all other absorbance data and a straightline is fitted between zero and the first standard. Aquadratic function is then fitted between every subse-quent pair of data points, a third fitting point beingcalculated using extrapolated slopes. This ’moving-quad-

hi. J. Adams et al. Acquisition and analysis of GFAAS data

O. 77

b

S I LVER

C oncn. (lolob) 38

Figure 3. Screen dump ofcalibration curve for silver.

ratic’ curve-fitting procedure ensures the curve followsthe standards and can be used for complexed shapedcalibration graphs. Once calculated, the general form ofthe calibration curve is displayed on the high-resolutiongraphics screen ofthe monitor. Ifany standard data pointis obviously in error, the procedure may be repeated afterediting the data by classifying the offending data as a nullrecord. If the curve is accepted then the unknown datapoints are fitted automatically to the curve and thesample concentration values computed and the resultsdisplayed on the screen. The calibration procedure can beundertaken with peak height or area data, as selected bythe user.

In addition to these data processing functions thesoftware allows the user to examine the GFAAS profile ofany of the records by loading the appropriate data fromthe disk-based graphics file. The display of this data, orthe displays of the analytical calibration curves, can beoutput to the printer. A printed ’screen-dump’ of thecalibration curve for the determination of silver by AAS isillustrated in figure 3. The facility to recall and displayabsorbance profiles on screen allows the operator to

quickly examine results from the unattended analysis ofabatch of samples to check, for example, that theabsorbance profiles of samples and standards are closelysimilar and that there are no spurious signals. Thisprovides the analyst with a degree of confidence thatcannot be obtained from a simple numerical value.

The functions performed by the DataCalc software on theApple computer are sufficient for most ofthe routine tasksperformed with the GFAAS system. Greater processingpower and integration of the AAS data into a morecomprehensive analytical scheme can be achieved byexporting the data and results to other software packagesand computers. This can be readily achieved by trans-mitting the data as an ASCII file via a serial RS232Cinterface from the Apple to a second computer. For mostof the time this host machine can be operated indepen-dently of the Apple system and will, typically, be runningcommercial spreadsheet, database, word processing andstatistics packages.

A Pascal program can be selected to transmit theanalytical data from the Apple machine to the PC system.The user.is prompted to select the transfer of absorbanceprofile graphics, data or the analytical spreadsheet data.Communication is achieved between the two microcom-puters using ’Kermit’, a de facto standard protocol forinter-computer information transfer, operating on thePC. Controlled by the PC the data is output from theApple and recorded as a text file on a PC disk. This data isavailable for subsequent analysis.

Applications

The main function of the computerized system is tocollect raw data from the atomic absorption instrument,compute the analytical calibration curve from the stan-dards and calculate the concentration of the analyte inunknown samples. In addition to this routine operation,however, the facilities offered by the microcomputer arevaluable during the development of new furnace analy-tical programs. An example of the use of the system in thedevelopment and application of a method for the analysisof silver in nitric acid extracts ofashed sphagnum moss isdescribed.

The first stage in the development of GFAAS methodsoften involves a series of measurements with standardsolutions of the analyte to determine the optimumconditions for the pyrolysis and atomisation steps of thefurnace program. Figure 4 shows the profiles for theatomization of 0.1 ng Ag (applied as AgNO3 in 2%HNO3) at 1800 C from a totally pyrolytic graphite tubeand platform combination following pyrolysis at differenttemperatures.

A comparison of the profile obtained at a pyrolysistemperature of 340 C with that at 490 C shows theappearance of an additional early peak. This is indicativeof a slight loss of silver from the platform to the tube wallduring pyrolysis. This deduction was confirmed by theatomization ofAg from the wall alone, see figure 4(d). Asthe pyrolysis temperature is increased further, to 685 C,there is in addition to loss from the wall to the platform, a

substantial loss ofanalyte from the atom cell resulting in adecrease in peak height andarea. The pyrolysis tempera-ture selected for further work on Ag, applied as nitrate,was 340 C.

The use of a host computer running a commercialspreadsheet program ’Symphony’ is illustrated in figure5. This shows an overlay of the absorbance profiles for theatomization of0" ng Ag at different temperatures. In thiscase the data has been re-scaled (DataCalc scales allpoints to fill the page) using the spreadsheet commands ofSymphony. The efect of increasing the atomizationtemperature on the relative positions and shapes of thesignals is clearly illustrated. As the atomization tempera-ture is increased peak height increases sharply and noplateau is reached in the range studied. Peak areameasurements exhibit a slight decrease as the tempera-ture is increased, probably due to increased diffusionlosses. For subsequent analyses a compromise atomiza-tion temperature of 1800 C held for a period of 5s was

133

M. ]. Adams et al. Acquisition and analysis of GFAAS data

0.34

lme (s)

Figure 4. Absorbance profiles for atomization of 0"1 ng Ag at1800 C. (a) following pyrolysis at 340; (b) 490; and (c) 680Cusing a totally pyrolytic tube and platform; and (d) followingpyrolysis at 340 C using a tube alone.

used. This example of the use of a powerful spreadsheetprogram such as Symphony serves to illustrate theprinciple ofdata transfer and use within a laboratory andhas many potential applications. Where a detailed studyor mathematical treatment of absorbance profiles is to beconducted, for example kinetic modelling of the atomiza-tion process, a variety of commercial programs areavailable. Similarly, the range of data-base packagescommercially available for microcomputers makes thein-house development of such programs largely unneces-sary. They can form the basis ofsolphisticated laboratorydata management systems which can readily link to thededicated microcomputer application as detailed here.

Conclusion

A microcomputer system has been described whichinterfaces to a commercial electrothermal atomic absorp-tion spectrometer for the direct acquisition and subse-

0,4

0.3

0.2

0.1

tlme

Figure 5. Overlay ofthe absorbance profilesfor the atomization of0"1 ng Ag. (a) at 1500; (b) 1800; (c) 2100; and (d) 2400 Cusing a totally pyrolytic tube and platform.

quent manipulation of the analytical data. As describedabove, an important advantage of this computerizedsystem is that a full record can be maintained of theabsorbance versus time profiles for all the solutionsanalysed. This information is invaluable in designing newanalytical procedures when it is necessary to monitor theeffects on the analytical signal of, for example, differentashing and atomizing temperatures. The computerizedstorage of the graphic trace information also permits theuser to check an analysis where a fault is suspected tohave occurred in the graphite tube system.

The absorbance peak height and area data are recordedin the computer system for calibration and analysis usingspreadsheet-type software. In many cases no furthermanipulation of this data is required other than thatsupplied by the in-house designed programs. Where theAAS analysis, however, forms a part ofa wider analyticalscheme involving other techniques and other labora-tories, the GFAAS data recorded by the microcomputersystem can be transferred via a simple asynchronousserial interface to other computer systems. An example ofGFAAS data manipulation using a commercial spread-sheet package on a PC computer has been presented.

References

1. VAN LOON, J. C., Selected Methods of Trace Metal Analysis(John Wiley and Sons, New York, 1985).

2. EWN, G.J. and ADAMS, M.J., Laboratory Practice, 33 (1984),116.

3. ADAMS, M.J. and EweN, G.J., Journal ofAutomatic Chemistry,6 (1984), 202.

4. WmTeSm, P. J., S’rocI,:DaI., T. J. and PRicv., W. J.,Spectrochimica Acta, 35B (1980), 795.

134