asimple and versatile interface to feed analogue data from...

Journal of Automatic Chemistry/Journal of Clinical Laboratory Automation, Vol. 9, No. 3 (July-September 1987), pp. 132-134

A simple and versatile interface to feedanalogue data from the output of analyticalinstruments to a BBC microcomputer

L. Ebdonf, J. I. Garcia Alonso*, S. J. Hill and A.HopkinsDepartment of Environmental Sciences, Plymouth Polytechnic, Drake Circus,Plymouth, Devon PL4 8AA, UK

An interface is described to convert analogue datafrom the recorderor other output of a typical analytical spectrometer, or otherinstrument, to an amplifiedform suitable forfeeding to the ADCofa microcomputer (BBC B+). The interface is simple and easyto construct, avoids complicated software, and can be used for avariety of applications. Excellent linearity was observed intransformed data and the interface is particularly suited tohandling transient signals. Examples of its use in atomicabsorption spectrometry, solution spectrophotometry and highperformance liquid chromatography are given.

Introduction

In spite of the extraordinary development of computersand computer-assisted analytical instrumentation thereis still a widespread use of chart-recorders to recordanalytical data. In a brief survey of leading journals inanalytical chemistry published during 1986 we observedthat chart-recorders were used in over 40% of the paperswith high performance liquid chromatography (HPLC)applications and in over 80% of the papers usingflow-injection analysis (FIA). This is surprising given thelimited data-handling facilities of chart-recorders, thetime-consuming and manual nature of the interpretationof results, the inherent noise problems and difficulties insignal filtering and the ready availability of inexpensivepersonal computers, competitively priced compared tochart-recorders. While there has been considerablegrowth in the use ofpurpose-designed computing integra-tors for chromatography, the lack of versatility inherentto many commercial integrators and computer-controlledinstruments makes the development of new analyticaltechniques which use discrete sample introduction diffi-cult. For example, the electrothermal vaporization ofanalyte into the inductively coupled plasma (ICP) ormicrowave induced plasma (MIP) generates a discreteanalyte emission signal not always totally suitable for achart-recorder because of the required speed of response,yet chart-recorders are still used in about 60% ofapplications.

Author to whom correspondence should be addressed.*On leave from Department of Analytical Chemistry, University ofOviedo, Spain.

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In recent investigations into the development of a newinterface for directly coupled high performance liquidchromatography- flame atomic absorption spectroscopy(HPLC-FAAS), based on a series of rotating Pt spirals,which transport discrete aliquots of sample to a flameatomizer [1-3], it was found that the use of a chart-recorder offered only limited data handling facilities.Ideally transient non-atomic absorption peaks arisingfrom introduction of the spirals into the flame would beeliminated, deviations in the background as the flamegases are disrupted corrected, small variations inresponse obtained from individual spirals compensatedfor and peak heights and areas computed. To achieve thisaim the analogue recorder output from an SP-9 AtomicAbsorption Spectrometer (Pve Unicam Ltd, Cambridge,UK) was interfaced to a BBC B+ microcomputer (AcornComputers Ltd, Cambridge, UK). In adctatlon, control ofvarious instrumental parameters, such as the rotation ofthe stepper motor [2], was incorporated into the software.We believe that this simple, cheap and reliable interfacehas many useful applications in those fields where the useof basic chart-recorders limits the value of the dataobtained.

Interface characteristics

The analogue voltage from the recorder output of theSP-9 spectrometer (10 mV) was amplified 100 times andthe signal was then fed to the analogue-to-digitalconverter (ADC) of the BBC microcomputer which canconvert signals up to 1.8 V into digital values between0-4095. Only one channel was used in the ADC (up tofour channels are available) facilitating 100 conversions/s. The digital values can be read and stored by a simpleBASIC program using the X ADVAL(1)/16 com-mand.

Figure shows the electronic circuit of the interface.Power supply (+5 V) was obtained from the computervia the analogue input port. The circuit consists ofa highimpedance MOS/FET operational amplifier, connectedin a non-inverting mode. Gain (R2 + R1)/R2.

Maximum output (P1) is limited to half the supplyvoltage less 0’ V, a potential divider network is used toreduce this to a maximum of 1.7 V (P2) in order to protectthe input to the ADC.

The performance of the interface was evaluated with theatomic absorption spectrometer. Figure 2 shows theexcellent linearity observed between absorbance and

L. Ebdon et al. Interface for feeding analogue data to a micro

+5v

p o

c5.6k

1Ok iFigure 1. Circuit diagram for the interface.

270

270.L

Ov

uO P2

transformed digital values both with and without scaleexpansion. The electronic noise of the interface wasdetermined to be constant at +2 units between levels of100 and 700 units. Figure 3 shows the transient atomicabsorption peaks obtained for mercury, cadmium, and

1200

1600

800

400

0.4 0.8 1.2

Absorbance

lead with the HPLC-FAAS coupling. As can be observed,the interface has no difficulty in following the peakprofiles, even for rapidly atomized elements such asmercury (i.e. very transient peaks).

Applications

Apart from possible educational applications, the newinterface can be used in many different fields, includingcollection of spectrophotometric and fluorimetric spectra

Time(s)

Figure 2. Linearity of the response of the interface shown in theconversion of digital units to absorbance (Q)) direct absorbance;([) 8.3 scale expansion.

Figure 3. Transient atomic absorption peaks for Hg, Cd and Pbfrom a platinum spiral introduced into the flame as displayed viathe interface and computer on a visual display unit.

133

L. Ebdon et al. Interface for feeding analogue data to a micro

800

._tl

Time (rain)

spectrophotometric detection at 475 rim. Eluent 85% methanolwith 2 x 10-4 M EDTA and 6 x 10-3 M acetate buffer at pH

4"1. Flow 0"4 ml rain-1, column 250 x 2 mm ShandonODS-hypersil 5 tm tmrticle size. Injection (20 tl) as dithizonecomplexes in methanol. (A) Dithizonw, (B) Methyl mercurydithizonate, (C) Mercury (II) dithizonate.

with ODS, 5 tm particle size. The output from thespectrophotometric detector (set at 475 nm) was fed tothe BBC micro computer and the signal integrated over

s. Once the chromatogram has been obtained andstored on disk, new possibilities are open for themanipulation, quantification and comparison of chro-matograms. Other possible applications are numericaltreatments of ’FIAgrams’ and in coupled techniqueswhere the data are obtained as transient peaks (forexample FIA-preconcentration-AA/ICP, electrothermalvaporization into plasmas, coupled chromatography-spectrometry).

Further information on the interface and computerprograms for the BBC can be obtained from the authors.

Acknowledgement

Dr J. I. Garcia Alonso wishes to thank the FICYT(Oviedo, Spain) for the provision ofa post-doctoral grant.

References

when numerical treatment of the data is necessary, andthe collection, storage and manipulation of chromato-graphic data. Figure 4 shows a chromatogram obtainedfrom the separation of CH3Hg+ and Hg2+ as dithizonecomplexes [4] on a minibore (2 mm id) column packed

39TH PITTSBURGH CONFERENCE AND EXPOSITION ON ANALYTICAL CHEMISTRY ANDAPPLIED SPECTROSCOPY22 to 26 February 1988: New Orleans, Louisiana, USA

The technical programme for the 1988 Pittcon will consist of 30 symposia and c. 900 contributed papers. Postersessions will supplement oral presentations on the latest developments in analytical chemistry and appliedspectroscopy.The progamme committee will consider 500-word abstracts for inclusion from now to 3 August 1987. Papers are

1. HILL, S., EBDON, L. and JONES, P., Analytical Proceedings, 23(1986), 6.

2. EBDON, L., HILL, S. andJoN.S, P.,Journal ofAnalyticalAtomicSpectrometry, 2 (1987), 205.

3. EBDON, L., HLL, S. andJoNEs, P.,Analyst, 112 (1987), 437.4. LAOSETH, W., Analytica Chimica Acta, 185 (1986), 249..

welcome in the following areas"

Atomic absorption spectroscopyAutomated analysisBiochemical analysisBiomedical pharmaceuticalClassical chemical analysisClinical chemistryComputer applicationsCountercurrent chromatographyElectrochemistryEmission spectroscopyField flow fractionationFlow injection analysisFluorescence-luminescenceGas chromatographyGel permeation chromatographyIndustrial hygieneInfrared spectroscopyIonchromatographyLaboratory roboticsLaser applications in spectroscopy

Liquid chromatographyMass spectrometryNear infraredNew instrumentationNuclear magnetic resonance spectroscopyParticle size analysisPesticide analysisPlasma emission spectroscopyPolymer analysisProcess stream analysisRaman spectroscopySelective ion electrodesSupercritical fluid chromatographySurface analysisThermal analysisThin layer chromatographyToxicological analysisTrace analysisUV-VIS spectrophotometryX-Ray diffraction/fluorescence spectroscopy

Abstracts should be sent to Mrs AlmaJohnson, Program Secretary, 12 Federal Drive, Suite 322, Pittsburgh, Pennsylvania 15235,USA.

134

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