Analyst, May, 1973, Vol. 98, $9. 343-346 343 A Simple Time-delay Accessory for Use with Micro-scale Sampling Atomic-absorption Techniques BY H. T. DELVES AND R. B. REESON (The Hospital for Sack Children and Institute of Child Health, Great Orwaond Street, London, W.C.l) A time-delay accessory for use with micro-scale sampling atomic- absorption spectrophotometry is described that facilitates the interpretation of chart recordings as a first step towards the automatic calculation of results. THE current concern with environmental pollution has resulted in large numbers of people being screened for excessive exposure to lead by means of blood-lead analyses. Micro-scale sampling and flameless atomic-absorption method~l-~ are becoming in- creasingly important for this application because of their low sample volume requirements.One such technique, the Delves cup method,3 has been used successfully at the Hospital for Sick Children for the past 3 years. This and other similar methods produce two or more signals from the combustion and atomisation of each sample, of which only one is the lead atomic-absorption signal. No difficulty arises in discriminating the lead atomic-absorption from the non-specific signals that precede it as they are adequately resolved in time. How- ever, the interpretation of chart recordings with multiple signals from each sample can be tiring and time consuming when the analysis rate approaches 200 tests per day. It was therefore decided to automate the interpretation of the chart recordings and the calculation of the results that are based not only on peak-absorption values but also on integrated absorption signals.The first stage in this project, which is reported here, was the development of a time-delay circuit to eliminate the recording of the non-specific absorption signals that are produced during the combustion of the sample. This elimination is essential for the successful integration of the lead signal and although it can be achieved by using background- correction accessories with a continuum source, these are expensive and require the use of double-beam instruments or single-beam instruments designed for that purpose. The device described here is simple, inexpensive and can be used with single- or double-beam instruments. This delay circuit will eliminate the recording of all the non-specific absorption signals from the combustion products of the sample that precede the lead signal.It will not eliminate any non-specific absorption signals that occur simultaneously with the lead signal. The delay circuit, which is shown schematically in Fig. 1, consists of two coupled inte- grated-circuit monostable multivibrators, which switch the mains supply to the recorder drive motor or to the recorder directly, and subsequently switch the input signal to the recorder. Initiation of the timing sequence is accomplished by closure of a microswitch that is actuated by the sample cup carrier. The total time of running of the recorder is fixed at 15 s, whereas the delay is continuously variable between 0-4 and 3-6 s; during this delay time a pre-set reference voltage is fed to the recorder to give a deflection equivalent to 100 per cent.transmission. A by-pass switch enables the chart recorder to be run continuously. RESULTS A number of blood-lead determinations were carried out by the method described previously .3 Fig. 2 shows that increasing the delay time before recording the absorption signals has no effect upon the peak lead atomic-absorption signal but decreases the non-specific signal to zero at delay times of greater than 2.4 s. The optimum delay time was 2.6 s, and this delay was used to establish the results shown in Fig. 3. These results show clearly that the chart recordings are simplified and are much easier to interpret. CONCLUSION The time-delay accessory described here facilitates the interpretation of chart recordings produced by micro-scale sampling atomic-absorption techniques.This development should @ SAC and the authors.344 DELVES AND REESON: A SIMPLE TIME-DELAY ACCESSORY FOR USE [Analyst, VOl. 98 TMay, 19731 WITH MICRO-SCALE SAMPLING ATOMIC-ABSORPTION TECHNIQUES 80 c; 5 70- u Q) a L < 60- C 2 g , 50- 5 m C - 2 40- 3 c(J w m 30- .- + ff $4 g 20- 10- 40 - 30 20 10 - - - 1.0 2-0 3.0 5 0 t ' " " ' " ' - Delay time/s Fig. 2. The effect of delay in the start of recording on absorption signals for blood-lead analysis. A, lead atomic-absorption signal ; and B, non-specific absorp- tion signal from combustion products of blood sample. Each point is a single determination on 10 1-11 of blood containing 140 pg of lead per 100 ml I A L 6 Sample I B !3 345 Sample II A Pb / \ L B Fig 3.Absorption signals from the micro-sampling atomic-absorption spectrophotometric determination of lead in blood. A, no time delay used; B, delay time of 2.6 s before recording signals; sample I, 140 pg of lead per 100 ml; and sample 11, 20 pg of lead per 100 ml346 DELVES AND REESON prove useful to workers carrying out large numbers of blood-lead analyses, and in particular, to those who use single-beam instruments that cannot be equipped with background-correction accessories with a continuum source. We thank Professor Barbara E. Clayton and Dr. G. Pampiglione for their interest in this work. One of us (H.T.D.) gratefully acknowledges financial support from the Medical Research Council. REFERENCES 1. 2. 3. Massman, H., Spectrochim. Acta, 1968, 23B, 215. West, T. S., and Williams, X. I<., Analytica China A d a , 1969, 45, 27. Delves, H. T., Analyst, 1970, 95, 431. Received October 16th, 1972 Accepted January 12th, 1973 Appendix LIST OF COMPONENTS Variable resistors- = 5 kS2, wire-wound = 1 kS1, pre-set VRl VR, Capacitors- Cl C, c37 c4 = 500 pF, 16 V working, electrolytic = 0-1 pF, 30 V working, ceraniic = 0.01 pF, 30 V working, ceramic Semicondwtors- IC,, IC, = Texas instruments, SN74121N (pin connections indicated) Tr,, Tr, = Texas instruments, 2N3904 D,, D2, D,, D,, D, = Texas instruments, 1N4001 Re jays- KLA = Radiospares, type 11A RLB = Radiospares, type “LP-Relay-12 V” Switches s,, s, = Radiospares, type “MT-SW-DPCO” s3 = Radiospares, type “Micro I1/T” Sundry componeflts- Nl = Radiospares, type “M-NEON-250 V-AMBER”