Anal. ch8m. 1988, 60, 287-288
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Peak Readout Device for Transient Signals from Flow Injection Analysis William E. Bauer,’ Adrian
P.Wade:
a n d S. R. Crouch*
Department of Chemistry, Michigan State University, East Lansing, Michigan 48824 While the well-equipped research and service laboratories of the industrialized nations utilize increasingly sophisticated techniques, there remains an ever-growing need for compact, rugged, inexpensive, and portable instruments that can perform analyses in isolated locations without the many conveniences of a modern laboratory. Examples of such requirements are many and occur in field hospitals, disaster relief centers, medical missionary posts, agriculture field test kits, and large teaching laboratories and colleges in developing and developed countries. With these applications in mind, an inexpensive circuit for detecting and displaying flow injection peak heights has been designed and evaluated. The circuit consists of two analog peak detectors, one for positive-going normal peaks and the other for negative-going peaks. The circuit can also respond to the peak-to-peak value of a voltage that goes both positive and negative. An inexpensive digital voltmeter or panel meter is used to display the maximum (or minimum) voltage held on a capacitor. The readout value is proportional to the flow injection analysis peak height. Flow injection analysis (FIA) is a sensitive and versatile techique that has become widely used, is relatively inexpensive, and allows rapid sample processing (1,2). Previously, there has been a perceived need for chart recorder output or microcomputer-controlled data acquisition. This has limited the applicability of FIA analyzers, since chart recorders and microcomputers are still relatively expensive and usually require that electricity be available. The readout reported here provides an accurate, inexpensive alternative to these, which can even be used with inexpensive light emitting diode (LED) based photometers and powered from dry-cell batteries or an automobile battery. The remaining components of an FIA system can also be chosen so as to minimize cost, while retaining utility. For simple FIA systems, propulsion of the carrier stream by a gravity feed system has proven quite adequate and is a more viable approach for field use. The gravity feed might be provided by a plastic medical drip bag (as used for dispensing saline solution and drugs in hospitals), raised to an appropriate height. This eliminates the need for a peristaltic pump and significantly reduces both the cost per apparatus and power requirements. The simple six-port (or dual four-port) sample injection valve normally used is retained and is operated manually. An alternative would be the 12-port design recently reported (3). Detector flow cells, whether incorporating photometric or electrochemical detection, are of flow-through design and are sturdy and essentially self-cleaning. Inexpensive, miniature photometric detectors (4-6) may be made to be extremely rugged and can be powered from batteries. Thus, by use of the readout circuit described here, a simple FIA apparatus becomes portable, totally independent of ac power, and costs less than $200. In this form, it is ideally suited to the difficult and isolated situations described above. EXPERIMENTAL SECTION Circuit Description. The circuit for the peak-height detector is shown in Figure 1. The top half of the circuit detects and holds the maximum height obtained by a normal peak. It alone Present address: Lucknow Christian College, Lucknow, U.P., 226018, India. 2Present address: The Chemistry Department, University of British Columbia, Vancouver, BC, Canada.
Table I. Electrical Components for the Peak Readout Device component
value or type
op amp
TL082 BIFET
R1
1000 R 100 R 10 WFelectrolytic IN4148 or other signal diodes 21/2 digit ICL7106 evaluation kit (Intersil)
RZ
c1, c2
D,, Dz
meter
is sufficient for the vast majority of analyses. The bottom half of the circuit detects and holds the minimum value and may be used when peaks present a negative response relative to the base line. Switch S3selects whether the output from the top or bottom half of the circuit is read by the meter and is required only if both halves of the circuit are used. The only difference between the two halves of the circuit is that in the lower half diode D2is reversed so that negative-going peaks are detected and held. Hence, we shall consider in detail only the upper half of the circuit. The first operational amplifier (op-amp), IC1, acts as a buffer. When S1 is closed the circuit acts as a voltage follower. With S1 open, the second op-amp, diode, capacitor combination acts as apositive voltage peak detector (negative,for bottom half), which holds the peak voltage for display by the meter. Any available digital voltmeter could be used. The operational amplifiers are BIFET devices because of their high input resistance and low bias current. None of the other component characteristics or layout are critical. The circuit is simple to construct, costs less than $30 (including meter) and may be powered from two 9-V alkaline batteries. Procedure. In normal operation only the top half of the circuit is used. First, the base line is established with S1 and SBaclosed. At this time any base-line irregularities (e.g., due to contaminated solutions, air bubbles, etc.) may be observed and, if possible, corrected. A steady base line will be seen as a stable meter reading, which may be noted. At this point the detector offset may be altered to adjust the output voltage to some convenient figure (e.g., 1V). Then, the sample is injected, is carried downstream and reaction takes place. As the product reaches the detector cell, the meter reading starts to increase rapidly. S1is then opened manually, and as the peak maximum is reached and passed, the meter holds a steady reading which indicates the maximum peak height. This value may then be noted. The peak height is the difference between the two noted values. When S1 is closed, the tail of the peak may be observed. This should be followed by reestablishment of a steady base line. The next experiment may then be carried out. The system as constructed is operated manually. It could be completely automated, but at additional expense and complexity. Negative peaks can occur when the sample plug has an absorbance less than that of the carrier stream after any reaction has taken place and may be observed when the unit is in “follower” mode (SI and Saaclosed). The bottom half of the circuit only is then used to quantify such peaks. Both halves of the circuit can be used for measuring signals having the shape of the differential of a Gaussian, which are due (e.g.) to refractive index differences between sample plug and carrier stream. These are directly observable when using photometric detection along the axis of the tube ( 4 ) . RESULTS AND DISCUSSION Typical response curves obtained with a chart recorder are represented in Figure 2a. Other possible curves are represented in parts b and c of Figure 2. The readout was found capable of dealing with all these possibilities. Two specific
0003-2700/88/0360-0287$01.50/00 1988 American Chemical Society
288
ANALYTICAL CHEMISTRY, VOL. 60, NO. 3, FEBRUARY 1, 1988
Detector
Meter
Gyy-’
9v
CONCLUSIONS
Figure 1. Circuit schematic for the peak height detector.
Time
instrument control and data acquisition system. This was being used to obtain a calibration curve for a recently published enzyme-based method for glucose determination (8). Ten different glucose concentrations were determined. The absorbance measurements taken using the readout were linear with concentration. A relative standard deviation of 1.0% was found for the method. As would be expected, the results from the new readout compared well with those obtained by the data acquisition system and were of comparable precision.
-
Figure 2. Typical FIA peak shapes: (a)simple case with two sample concentrations;(b) when low concentrations give a negative response; (c)single injection with significant refractive index difference between sample and carrier.
chemical systems were studied.
Determination of Chloride with Hg(SCN)z. This reaction (1)was carried out on a single-channel FIA system and followed an experimental method recently developed for an undergraduate teaching program (7). The photometric detector used was a tungsten-halogen lamp/silicon photodiode unit with logarithmic ratio output, which has been described elsewhere (6). It gave a voltage output which was linear with absorbance and was in the range 0.00-1.00 V. The readout was connected in parallel with a chart recorder. The recorded peaks were in the range 0.2 to 0.7 V. Each experiment took 15-35 s. A correlation coefficient value (R2)of 0.999 was obtained and the relative standard error of the readout results versus the chart recorder results was 0.7% for 18 measurements. The relative standard deviations (repeatability) of FIA peaks obtained with the new readout system were typically 0.6% and not limited by the readout system itself. Peak height values could be observed much more quickly with the digital meter than with the chart recorder. A few abnormal peaks were obtained and most could be identified from the meter display nearly as easily as from a recorder. Determination of Glucose. In a second experiment, the readout system was used in parallel with a microcomputer
These results indicated that this unit is a reliable alternative to a chart recorder or microcomputer data acquisition system for the collection of routine data from FIA. It should find wide applicability wherever cost, portability, and power requirements are of concern. The peak readout device is being applied to other FIA systems and can be used for other techniques where peak heights are important. The circuit could be improved by use of low-leakage polystyrene capacitors for C1 and Cz. Also, D, and Dzcould be low-leakage, epoxy-cased field effect transistors, since these would better hold the peak voltage (9).
ACKNOWLEDGMENT The authors thank C. L. M. Stults for making available her research operations for testing. W.E.B. thanks S. R. Crouch, C. G. Enke, and the Michigan State University Chemistry Department for the chance to effectively utilize a sabbatical leave.
LITERATURE CITED Ruzicka, J.; Hansen, E. Flow In]ection Analysis; Wlley: New York, 1981. Stewart, K. K. Anal. Chem. 1983, 55, 931A. Erickson, 9. C.; Kowalskl, 9. R.; Ruzicka, J. Anal. Chem. 1987, 5 9 , 1246-1248. Betterldge, D.;Dagless, E. L.; Fields, 8 . ; Graves, N. F. Analyst (London) 1978, 103, 897. Sly, J.; BetterMge, D.: Wlbberley, D.; Porter, D. G. J. Autom. Chem. 1982, 4 , 186. Patton, C. J.; Crouch, S. R. Anal. Chim. Acta 1986, 179, 189-201. Kraus, P. R.: Ratanathanawongs, S.K.; Stuits, C. L. M.; Patton, C. J.; Crouch, S. R., submitted for publication In J. Chem. Educ. Stults, C. L. M.:Wade, A. P.; Crouch, S. R. Anal. Chim. Acta 1987, 192, 155-163. Meiksln, 2. H.; Thackray, P. C. Electronic Design with Off-the-shelf Circuits, 2nd ed.; Prentice-Hall: Englewood Cliffs, NJ, 1984.
RECEIVED for review June 25,1987. Accepted September 29, 1987. This work was partially supported by National Science Foundation Grant No. CHE 83-20620 and by a grant from the Michigan State Research Excellence/Economic Development (REED) Program.
CORRECTION Electrochemical Pretreatment of Carbon Fibers for in Vivo Electrochemistry: Effects on Sensitivity and Response Time Jian-Xing Feng, Michael Brazell, Kenneth Renner, Richard Kasser, and Ralph N. Adams (Anal. Chem. 1987, 59, 1863-1867). Jian-Xing Feng’s permanent address should read, Nankai University, People’s Republic of China.
