Anal. Chem. 1981, 53, 2383 (6) Wall, L. L., Sr.; Gehrke, C. W. J . Assoc. Off. Anal. Chem. 1977, 60,
(9) Noel, R. J.; Hambleton, L. G. J . Assoc. Off. Anal. Chem. 1978, 59, 134-140.
a81-889. (7) Technicon CorDoration, Tarrytown, N.Y., Industrial Method No. 36975A. ( 8 ) Technicon Corporation, Tarrytown, N.Y., Industrlal Method No. 334-
74w.
2383
for review 13, 1981* Resubmitted and accepted September 18, 1981.
Electrometer Sulbstitute for a Flame Photometric Detector K. W. Michael Siu, James R. Hancock, and Walter A. Aue" Department of Chemistry, Walhousie University, Halifax, Nova Scotia, B3H 4J 1, Canada
Electrometers for gas chromatographic detection are, a t least in our hands, prone to malfunction; and they are expensive to repair or replace. Faced with such a dilemma, we recently assembled a simple amplifier from readily available components. I t served as an inexpensive electrometer substitute for use with our Shimadzu flame photometric detector (FPD). Due to the relatively high current output of the photomultiplier tube, the FPD lends itself well to low-cost amplification. This is possible through the use of inexpensive monolithic field-effect transistor (FET) operational amplifiers that became readily available after the mid 1970s. Futhermore, we are making use of the fact that, nowadays, many recorders have multirange capability. Of course, there is nothing fundamentally new in our approach; however, it is certainly not common knowledge that one can significantly reduce the high cost of a flame photometric detection system in this manner. 'The priice one pays for doing this-in terms of detector performance as well as in terms of electronic components-is minimal.
EXPERIMENTAL SECTION The heart of the electrometer substitute, shown in Figure 1, is a half-dollar FET operational amplifier, TL081CP (Texas Instruments). It converts the photomultiplier current to voltage; in this case a 1 nA current produces a 1 mV output. The operational amplifier is powered by two 9-V alkaline batteries as shown in Figure 2. The bucking is a continuously adjustable 0-2 V source--in this case a lab-made arrangement consisting of a battery, resistors, and potentiometers. The voltage ranges of the recorder (Linear Instruments, Irvine, CA, Model 385, 1 mV-5 V) replace the attenuation setting of a conventional electrometer.
1M
-
Figure 1. Circuit diagram.
electrometer
multi-range
substitute
Figure 2. Schematic setup.
RESULTS AND DISCUSSION To illustrate the stability and sensitivity of the electrometer substitute, Figure 3 shows 3- and 2-ng injections of diphenyl sulfide, monitored with the lab-made circuit and a regular FPD electrometer (Shimadzu Model EM-5S), respectively. The minimum detectable amount is about 2 times higher in the former (sulfur response if3 a quadratic function of the amount injected). Adding a conventional, first-order low-pass filter (lab made; less than $3) reduces the detection limit by nearly half (Le., to a level comparable to that of the regular electrometer). This, however, was not considered necessary for most practical purposes The thermal drift of the operational amplifier was specified as 10 pV/"C. Thus even under the most sensitive experimental sett,ing-a recoirder voltage range of 5 mV full scale-no noticeable drift was observed. The noise level of this electrometer substitute, when measured by grounding the
Figure 3. Chromatograms of 3-ng diphenyl sulfide using lab-made electrometer substitute (left) and of 2-ng diphenyl sulfide using the regular Shimadzu EM-SS electrometer (right).
input through a 1 M resistor, was about 4 p V . Component cost of the substitute electrometer-including chassis, connectors, and heavy-duty batteries-was less than $15 U.S. The less expensive assembly time involved 2 h of graduate student toil. RECEIVED for review June 17,1981. Accepted September 22, 1981.
