Anal. Chem. 1980,52, 2220-2223
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AIDS FOR ANALYTICAL CHEMISTS Microcomputer System for Potentiometric Stripping Analysis Anders GranGli,’ Daniel Jagner, and Mats Josefson Department of Analytical and Marine Chemistry, Chalmers University of Technology and University of Goteborg, S-4 12 96 Goteborg, Sweden
Potentiometric stripping analysis is a n electrochemical method for the determination of trace concentrations of some heavy metals (I). Like the anodic stripping voltammetry techniques it consists of a potentiostatic deposition (preelectrolysis) step and a subsequent stripping step. The deposition step in potentiometric stripping analysis is the same as is often used in anodic stripping voltammetry, Le., the simultaneous reduction and amalgamation of the trace metal analytes on a mercury-coated glassy carbon electrode, according to
The stripping step is, however, fundamentally different from that used in anodic stripping voltammetry in that the reduced analytes are reoxidized chemically and not by a linear or pulsed linear potential ramp, i.e.
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Mn+
I n anodic stripping voltammetry the current due to the reoxidation of the amalgamated metals is measured while in potentiometric stripping the change in redox potential of the working electrode is measured. The latter change has the form of a normal redox titration curve superimposed on a small capacitance background. There are two important advantages of potentiometric stripping analysis over anodic stripping techniques. T h e noncurrent stripping makes it possible t o use very simple instrumentation (2). Furthermore, since the changes in working electrode potential during stripping are governed by the chemical equilibrium a t the solution/electrode interface, very rapid consecutive reoxidation of several amalgamated trace elements without loss of resolution is possible. Several different oxidants can be used for the stripping of reduced elements according to reaction 2 above. In most applications hitherto ( 3 , 4 ) ,mercury(I1) ions have been employed as oxidants. Mercury(I1) ions, in the concentration range 2-25 mg L-l, have been added to the sample prior to preelectrolysis. In this way they have been used both for the in situ formation of the mercury film on the glassy carbon electrode and for the subsequent reoxidation of the amalgamated metals. In order to make mercury(I1) the predominant oxidant it is, however, necessary to remove the dissolved oxygen in the sample by inert gas bubbling. Otherwise, reoxidation would take place at such a high rate that a normal strip-chart recorder would not be able to register the stripping curve. Deoxygenation of the samples is often the most time-consuming part of a potentiometric stripping or an anodic stripping voltammetry experiment (5). T h e microcomputer system described in this paper, which has been constructed around a commercially available instrument for potentiometric stripping analysis, fulfills two tasks. It measures and stores the very rapid stripping curves Present address: Astra Lakemedel AB, S-151 85 Sodertalje, Sweden. 0003-2700/80/0352-2220$01 .OO/O
(time vs. redox potential) obtained in, for example, nondeaerated samples, and transfers them subsequently a t a slower rate to the strip-chart recorder for graphical display. The computer also measures t h e capacitance background, subtracts the background from t h e combined redox titration/background curve, and displays the difference at a data rate suitable to the strip-chart recorder. EXPERIMENTAL SECTION Main Features of the Instrumentation. The microcomputer system was constructed around a Radiometer ISS 820 ion scanning system (6) as is shown schematically in Figure 1. Only minor alterations of the ISS 820 system were necessary in order to incorporate the microcomputer system. As can be seen from Figure 1, the ISS 820 controls the preelectrolysis potential and time and also provides the strip-chart recorder on which the stripping curve, corrected for background, and, optionally, the uncorrected stripping curve are displayed. Main Features of t h e Measurements. The computerized registration of a potentiometric stripping curve consists of four cycles which are illustrated schematically in Figure 2. After the adjustment of the preelectrolysis potential and time on the ISS 820, the computer is started and the first cycle (I in Figure 2) is entered. The computer starts the preelectrolysis (see Figure 1). After 10 s the computer measures the preelectrolysis potential, disconnects the potentiostat, and allows the working electrode potential to reach a base level value, Ebaee. The time duration between the switch off of preelectrolysis and the registration of the base level potential, a s (see Figure 2), can be changed on the computer keyboard. A standard value of a = 5 s is normally used. After the registration of Ebase, the microcomputer starts the second cycle (I1 in Figure 2) by switching on the preeleckrolysis and transferring the command to the ISS 820. The computer senses the preelectrolysis on/off function of the ISS 820 until it signals that the preelectrolysis time has elapsed. The computer then starts to read the stripping potential at a frequency of 30.7 kHz. The primary stripping curve is registered simultaneously on the strip-chart recorder of the ISS 820. The computer registration of the stripping curve is stopped when the potential E, is reached (see Figure 2). The value for Estopis fed into the computer by specifying, on the computer keyboard, a value for the parameter according to (3)
A standard value of b of 100 mV is most frequently used. The registration of the stripping curve is stopped before the potential has been reached because the potential region between - b ) does not normally carry any analytical inand formation. When the EBtop potential has been reached the computer enters the third cycle (111 in Figure 2) by initiating a delay period of 2 s after which the preelectrolysis function is reactivated for a period of 10 s. The computer then stops the preelectrolysisand registers is reached again. the background curve until the potential Estop In the final cycle (IV in Figure 2) of the measurement process the computer subtracts the background curve from the combined analytical and background curve. The difference is then displayed, via the D/A converter of the microcomputer system, on the strip-chart recorder of the ISS 820. Since the primary stripping curve normally takes less than 1 s, and since the maximum speed of the strip-chart recorder is 1 cm s-l, the rate of data to the CZ 1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 13, NOVEMBER 1980
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Figure 3. Block scheme of the connection of the A I D converter to the microcomputer.
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Flgure 2. Main features of the computerized potentiometric stripping analysis procedure: I,preelectrolysis and base-level potentkals registered; 11, primary stripping curve registered; 111, background curve registered; I V , background corrected curve displayed at decreased rate.
recorder must be reduced. The parameter c, fed into the computer before the commencement of the analysis, controls the speed of data flow from the computer to the recorder. A c value of 100, which is often used in oxygen-containing samples, means that a potentiometric stripping curve registered during 100 ms will be transferred to the recorder during a period of 10 s. The graphical display of data also includes a number of options which are useful when new analytical procedures are developed for the computer system. Thus it is possible to display both the uncorrected curve and the curve corrected for background, as well as the differentials of these curves, a t a low speed for detailed investigation. The parameter d in Figure 2, fed into the computer before the commencement of analysis, is used to suppress graphical display of analytically uninteresting parts of the stripping curve. The value of d is often approximately the same as that of b. (4)
H a r d w a r e Details. The microcomputer used was an Intel SDK-85 (System Design Kit) with an Intel 8085 8-bit processor. The SDK-85 board contained 500 bytes RAM, 2 kbytes PROM, two timers, 60 bits of programmable 1/0and a current loop serial interface. The system was expanded with an 8-kbyte memory board (Sattco Data Board 4680). Fast data acquisition was provided by a 12-bit A/D converter (Datel ADC-HX) with a conversion time of 20 ws. Digital information was transferred to the strip-chart recorder by means of a 12-bit D/A converter (Datel DAC-HZ 12BGC). The microcomputer system controlled the ISS 820 by means of three optocouplers, one operating on the preelectrolysis on line, one on the reset line, and one on the paper feed on/off line (see Figure 1). The A/D converter was mapped to the computer via a 16-bit tristate buffer (see Figure 3). The digital values from the A/D were not stored in memory as such but were used as pointers in memory to store the time the redox potential dwells at a particular value. Since the A/D provides only 12 bits of address information, the remaining 4 bits of the buffer were connected with bit 1 to a logic 0 and bits 14, 15, and 16 to a logic 1 (see Figure 3). This provided the processor with a pointer to memory locations in the 8-kbyte RAM board used as data buffer. The end of conversion (EOC) signal from the A/D converter was connected to the 8085
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Figure 4. Time intervals in the real time data acquisition.
ready line through one bit on the tristate buffer. This forced the computer to wait in a read bus-cycle until the conversion was completed. Consequently, no software monitoring of the EOC was required (see Figure 4). A constant interval between successive potential measurements was provided by means of a programmable timer driven by the 3.072-MHz processor clock as shown in Figure 4. The timer triggered the sample-and-hold circuit and A/D converter sequentially a t a frequency of 30.72 kHz via a monostable multivibrator. In this way the triggering operation needed no software overhead during data acquisition. Software Details. All software was written in Intel 8085 Assembler by using the extensions published by Dehnhardt and Sorensen (7). The program occupied about 1 kbyte of PROM. A development system, Tektronix 8002, was used for software development. The command module handled the input parameters from keyboard. In addition to the parameters indicated in Figure 2, such as potential limitsand rate of graphical display, the command module could be programmed to clear the memory of experimental data collected in a previous experiment. If this command was omitted, the computer would add stripping and background data from several consecutive experiments to the data storage memory. In the stripping phase of a potentiometric stripping experiment the working electrode potential changes with time resulting in a "redox titration curve". On a stripping plateau the change in working electrode potential with time is much less than in the potential region of the "equivalence points" (see curve IV of Figure 2). Thus, if the potential region used during one stripping experiment is subdivided into potential intervals of equal size, the working electrode potential will dwell in some potential intervals for a longer period of time than in other potential intervals. Consequently, if the stripping potential is monitored a t a constant frequency, the number of potential measurements falling into one particular potential interval (referred to below as a count) is a measure of the time the electrode potential dwells in this interval, i.e., a time-potential curve can be registered (8). Since each potential measurement yields only 1 bit of data (one count), a large number of experimental points can be registered in a small data storage area. Since the 12-bit A/D converter used in the experiments had a full scale range equal to f2.5 V, the smallest potential region it was possible to make use of was 1.2 mV. This resolution is more than sufficient for a potentiometric stripping experiment. The maximum real time data acquisition rate was 30.72 kHz giving a time resolution of approximately 33 ps. An
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 13, NOVEMBER 1980
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Figure 5. Schematic illustration of how the data points are stored in computer memory. One interval corresponds to 1.2 mV on the potentiometric stripping curve.
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Table I. Subroutine Used for Real Time Data Acquisition label
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STRIP SHLD LXI DAD SHLD LHLD XCHG READ LHLD SPHL DAD JNC POP
INX
SKIP
PUSH JNOV LHLD SPHL
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argument
comment
TEMP save E-stop H, 0 get stackpointer SP STACKP save it TEMP put E-stop in DE-registers get new potential from AD AID place as stackpointer test if under limit of ED stop jump if so SKIP get old content of memH ory cell increment content of H memory cell store new content H if not overflow read again READ STACKP restore old stackpointer OFLOW if carry write overflow message
RET increased data acquisition rate would improve the counting statistics and consequently the accuracy and precision of the potentiometric stripping experiment. The eight machine code instructions needed to read the A/D converter and to increment the content of the corresponding memory location do, however, limit the rate of data acquisition. Then contents of the data buffer after the registration of a typical potentiometric stripping curve are shown schematically in Figure 5. The subroutine used for data acquisition is shown in Table I. The routine uses the stack-pointer for fast access to the 16-bit words using the POP and PUSH instructions. Furthermore a "jump if not 16-bit overflow" (JNOV) (7) has been incorporated in order to achieve rapid testing and to eliminate the need for three extra &bit instructions. In order to avoid disruption of the stack structure, the old stack-pointer has to be saved and properly restored after each data storage. During storage the interrupts must be disabled since they use the stack pointer to reference return points (see Table I and Figure 4). The background curve is measured by using the same subroutines as described above, the data being stored in other memory locations. To avoid the influence of statistical variations in consecutive memory locations of both the background and analytical curve during background correction, we used an algorithm such that the sum of the contents of four consecutive background memory locations are subtracted from the sum of the contents of the four corresponding memory locations. If the result of the
Flgure 6. Computerized potentiometric stripping analysis in a nondeaerated sample: (a) primary curve; (b) primary curve displayed at a rate 400 times slower than in (a); (c) background-corrected curve displayed at a rate 1000 times slower than in (a). 4 min of preelectrolysis; [Cd(II)] = [Pb(II)] = 250 ng L-', [Cu(II)] = 125 ng L-'.
subtraction is negative the value of the corrected curve is set equal to zero in the corresponding memory locations. The curve corrected for background is displayed on the strip-chart recorder in either of two ways. In the first mode the addresses of the memory array containing the data are used consecutively as the digital input to the D/A. The D/A is allowed to rest at each value for a time proportional to the number of counts contained in this location. The rate of display can be chosen by the operator to be between 4 and 1024 times slower than the registration of the primary curve. In this way the displayed curve will have the form of a potentiometric stripping curve (see curve IV of Figure 2). In the second display mode the memory locations are scanned consecutively at a constant rate, and the D/A converter displays a signal proportional to the number of counts in each location, Le., the derivative dt dE-' is displayed (see left part of Figure 5 ) .
RESULTS Figure 6 shows the results of a computerized potentiometric stripping experiment in which a sample containing 250 ng L-' of cadmium(II), 250 ng L-' of lead(II), 125 ng L-' of copper(II), and 10 mg L-' of Hg(I1) in 0.05 M hydrochloric acid has been preelectrolyzed a t -1.05 V vs. SCE for 4 min prior to stripping. T h e sample has not been deoxygenated. Curve a of Figure 6 shows the stripping curve as it was registered on the stripchart recorder. No stripping plateaus are discernible owing to the rapid stripping caused by the dissolved oxygen. Curve b shows the combined analytical/background curve as registered by the computer and displayed on the strip-chart recorder at a rate 400 times slower than that with which i t was originally measured. On this curve the stripping plateaus for cadmium and lead are discernible, but the equivalence point for the copper plateau is not visible. Curve c shows the potentiometric stripping curve corrected for background displayed a t a rate 1024 times slower than the primary curve. All stripping plateaus are well-defined and the detection limit, for 4 min of preelectrolysis, can be estimated to be 25 ng L-' of cadmium(II), 25 ng L-' of lead(II), and 10 ng L-' of copper(I1). These limits are more than 3 orders of magnitude better than those previously reported for noncomputerized potentiometric stripping analysis of nondeaerated samples (5).
CONCLUSIONS Computerization of potentiometric stripping analysis improves the detection limit for the determination of cadmium,
Anal. Chem. 1980, 52, 2223-2225
lead, and copper by almost 3 orders of magnitude. I t also simplifies t h e electrochemical determination of several trace elements in samples containing dissolved oxygen or other electroactive agents. Complicated sample pretreatment, often necessary in anodic stripping voltammetry, can thus be omitted.
LITERATURE CITED (1) Jagner, D.; Argn, K. Anal. Chim. Acta 1070, 700, 375-388. (2) Jagner, D. Anal. Chem. 1078. 5 0 , 1924-1929.
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(3) Jagner, D.; Danielsson, L. G.; Argn, K. Anal. Chim. Acta 1970, 706, 15-21. (4) Jagner. D.; Argn, K. Anal. Chim. Acta 1070, 707, 29-36. (5) Jagner, D. Anal. Chem. 1979, 57, 342-345. (6) Graabaek, A. M.; Jensen, 0. J. Ind. Res. D e v . 1070, 27, 124-127. (7) Dehnhardt, W.; Sorensen, V. M. Electronics 1070, 52, 144-146. (8) Mortensson, J.; Ouziel, E.; Skov, H. J.; Kryger, L. Anal. Chlm. Acta 1070, 7 72,297-304.
RECEIVED for review February 22, 1980. Accepted July 15, 1980.
Automated Sequential Sampler for Gas Chromatographic Determination of Trace Airborne Pesticides H. L. Gearhart" and R. L. Cook Department of Chemistty, Oklahoma State University, Stillwater, Oklahoma 74078
R. W. Whitney Department of Agricultural Engineering, Oklahoma State University, Stillwater, Oklahoma 74078
The determination of pesticide levels in air to date has been primarily directed at either (1)detection and measurement of individual or small groups of chemically related compounds in air near agricultural or industrial operations or (2) air sampling and measurement of multiclass pesticides. While development of a sampler capable of collecting a wide array of materials at the subparts per billion level is at present only visionary, successful attempts have been made with various types of single-sampling systems. These techniques have been reviewed in part (1-5) and in depth (6-8) by various authors. T h i s paper describes a new, sequential, accumulative air sampler specifically designed for operation within enclosures. T h e details of its design a n d operation are reported. T h e sampler is based on t h e principle of stripping and concentrating volatile compounds from air in a suitable concentration medium which is subsequently available for chromatographic analysis. An important and unique aspect of this device is ita dynamic capability for direct and continuous air flow rate measurement and integration and svnple volume totalization. T h e new sampler is totally self-contained, including a battery power supply, a n d i t is designed to sample sequentially ( u p to four separate samples) after being preprogrammed for volume per sample a n d time interval between samples. The sampler has t h e inherent portability for operation in small, remote enclosures without direction from occupants. I t is compatible with Tenax traps as well as other porous polymers such as Amberlite XAD, etc. It is adaptable for use with liquid concentration media as well.
EXPERIMENTAL SECTION Figure 1 presents a schematic diagram of the gas sampler. Four interchangeable sampling loops, in this case each containing -0.3 g of polymer trapping medium, are connected to an 8-position (Valco Instruments, Inc., Houston, TX), 16-port valve with zero volume fittings. Sampling loop construction is of nickel steel. The loops are 4-in. lengths of 1/4-in. tubing, plugged with silanized glass wool and silver soldered at either end to lengths of l/s-in. tubing fitted with Vespel ferrules. Alternate valve positions seal the ports to all sample loops as well as the valve entrance and exit. Sample air passes through a 140-pm stainless steel filter prior to entering the valve. The exit port of the valve is connected to a sampling pump via a manifold. A second, auxiliary source of air is provided to the pump through a matched resistance loop (identical pressure drop to the other valve loops), another 140-pm 0003-2700/80/0352-2223$0 1.OO/O
filter, and a solenoid operated shunt valve. Sample air exits the pump through a linear mass flow meter (Matheson Gas Co., La Porte, TX). Programmable control of the sample volume is achieved through an electronics package. Four thumb-wheel switches, each corresponding to one of the four sample loops, are set at the air volume in liters desired for the respective sample loop. A valve loop position indicator controls a multiplexer and communicates the appropriate setting to a digital to analog converter (DAC) and then to a comparator. Upon command from a digital valve sequence programmer (DVSP) (Valco Instruments, Inc., Houston, TX), the signal from the mass flow meter is integrated and continuously compared with the thumbwheel setting. When the two values are equal, the DVSP shuts the pump off and cycles the valve to the next position. Following completion of any programmed delay time and just prior to the next sampling period, the pump is turned on, the solenoid valve is opened, and air flowing through the resistance loop passes through the mass flow meter. This forces the flow meter output to achieve steady-state conditions prior to actual integration. The solenoid valve shuts off and the sampling valve is rotated simultaneously connecting the sample loop with the pump. Upon valve slider rotation to a given set point, the analog valve position indicator communicates the comparator (Model 4115/04, Burr-Brown Research Corp., Tucson, AZ)with the proper digital thumb-wheel switch setting by means of a 4-channel, 12-bit multiplexer and a digital/analog converter (DAC) (Model DAC 85, Burr-Brown Research Corp., Tucson, AZ). Simultaneously, sampling commences for that loop etc. The analog signal from the flow sensor was conditioned as shown in Figure 2. A chopper stabilized amplifier (Model 1538A, Burr-Brown Research, Tucson, AZ)was used to divide the input voltage by 5. A second chopper stabilized amplifier (Model 3291/14, Burr-Brown Corp., Tucson, AZ) was used in an integration configuration. The integrator resistance-capacitance time constant was calibrated (0.5 V = 1 L) by using a standard voltage source (O.C.I.-009 Calibrator, Olympic Controls, Inc., Houston, TX) and a Model 1090A digital oscilloscope (Nicolet Instruments Corp., Madison, WI). An error of
