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
