Although the aerosol sample taken with the PCPP electrostatic sampler is not entirely representative, as the sampling efficiency depends to some extent on the particle size, the true size distribution of the aerosol can be determined by making appropriate corrections using Figure 9. Further, because the absolute sampling efficiency of the sampler is known, the absolute concentration and size distribution of the
aerosol can be determined from the aerosol sample. This is generally impossible with the conventional electrostatic aerosolsampler. RECEIVED for review August 31, 1967. Accepted December 26,1967. Research supported under AEC contract AT(11-1)1248 and carries the publication number COO-1248-11 under this contract.
A Venting Apparatus and Cleaning Procedure for Electron Capture Detectors Robert R. Claeys and Tommy Farr Department of Agricultural Chemistry, Oregon State Uniuersity, Coruallis, Ore.
ONEof the problems encountered with a tritium source electron capture detector is cell contamination. The detector becomes noisy with time, with a concurring decrease in the linear response range and available standing current. Frequent cleaning of the detector is often necessary. Two developments have led to a large increase in the linear response range and to reduction in noise and base line drift. The first development is an improved cleaning procedure for tritium foils, and the second is the addition of a carrier gas venting apparatus. The carrier gas is shunted into the atmosphere instead of passing through the detector during idle periods or when the solvent and contaminating materials are eluted from the column. The new cleaning technique results in a cleaner foil, and the venting arrangement reduces foil contamination from column bleedoff or the elution of contaminants. EXPERIMENTAL Cleaning Procedure. The tritium foil is first removed from the cell and placed in a 25-ml boiling solution of 5 % KOH in 9 5 z ethanol. Disposable gloves and safety glasses must be worn. With the use of tweezers, the foil is scrubbed lightly with a pipe cleaner for 2 or 3 minutes. The foil is removed from the solution and rinsed with 10 ml of hot 95% ethanol using a disposable pipet. The foil is then cleaned with an aqueous, thiourea, jewelers' solution (Jeweluster manufactured by E-Z-EST Products Co., Inc., Oakland,
Calif.) by scrubbing with a cotton pipe cleaner for 2 to 3 minutes, and rinsed with 10 ml of hot ethanol. The detector cell and leads are also cleaned with this solution. All liquid wastes are combined for counting or disposal. The waste solution from cleaning a 250-mCi tritium foil has been found to contain 0.001 to 0.04 mCi of 3H. Apparatus. The venting apparatus, shown in Figures 1 and 2, was installed in a MicroTek 220 instrument equipped with a parallel plate detector and a Varian Aerograph 550 instrument equipped with a concentric tube detector. The toggle valve is mounted outside of the oven, preferably on the front panel. When the toggle valve is open the carrier gas is restricted from entering the detector by capillary tubing. Upon closing the toggle valve the carrier gas flows through the capillary tube to the detector. Arrangements must be made to pass auxilliary nitrogen gas directly into the detector. With instruments employing a parallel plate detector a purge gas provision may already be present. With instruments employing a common base for electron capture and flame ionization detectors the nitrogen gas can be admitted through the hydrogen inlet. The auxilliary nitrogen gas is adjusted to 20 ml/minute with a Nupro 2M needle valve (Nuclear Products Company, Cleveland, Ohio). After cleaning the tritium foil and installing of the venting apparatus, the linearity and sensitivity of both the parallel plate and concentric tube detectors were measured, and the results compared with previous detector responses. Both detectors were operated at a fixed dc voltage with nitrogen carrier gas.
G
B
I
Figure 1. Cross-sectional view of venting apparatus A. B.
C.
D. E.
F. G. H.
Gas chromatographic column Stainless steel Swagelok cao 1- X '/le-inch OD, i.047-inch ID, S.S. hypodermic tubing '/&ch OD copper tubing "4X +inch OD S.S. tubing 1- X l/l~-inchOD, 0.007-inch, S.S. capillary tubing Swagelok nuts Solder welds
DETECTOR BASE
TOGGLE VALVE
VOL. 40, NO. 4, APRIL 1968
847
PARALLEL PLATE DETECTOR INJECTiON PORT
Sensitivity: 1.6 x 10-9 omps/m\l Attenuotion: i / 3 2 0 0 --ct Cison foil -cC
Used foil
TOGGLE VALVE
Figure 2. Diagram of venting arrangement in a gas chromatograph
RESULTS AND DISCUSSION Various studies had been made to determine the cause of deteriorating detector performance with time. The chromatographic columns were conditioned for a week at an elevated temperature before use to reduce column bleeding. Extra precautions were taken to remove water from the carrier gas and from the samples injected onto the column. Septums were preheated in spare injection ports before use. The tritium foils were specially cleaned using 5% (w/v) KOH in methanol by either scrubbing with a pipe cleaner or sonication. These techniques did not eliminate the poor detector performance with time. In the new cleaning procedure alcoholic base aids in removing the column contaminants, while the thiourea solution polishes the surface of the foil. A large increase in the standing current has been observed with every tritium foil cleaned by this procedure, especially foils previously considered ready for disposal. In general, the standing current is nearly two to three times higher than that obtained by alcoholic base cleaning alone, but the sensitivity of the detector does not necessarily increase correspondingly with standing current. However, as shown in Figure 3, the linear response range of the cleaned foils is considerably greater than the response of the used foils measured prior to cleaning by the procedure described and the installation of venting apparatus. The loss of linearity between 0.3 and 0.6 ng of aldrin shown with the used foils had made previous quantitative analysis particularly troublesome. With the adoption of the venting arrangement the sensitivity and linear response of the electron capture detectors have remained relatively constant with time. Tritium foils have been used routinely in our laboratory for six months without the usual need for periodical cleaning and replacement of foils. Venting the solvent into the atmosphere is also recommended, The detector response will normally return to the base line within 10 to 20 seconds after the toggle valve is closed. When 10 ~1 or more of solvent is injected into the gas chromatograph, the electron capture detector should recover faster when the solvent is vented than when the solvent passes through the detector. Base line drift, especially in the first hour of analysis, is generally reduced by venting the solvent front.
848
ANALYTICAL CHEMISTRY
0.i
0 2 Nonagroms of Aldrin
0.3
C O N C E N T R I C T U B E DETECTOR Sensitivity: 6 4 x 10-10 amps/mV Atlenuolion. 1/16
0.2 Nonogroms of Aldrin
0.3
Figure 3. Response of electron capture detectors to aldrin Favorable results have been obtained by baking out the chromatographic column at an elevated temperature in the evening with the column effluent vented. Thus a clean column can be obtained at the beginning of each day. If the toggle valve is left closed for several days an appreciable drop in standing current may result. Increasing the auxilliary nitrogen flow for a day may aid in returning the standing current to its former level. The 1-inch capillary tubing in the venting apparatus is designed to restrict the flow of the carrier gas when the toggle valve is open, but not to cause an appreciable pressure drop at normal flow rates. A 1.0 to 1.5% drop in the carrier gas flow rate at 100 ml/minute occurs with instruments not equipped with flow controllers when the venting apparatus is in series with the carrier gas stream. Thus, venting the solvent front for different time intervals will not normally cause changes in rentention times. No increase in apparent tailing was observed with chlorinated hydrocarbon pesticides with the instruments cited using packed columns. RECEIVED for review November 27, 1967. Accepted January 15, 1968. Technical paper No. 2395 Oregon Agricultural Experiment Station, Corvallis. This investigation was supported in part by Public Health Service Research and Training Grant ES 00040-03.
