Programmed Temperature Gas Chromatography with Dual Electron Capture Detectors for Pesticide Residue Analysis T. J. Kneip,' T. H. Beasley, P. King, and W. K. Dean Mallinckrodt Chemical Works, St. Louis, Mo.
ISOTHERMAL GAS chromatography (1-5) is well accepted for the determillation of pesticide residues. A system is described here which offers many advantages in this use. The system provides the ability to obtain programmed temperature chromatograms with dual electron capture detectors and dual columns. Increased sensitivity is obtained by this method for the less volatile pesticides as compared to isothermal results. The sensitivity of pesticide residue analysis is customarily increased by concentration of the final solvent solution. However, concentration of solvents by factors greater than about two hundred (1000 ml to 5 ml) does not appear t o be practical. A procedure for the injection of large (0.1 ml) samples is also used to provide a convenient routine method requiring the evaporation of only 250-ml samples.
601
IO0
LINDANE 90
COLUMN
EXPERIMENTAL
200.C
BO 70
Apparatus. A Barber-Colman 5000 Series gas chromatograph was used with a battery detector power supply and the following modules: column bath, Model 5072; detector bath, Model 5002; injector-detector temperature controller, Model 5103; temperature programmer, Model 5084; electrometer, Model 5042; and recorder, Model 8300-26000-0-00. This unit was provided with two 6-foot by 5-mm. i.d. glass columns packed with 10% Dow Corning-200 on 100-120 mesh Gas-Chrom Q (Applied Science Laboratories, Inc., State College, Pa.). The columns were conditioned for four days at 250" C with a nitrogen flow of 125-150 ml per minute. All 0 rings and septums were baked overnight at 300" C in flowing nitrogen. Two foils were obtained with current plateaus matched to within 1 % (Barber-Colman Model 5120 electron attachment detector foils). The foils were cleaned with 5 % alcoholic potassium hydroxide with ultrasonic agitation. The detector bodies were cleaned with 20 Chem-Solv (Mallinckrodt) in methanol with ultrasonic agitation. The detector foils and bodies were rinsed by ultrasonic agitation in water, methanol, and toluene in that order. Reagents. All solutions were prepared with pesticides of 99+ % purity (Applied Science Laboratories, Inc., State College, Pa.) in solvents of special high purity (Mallinckrodt Chemical Works, St. Louis, Mo., Nanograde quality). The heptachlor epoxide used as a reference standard was obtained from Velsicol Chemical, Inc., Chicago, Ill. To maintain maximum sensitivity the prepurified nitrogen carrier gas was passed through titanium sponge (Number Present address, New York University, Institute of Environmental Medicine, Long Meadow Road, Tuxedo, N. Y. 10987 (1) J. Burke, J . Assoc. Ofic. Agr. Cliemisrs, 46, 198 (1963). (2) J. Burke and L. Giuffrida, Ibid., 47, 326 (1964). (3) H. L. Reynolds, J. Gas Chromatog., 2,219 (1964). (4) U. S. Department of Health, Education, and Welfare, Food and Drug Administration, "Pesticide Analytical Manual," 1, 2.32 (1964). (5) U. S. Department of Health, Education, and Welfare, Public Health Service, Guide to the Analysis of Pesticide Residues, 1, V. B. (1965). 1510
ANALYTICAL CHEMISTRY
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HEPTACHLOR EPOXIDE COLUMN 2 0 0 - C 80 240Y 70
4
60 50
5
IO
15
APPLIED
20
22 5
25
30
35
40
DC VOLTAGE
Figure 1. Normalized peak height cs. voltage applied to detector cell 501-94, Laboratory Equipment Corp., St. Joseph, Mich.) maintained at 400" C and Molecular Sieve 5A (Wilkens Instrument and Research, Inc., Walnut Creek, Calif.), The molecular sieve columns were located at the column inlets. Procedure. The less sensitive detector was used o n the samplc stream and the more sensitive detector on the reference stream. The reference detector signal was diminished to match the sample detector signal by reducing the reference column flow rate. The gas flow through the two detectors was maintained at the same level by introducing additional nitrogen at the reference detector purge inlet. N o drift problem was encountered and no rematching was required following programmed temperature determinations. N o adjustments were required between detector cleanings (usually three to four weeks) and only minor adjustments were necessary between column changes (usually three to four months). The use of temperature programming seems t o increase the useful column life by removing high boiling substances introduced in analyzing crude and intermediate production samples and research samples. RESULTS AND DISCUSSION
Voltages for maximum detector response to several pesticides were first determined a t 210" C detector temperature, 225 O C injector temperature, and isothermal column temperatures of both 200" and 240" C .
100
C
ELECTRON AFFINITY PxlO-" A F S 200'- 260. 3.C /Mln.
80
60
40
0 20
ii
W
z" (L
W
0
8
6
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I2
I5
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21
24
27
loo?
s0 : a 80
3
ELECTRON AFFINITY P X ~ O - AFS '~ 200.C ISOTHERMAL
60
-
100-
ELECTRON AFFINITY 9xlO-'O AFS 50°-260° 3 * C / M i n .
80. v) W
6
60,
Y
a OL Y
a 40, V Y
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Toluene, 100 pl 1. Lindane, 0.2 ng 2. Parathion, 2.0 ng 0.
3. Heptachlor epoxide, 0.2 ng 4. Endrin (dieldrin), 0.2 ng 5. Ethion, 2.0 ng VOL. 39, NO. 12, OCTOBER 1967
1511
Column condition 200" c 240 260 200-260 (at 3"imin)
Table I. Response (h) at a Detector Voltage of 22.5 V under Various Column Temperature Conditions h - Per cent full scale Heptachlor Lindane Aldrin epoxide Dieldrin pp-DDT Methoxychlor 29 23 18 15 7 1 44 41 33 30 25 4 43 44 33 34 29 5 41
46
The optimum detector voltage did not appear to be column temperature dependent over this temperature range and a voltage generally suitable for the detection of pesticides was sought. Figure 1 demonstrates the normalized peak height ( h ) cs. detector voltage curves for some low, medium, and high volatility chlorine-containing pesticides at a column temperature of 200" C. The normalized curve for heptachlor epoxide at 240" C column temperature is included to illustrate the observed, but unexplained, broadening of the h cs. detector voltage curve at the higher column temperature. For the analysis of a wide range of pesticides, a compromise voltage (22.5 V) was chosen, which was not optimum for any of the pesticides studied (see Figure 1). Therefore, the effect of column temperature on h was studied at this voltage. The results shown in Table I were obtained using a solution containing 0.2 ng of each pesticide in the injected sample. The increased sensitivity for components of lower volatility is shown in Figure 2 by comparison of the chromatograms at 200" C (isothermal) and 200" to 260" C (programmed temperature). Five to ten minutes are required to cool and equilibrate the instrument between determinations. The instrument time saved by temperature programming is apparent. The ability to program over a much Ionger temperature range is demonstrated in Figure 3 using a mixture of chlorine and phosphorus-containing pesticides. For routine testing it was required that quantitative determinations be made at a level of 10 ng per liter throughout the volatility range from Lindane to pp-DDT. No convenient and rapid method of evaporative concentration was found for concentration factors greater than about 200. Larger sample injections were tested with the programmed temperature
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ANALYTICAL CHEMISTRY
41
41
41
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system. It was found that a slow injection of 0.1 ml over a 10second period produced excellent results. Rapid injections of such large samples can sometimes generate artifact peaks. A study was performed to determine the precision of the determination for several chlorinated pesticides (Lindane, Aldrin, and Dieldrin) at 2 ng per liter and 10 ng per liter in petroleum ether. The pesticides were added at the above levels to five 250-ml samples of petroleum ether. These samples were concentrated to 5 ml with a Kuderna-Danish apparatus on a steam bath, and duplicate 0.1-ml injections were made from each concentrate. The precision of an individual determination was * 2 6 z relative at the 9 5 z confidence level for the range of 2 to 10 ng per liter, based on 18 degrees of freedom. The variation in retention times was determined as follows. A sample of heptachlor epoxjde was injected on 12 different days over a period of two months. The programmed temperature procedure was used with no adjustment of instrument parameters to adjust retenlion times. These samples were run as part of the routine sample schedule. The mean retention time was 12.1 minutes and the standard deviation was 1 0 . 2 8 minutes. The system has been routinely used in the testing of solvents varying in volatility between diethylether and toluene and in polarity between petroleum ether and acetonitrile. No significant effect has been observed due to variation of the properties of the solvent. RECEIVED for review May 24, 1965. Resubmitted July 14, 1967. Accepted July 14,1967.
