Table 11. Comparison of Syringe Sampling with Sampling by the Evacuated Gas Sampling Devicea
Table I. Carry-Over Study and Estimates of Precision for the Evacuated Gas Sampling Devicea 1st 0.5 LI1/l. std -
Compound
Meanb
S t d dev
10.0
d/l.std
Meanb
Std dev
Syringe method
2nd 0 . 5 u l / L std
hleanb
Compound
Std dev
&lead
Variance
Gas sampling device liieanb
L'anance
Acetone 0.207 0.004 3.862 0.062 0.213 0.006 Ethanol 0.047 0.005 0.562 0.011 0.055 0.007 1-Propanol 0.097 0.003 1.758 0.025 0.123 0.007 Chloroform 0.052 0.005 1.145 0.126 0.054 0.004 a All values are peak area/internal standard peak area. Mean of nine replicate 0.5-10.0-0.5 pl/l. sequences.
0.0095 0.849 0.0009 Acetone 0.964 0.372 0.0001 1-Propanol 0.332 0.0011 Chloroform 0.208 0.0025 0.171 0.0012 All values are peak area/internal standard peak area. Mean of 10 replicate analyses of a 2 ~ l / lcombined , standard.
Sample carry-over from a 10.0 to an 0.5 ~ l / l standard . (a 20-fold concentration difference) was very low for acetone, ethanol, and chloroform (Table I). Although a paired-sample t test ( 5 ) showed that the carry-over was significant for acetone and ethanol a t 0.05 significance level, the average carry-over was equivalent to only +0.02 and +0.08 pl/L for acetone and ethanol. respectively. The chloroform carryover was not significant at the 0.05 level. In the case of 1-propanol, the carry-over was significant at the 0.05 level and equivalent to +0.14 wl/L This error would be reduced if (a) the preceding sample concentration were less than 20X the following sample concentration or (b) if replicate samples were run at the low level. Previous work (not shown) has demonstrated that the carry-over decreases with each succeeding replicate, so that a mean value from a set of replicates would be less subject to carryover error than would the first sample in the set. The data from the 10.0 j J / L standards in Table I demonstrate the precision of the method a t a high concentration level. The standard deviations were less than 2% of the mean values for all compounds except chloroform, which showed a standard deviation of 11%of the mean, for reasons outlined above. Comparison of the gas sampling device with a syringe method of injection ('Table 11) showed that the variance of the gas sampling device was 10-fold smaller than that of the syringe method for acetone and 1-propanol. In the case of chloroform, the variance of the gas sampling device was about half that of the syringe method. Hence, the precision of the gas sampling device was better than that of the syringe for the three compounds tested. Inspection of the
rubber septum after the 10 syringe injections demonstrated serious physical damage to the septum, although no obvious leakage was noted in these tests. In both the syringe and valve tests, the absolute peak areas were not closely reproduced. In the case of the gas sampling device, for example, the isobutyl alcohol internal standard peak area varied from 38169 to 61825 area units. Both the syringe method and the gas sampling valve procedure were made more precise by including an internal standard, in agreement with the head space work of Kepner et al. ( 3 ) . We strongly recommend the use of internal standards for all quantitative head space analyses with the gas sampling device described here. The use of this apparatus is subject to the same limitations as were the head space procedures used by previous workers (1-4), namely, that the sensitivity is limited by the equilibrium vapor pressure of the particular compound in water.
ACKNOWLEDGMENT The authors thank E. P. Meier and J. J. Barkley, Jr., for their helpful suggestions during this investigation.
LITERATURE CITED C. Weurman, Food Techno/., 15, 531 (1961). S.Ozeris and R. Bassette, Anal. Chem., 35, 1091 (1963). R . E. Kepner, H.,,Maarse,and J. Strating, Anal. Chem., 36, 77 (1964). R. Bassette, S.Ozeris, and C. H. Whitnah, Anal. Chem., 34, 1540 (1962). (5) G. W. Snedecor and W. G. Cochran, "Statistical Methods", 6th ed., Iowa State University Press, Ames, iowa, 1967. pp 92-94.
(1) (2) (3) (4)
RECEIVEDfor review June 6, 1975. Accepted August 14, 1975.
Elimination of Some Integration Errors in Pesticide Residue Analyses J. M. Zehner and R. A. Simonaitis Stored-Product Insects Research and Development Laboratory, Agricultural Research Survey, USDA, Savannah, Ga. 3 1403
The method of determining residues of organophosphorus pesticides by gas chromatography with a Tracor flame photometric detector in the phosphorus mode is well documented ( I ) . Such analyses are done routinely by using automatic injection and integrated with a Hewlett-Packard 3370B integrator. When the sample is injected, the detector response passes through a minimum after the solvent elutes, and then gradually rises to base line without passing through a maximum. Because the response does not pass through a maximum before the organophosphate peak elutes, the integrator continues integrating until the pesticide peak (malathion) is eluted (see Figure 1, left). The integrator therefore records an erroneous area because the entire signal is included from the time of the minimum to
the time after the pesticide peak has eluted. Solutions to this problem could be adjustment of flow rate and column temperature. However, it is advantageous with respect to time of analysis and sensitivity, for the pesticide peak to elute as early as possible. Integrator start delay is another possible solution, but not all integrators have this capability. We found that the difficulty can be overcome by adding a low-boiling organosphosphate such as tributyl phosphate in sufficient amount to the solvent to achieve 20 ppm during the extraction procedure. The tributyl phosphate causes the detector response to pass through a maximum after the minimum caused by the solvent and return to the base line before the pesticide (malathion) peak elutes (see
ANALYTICAL CHEMISTRY, VOL. 47, NO. 14,DECEMBER 1975
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EXPERIMENTAL
Flgure 1 (Left). Typical chromatogram of organophosphate pesticide residue analysis observed with a flame photometric detector. (Right) Same as left with the addition of tributyl phosphate
The procedure used is as follows. Extraction. Weigh 40 g of representative samples and add 120 ml methylene chloride (Fisher Scientific) and 1 ml 0.1% tributyl phosphate (Aldrich Chemical Co.) in methylene chloride (wlv). Shake on wrist shaker for 3 hr. Filter for gas chromatographic analysis. Gas chromatographic Conditions. Instrument. HewlettPackard 5750B equipped with a flame photometric detector (FPD 100AT,Melpar, Ine.), Hewlett-Packard 3370B integrator, and H-P 7670A automatic sampler. Column.4-ft glass, 6-mmad., 4-mm i.d. packed with 5%OW101 (Applied Science Lab.) on Gas Chrom Q 801100 mesh (Applied Science Lab.); temperatures, column, 280 "C; injection port, 330 ' C ; detector, 220 'C. Gas Flours. Nitrogen carrier, 75 mllmin; hydrogen, 150 mllrnin; oxygen, 30 mllmin. Sample injection volume, 10 111. Calculation. Calculate the pesticide concentration by comparison of integrator counts obtained for the sample with those obtained from known malathion standard concentrations.
LITERATURE CITED Figure 1, right). Thus, an accurate area for the pesticide is recorded by the integrator. Tributyl phosphate is a suitable material for residue analyses of such pesticides as malathion, stirofos, chlorpyrifos, and other organophosphates on various agricultural commodities. The determination of malathion residues on rice is a typical illustration of the use of this technique.
(1)
S.S. Brody and J. E. Chaney. J. Gas Chromatogr., 4.42 (196%)
RECEIVEDfor review June 2, 1975. Accepted September 10, 1975. Mention of a pesticide or a commercial or proprietrary product in this paper does not constitute a recommendation or an endorsement of this product by the U.S.
p-.."-'----+
CORRECT11 Petroleum Review
In the Hydrocarbon section of the Petroleum Review, Anal. Chem., 47, 197R (1975), the authors inadvertently misquoted Wolfgang Zenker in paragraph 5. The correct statement is: "An infrared method was used by Zenker (3921)t o measure the methyl and methylene peak intensities in a homologous series of n-alkanes, n-alcohols, n-aldehydes, and n-carboxylic acids. He reports t h a t the methylene group absorptivity decreases with increasing polarity of the functional group."
Terpene-Sierane Release from Kerogen by Pyrolysls Gas Chromatography-Mass Spectrometry
In this article by E. J. Gallegos, Anal. Chem., 47, 1524 (1975), Figure 5 shows a duplicate IR spectrum rather than a Raman spectrum as indicated. A corrected Figure 5 presented here shows an improved quality IR spectrum and the missing Raman spectrum.
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ANALYTICAL CHEMISTRY, VOL. 47, NO. 14, DECEMBER 1975
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