Anal. Chem. 1997, 69, 2411-2417
Environmental Applications of Gas Chromatography/Atomic Emission Detection Donald F. Gurka,* Steven Pyle, and Richard Titus
National Exposure Research Laboratory, Characterization Research Division, U.S. Environmental Protection Agency, P.O. Box 93478, Las Vegas, Nevada 89193-3478
A gas chromatograph/atomic emission detector (GC/ AED) system has been evaluated for its applicability to environmental analysis. Detection limits, elemental response factors, and regression analysis data were determined for 58 semivolatile environmental contaminants. Detection limits for injected analytes ranged from 0.17 to 3.0 ng on the hydrogen 486-nm channel, from 1.0 to 5.0 ng on the nitrogen 174-nm channel, from 0.65 to 11.7 ng on the oxygen 777-nm channel, from 0.071 to 3.0 ng on the chlorine 479-nm channel, and from 0.023 to 0.038 ng on the sulfur 181-nm channel. Mean elemental response factors (ERFs) measured on these channels, relative to the carbon 496-nm channel, were hydrogen, 0.084 (mean %RSD ) 6.6); nitrogen, 0.246 (mean %RSD ) 19); oxygen, 0.459 (mean %RSD ) 16); and chlorine, 0.417 (mean %RSD ) 3.6). The higher precision obtained for hydrogen and chlorine, relative to that for nitrogen and oxygen, is attributed to the ability to scan these elemental channels in the same GC run as the carbon 496-nm channel (diode array wavelength range limitation of ∼40 nm/run). Mean ERFs of standard compounds were used to determine the molecular formulas of chlorinated hydrocarbons and chlorinated organosulfur compounds in a contaminated environmental soil sample. These formulas are in good agreement with the molecular weights and chlorine isotopic data obtained from low-resolution gas chromatography/mass spectrometry. Environmental sample screening is typically performed by GC/ MS, a technique which is both sensitive and effective;1 however, this technique is also very sensitive to unsubstituted and substituted saturated hydrocarbons, compounds which are common environmental sample constituents. Indeed, an EPA study, in which the most intense GC/MS total ion chromatogram (TIC) peaks from 3000 environmental analyses were manually checked, indicated that greater than 50% of the detections were saturated hydrocarbons.2 Because these are not hazardous compounds, it was concluded that a screening technique which could selectively “flag” those TIC peaks corresponding to heteroatom-containing chemicals would facilitate subsequent GC/MS analysis using the following approach: (i) screen sample by GC/AED and flag chromatogram peaks corresponding to heteroatomics; (ii) re(1) Message, G. M. Practical Aspects of Gas Chromatography/Mass Spectrometry; John Wiley and Sons, Inc.: New York, NY, 1984. (2) Donnelly, J. R.; Abdel-Hamid, M. S.; Jeter, J. L.; Gurka, D. F. J. Chromatogr. 1993, 642, 409-415.
analyze sample by low-resolution GC/MS, focusing on GC/AED flagged peaks (if library mass spectrum is available, confirm tentative GC/MS identification with elemental data; if spectrum is unavailable, use combination of low-resolution spectral information with elemental data to assign tentative identity or partial structure); and (iii) use of GC/AED data to select standards with elemental distribution similar to that of flagged heteroatomics for analyte semiquantitation (completely or partially identified analytes should be quantifiable to (10-20%). The gas chromatograph/atomic emission detector (GC/AED) system promises to be a valuable addition to the array of tools for screening volatile and semivolatile environmental compounds. The GC/AED uses a microwave-induced helium plasma to fragment, atomize, and ionize gas chromatographic eluents and exhibits high selectivity (relative to carbon) and picogram per second elemental sensitivity for both metals and nonmetals.3 The GC/AED system offers potential as an elemental screening tool for complex environmental samples containing pesticides and halogenated hydrocarbons. GC/AED is a technique for routine determination of empirical formulas when mass spectrometric techniques are unavailable or uneconomical. It is also a means of reducing the number of internal standards required for quantification. The application of GC/AED as a routine environmental screening tool has not yet been reported for actual environmental samples. There is disagreement regarding the prospects for empirical formula determinations,4-6 and it is postulated that GC/ AED elemental ratios are best used in conjunction with mass spectrometric molecular weights.7 There is also disagreement whether GC/AED can be used to minimize the number of standards required for quantification. The disagreement ranges from whether to require only a single internal standard for a chemical family with the same elemental distribution8 to the need for standards with the same molecular structure and retention time as those of the analyte to be quantified.9 The literature disagreement may arise from several factors. First, many laboratories have assembled their own helium microwave plasma systems,10,11 while others use integrated com(3) Quimby, B. D.; Sullivan, J. J. Anal. Chem. 1990, 62, 1027-34. (4) Valente, A. L. P.; Uden, P. C. Analyst 1990, 115, 525-529. (5) Jelink, J. T.; Venema, A. J. High Resolut. Chromatogr. 1990, 13, 447-450. (6) Pedersen-Bjergaard, S.; Asp, T. N.; Vedde, J.; Greibrokk, T. J. Microcolumn Sep. 1992, 4, 163-170. (7) Hooker, D. B.; DeZwaan, J. Anal. Chem. 1989, 61, 2207-2211. (8) Andersson, J. T.; Schmid, B.; Fresen, J. Anal. Chem. 1993, 346, 403-409. (9) Jana´k, K.; Colmsjo ¨, A.; O ¨ stman, C. J. Chromatogr. Sci. 1995, 611-621. (10) Gelencse´r, A.; Szepvolgyi, J.; Hlavay, J. J. Chromatogr. 1993, 654, 269277.
S0003-2700(96)01273-5 This article not subject to U.S. Copyright. Publ. 1997 Am. Chem. Soc.
Analytical Chemistry, Vol. 69, No. 13, July 1, 1997 2411
mercially available GC plasma systems.12,13 This might affect the between-laboratory reproducibility of GC/AED data. Second, molecular effects such as incomplete conversion to elemental ions,14 molecular weight or GC retention time effects,9 and chromatographic peak shape15 may also affect performance. Third, sample effects such as difficult-to-correct ion backgrounds resulting from the high-energy plasma or trace gaseous impurities16,17 and concentration effects on response15 may also affect data comparability. Finally, some laboratories may not have analyzed a sufficient number of analytes to draw meaningful conclusions or may have inadvertently selected “well-behaved” analytes. In this study, 58 typical environmental contaminants were analyzed by GC/AED. Detection limits and elemental response factors (ERFs) were determined for hydrogen, nitrogen, oxygen, and chlorine. Regression analyses were carried out from the low nanogram to picogram detection limit to the upper loading limit of the GC column (about 50 ng). Using the ERFs of standard compounds, the empirical formulas were determined for 19 chlorinated hydrocarbons and eight chlorinated sulfur-containing hydrocarbons in a complex environmental sample. EXPERIMENTAL SECTION GC/AED. The atomic emission detector (AED) used in this study was a Hewlett-Packard Model 5921A equipped with a photodiode array (PDA) having a nominal wavelength range of 160-800 nm and a spectral resolution of 0.1 nm at 400 nm. The concave holographic grating has a flat focal plane along which the PDA can be moved. Plasma power is supplied by a Panasonic 2Mz11A microwave magnetron tube. The AED and PDA have been described in detail elsewhere.18 Extract solvent was vented, before entering the plasma, under computer control. After venting, the GC ramp was initiated and data collection began. The 1.0-mm-i.d. × 42-mm long fused-silica discharge tube was watercooled and fitted with a fused-silica window, which was purged with 35 mL/min of reagent grade helium. PDA wavelength calibration and reagent gas selection were under computer control. Groups of elements were scanned, without background correction, for carbon, hydrogen, chlorine, and bromine (group I) at 496, 486, 479, and 478 nm, respectively. Sulfur and nitrogen (group II) were scanned at 181 and 174 nm, respectively, and oxygen (group III) was scanned at 777 nm. Data Acquisition/Data Handling. The instrumental parameters, GC program, and elemental group (I, II, or III) wavelengths were entered into the computer. After injection and solvent venting, the GC ramp, reagent gas selection, and wavelength changes for each element group were under computer control. After elemental chromatograms were integrated, data were transferred to a Hewlett-Packard 486 Vectra personal computer. Integration area counts and concentration data were subjected to regression analysis using the Quattro Pro statistics package. Detection limits were determined by extrapolation of dilution curve (11) Yieru, H.; Qingyu, O.; Weile, Y. J. Chromatogr. Sci. 1990, 28, 584-588. (12) Kovacic, N.; Ramus, T. L. J. Anal. At. Spectrom. 1992, 7, 1000-1005. (13) Petersen-Bjergaard, T. N. A.; Greibrokk, T. Anal. Chim. Acta 1992, 265, 87-92. (14) Windsor, D. L.; Denton, M. B. Anal. Chem. 1979, 51, 1116-1119. (15) Janak, K.; Ostman, C.; Carlsson, H.; Bemgaard, A.; Colmsjo, A. J. High Resolut. Chromatog. 1994, 17, 135-140. (16) Bradley, C.; Carnahan, W. Anal. Chem. 1988, 60, 858-863. (17) Freeman, J. E.; Hieftje, G. M. Spectrochim. Acta, Part B 1985, 40, 475. (18) Sullivan, J. J.; Quimby, B. D. Anal. Chem. 1990, 62, 1034-1043.
2412
Analytical Chemistry, Vol. 69, No. 13, July 1, 1997
chromatograms to signal/noise ) 3, measured peak to peak. Elemental response factors were calculated at each of five concentration levels for each analyte, and the mean and %RSD were determined. A Hewlett-Packard Model 5890 gas chromatograph, equipped with a Hewlett-Packard Model 7673 autoinjector, was interfaced to the AED. A 30-m × 0.32-mm capillary column coated with 0.25µm HP-5 was used for all analyses. The column coating was removed from the last 4 in. of the GC column, which was then inserted into the discharge tube to within 15 mm of the plasma. The GC injector and the AED transfer line and discharge cavity block were maintained at 280 °C. Autoinjections were splitless with a purge delay of 30 s. The GC was programmed from 45 to 280 °C at 10 °C/min with an initial hold time of 4 min. The hydrogen, oxygen, and 10% methane-in-nitrogen reagent gases were of ultra-high-purity grade (g99.995%). Ultrapure helium was used at a GC column flow of 1.0 mL/min and also served as a makeup gas. Helium makeup plus reagent gas flow to the discharge tube was approximately 67.5 mL/min for nitrogen and sulfur determination and 75 mL/min for oxygen, hydrogen, chlorine, bromine, and carbon determination. Gas flows were optimized, under computer control, to the manufacturers’ specifications. A UOP Model P-100-1 helium purifier was used inline between the helium tank and the plasma cavity. The purifier lowered the nitrogen 746-nm, carbon 496-nm, oxygen 777-nm, and hydrogen 486-nm channel background counts by at least 50%. GC/MSD. Samples and standards were analyzed with a Hewlett-Packard Model 5890 gas chromatograph interfaced to a Model 5970A mass-selective detector. The detector was scanned from 50 to 550 Da in 1.3 s at nominal 1 amu resolution. The inlet and detector were set at 250 and 280 °C, respectively. Source was operated at 70 eV and maintained at 150 °C. The GC column and chromatographic conditions and settings were the same as those used for GC/AED runs, except that splitless manual injections were used. Standards/Samples. Supelco base-neutral, phenol, and pesticide standards were used for the detection limit, regression analysis, and elemental response factor data. For regression analyses, five concentration levels were prepared and analyzed over the range 50-1.0 ng. Each analysis was performed in triplicate at five different concentration levels. The environmental soil sample was collected from an EPA Superfund site and was extracted with methylene chloride. The dried methylene chloride extract was used without further cleanup. RESULTS AND DISCUSSION Detection Limits. Table 1 lists 58 (n ) 58) analyte detection limits for a variety of environmental contaminants including anilines, chlorinated and nitrated hydrocarbons, phenols, pesticides, and phthalates which elute over a GC chromatogram range of 29 min. The representativeness of this selection is further enhanced by the inclusion of a variety of functional groups (aliphatic and aromatic chlorine, nitro, nitroso, hydroxy, aliphatic and aromatic ether, and oxygen and sulfur esters). This selection of compounds should be a good test of the literature claims of molecular-independent, but elemental distribution-dependent, plasma emission response factors.19,20 (19) Scha¨fer, W. J. High Resolut. Chromatog. 1993, 16, 674-676. (20) Olson, N. L.; Carrell, R.; Cummings, R. K.; Rieck, R. LC/GC 1994, 12, 142-154.
Table 1. Detection Limitsa compd no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58
analyte
retention time, min
bis(2-chloroethyl) ether 2-chlorophenol 1,3-dichlorobenzene 1,4-dichlorobenzene 1,2-dichlorobenzene benzyl alcohol bis(2-chloroisopropyl) ether 2-methylphenol N-nitroso-di-n-propylamine nitrobenzene hexachloroethane 4-methylphenol isophorone 2-nitrophenol 2,4-dimethylphenol bis(2-chloroethoxy)methane 2,4-dichlorophenol 1,2,4-trichlorobenzene impurity 2-chloroaniline hexachlorobutadiene 4-chloro-3-methylphenol 2-methylnaphthalene 2-chloronaphthalene 2,4,5-trichlorophenol 2,4,6-trichlorophenol impurity 2-nitroaniline 2,6-dinitrotoluene dimethyl phthalate 3-nitroaniline 2,4-dinitrophenol 2,4-dinitrotoluene dibenzofuran 4-nitrophenol 4-nitroaniline dimethyl phthalate 4,6-dinitro-2-methylphenol 4-chlorophenyl phenyl ether N-nitrosodiphenylamine R-BHC 4-bromophenyl phenyl ether β-BHC hexachlorobenzene γ-BHC pentachlorophenol δ-BHC heptachlor di-n-butyl phthalate aldrin heptachlor epoxide R-endosulfan coelutants dieldrin, DDE endrin β-endosulfan endrin aldehyde, DDD endosulfan sulfate butyl benzyl phthalate DDT p,p′-methoxychlor bis(2-ethylhexyl) phthalate di-n-octyl phthalate
8.3 8.6 8.9 9.0 9.4 9.7 10.0 10.0 10.1 10.2 10.3 10.4 10.9 11.0 11.6 11.6 11.8 12.0 12.0 12.2 12.8 13.8 13.8 14.6 14.6 14.7 14.9 15.1 15.7 15.9 16.1 16.3 16.7 16.8 17.0 17.6 17.7 17.8 17.8 18.1 18.9 19.0 19.2 19.3 19.6 19.7 19.7 21.4 21.9 22.1 22.9 23.6 24.1 24.4 24.5 24.8 25.3 25.5 25.6 26.6 27.3 28.8
mean detection limit, ng range a
hydrogen 0.50 0.26 1.0 1.0 0.75 1.0 0.50 0.20 0.40 1.0
detection limit (ng of analyte injected) nitrogen oxygen chlorine 1.0 1.0
2.0 1.3
5.0 1.0 1.0 2.0 1.5
sulfur
0.25 0.50 0.30 0.30 0.20 0.40
0.10 0.30 0.17 0.70 0.36 0.55 1.0 2.0 1.0 3.0
1.0
2.0 2.0 1.0 3.0 1.0 3.0
0.50 0.71 0.23
4.0 5.0 4.0
3.0 0.10 1.0
4.0 6.0
0.4 0.7
0.37 1.5 2.0 1.0 1.0 0.50 2.0 3.0 3.0 0.53
1.0 1.0
3.0 0.50 1.3 0.50 0.75 0.77 0.86 1.0
4.0
2.0 2.0 2.0
1.4 3.0
3.0 1.3 0.65 4.0 2.0 3.0 2.0 5.0 0.68 1.5 0.71 3.0
0.50 0.25
4.0
0.77 1.5 1.4 1.0 0.50 0.50 1.0 1.0 0.32 0.73 1.0 0.50 0.70 1.0 0.50 1.0 0.50 0.60
0.25 0.11 0.16 0.5 0.31 0.13
1.0 2.0 2.0 9.4 2.0 11 2.0 5.0 2.5 2.5
0.97 0.17-3.0
2.1 1.0-5.0
2.8 0.65-11
0.14 0.12 0.14 0.10 0.70 0.18 0.18 0.20
0.023 0.027 0.038
0.25 0.63
0.41 0.10-3.0
0.029 0.023-0.038
Based on chromatogram peak S/N ) 3 (measured peak to peak).
The analyte detection limits for the nonmetals hydrogen, nitrogen, oxygen, chlorine, and sulfur are 0.97 (n ) 54), 2.1 (n ) 12), 2.8 (n ) 41), 0.41 (n ) 33), and 0.029 (n ) 3) ng, respectively. The hydrogen, nitrogen, oxygen, chlorine, and sulfur detection
limit ranges are 0.17-3.0, 1.0-5.0, 0.65-11, 0.07-3.0, and 0.0230.028 ng, respectively. These correspond to range factors of 18, 5.0, 17, 43, and 1.2, respectively. When corrected for the number of element atoms, these range factors change to 10, 5.0, 25, 35, Analytical Chemistry, Vol. 69, No. 13, July 1, 1997
2413
Table 2. Regression Analysis Data
analyte
retention time, min
bis(2-chloroethyl) ether 2-chlorophenol 1,3-dichloromethane 1,4-dichlorobenzene 1,2-dichlorobenzene bis(2-chloroisopropyl) ether N-nitroso-di-n-propylamine nitrobenzene hexachloroethane isophorone 2-nitrophenol 2,4-dimethylphenol bis(2-chloroethoxy)methane 2,4-dichlorophenol 1,2,4-trichlorobenzene hexachlorobutadiene 4-chloro-3-methylphenol 2-chloronaphthalene 2,4,6-trichlorophenol 2,6-dinitrotoluene dimethyl phthalate 2,4-dinitrophenol 2,4-dinitrotoluene 4-nitrophenol diethyl phthalate 4,6-dinitro-2-methylphenol 4-chlorophenyl phenyl ether N-nitrosodiphenylamine R-BHC β-BHC hexachlorobenzene γ-BHC pentachlorophenol δ-BHC heptachlor di-n-butyl phthalate aldrin heptachlor epoxide R-endosulfan dieldrin, DDE endrin aldehyde, DDD endosulfan sulfate butyl benzyl phthalate DDT endrin ketone bis(2-ethylhexyl) phthalate di-n-octyl phthalate
8.3 8.6 8.9 9.0 9.4 10.0 10.1 10.2 10.3 10.9 11.0 11.6 11.6 11.8 12.0 12.8 13.8 14.6 14.7 15.7 15.9 16.3 16.7 17.0 17.7 17.8 17.8 18.1 18.9 19.2 19.3 19.6 19.7 19.7 21.4 21.9 22.1 22.9 23.6 24.1 24.8 25.3 25.5 25.6 26.1 27.3 28.8
slope range, mean %RSD a
hydrogen 486-nm slope r2
nitrogen 174-nm slope r2
oxygen 777-nm slope r2 34.1
25.8 25.6 27.5 38.1 110 37.6
0.9891 0.9839 0.9836 0.9876 0.9915 0.8705
109 35.4 88.7 58.3 25.8 14.4
0.9746
19.7 342 79.7
0.7986 0.9507 0.9899
0.9902 0.9755 0.9711 0.9331 0.9768 0.9413
27.6 99.8 39.3 54.6 29.0
0.8479 0.9883 0.9745 0.8914 0.9616
47.5 43.9
0.9430 0.9794
47.8
0.9430
32.1 56.4 17.3 31.1 24.4 63.8 26.8 49.6 71.3 18.5 17.6
0.9824 0.9958 0.8489 0.9724 0.5237 0.9947 0.9660 0.9934 0.9925 0.9939 0.9818
27.8 101 108 118 108
0.8304 0.9883 0.9824 0.9790 0.9824
81.8
0.9898
28.9
0.9490
18.6
0.9882
18.9 11.6 87.8 19.7 11.3 12.6 39.7 49.2 12.8 67.8 21.9 21.0 111 108
0.9724 0.9853 0.9960 0.9909 0.9735 0.9768 0.9909 0.9416 0.9390 0.8139 0.9863 0.9795 0.9807 0.9532
11-111
0.9558
46.0 27.1
0.8880 0.8508
34.5
0.8167
5.1 34.8
0.9326 0.8474
22.3
0.7277
37.0
0.8098
5.1-46
0.8390
17.4
0.7452
75.9
0.9529
8.4 31.0 10.6 10.9 28.5 65.5
0.9007 0.9888 0.9303 0.8801 0.9450 0.9542
18.8 52.9 47.6
0.9930 0.9423 0.8959
8.4-342
0.8991
chlorine 479-nm slope r2 57.4 32.3 58.4 58.1 60.9 26.8
0.9830 0.9931 0.9887 0.9864 0.9863 0.9790
112
0.9888
26.8 59.7 73.0 101 31.7 25.7 27.8
0.9790 0.9908 0.9858 0.9918 0.9819 0.9558 0.8304
85.0 83.5 95.6 82.5 70.2 84.4 76.3
0.9934 0.9932 0.9906 0.9929 0.9891 0.9928 0.9934
66.3 72.0 59.3 114 125a 54.2
0.9928 0.9918 0.9894 0.9932 0.9986 0.9882
54.0 71.9
0.9882 0.9980
26-114
0.9830
carbon 496-nm slope r2 333
0.9959
470 469 490 219 505 584 110 746 496 741 340 421 400 179 598 743 363 641 641 367 621 439 730 412 960 262 235 230 431 225 238 240 299 726 366 283 244 839 899 227 753 418 396 754 746
0.9963 0.9956 0.9965 0.9953 0.9947 0.9950 0.9570 0.9966 0.9791 0.9790 0.9926 0.9691 0.9965 0.9965 0.9685 0.9969 0.9685 0.9971 0.9971 0.9668 0.9963 0.9319 0.9962 0.9672 0.8467
110-960
0.9832
0.9974 0.9973 0.8757 0.9955 0.9659 0.9982 0.9972 0.9967 0.9971 0.9966 0.9962 0.9971 0.9980 0.9975 0.9944 0.9959 0.9964 0.9906 0.9907
Excluded from slope range because of coelution.
and 1.2, respectively. This indicates that the order of decreasing sensitivity to molecular structure is Cl > O > H > N . S. These detection limit ranges also suggest that errors using molecular-independent quantification (use of a single standard to quantify a series of compounds having similar elemental distribution but different molecular structure) will be larger for chlorine than for nitrogen, oxygen, or hydrogen. A second implication is that compound detection limits will be determined by the least sensitive heteroatom and, for empirical formulas of the type CxCly, CxClyHz, CxHzNb, and CxHzOc, will be 0.41, 0.97, 2.1, and 2.8 ng injected, respectively. Except for the phthalate oxygen channel, there are no significant trends for hydrogen, nitrogen, oxygen, and chlorine as a function of increasing retention time. Regression Analysis. Table 2 shows the regression analysis data for the hydrogen 486-nm, nitrogen 174-nm, oxygen 777-nm, chlorine 479-nm, and carbon 496-nm channels. The order of 2414
Analytical Chemistry, Vol. 69, No. 13, July 1, 1997
decreasing regression precision, as measured by the coefficient of determination (r2), is C = Cl < H < O < N. The poorer precision for nitrogen may result from chromatographic tailing resulting from the greater polarity of these compounds relative to the neutral analytes. The regression slope range ratios for hydrogen, nitrogen, oxygen, chlorine, and carbon are 9.0, 9.0, 41, 4.4, and 8.7, respectively; if the oxygen channel slope of 342 for N-nitroso-din-propylamine is discarded as an outlier, the range ratio drops to 14 (this can be justified on the basis of the known thermal instability of N-nitrosoamines21). The magnitude of these ratios suggests little sensitivity of the elemental regression slope to molecular structure, unlike earlier reported molecular regression (21) Fine, D. H. N-Nitroso Compounds in the Environment. In Advances in Environmental Science and Technology; Pitts, J., Metcalf, R., Eds.; John Wiley and Sons, Inc.: New York, NY, 1980; Vol. 10, p 40.
data for mass spectral and infrared detectors.22 Relative insensitivity to molecular structure is consistent with earlier reports that a single standard, with similar elemental distribution, can be used to semiquantitate a family of compounds.19,20 Effects of Carbon Bond Hybridization. Next the analytes were separated into SP3 and SP2 bond hybridization classes by means of the carbon type to which the functional group is bonded. The detection limits and regression slopes were then corrected for the number of atoms per molecule. Then the mean detection limits, in nanograms, were hydrogen, 0.12 (n ) 14); chlorine, 0.067 (n ) 14); oxygen, 2.16 (n ) 10); nitrogen, 1.4 (n ) 12); and sulfur, 0.029 (n ) 3). Following this, there was a slight trend to higher hydrogen detection limits, as a function of retention time, for both SP3 and SP2 hybridization, but there was no discernable trend for oxygen or chlorine detection limits. The order of decreasing detection limits (O > N > H > Cl > S), with the exception of oxygen, parallels the order of decreasing first ionization energies, which are 314, 335, 314, 300, 260, and 239 kcal/mol, respectively.23 Problems associated with oxygen-selective analysis by atmospheric pressure microwave plasma systems have been summarized by Bradley and Carnahan.24 For regression slopes, there is no significant trend with retention time for SP3 or SP2 carbon hybridization for the hydrogen channel. In addition, there is no significant retention time trend for chlorine bound to SP3 carbon, but there is a slight trend for SP2 carbon. The oxygen channel regression results are complicated by analytes which contain oxygen bound to sulfur, aromatic carbon, and nitrogen. If oxygen bound to carbonyl carbon is considered to be SP3 hybridization, rather than SP2, there are no significant trends in the regression slopes of elements bound to SP3 or SP2 carbon as a function of retention time. Elemental Response Factors. A mean elemental response factor (ERF) was calculated from the average of ERFs for each concentration level used for regression analysis using the expression
ERF ) (elemental area counts/carbon 496-nm area
Figure 1. Total ion chromatogram of soil extract (a), AED chlorine 479-nm channel (b), and AED sulfur channel at 181 nm (c).
counts) × (no. of carbon atoms/no. of element atoms) The order of increasing %RSD (mean value) was Cl < H , O = N, and the order of increasing ERF was H < N < Cl = O; thus, although the hydrogen response is weakest, it is precise. Empirical Formula Determination. To test the capability of GC/AED for environmental sample characterization, a soil sample, containing organochlorine and organochlorine-sulfur compounds, was analyzed. The total ion chromatogram and the AED chlorine and sulfur channel chromatograms are shown in Figure 1. The sample had previously been analyzed, and sulfurcontaining constituents could not be identified with confidence using low-resolution GC/MS; it was thus necessary to obtain an accurate mass by high-resolution techniques. Because routine use of accurate mass approaches is economically prohibitive, a subsequent attempt was undertaken to see if this sample could be characterized by GC/AED alone or in combination with lowresolution GC/MS. Following GC/AED analysis, calculated atoms were obtained by using the mean ERFs listed in Table 3. The (22) Gurka, D. F.; Hiatt, M.; Titus, R. Anal. Chem. 1984, 56, 1102-1110. (23) Harvey, K. B.; Porter, G. B. Physical Inorganic Chemistry; Addison Wesley: Reading, MA, 1963. (24) Bradley, C.; Carnahan, J. W. Anal. Chem. 1988, 60, 858-863.
theoretical and calculated numbers of C, H, Cl, and S atoms are shown in Table 4. The percent deviations between theoretical and calculated atoms for the chlorobenzenes are carbon, 0-4%; hydrogen, 4-25%; and chlorine, 0-0.2%. For organochlorine sulfides, the carbon, hydrogen, chlorine, and sulfide deviations computed from theory are 1-5%, 3-59%, 0-5%, and 0-12%, respectively. Typically, the deviation between computed and theoretical numbers of atoms was not close to one-half atom, and thus one was not faced with the problem of integer multiples of the simplest formula. Because halogen comprised 44-62% of the molecular weight of these analytes and the molecular weights of 15 of the 17 sample analytes exceeded 200, analytes which are integer multiples of the simplest molecular formula were not likely to pass through a gas chromatograph (molecular weight cutoff, ∼450 for neutral compounds). CONCLUSION The sensitivity and precision of GC/AED for trace analysis compare favorably with reports for GC/FT-IR and full-scan GC/ MS.25 Nanogram analyte detection limits for carbon, hydrogen, chlorine, nitrogen, and oxygen translate to picogram elemental Analytical Chemistry, Vol. 69, No. 13, July 1, 1997
2415
Table 3. Elemental Response Factors (ERFs)a analyte
retention time, min
bis(2-chloroethyl) ether 2-chlorophenol 1,3-dichlorobenzene 1,4-dichlorobenzene 1,2-dichlorobenzene benzyl alcohol bis(2-chloroisopropyl) ether 2-methylphenol N-nitroso-di-n-propylamine nitrobenzene hexachloroethane 4-methylphenol isophorone 2-nitrophenol 2,4-dimethylphenol bis(2-chloroethoxy)methane 2,4-dichlorophenol 1,2,4-trichlorobenzene 2-chloroaniline hexachlorobutadiene 4-chloro-3-methylphenol 2-methylnaphthalene 2-chloronaphthalene 2,4,5-trichlorophenol 2,4,6-trichlorophenol 2-nitroaniline 2,6-dinitrotoluene dimethyl phthalate 3-nitroaniline 2,4-dinitrophenol 2,4-dinitrotoluene dibenzofuran 4-nitrophenol 4-nitroaniline diethyl phthalate 4,6-dinitro-2-methylphenol 4-chlorophenyl phenyl ether N-nitrosodiphenylamine R-BHC 4-bromophenyl phenyl ether β-BHC hexachlorobenzene γ-BHC pentachlorophenol δ-BHC heptachlor di-n-butyl phthalate aldrin heptachlor epoxide R-endosulfan endrin β-endosulfan endosulfan sulfate butyl benzyl phthalate DDT endrin ketone p,p′-methoxychlor bis(2-ethylhexyl) phthalate di-n-octyl phthalate
8.3 8.6 8.9 9.0 9.4 9.7 10.0 10.0 10.1 10.2 10.3 10.4 10.9 11.0 11.6 11.6 11.8 12.0 12.2 12.8 13.8 13.8 14.6 14.6 14.7 15.1 15.7 15.9 16.1 16.3 16.7 16.8 17.0 17.6 17.7 17.8 17.8 18.1 18.9 19.0 19.2 19.3 19.6 19.7 19.7 21.4 21.9 22.1 22.9 23.6 24.4 24.5 25.3 25.5 25.6 26.1 26.6 27.3 28.8
H486 (%RSD)
mean ERF, mean %RSD a
0.093 (19.9) 0.079 (20.3) 0.085 (1.1) 0.089 (6.84) 0.094 (9.72) 0.14 (27.9) 0.079 (20.3) 0.089 (10.6) 0.093 (7.80) 0.084 (5.88)
N174 (%RSD)
0.271 (18.0) 0.282 (11.5)
O777 (%RSD)
C1479 (%RSD)
0.456 (19.1) 0.450 (14.0)
0.420 (6.77) 0.469 (3.38) 0.434 (5.62) 0.416 (8.40) 0.500 (7.77)
0.490 (30.7) 0.450 (14.0) 0.490 (12.5) 0.469 (8.20) 0.500 (14.5)
0.413 (11.5)
0.418 (4.78) 0.088 (10.2) 0.091 (7.82) 0.084 (7.95) 0.080 (10.6) 0.090 (7.0) 0.080 (6.14) 0.084 (6.41) 0.079 (9.81) 0.088 0.083 (6.23) 0.086 (5.03) 0.083 (7.21) 0.086 (2.53) 0.080 (7.20) 0.084 (2.11) 0.089 (5.63) 0.080 (8.60) 0.080 0.081 (10.61) 0.086 (6.89) 0.076 0.065 (0.55) 0.086 (12.1) 0.082 (11.26) 0.092 (5.79) 0.093 (4.85) 0.083 (1.87) 0.083 (6.15) 0.082 (0.58)
0.248 (22.7)
0.473 (7.46) 0.536 (18.2) 0.482 (12.6) 0.477 (13.5) 0.501 (10.6) 0.409 (17.7)
0.230 (18.7) 0.477 (15.4)
0.229 (16.7) 0.272 (16.6) 0.226 (19.5) 0.251 (27.8) 0.279 (16.2) 0.212 0.199 (30.6) 0.256 (13.1)
0.489 (16.2) 0.484 (16.5) 0.420 (15.4) 0.485 (25.9) 0.462 (4.95) 0.412 (17.7) 0.492 (26.8) 0.471 (24.8) 0.553 (18.3) 0.473 (24.7) 0.340 (51.7) 0.473 (11.1) 0.479 (18.9) 0.515 (8.54)
0.432 (4.22) 0.426 (4.01) 0.420 (5.01)
0.430 (3.13) 0.402 (1.03)
0.475 (6.21)
0.083 (1.60) 0.446 (13.7) 0.084 (2.14) 0.084 (0.52) 0.089 (4.38) 0.085 (4.17) 0.083 (1.15) 0.078 (0.98) 0.080 (1.09) 0.081 (1.33) 0.080 (1.78) 0.071 (6.10) 0.080 (0.80) 0.081 (0.88) 0.083 (2.97) 0.092 (7.46) 0.091 (6.57) 0.084 (6.64)
0.426 0.418 (3.95) 0.434 (5.62) 0.377 (5.10) 0.432 (3.90) 0.415 (3.87)
0.406 (2.06) 0.422 (4.46) 0.400 (1.66) 0.410 (4.72) 0.400 (1.59) 0.403 (1.31)
0.459 (6.18) 0.381 (13.7) 0.406 (17.6) 0.349 (15.8) 0.404 (17.4) 0.477 (16.5) 0.537 (16.0) 0.354 (15.1) 0.434 (14.4) 0.435 (14.3) 0.246 (19.2)
0.459 (16.4)
0.405 (0.80) 0.402 (0.97) 0.404 (1.17) 0.409 (0.85) 0.396 (0.83) 0.415 (2.15) 0.401 (1.16) 0.406 (0.67) 0.400 (2.05)
0.417 (3.58)
(Elemental area counts/carbon 496 area counts) × (no. of carbon atoms/no. of element atoms).
detection limits. Precision worsens as measured by regression coefficient of determination or ERF in the order C = Cl > H > O > N. Detection limits decrease in the order O > N > H > Cl > S, which approximates the order of decreasing first ionization energies. The orbital hybridization type of the carbon bound to (25) Gurka, D. F.; Farnham, I.; Potter, B. B.; Pyle, S.; Titus, R.; Duncan, W. Anal. Chem. 1989, 61, 1584-1589.
2416
Analytical Chemistry, Vol. 69, No. 13, July 1, 1997
the functional group does not significantly affect the detection limit or regression analysis results. There is no significant trend with increasing retention times or molecular weights (proportional to retention time), except for the phthalate family. The regression slope ranges are insensitive to molecular structure. The combination of low-resolution GC/MS and GC/AED offers an alternative to high-resolution GC/MS for molecular formula determinations,
Table 4. Empirical Formula Determination for Environmental Sample Analytes
peak no.
retention time, min
1
13.1
2
compound type
molecular weight
atom type
trichlorobenzene
180
13.6
trichlorobenzene
180
3
14.5
dichlorobromobenzene
224
4
15.7
tetrachlorobenzene
214
5
16.4
tetrachlorobenzene
214
6
17.5
tetrachlorotoluene
228
7
18.1
pentachlorobenzene
248
8
20.2
pentachlorotoluene
262
9
20.6
hexachlorobenzene
282
10
26.8
tetrachlorodiphenyl sulfide
322
11
28.0
pentachlorodiphenyl sulfide
356
12
28.2
pentachlorodiphenyl sulfide
356
13
29.2
hexachlorodiphenyl sulfide
390
14
29.6
hexachlorodiphenyl sulfide
390
15
30.0
hexachlorodiphenyl sulfide
390
16
31.6
heptachlorodiphenyl sulfide
424
17
34.9
octachlorodiphenyl sulfide
458
C H Cl C H Cl C H Cl Br C H Cl C H Cl C H Cl C H Cl C H Cl C Cl C H Cl S C H Cl S C H Cl S C H Cl S C H Cl S C H Cl S C H Cl S C H Cl S
provided that analyte molecular weights are high enough (volatility restriction) to eliminate integer formula multiples and that analytes are chromatographically resolved. Molecular formulas may be determined with previously determined ERFs.
ACKNOWLEDGMENT The U.S. Environmental Protection Agency (EPA), through its Office of Research and Development (ORD), funded and
no. of atoms theory calcd 6 3 3 6 3 3 6 3 2 1 6 2 4 6 2 4 7 4 4 6 1 5 7 3 5 6 6 12 6 4 1 12 5 5 1 12 5 5 1 12 4 6 1 12 4 6 1 12 4 6 1 12 3 7 1 12 2 8 1
5.9 3.2 3.0 6.0 3.3 3.0 5.7 2.9 2.0 1.0 6.0 2.3 4.0 6.1 2.5 3.9 6.8 4.3 4.1 6.0 1.1 5.0 6.8 3.6 5.1 5.8 6.1 12.0 5.8 4.2 0.88 12.0 5.1 5.1 1.0 11.6 4.8 5.2 0.97 11.7 4.8 6.1 1.0 12.0 4.2 6.1 1.0 12.0 4.2 6.0 1.0 12.1 3.4 7.0 1.0 11.4 3.2 8.1 1.1
performed the research described here. It has been subjected to the Agency’s peer review and has been approved as an EPA publication.
Received for review December 16, 1996. Accepted March 27, 1997.X AC9612739 X
Abstract published in Advance ACS Abstracts, May 1, 1997.
Analytical Chemistry, Vol. 69, No. 13, July 1, 1997
2417
