Anal. Chem. 2003, 75, 3115-3121
Determination of Nitrated Phenolic Compounds in Rain by Liquid Chromatography/Atmospheric Pressure Chemical Ionization Mass Spectrometry Martin Kohler* and Norbert V. Heeb
Swiss Federal Laboratories for Materials Testing and Research (EMPA), Laboratory of Organic Chemistry, ¨ berlandstrasse 129, 8600 Du¨bendorf, Switzerland U
A sensitive, specific, and rapid analytical method based on liquid-liquid extraction and liquid chromatography/ atmospheric pressure chemical ionization mass spectrometry has been developed for trace analysis of nitrated phenolic compounds in rain samples. Selective detection in the low nanogram-per-milliliter range was achieved on the basis of selected ion monitoring of the respective phenolate anions [M - H]-. The presence of alkylated (C1-C3) and nonalkylated nitrophenols (C0) was confirmed by their characteristic neutral loss of nitrogen dioxide upon collision-induced dissociation in tandem mass spectrometry mode. In individual rain samples, 27 C0-C3-nitrophenol isomers as well as 16 C0-C3-dinitrophenol isomers were detected. Total levels of nitro- and dinitrophenol isomers were estimated on the basis of 2-nitrophenol and 2,6-dinitrophenol. Concentrations between 0.66 and 2.0, 12-29, 12-36, and 5.5-12 µg/L were obtained for the C0-, C1-, C2-, and C3-nitrophenols, respectively. Lower levels of 0.19-1.4, 0.39-2.1, 0.0530.55, and 0.081-0.10 µg/L were estimated for the corresponding C0-, C1-, C2-, and C3-dinitrophenols. The highest number of individual isomers was found in winter rain samples, and distinctive isomeric patterns were observed for individual samples. Mono- and dialkylated nitrophenols and non- and monoalkylated dinitrophenols represent the major part of nitro- and dinitrophenol species. Comparing the pattern of C0-, C1-, and C2nitrophenols in rain with the corresponding C0-, C1-, and C2-benzene pattern in ambient air suggests that atmospheric oxidation and nitration processes of alkylbenzenes are favored, as compared to those of benzene. Nitrated phenolic compounds are secondary air pollutants formed by atmospheric oxidation and subsequent nitration of benzene and alkylated benzenes originating from various combustion processes as well as from solvent and fuel evaporation.1-6 * Corresponding author. Phone: +41 1 823 4334. Fax: +41 1 823 4041. E-mail:
[email protected]; WWW: http://www.empa.ch. (1) Nojima, K.; Fukaya, K.; Fukui, S.; Kanno, S. Chemosphere 1975, 4, 77. (2) Nojima, K.; Fukaya, K.; Fukui, S.; Kanno, S.; Nishiyama, S.; Wada, Y. Chemosphere 1976, 5, 25. (3) Atkinson, R.; Carter, W. P. L.; Darnall, K. R.; Winer, A. M.; Pitts, J. N. Int. J. Chem. Kinet. 1980, 12. (4) Grosjean, D. Environ. Sci. Technol. 1985, 19, 968. (5) Leone, J. A.; Flagan, R. C.; Grosjean, D.; Seinfeld, J. H. Int. J. Chem. Kinet. 1985, 17, 177. 10.1021/ac0264067 CCC: $25.00 Published on Web 05/20/2003
© 2003 American Chemical Society
Nitrated phenolic compounds are present in ambient air at low nanogram-per-cubic meter levels.7 Because of their high water solubility, they are efficiently removed from the atmosphere by precipitation, reaching concentrations in the microgram-per-liter range in rain, snow, and fog.7-12 Sources of nitrated phenolic compounds include automobile exhaust gas13 and the application of certain herbicides,14 such as 2-methyl-4,6-dinitrophenol (DNOC). The U.S. Environmental Protection Agency (EPA) has classified 2-methyl-4,6-dinitrophenol and 2,4-dinitrophenol as priority toxic pollutants,15 and nitrated phenolic compounds present in the atmosphere have been suggested as possible contributors to forest decline.16 In addition, nitrated phenols are expected to induce secondary aerosol formation and may be involved in atmospheric oxidation of nitrogen oxide (NO) to nitrous acid (HONO).17 Trace analysis of nitrated phenolic compounds in rain samples presents challenges, such as low concentrations, a large number of isomers being present, and the lack of commercially available reference compounds for the vast majority of the nitrated alkylphenols occurring in environmental samples. For these reasons, there is still little data available on environmental levels of alkylated nitro- and dinitrophenols. Methods such as liquid chromatography with photodiode array detection (LC/UV),14 supercritical fluid chromatography (SFC),18 capillary zone electrophoresis (CZE),19 (6) Tremp, J. Ph.D. Thesis, Federal Institute of Technology, 1992. (7) Lu ¨ ttke, J.; Levsen, K. Atmos. Environ. 1997, 31, 2649. (8) Schu ¨ ssler, W.; Nitschke, L. Chemosphere 2001, 42, 277. (9) Alber, M.; Bo ¨hm, H. B.; Brodesser, J.; Feltes, J.; Levsen, K.; Scho ¨ler, H. F. Fresenius’ J. Anal. Chem. 1989, 334, 540. (10) Lu ¨ ttke, J.; Levsen, K.; Acker, K.; Wieprecht, W.; Moller, W. Int. J. Environ. Anal. Chem. 1999, 74, 69. (11) Collett, J. L.; Hoag, K. J.; Rao, X.; Pandis, N. S. Atmos. Environ. 1999, 33, 4833. (12) Richartz, H.; Reischl, A.; Trautner, F.; O., H. Atmos. Environ. 1990, 24A, 3067. (13) Tremp, J.; Mattrel, P.; Fingler, S.; Giger, W. Water Air Soil Poll. 1993, 68, 113. (14) Bo ¨hm, H. B.; Feltes, J.; Volmer, D.; Levsen, K. J. Chromatogr., A 1989, 478, 399. (15) California Environmental Protection Agency; Establishment of Numeric Criteria for Priority Toxic Pollutants for the State of California. Water Quality Standards; 2000. (16) Rippen, G.; Zietz, E.; Frank, R.; Knacker, T.; Klo¨pffer, W. Environ. Technol. Lett. 1987, 8, 475. (17) Arens, F.; Gutzwiller, L.; Baltensperger, U.; Ga¨ggeler, H. W.; Ammann, M. Environ. Sci. Technol. 2001, 35, 2191. (18) Pocurull, L.; Marce´, R. M.; Borrull, F.; Bernal, J. L.; Toribio, L.; Serna, M. L. J. Chromatogr. 1996, 67, 755. (19) Kaniansky, D.; Krcmova´, E.; Madajova´, V.; Masa´r, M.; Mara´k, J.; Onuska, F. I. J. Chromatogr., A 1997, 772, 327.
Analytical Chemistry, Vol. 75, No. 13, July 1, 2003 3115
or nuclear magnetic resonance spectroscopy (NMR)20 provide either limited specificity or limited sensitivity. Because of the high polarity and the thermal sensitivity of nitrated phenolic compounds, analytical methods based on gas chromatography (GC) usually require a derivatization step prior to analysis.21-23 A series of own experiments showed that quantitative derivatization of nitrophenols is not always straightforward. Low derivatization yields were encountered particularly for ortho-substituted nitrophenols and dinitrophenols. Incomplete reaction of individual nitrophenol isomers with the derivatization reagent has been reported by the EPA, as well.24 For these reasons, direct analysis based on liquid chromatography is an attractive alternative for the separation of underivatized nitrated phenols. Combined with the high sensitivity and high selectivity of atmospheric pressure chemical ionization mass spectrometry (APCI-MS), LC/APCI-MS is a useful technique for the determination of phenolic compounds, including nitrophenols.25,26 In this paper, we present a sensitive, specific, and rapid analytical method for the determination of nitrated phenolic compounds in rain samples. Using LC/APCI-MS, alkylated (C1C3) and nonalkylated (C0) nitro- and dinitrophenols can be determined in one step. Due to the lack of authentic reference compounds, concentrations of the chromatographically resolved isomers were estimated based on 2-nitrophenol and 2,6-dinitrophenol. For three selected rain samplessa summer and two winter rain samplesstotal contents of C0-C3-nitro- and dinitrophenols are presented, and their chromatographic patterns are discussed. In addition, the presence of nitrated phenolic compounds in rain samples is confirmed using tandem mass spectrometry (LC/APCIMS/MS), relaying on their characteristic loss of a NO2 neutral upon collisional induced dissociation (CID). EXPERIMENTAL SECTION Reference Compounds. Reference compounds (2-nitrophenol, 2,6-dinitrophenol, 2,6-dimethyl-4-nitrophenol, and 2,6-dinitro4-methylphenol, all from Sigma-Aldrich) were prepared as 10 ng/ µL solutions in methanol (Merck). Isotopically labeled 13C6pentachlorophenol (Cambridge Isotope Laboratories) was used as an internal standard, since no 13C-labeled nitrophenols are yet commercially available, and 2-bromo-4-chlorophenol (SigmaAldrich) was used as the recovery standard. Samples. One urban summer rain sample (sample A, taken on September 2, 1997, in the city of Winterthur) and two urban winter rain samples (samples B and C, both taken on February 22, 1998, in the cities of Winterthur and Zu¨rich, respectively) were collected and stored at 4 °C prior to analysis (analyte losses at this temperature were reported to be minimal8). Liquid-Liquid and Solid-Phase Extraction. Liquid-Liquid Extraction. Aliquots (500 mL) of each sample were acidified (pH
