Anal. Chem. 2003, 75, 1792-1797
Determination of Alkyltrimethylammonium Chlorides in River Water by Gas Chromatography/ Ion Trap Mass Spectrometry with Electron Impact and Chemical Ionization Wang-Hsien Ding* and Pei-Chuan Tsai
Department of Chemistry, National Central University, Chung-Li, 32054, Taiwan
This work describes a modified method to analyze alkyltrimethylammonium chlorides (ATMACs) in river water samples. The proposed method involves adding solid potassium iodide to water sample (pH adjusted to 10.0) as a counterion to enhance the extraction of ATMAC residues by dichloromethane liquid-liquid extraction. The iodide-ATMA+ ion pairs were demethylated to their corresponding nonionic alkyldimethylamines (ADMAs) by thermal decomposition in a GC injection port. The corresponding ADMAs were then identified and quantitated by gas chromatography/ion trap mass spectrometry (GC/ MS) in electron impact and low-pressure positive ion chemical ionization (PICI) modes. A relatively high abundance of ADMAs was detected at a demethylation temperature above 300 °C in the injection port. Experimental results indicate that the proposed method is precise and sensitive in ATMACs analysis and allows quantitation at e0.01 µg/L in 500 mL of the water samples. The enhanced selectivity of quasi-molecular ion chromatograms of C12-C18-ADMA, obtained using methanol PICIMS, enables ATMAC residues to be identified at trace levels in environmental samples. Recovery of the ATMACs in various spiked water samples ranged from 70 to 94% while RSD ranged from 3 to 12%. The concentrations of total measured ATMAC residues in river water samples ranged from nondetectable to 1.24 µg/L. Alkyltrimethylammonium chlorides (ATMACs) are widely used cationic surfactants, which are a mixture of linear alkyl homologues of dodecyl- (C12-), tetradecyl- (C14-), hexadecyl- (C16-) and octadecyl- (C18-) trimethylammonium chlorides. They are applied in many industrial applications and pharmaceutical/ cosmetic preparations as softeners, antistatics, and bactericides. They are also the main ingredient of hair conditioners, which impart softness, manageability, and antistatic properties to hair. Their acute toxicity (effect/lethal concentration) to fish varied from 0.36 to 8.6 mg/L (LC50 for golden orfe, Idus melanotus) to 0.73-23 mg/L (LC50 for water snail, Planorbis corneus).1,2 The toxicity (96-h EC50) in algal growth studies ranged from 0.12 mg/L * Corresponding author. Tel: 011-886-3-4227151 ext 5905. Fax: 011-886-34227664. E-mail:
[email protected]. (1) Knauf, W. Tenside Deterg. 1973, 10, 251-255.
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for Micro-cystis aeruginosa to 0.2 mg/L for Selenastrum capricornutum and Navicula pelliculosa.3 After use, ATMACs are normally discharged via wastewater treatment facilities to surface waters. They can interfere with the ecosystem since they are toxic to aquatic organisms. Their toxicity and persistence are such that an accurate and sensitive analytical method must be developed to study the occurrence and fate of trace level ATMAC residues in aquatic environment. Numerous methods for analysis of cationic surfactants have been developed for water quality investigation in water treatment industries. Cationic surfactants are commonly treated with anionic dyes, to form ion-pair complexes, which can be extracted by solvents and followed by spectrophotometry.4-6 However, these methods lack specificity to individual homologues and are affected by several interfering compounds. High-performance liquid chromatography (HPLC) is the most promising method for analyzing these cationic surfactants. However, the inability of mono and dialkyl quaternary ammonium surfactants to absorb UV is such that electrical conductivity detection7-10 and postcolumn detection11-13 are typically employed in HPLC, giving detection limits for these surfactants of around 5-30 µg/L. Fast atom bombardment mass spectrometry (FAB-MS) techniques has been applied to analyze relatively high concentrations of mono and dialkyl quaternary ammonium surfactants in a variety of matrixes, particularly in pharmaceutical formulations and ophthalmic products.14-16 Matrix (or surface)-assisted laser desorption/ ionization time-of-flight mass spectrometry (MALDI (or SALDI)(2) Boethling, R. S.; Lynch, D. G. In The Handbook of Environmental Chemistry; Hutzinger, O., Ed.; Springer-Verlag: New York, 1992; Vol. 3, Part F, pp 164196. (3) Lewis, M. A.; Hamm, B. G. Water Res. 1986, 20, 1575-1582. (4) Llenado, R. A.; Jamieson, R. A. Anal. Chem. 1981, 53, 174R-188R. (5) Sakai, T.; Ohno, N. Talanta 1986, 33, 415-419. (6) Motomizu, S.; Oshima, M.; Gao, Y.; Ishihara, S.; Uemura, K. Analyst 1992, 117, 1775-1780. (7) Wee, V.T.; Kennedy, J. M. Anal. Chem. 1982, 54, 1631-1633. (8) Wee, V. T. Water Res. 1984, 18, 223-225. (9) Levsen, K.; Emmrich, M.; Behnert S. Fresenius’ J. Anal. Chem. 1993, 346, 732-737. (10) Matthijs, E.; De Henau, H. Vom Wasser 1987, 69, 73-83. (11) De Ruiter, C.; Hefkens, J. C. H. F.; Brinkman, U. A. Th.; Frei, R. W.; Evers, M.; Matthijs, E.; Meijer, J. A. Int. J. Environ. Anal. Chem. 1987, 31, 325339. (12) Gort, S. M.; Hogendoorn, E. A.; Baumann, R. A.; van Zoonen, P. Int. J. Environ. Anal. Chem. 1993, 53, 289-296. (13) Fernandez, P.; Alder, A. C.; Suter, M. J. F.; Giger, W. Anal. Chem. 1996, 68, 921-929. 10.1021/ac020536y CCC: $25.00
© 2003 American Chemical Society Published on Web 03/13/2003
TOFMS) techniques have also been used to analyze quaternary ammonium compounds (QACs) in disinfectant formulations, oral rinse products, and aqueous solutions.17-19 Although FAB-MS and MALDI (or SALDI)-TOFMS can identify the homologues of QACs, these direct MS methods are not sufficiently specific for routine quantitative analysis. Combining HPLC and MS with electrospray may represent a powerful method for determining trace amounts of QACs in environmental samples,20-22 but the required equipment is expensive and not easily available. Gas chromatography (GC) or gas chromatography/mass spectrometry (GC/MS) is not only more readily available in many environmental laboratories, but it does provide a higher chromatographic resolution with a capillary column than do the LC/MS methods. GC has been used to determine long-chain cationic surfactants by converting them into the corresponding tertiary amines by thermal decomposition in the injection port23-25 or by the Hofmann elimination reaction to decompose them.26-29 A promising demethylation of alkyltrimethylammonium bromides in Bayer process liquors with potassium iodide in the injection port has been recently reported.30 However, quantitative determination of trace levels of C12-C18ATMACs by GC/MS in environmental samples has yet to be achieved. Recently, an ion trap GC/MS system with low-pressure positive ion chemical ionization (PICI) capability was developed for quick switching between electron impact ionization (EI) and PICI scans without compromises in spectral quality. This technique has been applied to many environmental analyses routinely.31-35 In this system, a variety of organic solvents can also be introduced into the ion trap as chemical ionization (CI) reagent gases to improve the PICI mass spectra for structural elucidation. The general (14) Mambagiotti-Alberti, M.; Pinzauti, S.; Moneti, G.; Agati, G.; Giannellini, V.; Coran, S. A.; Vincieri, F. F. J. Pharm. Biomed. Anal. 1984, 2, 409-415. (15) Pinzauti, S.; Mambagiotti-Alberti, M.; Moneti, G.; La Porta, E.; Coran, S. A.; Vincieri, F. F.; Gratteri, P. J. Pharm. Biomed. Anal. 1989, 7, 1611-1616. (16) Coran S. A.; Mambagiotti-Alberti, M.; Giannellini, V.; Moneti, G.; Pieraccini, F.; Raffaelli, A. Rapid Commun. Mass Spectrom. 1998, 12, 281-284. (17) Thompson, B.; Wang, Z.; Paine, A.; Rudin, A.; Lajoie, G. J. Am. Oil Chem. Soc. 1995, 72, 11-15. (18) Morrow, A. P.; Kassim, O.; Ayorinde, F. O. Rapid Commun. Mass Spectrom. 2001, 15, 767-770. (19) Chen, Y. C.; Sun, M. C. Rapid Commun. Mass Spectrom. 2001, 15, 25212525. (20) Radke, M.; Behrends, T.; Forster, J.; Hermann, R. Anal. Chem. 1999, 71, 5362-5366. (21) Ferrer, I.; Furlong, E. T. Environ. Sci. Technol. 2001, 35, 2583-2588. (22) Ferrer, I.; Furlong, E. T. Anal. Chem. 2002, 74, 1275-1280. (23) Metcalfe, L. D. J. Am. Oil Chem. Soc. 1984, 61, 363-366. (24) Valls, M.; Bayona, J. M. Fresenius J. Anal. Chem. 1991, 339, 212-217. (25) Haskins, N. J.; Mitchell, R. Analyst 1991, 116, 901-903. (26) Takano, S.; Takasaki, C.; Kunihiro, K.; Yamanaka, M. J. Am. Oil Chem. Soc. 1977, 54, 139-143. (27) Suzuki, S.; Nakamura, Y.; Kaneko, M.; Mori, K.; Watanabe, Y. J. Chromatogr. 1989, 463, 188-191. (28) Suzuki, S.; Sakai, M.; Ikeda, K.; Mori, K.; Amemiya, T.; Watanabe, Y. J. Chromatogr. 1986, 362, 227-234. (29) Ding, W. H.; Liao, Y. H. Anal. Chem. 2001, 73, 36-40. (30) Hind, A. R.; Bhargava, S. K.; Grocott, S. C. J. Chromatogr., A 1997, 765, 287-293. (31) Cairns, T.; Chiu, K. S.; Siegmund, E. G. Rapid Commun. Mass Spectrom. 1992, 6, 331-338. (32) Mattern, G. C.; Singer, G. M.; Louis, J.; Robson, M.; Rosen, J. D. J. Agric. Food Chem. 1990, 38, 402-407. (33) Mattern, G. C.; Louis, J. B.; Rosen, D. J. J. Assoc. Off. Anal. Chem. 1991, 74, 982-986. (34) Ding W. H.; Lo, J. H.; Tzing S. H. J. Chromatogr., A 1998, 818, 270-279. (35) Ding W. H.; Tzing S. H. J. Chromatogr., A 1998, 824, 79-90.
requirements for the solvents are a moderate vapor pressure and small molecular weight. PICI-MS has proved to be a useful technique for the characterization of nonionic and anionic surfactant residues as well as their degradation products in the environmental samples, which is highly complementary to the EIMS technique for compounds with a weak or absent molecular ion.34-38 This study develops a novel method to routinely determine C12-C18-ATMACs in aqueous samples by applying liquid-liquid extraction and ion trap GC/MS analysis of their corresponding alkyldimethylamines (ADMAs) using various MS techniques. Additionally, following EI and methane PICI-MS studies, this work examines the feasibility of using methanol as a CI reagent gas to facilitate molecular weight determination of the corresponding ADMAs in water samples for the first time. Our results further demonstrate the effectiveness of the method in determining ATMACs at trace levels in environmental samples. EXPERIMENTAL SECTION Chemicals and Reagents. Unless stated otherwise, all highpurity chemicals and solvents were purchased from Aldrich (Milwaukee, WI), Tedia (Fairfield, OH), and Merck (Darmstadt, Germany) and were used without further purification. Dodecyltrimethylammonium chloride (99% purity) was purchased from Sigma (St. Louis, MO). Hexadecyl- and octadecyltrimethylammonium chloride (all above 98% purity) were purchased from ChemServices. Tetradecyltrimethylammonium chloride and undecyldimethylamine (C11H23N(CH3)2, as internal standard) were purchased from Aldrich. A standard mixture containing 100 mg/L of each ATMAC compound in methanol was prepared. Reagent grade potassium iodide was purchased from Mallinckrodt. Deionized water was further purified with a Minipore water purification device (Bedford, MA). Sample Collection. Samples of river water were collected from Kao-Ping and Dong-Kang Rivers in southern Taiwan. Untreated municipal wastewaters are discharged directly into these rivers. Duplicate 500-mL samples were collected and shipped to the laboratory in ice-packed containers. Upon arrival, the samples were immediately adjusted to pH 2-3 by adding concentrated HCl and then stored at 4 °C until analysis. The selected water quality data of pH, specific conductance, dissolved oxygen, and suspended solid were 7.2, 1200 µS/cm, 6.3 mg/L, and 63 mg/L for Kao-Ping River and 7.4, 1300 µS/cm, 4.8 mg/L, and 20 mg/L for Dong-Kang River, respectively. These values were measured, using appropriate on-site instrumentation, by the National Institute of Environmental Analysis (NIEA) staff during sampling. Sample Preparation. Water samples were preconcentrated with RP-18, strong cationic exchange (SCX) solid-phase extractions (SPE) and liquid-liquid extraction. For RP-18 SPE, filtered water sample was mixed with linear alkylbenzenesulfonates (i.e., 4-C8LAS) and then extracted according to a procedure described elsewhere.29 Briefly, an acidified water sample (500 mL) was (36) Ding, W. H.; Fujita, Y.; Aeschimann, R.; Reinhard, M. Fresenius J. Anal. Chem. 1996, 354, 48-55. (37) Stephanou, E.; Reinhard, M.; Ball, H. A. Biomed. Environ. Mass Spectrom. 1988, 15, 275-282. (38) Ding, W. H.; Fujita, Y.; Reinhard, M. Rapid Commun. Mass Spectrom. 1994, 8, 1016-1020.
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adjusted to pH 7.0 with 1 N NaOH, mixed with 4-C8-LAS, and then passed through the cartridge (Bakerbond spe C18, J.T. Baker) at a flow rate of 2-5 mL/min. The ATMAC residues were eluted with 10 mL of methanol-ethyl acetate (1:1, v/v) eluent. For SCX SPE extraction, an acidified 500-mL filtered water sample (adjusted to pH 9.0) passed through the SCX cartridge (Supelclean LC-SCX, Supelco, Bellefonte, PA) at a flow rate of 2-5 mL/min. The ATMAC residues were then eluted with 10 mL of NH4OHmethanol (5:95, v/v) eluent. The extract was then evaporated to dryness, and the residue was redissolved in 100 µL of dichloromethane containing 20 µg/L undecyldimethylamine, as internal standard and made ready for thermal demethylation GC/MS analysis. For optimal extraction (see Results and Discussion), a liquidliquid extraction method was applied to quantitatively extract water sample by adding with solid potassium iodide. Herein, KI served as a counterion to exchange chloride from ATMACs, which may enhance the extraction of alkyltrimethylammonium ions (ATMA+) by forming iodide ion-pair complexes (iodide-ATMA+) and provide the effective demethylation results in the injection port.30 Before extraction, 2 g of solid KI was added to 500 mL of unfiltered water sample, and the pH of the solution was adjusted to 10.0 with 1 N NaOH. The iodide-ATMA+ ion pair was then consecutively extracted with dichloromethane (20 mL each)sliquid-liquid extraction three times in a separatory funnel. The organic extracts were combined and dried with sodium sulfate. The volume of the extract was then reduced to 1 mL by rotary evaporation (35 °C,
