Anal. Chem. 2004, 76, 781-787
Attenuated Total Reflectance FTIR Detection and Quantification of Low Concentrations of Aqueous Polyatomic Anions Gretchen N. Hebert, Matthew A. Odom, Stephanie C. Bowman, and Steven H. Strauss*
Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523
Development of a new quantitative method for determining low concentrations of aqueous polyatomic anions using attenuated total reflectance (ATR) FTIR spectroscopy is described. Evaporated thin-film coatings of anionselective tetraalkylated ferrocenium salts were applied to the surface of ATR crystals, which enabled anion detection limits to be lowered up to 23 000-fold below those achieved using the commercially available spectrometer with identical uncoated ATR crystals. Linear calibration curves based on d(absorbance)/dt, which is related to the rate of anion exchange in the thin film, were established in the 0.04-30 µM range. Limits of detection (10-min analyses) for perchlorate, chlorate, trifluoromethanesulfonate, perfluoro-n-butanesulfonate, perfluoro-n-octanesulfonate, tetrafluoroborate, hexafluorophosphate, and pinacolylmethylphosphonate in aqueous solution were 0.03, 0.2, 0.05, 0.07, 0.06, 0.06, 0.6, and 0.7 µM, respectively, using the thin-film coatings. This simple detection/quantification method afforded good reproducibility with relatively fast detection times. Detection, identification, and quantification of aqueous anions, such as phosphonates, perchlorate, and perfluoroalkanesulfonates, are of interest given that they can cause problems to human health and persist in the environment. For example, the pinacolylmethylphosphonate anion (C7H16O3P-, PMPA-) is a hydrolysis product of the G-type nerve agent Soman.1 Although PMPA- itself is not toxic, identifying PMPA- in water would indicate the possible manufacture or release of Soman in the vicinity. The perchlorate anion (ClO4-) interferes with thyroid gland function in infants.2 Perchlorate is on the Contaminant Candidate List and the Unregulated Contaminant Monitoring Regulation List published by the U.S. Environmental Protection Agency (EPA), and in 2000 the EPA established a minimum reporting level for perchlorate in drinking water of 0.04 µM. In 2002, the EPA released a risk assessment draft that proposed a reference dose of 0.03 µg/kg day-1 for ClO4-.3 Perchlorate has been detected at 0.18 µM or higher in a variety of lakes, rivers, and groundwater sources in the southwestern United States.4 * Phone: (970)-491-5104. Fax: (970)-491-1801. E-mail: strauss@ lamar.colostate.edu. (1) Epstein, J.; Bauer, V. E.; Saxe, M.; Demek, M. M. J. Am. Chem. Soc. 1956, 78, 4068-4071. (2) Clark, J. J. J. In Perchlorate in the Environment; Urbansky, E. T., Ed.; Kluwer/ Plenum: New York, 2000, pp 15-29. 10.1021/ac034915i CCC: $27.50 Published on Web 12/19/2003
© 2004 American Chemical Society
The perfluoro-n-octanesulfonate anion (C8F17SO3-, PFOS-) has been detected in surface waters, groundwater sources, human blood, and tissues of animals around the globe.5 This is due in part to its ubiquitous presence as a component in many commercial products, including antistatic agents and fire-fighting aqueous film-forming foams (AFFFs). The widespread presence and persistence of PFOS- in the environment as well as the discovery of its toxicity6,7 prompted the 3M Company to discontinue its production in 20008 and later for the EPA to regulate its production and use in the United States.9 In the past, Fourier transform infrared (FTIR) spectroscopy has not been viewed as a useful method for the detection of trace amounts of analytes in water matrixes.10 This is due to the intense OH stretching and bending bands that extend over large regions of the IR spectrum. With the development of attenuated total internal reflectance (ATR) FTIR spectroscopy, this problem has been mitigated to some extent, and detection of aqueous analytes at millimolar concentrations is possible. The sensitivity of ATRFTIR can be improved by applying a thin analyte-absorbing film to the surface of the ATR crystal.11-16 Several reviews on the detection of analytes in aqueous solution using ATR crystals coated with polymeric or sol-gel materials have been published.17,18 Sulfate and several carboxylates have been detected (3) Perchlorate environmental contamination: toxicological review and risk characterization; Second external review draft, NCEA-1-0503; U.S. EPA, Office of Research and Development, National Center for Environmental Assessment, U.S. Government Printing Office: Washington, DC, 2002, and references therein. (4) Urbansky, E. T. Environ. Sci. Pollut. Res. Int. 2002, 9, 187-192. (5) Moody, C. A.; Hebert, G. N.; Strauss, S. H.; Field, J. A. J. Environ. Monit. 2003, 5, 341-345, and references therein. (6) Case, M. T.; York, R. G.; Christian, M. S. Int. J. Toxicol. 2001, 20, 101109. (7) Seacat, A. M.; Thomford, P. J.; Hansen, K. J.; Olsen, G. W.; Case, M. T.; Butenhoff, J. L. Toxicol. Sci. 2002, 68, 249-264. (8) Tullo, A. Chem. Eng. News 2000, May 22, 9-10. (9) Federal Register 2002, 67, 11008-11030. (10) Griffiths, P. R.; de Haseth, J. Fourier Transform Infrared Spectrometry; John Wiley & Sons: New York, 1986. (11) Rivera, D.; Poston, P. E.; Uibel, R. H.; Harris, J. M. Anal. Chem. 2000, 72, 1543-1554. (12) Rivera, D.; Harris, J. M. Anal. Chem. 2001, 73, 411-423. (13) Haibach, F. G.; Sanchez, A.; Floro, J. A.; Niemczyk, T. M. Appl. Spectrosc. 2002, 56, 398-400. (14) Han, L.; Niemczyk, T. M.; Haaland, D. M.; Lopez, G. P. Appl. Spectrosc. 1999, 53, 381-389. (15) Howley, R.; MacCraith, B. D.; O’Dwyer, K.; Masterson, H.; Kirwan, P.; McLoughlin, P. Appl. Spectrosc. 2003, 57, 400-406. (16) Howley, R.; MacCraith, B. D.; O’Dwyer, K.; Kirwan, P.; McLoughlin, P. Vib. Spectrosc. 2003, 31, 271-278.
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at concentrations as low as 1 µM using Fe2O3 and TiO2 thin films, respectively, on zinc selenide (ZnSe) ATR crystals.19,20 The lowest concentration of perchlorate reported by ATR-FTIR was 1 mM using a Cr2O3 film on a ZnSe ATR crystal.21 See Supporting Information for more details. We previously reported that coated ATR crystals could increase the sensitivity of a commercially available FTIR spectrometer by orders of magnitude for cyanide, perchlorate, and PFOS- in water.22 For the perchlorate and PFOS- analyses, the ATR crystals were coated with thin-films of 1,1′,3,3′-tetrakis(2-methyl-2-nonyl)ferrocenium nitrate (DEC+NO3-), a homologue of the robust, selective, redox-recyclable anion extractant with 2-methyl-2-hexyl substituents previously developed in our laboratory for the efficient extraction and recovery of weakly hydrated anions from water.23-25 We also quantified trace amounts of perchlorate in hydroponic nitrate fertilizers by the method of standard additions.26 In this report, we demonstrate the detection of e0.7 µM perchlorate, chlorate, three perfluoroalkanesulfonates, tetrafluoroborate, hexafluorophosphate, and pinacolylmethylphosphonate in water in 10 min using ATR crystals coated with thin films of salts of the organometallic ion-exchange cation DEC+. Calibration curves based on d(absorbance)/dt (dA/dt) for three of the anions are linear over at least 1 order of magnitude of concentration starting at or near the quantification limit. We also present an analysis of ATR-FTIR-determined detection limits. EXPERIMENTAL SECTION Apparatus. IR spectra were recorded using an ATR-FTIR spectrometer (ReactIR-1000, Applied Systems Inc., Millersville, MD) equipped with a silicon (SiComp) or diamond (DiComp) ATR probe (Applied Systems Inc, Millersville, MD) and an MCT detector. The spectral window was 4000-650 cm-1 with a nominal spectral resolution of 8 cm-1. The electronic gain was 1 (SiComp probe) or 2 (DiComp probe). Happ-Ganzel apodization was used with no postrun spectral smoothing. The SiComp probe consisted of a 30-bounce silicon ATR crystal mated to a ZnSe optical focusing element and was housed in a 5.2-cm-long × 2.5-cm-diameter cylindrical stainless steel conduit. The DiComp probe consisted of an 18-bounce diamond ATR crystal mated to a ZnSe optical focusing element and housed in a 1.3-cm-thick × 7.6-cm-diameter stainless steel DuraDisk (Applied Systems Inc., Millersville, MD). The wetted surface of both the silicon and diamond ATR crystals was a circular area 0.9 cm in diameter. Reagents. The reagents sodium chlorate; potassium tetrafluoroborate; potassium hexafluorophosphate; lithium trifluoromethane(17) Janotta, M.; Mizaikoff, B. Proc. SPIE-Int. Soc. Opt. Eng. 2002, 4616, 1-8, and references therein. (18) Spichiger-Keller, U. E. Sens. Actuators, B 1997, 38-39, 68-77. (19) Hug, S. J. J. Colloid Interface Sci. 1997, 188, 415-422. (20) Weisz, A. D.; Rodenas, L. G.; Morando, P. J.; Regazzoni, A. E.; Blesa, M. A. Catal. Today 2002, 76, 103-112. (21) Degenhardt, J.; McQuillan, A. J. Langmuir 1999, 15, 4595-4602. (22) Strauss, S. H.; Odom, M. A.; Hebert, G. N.; Clapsaddle, B. J. J. Am. Water Works Assoc. 2002, 94, 109-115. (23) Clark, J. F.; Clark, D. L.; Whitener, G. D.; Schroeder, N. C.; Strauss, S. H. Environ. Sci. Technol. 1996, 30, 3124-3127. (24) Chambliss, C. K.; Martin, C. R.; Strauss, S. H.; Moyer, B. A. Solvent Extr. Ion Exch. 1999, 17, 553-584, and references therein. (25) Clapsaddle, B. J.; Clark, J. F.; Clark, D. L.; Gansle, K. M.; Gash, A. E.; Chambliss, C. K.; Odom, M. A.; Miller, S. M.; Anderson, O. P.; Hughes, R. P.; Strauss, S. H. Unpublished work. (26) Collette, T. W.; Williams, T. L.; Urbansky, E. T.; Magnuson, M. L.; Hebert, G. N.; Strauss, S. H. Analyst 2003, 128, 88-97.
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sulfonate; pinacolylmethylphosphonic acid; sodium nitrate; dichloromethane; and the lithium, ammonium, sodium, and potassium salts of perchlorate were reagent grade or better and were used as received. The polyalkylated ferrocenium salts 1,1′,3,3′-tetrakis(2-methyl-2-nonyl)ferrocenium nitrate (DEC+NO3-) and DEC+Clwere synthesized by literature methods.23,25 Potassium perfluoron-octanesulfonate (K(PFOS)) was synthesized from perfluorooctanesulfonyl fluoride (3M Company, St. Paul, MN) by adding it to potassium hydroxide in water and recrystallizing the resultant salt five times from water to a final purity of >99%.27 Potassium perfluoro-n-butanesulfonate (K(PFBS)) was synthesized from perfluorobutanesulfonyl fluoride (3M Company, St. Paul, MN) in the same manner. CAUTION: the preparation of K(PFOS) and K(PFBS) must be carried out in a fume hood by trained personnel because of the generation of hydrogen fluoride. Procedure. All aqueous solutions, made from K+, Li+, or Na+ salts of each anion except for PMPA-, where H(PMPA) was used, were prepared in class A volumetric glassware using distilled deionized water (Barnstead NANOpure, Dubuque, IA) that had an initial resistivity of 18 MΩ cm. Experiments were performed at 24 ( 1 °C unless otherwise noted. Since the pKa of H(PMPA) is 2.4,28 the concentration ratio [PMPA-]/[H(PMPA)] is ∼400 when [PMPA-] + [H(PMPA)] ) 10 µM. Uncoated-probe experiments involved contacting the unmodified silicon or diamond ATR crystal with 100 mL of water with stirring (silicon probe at ∼200 rpm, diamond probe at ∼60 rpm) and collecting a background spectrum (1660 co-added scans) over a 10-min time period. An aliquot of a stock solution was added to the water with stirring, and a 10-min sample spectrum was collected. Control experiments with colored dyes showed that mixing of the aliquot occurred within 10 s for both probes. Coated-probe experiments involved treating the exposed surface of the ATR crystal with 20 ( 3 µL of a fresh (
