Anal. Chem. 2007, 79, 7198-7200
Dicationic Ion-Pairing Agents for the Mass Spectrometric Determination of Perchlorate P. Kalyani Martinelango†
Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061 Purnendu K. Dasgupta*
Department of Chemistry and Biochemistry, The University of Texas at Arlington, Arlington, Texas 76019-0065
Perchlorate and other hydrophobic ions can be measured with high sensitivity and selectivity by forming a positively charged ion pair with a dicationic agent. A commercially available reagent, 1,6-bis(trimethylammonium)hexane dibromide (Br(N(CH3)3)(CH2)6(N(CH3)3)Br) allows for the determination of perchlorate by electrospray ionization mass spectrometry as the [(N(CH3)3)(CH2)6(N(CH3)3)ClO4]+ ion. Limits of detection (LODs) are better than those previously observed with custom-synthesized dicationic agents. An LOD of 20 ng/L is readily attainable with a single-quadrupole mass spectrometer. Ever since the discovery of perchlorate in the waterways of Arizona, California, and Nevada, there has been intense interest regarding its detection and quantification in various matrices. Consequently, techniques to quantitate low levels of perchlorate have dramatically improved. The presence of perchlorate in ground and surface waters, in a variety of vegetables and fruits, agricultural products such as tobacco, and in dairy and human milk is now being routinely reported in the United States.1-9 Ion chromatography (IC)-mass spectrometry (MS) is often the method of choice: with state-of-the-art liquid chromatography (LC) or IC-MS/MS instrumentation it is possible to achieve 5-25 ng/L limits of detection (LODs) in water,10 urine,11 amniotic fluid,12 * Corresponding author. E-mail:
[email protected]. † Present address: Environment Canada, Environmental Technology Center, Analysis & Air Quality Division, Ottawa ON K1A 0H3, Canada. (1) Susarla, S.; Wolfe, N. L.; McCutcheon, S. C. Abstr. Pap.sAm. Chem. Soc. 1999, 218, 13-ENVR, Part 1. (2) Lewis, S. L.; Susarla, S.; Wolfe, N. L.; McCutcheon, S. C. Abstr. Pap.sAm. Chem. Soc. 1999, 218, 12-ENVR, Part 1. (3) Hogue, C. Chem. Eng. News 2003, 81, 11. (4) Krynitsky, A. J.; Niemann, R. A.; Nortrup, D. A. Anal. Chem. 2004, 76, 6618-6622. (5) Ellington, J. J.; Wolfe, N. L.; Garrison, A. W.; Evans, J. J.; Avants, J. K. Environ. Sci. Technol. 2001, 35, 3213-3218. (6) Jackson, W. A.; Joseph, P.; Laxman, P.; Tan, K.; Smith, P. N.; Yu, L.; Anderson, T. A. J. Agric. Food. Chem. 2005, 53, 369-373. (7) Sanchez, C. A.; Krieger, R. I.; Khandaker, N.; Moore, R. C.; Holts, K. C.; Neidel, L. L. J. Agric. Food Chem. 2005, 53, 5479-5486. (8) Kirk, A. B.; Smith, E. E.; Tian, K.; Anderson, T. A.; Dasgupta, P. K. Environ. Sci. Technol. 2003, 37, 4979-4981. (9) Kirk, A. B.; Martinelango, P. K.; Tian, K.; Dutta, A.; Smith, E. E.; Dasgupta, P. K. Environ. Sci. Technol. 2005, 39, 2011-2017. (10) Burrows, R. Advanced Mass Spectrometric Techniques for DOD Analytes of Interest.www.dtic.mil/ndia/2004Chemistry/Burrows_AFCEE_2004_MS_techniques.pdf.
7198 Analytical Chemistry, Vol. 79, No. 18, September 15, 2007
wine,13 and foods.14 The LODs attainable in single-quadrupole mass spectrometers that routinely constitute the heart of most LC/IC-electrospray ionization (ESI)-MS instrumentation are more modest and sometimes inadequate for ultratrace analysis. We reported a sensitive method for the IC-ESI-MS determination of perchlorate using a dicationic agent, D2+, which forms a charged DClO4+ ion pair with perchlorate.15 Perchlorate is measured as this aggregate in the positive-ion mode and at much higher m/z (304.7-582.9) compared with ClO4- alone (99-101). An optimum signal-to-noise (S/N) ratio is achieved at a very low ionization voltage as the aggregate carries an intrinsic charge. We evaluated 10 dicationic agents for their effectivenesssthough some were better than others, all provided a significantly more sensitive means of measuring perchlorate than measuring ClO4directly. The approach is applicable to not just perchlorate but other anions like SCN- and I- as well.16 However, all of the dicationic agents used in that work were synthesized15,17 in-house. Few analytical laboratories, especially those having to perform routine analysis for regulatory reasons or otherwise, will ever use a procedure that will require synthesis of a reagent. On the other hand, since our original work, we have discovered that some dicationic agents are readily commercially available. In this note, we compare two commercially available dicationic reagents for their suitability for this application and show that some perform as well as the best of the custom-synthesized reagents. EXPERIMENTAL SECTION Reagents. All the dicationic reagents contain tetraalkylammonium end groups and a straight aliphatic hydrocarbon chain with the general structure Me3N+-(CH2)n-NMe3+. Compound I (n ) 12), the benchmark compound used in previous work15 was (11) Valentı´n-Blasini, L.; Mauldin, J. P.; Maple, D.; Blount, B. C. Anal. Chem. 2005, 77, 2475-2481. (12) Blount, B. C.; Valentı´n-Blasini, L. Anal. Chim. Acta 2006, 567, 87-93. (13) El Aribi, H.; Le Blanc, Y. J. C.; Antonsen, S.; Sakuma, T. Anal. Chim. Acta 2006, 567, 39-47. (14) Krynitsky, A. J.; Niemann, R. A.; Williams, A. D.; Hopper, M. L. Anal. Chim. Acta 2006, 567, 94-99. (15) Martinelango, P. K.; Anderson, J. L.; Dasgupta, P. K.; Armstrong, D. W.; Al-Horr, R. S.; Slingsby, R. W. Anal. Chem. 2005, 77, 4829-4835. (16) Martinelango, P. K.; Tian, K.; Dasgupta, P. K. Anal. Chim. Acta 2006, 567, 100-107. (17) Anderson, J. L.; Ding, R.; Ellern, A.; Armstrong, D. W. J. Am. Chem. Soc. 2005, 127, 593-604. 10.1021/ac0709899 CCC: $37.00
© 2007 American Chemical Society Published on Web 08/22/2007
Table 1. Response Behavior with Different Dicationic Agents as Postcolumn Reagentsa
a
postcolumn reagent
primary ion m/z monitored
I, fluoride, 2.5 µM I, fluoride, 5 µM II, bromide, 5 µM II, fluoride, 5 µM III, bromide, 5 µM III, fluoride, 5 µM
384.8 384.8 356.8 356.8 300.7 300.7
III, fluoride, 2.5 µM III, fluoride, 5 µM III, fluoride, 10 µM
300.7 300.7 300.7
slope (million count‚s/ppb)
intercept (million count‚s)
n
coefficient of determinationb
LODc ng/L
0.2-10 µg/L perchlorate 1.198 ( 0.044 0.198 ( 0.270 0.742 ( 0.005 0.042 ( 0.030 0.894 ( 0.035 0.132 ( 0.176 1.089 ( 0.040 0.091 ( 0.212 0.840 ( 0.007 0.073 ( 0.041 1.120 ( 0.033 0.181 ( 0.165
9 9 11 9 12 12
0.9997 0.9920 0.9864 0.9906 0.9992 0.9914
190 50 45 91 22 26
0.2-100 µg/L perchlorate 0.380 ( 0.005 0.380 ( 0.214 0.562 ( 0.002 0.338 ( 0.082 0.445 ( 0.002 0.379 ( 0.087
18 18 18
0.9994 0.9998 0.9996
15 22 30
The uncertainties shown are (1 standard deviation. b Linear r2 values. c Based on S/N ) 3.
synthesized as the diiodide from the R,ω-diamine and excess CH3I according to the literature.18 Compounds II (n ) 10) and III (n ) 6) were both purchased as the dibromide (www.sigmaaldrich.com). In order to maximize the production of the dication D2+ in the mass spectrometer from the dihalide, the bromide or iodide salt was converted to the fluoride form DF2 by anion exchange.15 The commercial dicationic agents II and III were evaluated in both the bromide and the fluoride forms to ascertain the value of the additional ion-exchange step. Methods. Ion chromatography with suppressed conductivity detection was carried out on a Dionex DX-600 ion chromatograph with a GS50 gradient pump, an ASRS-Ultra suppressor, and a CD25 conductivity detector. PeakNet, version 6.2, was used for system control, and area-based analyte quantification (external standard mode, five-point calibration) was used. The preconcentrationpreelution method developed by Tian et al.19 was used. The separation of perchlorate was done on a Dionex Ionpac AG16-AS 16 guard-separator column pair (all 4 mm bore). A TAC-LP1 anion concentrator (Dionex) was used as a preconcentrator column. Aqueous standards or samples (1 mL) were loaded on the preconcentrator column and preeluted with 2.0 mL of 10 mM NaOH and then switched to the main separation column. Sodium hydroxide was used as the eluent at a concentration of 100 mM and a flow rate of 1 mL/min. Preelution removes to a large extent common anions like chloride, nitrate, sulfate, etc. that are not as strongly retained as perchlorate. MS studies were performed using a ThermoQuest Finnigan AQA single-quadrupole MS in the positive-ion mode with Xcalibur (version 1.1) software. The dicationic agent was introduced as a postcolumn reagent (PCR). The chromatographic effluent was split at a tee: a flow of 0.4 mL/min was allowed to go to waste via an appropriate restrictor. The dicationic postcolumn reagent was introduced at a second tee into the remaining 0.6 mL/min stream at a flow rate of 0.06 mL/min. This results in a 11-fold dilution in the mixture entering the MS compared with the original concentration; the concentration cited throughout the paper is the final concentration entering the MS. The mixture flowed through a 0.8 mm i.d. × 1.15 m mixing coil before entry into the MS. The (18) Blomquist, A. T.; Hallam, B. F.; Josey, A. D. J. Am. Chem. Soc. 1959, 81, 678-680. (19) Tian, K.; Dasgupta, P. K.; Anderson, T. A. Anal. Chem. 2003, 75, 701706.
Figure 1. Comparison of MS chromatograms of 1 µg/L perchlorate using different dicationic agents.
electrospray voltage and desolvation temperature were maintained at 3.0 kV and 350 °C, respectively. The ionization voltage used was +2 V. The D35ClO4+ ion was monitored. For the quantitation of perchlorate in the analyzed samples, the isotope dilution technique with NaCl18O4 (www.iconisotopes.com) was used to spike samples in known concentration, and the D35Cl18O4+ ion was also monitored. RESULTS AND DISCUSSION Although in our original work we used a dicationic agent concentration of 9.1 µM entering the MS (PCR concentration 100 µM), initial results showed that both the signal and the S/N ratio are better at lower reagent concentrations as less ionization suppression occurs. These results also led us to confine our experiments to 2.5 and 5 µM reagent concentrations. Figure 1 shows the selected ion chromatogram traces resulting from the injection of 1 ug/L perchlorate using I-III as PCR. The data for the correspondence to linear response at low concentration (0.210 µg/L) are shown in Table 1. Experiments with I in the fluoride form indicated that although the signal is ∼50% higher at a reagent concentration of 2.5 µM, the reproducibility is much better at the Analytical Chemistry, Vol. 79, No. 18, September 15, 2007
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Table 2. Comparison of I (as Fluoride) and III (as Bromide) as PCR PCR
2.5 µM III
5 µM III
10 µM III
10 µM I
0.2 µg/L stda 1 µg/L std 5 µg/L std 10 µg/L std 20 µg/L 100 µg/L HB-0c HB-0.2 HB-1 HB-5 HB-10 HB-20 tap water groundwater milk seaweed
0.25 ( 0.01 1.10 ( 0.04 5.58 ( 0.04 10.7 ( 0.16 20.5 ( 0.32 99.8 ( 2.26 0.00 0.26 ( 0.01 1.20 ( 0.02 5.42 ( 0.05 10.8 ( 0.15 19.4 ( 0.05 1.00 ( 0.01 3.49 ( 0.06 3.80 ( 0.08 na
0.27 ( 0.01 1.21 ( 0.05 6.06 ( 0.21 10.6 ( 0.16 20.7 ( 0.40 99.8 ( 0.79 0.00 0.27 ( 0.01 1.17 ( 0.03 5.48 ( 0.10 10.9 ( 0.20 21.4 ( 0.08 1.02 ( 0.05 3.51 ( 0.08 3.93 ( 0.04 0.44 ( 0.02
0.34 ( 0.04 1.30 ( 0.06 5.68 ( 0.23 11.0 ( 0.14 21.7 ( 0.28 99.5 ( 0.61 0.00 0.29 ( 0.05 1.25 ( 0.07 5.73 ( 0.22 11.00.14 21.9 ( 0.14 1.01 ( 0.02 3.62 ( 0.03 3.96 ( 0.08 0.49 ( 0.03
0.24 ( 0.03 1.10 ( 0.04 4.86 ( 0.08 10.0 ( 0.22 20.0 ( 0.13 nab 0.00 na 1.18 ( 0.09 5.34 ( 0.10 10.7 ( 0.17 21.0 ( 0.36 1.02 ( 0.02 3.52 ( 0.04 4.14 ( 0.05 0.45 ( 0.01
a Quantitation of all standards are based on a single 0.2-100 µg/L external calibration plot run prior to the experiment. Values based on isotopic internal standards were statistically indistinguishable in most cases. All analyses are conducted in triplicate. b No analysis was conducted. c HB-x refers to an x µg/L perchlorate spike in synthetic matrix containing 2000 ppm each of Cl-, CO32-, and SO42-.
higher concentration of 5 µM, which in fact leads to a substantially superior LOD. Similar observations were made for other reagents (data not shown). The differences between the bromide and the fluoride forms of II and III are not significant in terms of the increase in signal (17-33% gain is observed on going to fluoride). However, this gain is more than offset by the better reproducibility observed with the bromide salt, leading to a LOD of 20 ng/L perchlorate with 5 µM III as the bromide, directly as bought offthe-shelf. Complex Matrices. On the basis of the above results, the use of III (as the bromide) was further explored. The effect of matrix ions in the samples was evaluated by preparing a synthetic matrix containing 2000 mg/L each of Cl-, CO32-, and SO42-. With either 2.5 or 5.0 µM III (as bromide), the signal suppression was negligible (
