Anal. Chem. 1998, 70, 353-359
Electrospray Ion Chromatography-Tandem Mass Spectrometry of Oxyhalides at Sub-ppb Levels L. Charles* and D. Pe´pin†
Faculte´ s de Me´ decine et de Pharmacie, Institut Louise Blanquet, 63000 Clermont-Ferrand, France
An electrospray ion chromatography-tandem mass spectrometry (IC-MS/MS) method has been developed for simultaneous analysis of oxyhalide ions (chlorite, chlorate, bromate, iodate) in water. Oxyhalide ions are extracted, from pretreated sample, in solid phase extraction (SPE), using an ion exchange column. Elution with water/ methanol ammonium nitrate eluent is performed on-line with negative ion electrospray mass spectrometry detection. Sample pretreatment is required in order to remove any major ions that displace oxyhalides during SPE, consisting of eliminating SO42-, Cl-, and HCO3- with ion exchange resins. Oxyhalide ion fragmentation yields mass spectra with successive oxygen losses. Isotope contribution allows the selection of two precursor ions per analyte (except IO3-), highly enhancing the selectivity of the method. Eluent ion choice is discussed, particularly because most of the tested ammonium salts were shown to be chlorate contaminated. In water samples, the limits of quantitation are 0.05 (BrO3- and ClO3-), 0.5 (IO3-), and 1.0 µg/L (ClO2-). In some ammonium salts, the microgram per kilogram level is reached for chlorate impurity quantitation. The major route of environmental exposure to oxyhalides is drinking water. Chlorite (ClO2-) and chlorate (ClO3-) are formed using chlorine dioxide (ClO2) as disinfectant.1-3 Chlorate can also be found in freshwater as pesticide residues. Bromate (BrO3-) is a byproduct of bromide-containing water ozonation.4 Iodide ions may be found in brackish waters and, to a lesser extent, in freshwater, and they may form iodate (IO3-) during ozonation. Condie5 has shown that chlorate and chlorite have toxicological effects. No guideline value was proposed for chlorate because available data on chlorate effects on humans are insufficient, but for chlorite 0.2 mg/L was designated as a provisional guideline value. Kurokawa’s study6 has led the International Agency for Research on Cancer to classify bromate as a group 2B carcinogen and to associate tumor risks with bromate levels above 0.05 µg/ L. Because of serious problems in bromate analysis at sub-ppb †
Chaire d’Hydrologie et Hygie`ne. (1) Aieta, E. M.; Berg, J. D. J. Am. Water Works Assoc. 1986, 78, 62-72. (2) Zika, R. G.; et al. Water Chlorination Chemistry: Environmental Impact and Health Effects; Lewis Publ.: Chelsea, MI, 1985; Vol. 5. (3) Gordon, G.; Tachiyashiki, S. Environ. Sci. Technol. 1991, 25, 468-474. (4) Haag, W. R.; Hoigne, J. Environ. Sci. Technol. 1983, 17, 261-267. (5) Condie, L. W. J. Am. Water Works Assoc. 1986, 78, 73-78. (6) Kurokawa, Y.; Maekawa, A.; Takahashi, M.; Hayashi, Y. Environ. Health Perspect. 1990, 87, 309-335. S0003-2700(97)00718-X CCC: $15.00 Published on Web 01/15/1998
© 1998 American Chemical Society
levels, the maximum allowable level of bromate presently proposed by the U.S. EPA is 10 µg/L. Although no data are available for iodate ions, they may also be toxic. Numerous techniques were used to analyze oxyhalide compounds. Titrimetry and colorimetry are subject to interference and contamination,7 whereas amperometric, iodometric, and DPD methods lack the ability to determine some of these analytes.8 Other methods, such as polarography,9 capillary electrophoresis,10 and pulsed electrochemical detection,11 are not sensitive enough. Improvements in pretreatment strategies and columns made ion chromatography a method of choice to speciate this type of anions, but suppressed conductivity detection is nonspecific. Two kinds of interference may be present: first, analyte coelution, and second, a highly concentrated anion peak, which may broaden an adjacent peak. Our previous work on bromate analysis by electrospray ion chromatography-tandem mass spectrometry12 showed that this coupling was a very attractive technique for inorganic species analysis in water for providing great specificity and sensitivity. This method is then proposed to be adapted for a simultaneous analysis of the main oxyhalide compounds. EXPERIMENTAL SECTION Apparatus. Ion chromatography was performed with an LC 200 binary pump (Perkin Elmer, Norwalk, CT), and direct introduction of the samples was done with an infusion pump (Model 22) from Harvard Apparatus (South Natick, MA). Flow injection analysis (FIA) was carried out using a Rheodyne 81-25 valve (Cotati, CA); an injection volume of 20 µL was used. Both introduction systems were coupled to a Sciex API III Plus triple-quadrupole mass spectrometer (Thornill, ON, Canada), equipped with an atmospheric pressure ionization (API) source, via an ionspray interface. The starting resolution for both quadrupoles was set at 0.7 amu fwhh. Mass calibration was performed on poly(propylene glycol) (PPG) solution as documented in the operator’s manual.13 The interface temperature was held at 54 °C. (7) Pfaff, J. D.; Brockhoff, C. A. J. Am. Water Works Assoc. 1990, 82, 192-195. (8) Aieta, E. M.; Roberts, P. V.; Hernandez, M. J. Am. Water Works Assoc. 1984, 76, 64-70. (9) Denis, M.; Masschelein, W. J. Analusis 1983, 11 (2), 79-83. (10) Bondoux, G.; Delsenne, F. Conf. 11eme Journ. Inf. Eaux, Poitiers, France, Sept 28-30, 1994; pp 1-13. (11) Kuo, C. Y.; Krasner, S. W.; Stalker, G. A.; Weinberg, H. S. Proc. Water Qual. Technol. Conf. 1992, 1993, 503-525. (12) Charles, L.; Pe´pin, D.; Casetta, B. Anal. Chem. 1996, 68, 2554-2558. (13) Perkin-Elmer Sciex Instruments, 8th issue, 1994.
Analytical Chemistry, Vol. 70, No. 2, January 15, 1998 353
All gases were purchased from Air Gaz (Saint-Denis, France). Ultrahigh-purity (UHP, 99.999%) nitrogen was used as the curtain gas in the API source at a flow rate of 0.6 L/min, and zero-grade air was the nebulizing gas, at a flow rate of 0.8 L/min. Helium used for sample degassing was of 99.998% purity. Tandem mass spectrometry (MS/MS) measurements were based on collision-induced dissociations, with a collision-activated dissociation (CAD) energy of 20 eV. UHP argon was the target gas, at a collision gas target (CGT) value of 300 × 1015 molecules/ cm2. The API III Hyperspec workstation and API software version 2.6 were used on a Power Macintosh 8100/80 for instrument control, data acquisition, and data processing. Materials and Reagents. Removal of sulfate, chloride, and bicarbonate anions from samples was performed using respectively On Guard-Ba, On Guard-Ag, and On Guard-H cartridges obtained from Dionex (Sunnyvale, CA). These cartridges were also used for eluent salt solutions pretreatment. Cartridges were placed on a vacuum manifold from Supelco (Bellefonte, PA). Ion chromatography of oxyhalide was carried out with an IonPac AG9-SC guard column (length, 50 mm; internal diameter, 4 mm; polymeric packing, pellicular configuration) from Dionex. Methanol (HPLC grade), sodium carbonate (Na2CO3, 99.9%), and sodium bicarbonate (NaHCO3, 99.5%) were purchased from Merck (Darmstadt, Germany). Ammonium sulfate ((NH4)2SO4) was purchased as 99% pure salt from Sigma Chemical Co. (St. Louis, MO), while ACS grade salt was purchased from Alfa (Karlsruhe, Germany) and as 99.5% pure salt from Merck. Ammonium nitrate (NH4NO3, ACS grade) was from Alfa. Ammonium dihydrogen phosphate (NH4H2PO4, 99.999%), ammonium iodide (NH4I, ACS grade), ammonium oxalate monohydrate ((NH4)2C2O4‚H2O, 99.99%), and ammonium thiocyanate (NH4SCN, ACS grade) were from Aldrich Chemical Co. (Milwaukee, WI); ammonium molybdate ((NH4)6Mo7O24‚4H2O, ACS grade) and magnesium chloride (MgCl2‚6H2O, 99.5%) were from Merck. Potassium bromate (KBrO3, 99.8%), potassium chlorate (KClO3, 99%) and sodium chlorite (NaClO2, ACS grade 80%) were purchased from Alfa and potassium iodate (KIO3, 99.5%) from Fluka (Buchs, Switzerland). Water was obtained from a Milli-Q water purification system (Millipore, El Paso, TX). Procedure. (i) Ion Chromatographic Conditions. First kept in a carbonate buffer (2.0 mM Na2CO3, 0.75 mM NaHCO3), the stationary phase was equilibrated in 10:90 water/methanol (v/v) at 2 mL/min for 5 min. A 5 mL sample volume loading was then performed for preconcentration, followed by reequilibration of the column in the water/methanol mixture. During elution, the flow rate was decreased to 1 mL/min. Using a zerodead-volume tee connector, the column effluent was split, and the flow rate in the electrospray was 50 µL/min. The eluent composition was 10:90 water/methanol (v/v), 65 mg/L NH4NO3. After elution, the column was flushed with a carbonate buffer (200 mM Na2CO3, 75 mM NaHCO3) at 2 mL/min for 3 min and then with the storage eluent (2.0 mM Na2CO3, 0.75 mM NaHCO3) for 5 min, the effluent being sent to the waste. (ii) ESI-MS/MS Data Acquisition. Mass spectra of each oxyhalide were first achieved in product scan mode over m/z 20200 (Figure 1). 35Cl and 37Cl isotopic contribution yields chlorite and chlorate spectra from respectively m/z 67 and 69 (ClO2-) and 354 Analytical Chemistry, Vol. 70, No. 2, January 15, 1998
Figure 1. Oxyhalide fragmentation spectra acquired in product scan mode with precursor ions (a)37ClO2- and 35ClO2-; (b) 37ClO3- and 35ClO -; (c) 81BrO - and 79BrO -; (d) 127IO -. 3 3 3 3
m/z 83 and 85 (ClO3-); m/z 127 and 129 correspond to BrO3mass with 79Br and 81Br contribution; and iodate spectrum was from the fragmentation of m/z 175 (IO3-). Spectra show successive oxygen losses; therefore, the signal was recorded in the multiple reaction mode (MRM). Recorded transitions were 67/ 51 and 69/53 for chlorite; 83/67 and 85/69 for chlorate; 127/111 and 129/113 for bromate; and 175/159 and 175/143 for iodate. Dwell time was 300 ms, and pause time 0.052 ms. (iii) Sample Pretreatment. The resin cartridges were prepared independently with a 5 mL deionized water flush at a maximum flow rate of 2 mL/min. Three cartridges were successively connected in the order Ba-Ag-H and then installed on the vacuum manifold. A 10 mL sample was loaded at a maximum flow rate of 2 mL/min. The first 3 mL was discarded, and a minimum of 6 mL was collected in a tube. A sparge with helium gas at 5 psi for 5 min was performed to remove dissolved carbon dioxide.14 RESULTS AND DISCUSSION In an earlier work,12 an electrospray ion chromatographytandem mass spectrometry (IC-MS/MS) method was developed for the analysis of bromate at sub-ppb levels in water. We proposed here to apply this coupling for quantitative determination of several oxyhalides of particular interest in water disinfection. According to the bromate method, ions were first extracted from the sample in a solid phase extraction step, using AG9-SC stationary phase. Sorbed ions were then eluted from the column (14) Dionex Corp. Application Note 101, 1995; pp 1-6.
Figure 2. Influence of the methanol content in the mobile phase on oxyhalide sensitivity.
with water/methanol (10:90 v/v), 27.5 mg/L (NH4)2SO4, an eluent optimized to be ESI compatible. Electrospray-Tandem Mass Spectrometry (ESI-MS/MS). Oxyhalides are ionic species of particular interest in mass spectrometry because of the natural occurrence of bromine and chlorine in two isotopic forms and because of a typical fragmentation behavior of inorganic oxide. In negative mode, oxychlorides and oxybromides are detected at two m/z values in MS (single mode): m/z 67 and 69 for ClO2-; m/z 83 and 85 for ClO3-; and m/z 127 and129 for BrO3-. IO3- is detected at m/z 175. All these precursor ions can be selected in the first quadrupole to be fragmented in the collision cell. The MS/MS spectra were obtained in product scan mode (Figure 1). The fragmentation consists of successive oxygen losses. This fragmentation added to the isotopic distribution confers a very high selectivity to the analysis. Therefore, the signal was recorded in MRM, consisting of spectral transition monitoring. This acquisition mode is limited to eight signals: for each compound, two signals were selected so as to include isotopic distribution and oxygen loss. Then, the recorded transitions were 35ClO2-/35ClO- (67/51), 37ClO2-/37ClO(69/53), 35ClO3-/35ClO2- (83/67), 37ClO3-/37ClO2- (85/69), 79BrO -/79BrO - (127/111), and 81BrO -/81BrO - (129/113). Io3 2 3 2 date signal was from IO3-/IO2- (175/159) and IO3-/IO- (175/ 143) transitions. Two chromatograms were available for each compound, the most sensitive transition being used for quantitation. Because this acquisition mode is extremely specific to each analyte, no spectral interference can occur. This allows the resolution of the first quadrupole to be adjusted to a lower value, enhancing sensitivity. Eluent Composition. Influence of Organic Solvent. FIA of individual compound standard solutions in deionized water showed that mobile phase enrichment with methanol enhanced sensitivity (Figure 2). From 10 to 90% of methanol in the mobile phase, the net signal intensity increases from 6- to 20-fold, depending on the analyte. As a 90% methanol mobile phase provides optimal spray conditions, a solvent exchange was choosen rather than a 1:10 dilution of the aqueous sample with methanol. Solvent exchange was performed in a solid phase extraction step (SPE) on the AG9SC column. Using anionic exchange properties of the AG9-SC column, oxyhalides adsorb on the stationary phase as the aqueous sample is loaded. Adsorbed anions were redissolved in the optimized water/methanol mixture during the elution step, the
stationary phase being solvent compatible (SC). The optimal required solvent composition in the column effluent allowed SPE on-line with MS/MS detection. In this configuration, the solvent exchange step can become a preconcentration step, loading a 5 mL sample volume. Moreover, optimizing the choice of the eluent anion, oxyhalides can be separated in a chromatographic step. Choice of the Eluent Anion. Classical IC eluents are not compatible with electrospray coupling. During the evaporation process, inorganic nonvolatile salts (Na2CO3, NaHCO3) remain as residue in a dry particle, and a deposit rapidly blocks the orifice of the spectrometer. A new anion eluent has been investigated, considering that the performance of the AG9-SC column is limited by ion exchange competition. An anion, having a high affinity for the column resin and being present in much higher concentration, can cause complete displacement of the analyte of interest and then acts as an eluent. In our first bromate analysis method,12 sulfate was selected as the eluent anion. Sulfate was shown to desorb other oxyhalides as well. But blank analysis with a 10:90 water/methanol (v/v), 27.5 mg/L (NH4)2SO4 eluent yielded chromatograms with a chlorate peak. Retention time of the peak and, above all, spectral transitions (83/67 and 85/69) observed at this retention time demonstrate a chlorate contamination in the reagents. The four potential contamination sources were then examined: carbonate buffers, water, methanol, and ammonium sulfate salt. Because carbonate buffers used for column flushing or storage have a high elutive strength, eventual chlorate anions cannot stay on the resin as sorbed ions. To check their purity in terms of chlorate, water and methanol have been distilled: blank analysis still showed a chlorate peak. The last hypothesis was the sulfate salt contamination. Ammonium sulfate salts from various manufacturers have been tested, but all of them were chlorate contaminated. A new anion then had to be investigated. Compounds selected as potential ionic eluents were anions usually strongly retained on the stationary phase, like nitrate (NO3-), oxalate (C2O42-), hydrogen phosphate (HPO42-), and molybdate (Mo7O246-) because of their higher valency, or like iodide (I-) and thiocyanate (SCN-) because of a steric effect. Ammonium salt solutions of these anions have been tested as eluents because, being volatile, ammonium cation is well suited for electrospray coupling. First, chlorate standard solution in deionized water was analyzed with every eluent to establish the chlorate retention time in each case. Then, eluent purity was checked with blank analysis. Among the tested ammonium salts, only nitrate was shown to be chlorate-free. Optimization of the Eluent Anion Concentration. Nitrate affinity toward the resin is slightly higher than that of chlorate, and its ability to elute chlorate is mainly due to a concentration effect. Chlorate peak broadens at low nitrate concentration, whereas a 100 mg/L NO3- eluent provides a thinner peak with a higher signal-to-noise ratio (Figure 3). To work in these conditions, ion spray voltage (ISV) and coelution effect had to be assessed. The ISV value has to be lowered to avoid corona discharge effects in negative mode electrospray when high salt content eluent is used. Lowering the ISV did not significantly decrease the signal sensitivity. Chlorite and iodate coelute at 8.4 min when using a 100 mg/L NO3- eluent. In terms of analyte discrimination, coelution is not a problem in MS/MS detection because each compound has specific spectral transitions. A chromatogram is Analytical Chemistry, Vol. 70, No. 2, January 15, 1998
355
Figure 3. Influence of the nitrate content in the mobile phase on chlorate peak width.
Figure 5. Chromatograms of a standard mixture in deionized water, using a 50 mg/L NO3- eluent: (a) chlorite 67/51 (3 µg/L); (b) chlorate 83/67 (0.2 µg/L); (c) bromate 127/111 (0.2 µg/L); (d) iodate 175/159 (2.0 µg/L). Table 1. Limits of Quantitation (LOQs) of Oxyhalide Anions in Water
-
ClO2 IO3BrO3ClO3-
Figure 4. (a) Influence of increasing amounts of iodate on chlorite sensitivity during coelution. (b) Influence of increasing amounts of chlorite on iodate sensitivity during coelution. N, mole number.
recorded for each transition so that each analyte can be studied qualitatively and quantitatively. However, in ionspray ionization mode, competition may occur between two analytes, in terms of ionization rate and/or ion evaporation. Experiments were conducted in which the concentration of one analyte was held constant and the other increased. Figure 4a shows the influence of increasing amounts of iodate on chlorite sensitivity at three concentration levels (1.3, 5.0, and 6.7 µg/L), whereas the evolution of iodate sensitivity (at 1.0, 5.0, and 6.7 µg/L) with increasing chlorite amounts is reported in Figure 4b. At the lower concen356 Analytical Chemistry, Vol. 70, No. 2, January 15, 1998
selected transition
LOQ ( σ (µg/L)
accuracy (%)
67/51 175/159 127/111 83/67
1.0 ( 0.1 0.5 ( 0.1 0.05 ( 0.01 0.05 ( 0.01
1 3 4 1
tration level, the experiment shows that no significant signal variation of the first analyte is observed when increasing the amount of the second one (Figure 4). But at the higher concentration levels, sensitivity is affected. A 20% loss in chlorite signal at 5 µg/L is observed when the iodate mole number is 2.5 times that of chlorite. But at 6.7 µg/L, chlorite response decrease is less important (7%), even with a higher mole number ratio (N(IO3-)/N(ClO2-) ) 3.8) (Figure 4a). As shown in Figure 4b, a 20% loss in iodate sensitivity is observed with mole number ratios N(ClO2-)/N(IO3-) of 16.7 and 12.5 for concentration levels of 5.0 and 6.7 µg/L, respectively. These results mean that (a) competition may occur between these two compounds when coeluting and (b) this competition depends not only on the relative concentrations of the analytes but also on their respective concentration levels. Therefore, the upper limit at which problems would occur in electrospray ionization of these compounds cannot be defined. Because in real water samples disparate concentrations of chlorite and iodate may be present, coelution must be avoided to obtain reliable results. Chlorite and iodate can be separated with a 50 mg/L NO3- eluent (10.5 and 10.7 min, respectively). This eluent will finally be selected, even if it causes chlorate peak broadening (Figure 5). Sample Pretreatment. The preconcentration step involves sample pretreatment to eliminate ions that can disturb oxyhalide trapping. Sulfate, chloride, and bicarbonate ions are commonly present in water samples at milligram per liter levels. They have
Figure 6. (a) Peak area (67/51) calibration over concentration range 1.0-10.0 µg/L for chlorite. (b) Peak area (83/67) calibration over concentration range 0.05-10.00 µg/L for chlorate. (c) Peak area (127/111) calibration over concentration range 0.05-1.00 µg/L (c-1) and 1.0010.00 (c-2) for bromate. (d) Peak area (175/159) calibration over concentration range 0.5-10.0 µg/L for iodate.
a high affinity for the column stationary phase and can easily cause oxyhalide displacement during the extraction-preconcentration step. Sulfate was removed as BaSO4 precipitate when the sample was loaded through a barium cartridge (On Guard-Ba). Studies indicate that, for consistent sulfate removal, a sample must have a sufficient amount of a divalent cation to displace the divalent barium from the resin so that it can react with sulfate.14 In some cases, 100 µL of a magnesium chloride solution (0.5 M Mg2+) was added to the initial sample. Chloride was suppressed from the sample as AgCl precipitate in a silver cartridge (On GuardAg). Bicarbonate was eliminated by sample acidification in a
hydrogen cartridge (On Guard-H); the resulting dissolved carbon dioxide was removed from the pretreated sample by sparging with helium gas. This pretreatment is very efficient as long as the cartridge maximum capacities are not exceeded. These capacities (2.5 mequiv for each cartridge) were shown to be sufficient for most water sample desalting. Analytical Results. Under classical ion chromatographic conditions using AG9-SC column, oxyhalides elute in the order iodate, chlorite, bromate, chlorate. As mentionned by Rabin and Stillian,15 the presence of methanol in the eluent modifies the retention properties of the column, with chlorite eluting before Analytical Chemistry, Vol. 70, No. 2, January 15, 1998
357
Table 2. Chlorate Peak Area in Blank Analysis Using Contaminated Ammonium Salt in the Eluent eluent ion type -
NO3 HPO42ISCNSO42MoO246C2O42-
Figure 7. Tap water sample analysis. Sample composition: 25.8 mg/L Ca2+; 9.4 mg/L Mg2+; 13.7 mg/L Na+; 3.7 mg/L K+; 11.9 mg/L Cl-; 6.7 mg/L NO3-; 19.3 mg/L SO42-; 129.4 mg/L HCO3-. Chlorate was quantified at 1.95 µg/L.
iodate. Limits of quantitation (LOQs) and external standard calibrations were determined by analyzing individual standard solutions because some oxyhalide salts (mainly NaClO2, ACS purity 80%) contained chlorate impurities. LOQ was defined as the concentration which yields a signal as 10σ the blank. A repeatability experiment at LOQ level was done with n ) 7. The results are reported in Table 1. This method is very sensitive for oxyhalide analysis. Particularly, bromate sensitivity has been enhanced compared to our first results,12 and the bromate toxicological threshold (0.05 µg/L) can be reached. Linear working ranges were determined for each analyte. The upper limit was 10 µg/L, considering that other techniques, like IC-suppressed conductivity, were relevant from about 2 to 5 µg/ L. Linearity and correlation coefficients are given in Figure 6. It should be noted that, for bromate, the working range from LOQ (0.05 µg/L) to 10 µg/L had to be divided in two linear ranges (Figure 6c-1 and c-2). A tap water sample analysis is shown in Figure 7. Chlorate was found at 1.95 µg/L. We observed a shift in retention times: the sample contains nitrate (6.7 mg/L), which was not eliminated during the pretreatment, and began to elute the analytes during the preconcentration-extraction step. But MS/MS acquisition mode ensures the identity of the observed analyte. Multiple spiked additions of chlorate in the sample before pretreatment were performed, first to confirm the retention time shift and second to calculate the recovery of chlorate (98%) during pretreatment. Chlorate Impurity Quantitation. When investigating the eluent anion for oxyhalide analysis, many ammonium salts were shown to be chlorate contaminated. Chlorate peak areas measured in blank analysis using these different eluents are reported in Table 2. It cannot be said here that the higher the peak (15) Rabin, S.; Stillian, J. J. Chromatogr. A 1994, 671, 63-71.
358 Analytical Chemistry, Vol. 70, No. 2, January 15, 1998
chlorate contamination -
concn (mg/L)
ClO3 retention time (min)
peak area
50 50 70 20 20 70 20
11.2 12.3 13.4 16.5 17.1 18.0 19.3
none 9642 2884 870 16688 11583 20119
area, the higher the chlorate contamination in the eluent. The chlorate chromatographic response in blank analysis is directly connected to the amount of chlorate accumulated on the column during the elution step. Accumulation time of chlorate from the eluent depends on the eluent anion nature, as chlorate retention time varies in accordance with the eluent strength. To quantify chlorate contamination, ammonium salt solutions should be considered as samples and analyzed in IC-MS/MS as water samples. Because they are highly mineralized, these particular samples must be specifically pretreated to eliminate the matrix anions. Sulfate was removed from ammonium sulfate solution with a Ba cartridge, after addition of 100 µL of a MgCl solution (0.5 M Mg2+) in the sample. To eliminate chloride contributed to the sample by the magnesium chloride, a Ag cartridge was placed after the Ba cartridge. Finally, a H cartridge allowed the trapping of any eluted silver ions. According to instructions for On Guard cartridges use,16 phosphate ions should be removed with an Ag cartridge. But this treatment was not very efficient. As barium phosphate (BaHPO4) is practically insoluble in water, phosphate was eliminated from ammonium phosphate solution with a Ba cartridge. The same treatment configuration as for sulfate removal described above was applied. Molybdate, iodide, and thiocyanate were removed from respectively ammonium molybdate, ammonium iodide, and ammonium thiocyanate solution as silver salt precipitates by mean of a Ag cartridge. Any eluted silver ions were trapped in a H cartridge. The different pretreated salt solutions were analyzed and quantified as samples using an external standard calibration. Chlorate was found as a contaminant in the range 5-50 mg/kg ammonium salt. The chlorate LOQ in ammonium salts mainly depends on the maximum salt concentration which can be pretreated; that is to say, cartridge maximum capacity. According to chlorate LOQ in water, chlorate LOQs in ammonium salts were calculated in a standard configuration (one desalting cartridge of each type), and the results are reported in Table 3. The method is then shown to be relevant to quantitate trace level contamination in salts, as long as the sample matrix can be eliminated. If no pretreatment is available for matrix anion removal, as is the case for ammonium oxalate solution, it can be still possible to evaluate the chlorate contamination level. This (16) Dionex Corp. Installation Instructions and Troubleshooting Guide for OnGuard Cartridges, 1995; Revision 07, p 5.
Table 3. Limits of Quantitation (LOQs) of Chlorate in Ammonium Salt
ammonium salt
removed ion
cartridge maximum capacity type mequiv g/La
(NH4)2SO4 NH4H2PO4 (NH4)6Mo7O24‚ 4H2O NH4I NH4SCN
SO42HPO42Mo7O246-
Ba2+ Ba2+ Ag+
2.5 2.5 2.5
12.0 11.9 44.0
3.0 3.5 1.0
ISCN-
Ag+ Ag+
2.5 2.5
31.8 14.5
1.5 2.5
a
chlorate LOQ (µg/kg)
Calculated using a 10 mL loaded sample volume.
was done in a different way: ammonium oxalate salt was dissolved in 10:90 water/methanol (v/v) and used as the eluent in which chlorate multiple spiked addition was performed. As discussed earlier, in blank analysis, the chlorate chromatographic response is directly connected to the chlorate quantity which has accumulated on the column during elution. If the contaminated eluent is spiked with growing amounts of chlorate, the resulting peak area will increase proportionally. The chlorate contamination
evaluated this way in ammonium oxalate salt was in the same range as in the other ammonium salts. CONCLUSION This method allows simultaneous oxyhalide quantitation at trace levels in disinfected drinking water. It is very specific and very sensitive; particularly, the bromate toxicological threshold (0.05 µg/L) is reached. When specific pretreatment is available for matrix anion removal, this coupling is also shown to be powerful in monitoring oxyhalide trace contamination in commercial salts. Extending its application field to some other inorganic anions, this IC-MS/MS coupling could be well suited for impurity checking in water, organic solvents, salts, or raw materials used in industrial areas, like the semiconductor industry, where ionic contamination, at low-ppb levels, is one of the major causes of defects. Received for review July 7, 1997. Accepted October 22, 1997.X AC9707186 X
Abstract published in Advance ACS Abstracts, December 15, 1997.
Analytical Chemistry, Vol. 70, No. 2, January 15, 1998
359
