Anal. Chem. 1996, 68, 2554-2558
Electrospray Ion Chromatography-Tandem Mass Spectrometry of Bromate at Sub-ppb Levels in Water L. Charles,*,† D. Pe´pin,‡ and B. Casetta§
Institut Louise Blanquet and Chaire d’Hydrologie et Hygie` ne, Faculte´ s de Me´ decine et de Pharmacie, 63000 Clermont-Ferrand, France, and Perkin Elmer Sciex, via Tiepolo 24, I-20052 Monza (MI), Italy
An electrospray ion chromatography-tandem mass spectrometry (IC-MS/MS) method has been developed for the analysis of bromate ions in water. This IC-MS/MS method improves the limit of detection of bromate ions by a factor of 10. The method consists of solid phase extraction with an ion exchange column and elution of the analyte with water/methanol ammonium sulfate eluent online with a negative ion electrospray mass spectrometry detection. SPE requires sample pretreatment to remove any major ions that displace bromate, consisting of eliminating SO42-, Cl-, and HCO3- ions respectively with barium-form, silver-form, and acid (H+-form) exchange resins. The methanolic sulfate eluent permits IC-MS coupling via an electrospray interface. BrO3- was selected in the first quadrupole (Q1) at two m/z values, 127 and 129, according to the isotope contributions of 79Br and 81Br. After fragmentation in the collision cell (second quadrupole, Q2), the third quadrupole (Q3) analyzes the product ions as (M - O)-, (M - 2O)-, and (M - 3O)-. Among the six recordable transitions, four were selected, the other two yielding high background. A lowered resolution raised sensitivity by a factor of up to 3. The limit of quantitation of this method was 0.1 µg/L. In France, drinking water disinfection is mainly done by chlorination, the main disadvantages of which are typical bad taste and odor and byproducts like the trihalomethanes (THMs), heavy carcinogens. As an alternative, ozonation is performed to disinfect water and to remove organic and mineral micropollutants, producing low THM levels. Nevertheless, one of the main ozonation byproducts comes from the oxidation of bromide (naturally present at trace level in most waters), that is to say, bromate. Since 1990, toxicological studies1 have led the International Agency for Research on Cancer (IARC) to classify bromate as a group 2B carcinogen and associate renal tumor risks with bromate concentrations above 0.05 µg/L. The maximum allowable level of bromate presently proposed by the U.S. EPA is 10 µg/L, but there are serious problems in analysis at lower levels. The U.S. EPA plans to convene a second regulatory negotiation in 1998, at which time it is anticipated that the bromate practical quantitation limit (PQL) will be lower, more health effects on this disinfection byproduct will be available, and the treatment to control bromate †
Institut Louise Blanquet, Faculte´s de Me´decine et de Pharmacie. Chaire d’Hydrologie et Hygie`ne, Faculte´s de Me´decine et de Pharmacie. Perkin Elmer Sciex. (1) Kurokawa, Y.; Maekawa, A.; Takahashi, M.; Hayashi, Y. Environ. Health Perpect. 1990, 87, 309-335. ‡ §
2554 Analytical Chemistry, Vol. 68, No. 15, August 1, 1996
formation will have been appropriately developed and field-tested.2 Numerous techniques are available to analyze bromate. Some of them are not sensitive enough, such as polarography,3 capillary electrophoresis,4 and pulsed electrochemical detection.5 Others are nonspecific, such as the colorimetric method with nitrite and chlorite.6 Ion chromatography has been severely limited, but improvements in pretreatment strategies, ion chromatography columns, and suppressed conductivity detection has led to its detection at the 5-10 µg/L level, according to U.S. EPA Method 300.0.7 Preconcentration techniques are necessary to reach ppb concentrations, the preconcentrated volume directly determining the limit of detection (LOD) of the method. Some authors8,9 have achieved LODs of about 1 µg/L with a 5 mL sample preconcentration. But the conductivity detector is nonspecific. A new approach is to combine the advantages of ion chromatography with a mass spectrometer. This coupling is performed via an electrospray interface and satisfies selectivity and sensitivity requirements. This study is the first application of electrospray to bromate analysis. 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, Ontario, 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.10 (2) Krasner, S. W.; Glaze, W. H.; Weinberg, H. S.; Daniel, P. A. AWWA 1993 Annual Conference 6, San Antonio, TX, June 10, 1993; pp 55-83. (3) Denis, M.; Masschelein, W. J. Analusis 1983, 11 (2), 79-83. (4) Bondoux, G.; Delsenne, F. Conf. 11eme Journ. Inf. Eaux, Poitiers, France, Sept 28-30, 1994; pp 1-13. (5) Kuo, C. Y.; Krasner, S. W.; Stalker, G. A.; Weinberg, H. S. Proc. Water Qual. Technol. Conf. 1992, 1993, 503-525. (6) Gordon, G.; Bubnis, B.; Sweetin, D.; Kuo, C. Y. Ozone: Sci. Eng. 1994, 16, 79-87. (7) Pfaff, D. J.; Brockhoff, C. A.; O’Dell, J. W. EPA Method 300.0; U.S. Environmental Protection Agency: Washington, DC, August 1991. (8) Joyce, R. J.; Dhillon, H. S. J. Chromatogr., A 1994, 671, 165-171. (9) Weinberg, H. J. Chromatogr., A 1994, 671, 141-149. (10) Perkin-Elmer Sciex Instruments, 8th issue, 1994. S0003-2700(96)00175-8 CCC: $12.00
© 1996 American Chemical Society
Table 1. Ion Chromatography Parameters conditioning column conditioning water/methanol (10:90 v/v) for 5 min conditioning flow 2 mL/min sample loading volume flow
5 mL 2 mL/min
analytical column length internal diameter packing
IonPac AG9-SC (SC, solvent compatible) 50 mm 4 mm polymeric packing (pellicular configuration)
elution eluent
composition purpose water/methanol (10:90 v/v) better nebulization 27.5 mg/L (NH4)2SO4 bromate desorption eluent flow rate 1 mL/min detection regeneration column regenerants
regenerants flow rate
ESI-MS/MS, negative MRM (1) 200 mM Na2CO3-75 mM NaHCO3 for 3 min (2) 2.0 mM Na2CO3-0.75 mM NaHCO3 for 10 min 2 mL/min
The interface temperature was held at 54 °C. 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 15 eV. A UHP argon/nitrogen mixture was chosen as the target gas, at a collision gas target (CGT) value of 350 × 1015 molecules/cm2. The API III Hyperspec workstation and API software version 2.6 were on a Power Macintosh 8100/80 for instrument control, data acquisition, and data processing. Materials and Reagents. Removal of sulfate, chloride, and bicarbonate anions was performed using respectively On GuardBa, On Guard-Ag, On Guard-H cartridges obtained from Dionex (Sunnyvale, CA). They were placed on a vacuum manifold from Supelco (Bellefonte, PA). Ion chromatography of bromate was carried out with an IonPac AG9-SC guard column from Dionex. Methanol (HPLC grade), sodium carbonate (Na2CO3), and ammonium sulfate ((NH4)2SO4) were purchased from Merck (Darmstadt, Germany). Water was obtained from a Milli-Q water purification system (Millipore, El Paso, TX). Sodium bicarbonate (NaHCO3) was from Prolabo (Paris, France), and potassium bromate (KBrO3) was from Riedel de Hae¨n (Hannover, Germany). Pure salts were used as received. Procedure. (i) Sample Pretreatment. The resin cartridges were prepared independently with a 5 mL deionized water flush at a maximum flow rate of 2 mL/min. The 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.11 (11) Dionex Corp. Application Note 101; 1995, pp 1-6.
Figure 1. Bromate fragmentation spectra acquired in product scan mode with precursor ion 81BrO3- (underlined) and 79BrO3-. Table 2. MS/MS Data Acquisition Parameters (Multiple Reaction Monitoring) dwell time pause time duration
100.0 ms 0.052 ms 30 min
Q1 mass
Q3 mass
127.0 127.0 127.0 129.0
111.0 95.0 79.0 113.0
(ii) Ion Chromatographic Conditions. The chromatographic steps are listed in Table 1. The column was connected to the detector only during elution. To obtain the appropriate flow rate (50 µL/min) for electrospray ionization (ESI), the column effluent was split (ratio of 1:20) using a zero-dead-volume tee connector. (iii) ESI-MS/MS Data Acquisition. Mass spectra of bromate were first achieved in product scan mode over m/z 20-135 (Figure 1). The precursor ions chosen to be fragmented in the collision cell were m/z 127 and 129, corresponding to the BrO3mass with respectively 79Br and 81Br isotopic contribution. BrO3- can lose one, two, or three oxygen atoms during fragmentation; therefore, the signal was recorded in the multiple reaction monitoring mode (MRM). Acquisition parameters during ion chromatography are presented in Table 2. RESULTS AND DISCUSSION Classical ion chromatography eluents (such as borate or carbonate/bicarbonate solutions) are incompatible with an ESIMS/MS detection system. These high-ionic-strength eluents suppress analyte ion signals and/or raise background ion counts, increasing LODs. During the evaporation process, bicarbonate or borate ions are not all emitted from the droplets, and most of them remain as involatile residue in a dry particle. A deposit is observed which rapidly blocks the orifice of the spectrometer. Operating with Na2CO3/NaHCO3 eluent, the sensitivity with ESIAnalytical Chemistry, Vol. 68, No. 15, August 1, 1996
2555
Figure 2. Influence of the methanol content in the mobile phase on bromate sensitivity.
MS/MS is one-fifth that obtained without matrix ions (in deionized water). This observation has been made for other compounds.12 Reducing the electrolyte concentration in the chromatographic eluent by passing it through an ion exchange fiber allows maximum sensitivity.13 The proposed alternative is to select an electrolyte that is a good ionic eluent for bromate and also ESI compatible. Eluent Composition. The presence of methanol plays a significant role in response stability. FIA of bromate standard solutions in deionized water showed that mobile phase enrichment with methanol increased the detected signal: with 30-90% of methanol in the mobile phase, the net signal intensity increases 10-fold (Figure 2). Better nebulization and subsequent vaporization occur at these optimal spray conditions. To avoid having a 1:10 dilution of the sample with methanol, the alternative is solvent exchange: bromate ions are extracted from the aqueous sample and redissolved in the water/methanol (10:90 v/v) eluent. Using anionic exchange properties, this solid phase extraction (SPE) is performed with the AG9-SC column. The stationary phase is solvent compatible (SC): the eluent used to desorb bromate can contain, among others, 90% of methanol. The optimal required solvent composition in the column effluent allows SPE on-line with ESI-MS/MS detection. The performance of the AG9-SC concentrator column is limited by ion exchange competition. The column resin can trap only a certain quantity of analyte. Once the column capacity is exceeded, the trapping will not be quantitative. The processes become more complicated when concentrating ions having widely different affinities for the resin. An anion such as sulfate, which has a high affinity for the resin and which is present in much higher concentration, can cause displacement of bromate. Particularly above 20 mg/L sulfate in the sample, bromate is completely displaced.8 Sulfate perturbs bromate sorption and must be eliminated from the sample. But sulfate can be useful when bromate has to be desorbed. So, with an eluent containing 20 mg/L SO42-, sorbed bromate can be eluted. Figure 3 shows a chromatogram of a bromate standard in deionized water, using a water/methanol (10:90 v/v), 27.5 mg/L (NH4)2SO4 eluent. No blocking of the orifice nor deterioration in sensitivity was observed: sulfate was shown to be ESI compatible, even at this relatively high concentration. (12) Siu, K. W. M.; Guevremont, R.; Le Blanc, J. C. Y.; Gardner, G. J.; Berman, S. S. J. Chromatogr., A 1991, 554, 27-38. (13) Conboy, J. J.; Henion, J. D.; Martin, M. W.; Zweigenbaum, J. A. Anal. Chem. 1990, 62, 800-807.
2556 Analytical Chemistry, Vol. 68, No. 15, August 1, 1996
Figure 3. Chromatogram of a 1 µg/L bromate standard in deionized water using a 20 mg/L SO42- water/methanol (10:90 v/v) eluent on AG9-SC column (detection in MRM 127/111-127/95-127/79-129/ 113).
With the SPE configuration, the solvent exchange step becomes a concentration step, and the introduced volume lowers the LOD. A useful comparison of different techniques and especially their sensitivities is possible only with the same preconcentration volume, usually 5 mL. Sample Pretreatment. The analytical working conditions involve sample pretreatment to eliminate ions that can disturb concentration or detection steps. As discussed above, bicarbonate ions hinder ESI-MS/MS detection; moreover, they are able to displace bromate from the stationary phase, proved by their intensive use as an ion chromatography eluent. Therefore, they are eliminated by sample acidification with an H cartridge; the resulting dissolved carbon dioxide is eliminated by sparging with helium gas. Because it is used in the chromatographic eluent, the original sulfate concentration in the sample must be below 20 mg/L. Thus, sulfate removal is performed with the formation of BaSO4 precipitate after percolating the sample through a Ba cartridge.11 Chloride is suppressed from the sample as well because it presents a high affinity for the column resin. To ensure good reproducibility of bromate recovery, chloride is then precipitated as AgCl in a Ag cartridge. A drawback to Ag cartridges is the leaching of silver from the cartridge into the sample matrix. To avoid the accumulation of silver on the column, the H cartridge is placed after the Ag cartridge to trap any eluted silver ions. Care must be taken concerning highly mineralized samples which exceed the cartridge capacity. In a standard configuration (one cartridge of each type in-line), the analytical ranges are defined in Table 3. The bromate recovery efficiency is high (95-100%) but depends on the sample matrix composition. 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.11 For waters which have not sufficient calcium and magnesium to initiate the barium displacement, sulfate remains in the treated sample and perturbs
Table 3. Pretratment Cartridge Capacity Calculated from the Data Given by the Manufacturer maximum capacity cartridge Ba2+ Ag+ H+
removed anion 2-
SO4 ClHCO3-
mequiv
g/La
2.5 2.5 2.5
12 8.875 13.4
a The H cartridge capacity calculation assumes that the cartridge neutralizes 10 mL of 0.2 M NaOH and that HCO3- is the only monovalent anion.
Figure 5. Chromatogram of bromate at 0.1 µg/L. The LOQ was determined at 10σ the blank. Table 4. Statistical Results of Repeatability Experiments for 0.1 µg/L BrO3- - σ: Standard Deviation - Relative Standard Deviation n
peak area
calcd concn (µg/L)
1 2 3 4 5 6 7
6325 6312 5872 6094 6439 6276 6590
0.0984 0.0982 0.0898 0.0940 0.1006 0.0975 0.1034
mean σ RSD
Figure 4. Background noise level produced by the transition 129/ 97.
quantitative bromate trapping on the concentrator column. The bromate recovery must then be specified for each type of sample, so multiple spike additions are necessary. If it is not correct, a divalent cation must be added in excess in the initial sample. Ion Chromatography. In this study, ion chromatography is used for the ion exchange properties of the stationary phase and not in its first application consisting of compound separation. The different chromatographic steps listed in Table 1 can be classified in three groups: loading of the sample (A), elution of the analyte (B), and column property regeneration (C). Though first kept in a carbonate buffer (2.0 mM Na2CO3, 0.75 mM NaHCO3), the stationary phase is conditioned in 10:90 water/ methanol (v/v) (step A). This eliminates excess free salts and reequilibrates the column after sample loading. During elution (step B), ion exchange competition between eluent sulfate and sorbed bromate takes place. As mentioned above, sulfate is able to displace the bromate. After bromate elution, sulfate ions are purged from the resin with a second carbonate buffer (200 mM Na2CO3, 75 mM NaHCO3), the column being disconnected from the detector (step C). Finally, initial conditions are reached again by loading the initial carbonate buffer. The next main improvement of the method will be the automation of all eluent circuits with four to six port valves to connect and disconnect automatically the eluent flow to the detector. This is being done in our laboratory.
0.0974 µg/L 0.0018 µg/L 0.019
Electrospray-Tandem Mass Spectrometry (ESI-MS/ MS). Bromate is an ionic species of particular interest in mass spectrometry because of the natural occurrence of bromine in two isotopic forms, 79Br and 81Br, in equal proportions. The bromate ion is detected as BrO3- at m/z 127 and 129. Both ions are selected in the first quadrupole (Q1) to be fragmented in the collision cell (second quadrupole, Q2). The MS/MS spectra are obtained in product scan mode. Figure 1 shows that bromate can lose one, two, or three oxygen atoms during fragmentation, yielding the following species: BrO2-, BrO-, and Br-. According to the isotopic contribution, this totals six transitions potentially recordable in multiple reaction monitoring mode (MRM): 127/ 111, 127/95, 127/79, 129/113, 129/97, and 129/81. When the six transitions are recorded, a high background appears after bromate elution, due to 129/81 and 129/97 (Figure 4). Thus, only four transitions are selected in MRM to obtain a well-resolved chromatographic peak, as shown in Figure 3. The origin of the interference for transition 129/97 is
This reaction scheme for 129/97 interference explains why the Analytical Chemistry, Vol. 68, No. 15, August 1, 1996
2557
high background noise level appears only after 18 min, the bromate retention time: sulfate ions are not present in the column effluent while occupying all resin sites while playing their eluent role. The combination with methanol in the spray occurs only after this time. The monitoring of the four transitions confers a very high selectivity to the analysis. No spectral interference can occur, the acquisition mode parameters being extremely specific to the bromate ion. This allows the resolution of the Q1 quadrupole to be adjusted to a lower value, enhancing sensitivity. Q1 resolution is then opened up to distinguish the ions m/z 127 from 129 to avoid the contribution of the undesirable 129/97 and 129/81. These new operating conditions raise sensitivity by a factor of 3 in comparison to the standard Q1 resolution factor. In this analytical configuration, the limit of quantitation (LOQ) of the method is 0.1 µg/L (Figure 5). A repeatability experiment (n ) 7) at this concentration level shows 1.9% relative standard deviation (RSD), and the accuracy is 2.6% (Table 4).
2558
Analytical Chemistry, Vol. 68, No. 15, August 1, 1996
CONCLUSION This method is sensitive enough to detect bromate ions formed during the ozonation of a bromide-containing water. Originally developed to study organic molecules, especially high-weight molecular compounds, ESI-MS/MS detection is also well suited to analyze very polar compounds and ionic species. Coupling of ion exchange chromatography to mass spectrometry via an electrospray interface is an attractive analytical technique for inorganic species analysis.
Received for review February 23, 1996. Accepted May 10, 1996.X AC960175K
X
Abstract published in Advance ACS Abstracts, June 15, 1996.
