ЖУРНАЛ АНАЛИТИЧЕСКОМ ХИМИИ, 2009, том 64, № 9, с. 959-964
^=ОРИГИНАЛЬНЫЕ СТАТЬИ
УДК 543
DETERMINATION OF SULFONYLUREA HERBICIDES IN WATER USING SOLID-PHASE EXTRACTION FOLLOWED BY LIQUID CHROMATOGRAPHY WITH ELECTROSPRAY ION TRAP
MASS SPECTROMETRY
© 2009 Xihui Ouyang1, Wei Zhang1, Jun Xu1, Na Chang1, Canping Pan1,
Jingping Zhang2, Weimin Niu2
1 Department of Applied Chemistry, College of Science, China Agriculture University
Beijing 100193, China 2Wuxi Municipal Center for Disease Control and Prevention Wuxi, 214023, China Received 16.01.2008; in final form 12.02.2009
A method has been developed for confirmation and quantitation of ten sulfonylurea herbicides (nicosufuron, thifen-sulfuron-methyl, metsulfuron-methyl, sulfometuron-methyl, chlorsulfuron, ethametsulfuron-methyl, tribenuron, bensulfuron-methyl, pyrazosulfuron-ethyl and chlorimuron-ethyl) in water samples. Herbicides were extracted from water by off-line solid-phase extraction (SPE). Different types of absorbents were evaluated: silica-based ODS-C18 and two polymeric sorbents, Cleanert HXN and Oasis HLB. Analyte determination and quantitation was performed by liquid chromatography with electrospray mass spectrometry (LC-ESI-MS) instrumentation, equipped with ion trap mass filter. Confirmatory analysis was carried out by LC/MS/MS. MS data acquisition was performed by a single or two-ion extracted ion monitoring program. The ten herbicides were measured in fortified tap water. Average recoveries of the nine analytes (except for tribenuron) from water samples were in the range of 77-109%, and the RSD ranged from 0.3 to14.5%. The limits of detection (LODs) varied from 6 to 34.8 ng/L.
Sulfonylurea herbicides were first introduced in 1982 by DuPont Agricultural Products. Sulfonylurea herbicides are labile and weakly acidic compounds. In recent years, sulfonylurea herbicides have become very popular worldwide because of their low application rates (typically in the range 10-40 g/ha), low toxicity to mammals, and unprecedented herbicidal activity. Therefore, very low concentrations (parts-per-billion) of these herbicides are to be expected in environmental samples. Determination of these compounds in water and soil were carried out using the liquid chromatograph with ultraviolet detector (LC-UV) [1-6], capillary electrophoresis with ultraviolet detector (CE-UV) [7-9], LC with mass spectrometry (MS) [10-16], immunoassay, bioassay, and other specific methods [17]. Sulfonylureas are generally not directly amenable to separation by gas chromatography (GC) because of their extremely low volatility and thermal instability. Sulfonylureas can be separated using reversed-phase high-performance liquid chromatography (HPLC), but they are not readily identified by standard HPLC detectors. These substances generally do not have extremely strong UV or visible light absorption that would allow selective and high-sensitive detection. For confirmatory and quantitative trace analysis of compounds having thermal instability or extremely low volatility, such as the compounds of interest in this study, the use of LC/MS/MS is required. LC/MS/MS offers chromatographic separation without thermal decomposition, high sensitivity, mass selectivity and structural information.
The herbicides investigated here are in widespread use in China, but there have been few reports describing measuring methods or simultaneous estimating quantities of these herbicides. Therefore, it is necessary to quickly develop suitable analytical methods, which have high selectivity, high sensitivity and are simple. We report here the results of analysis of water samples obtained by SPE on C18, HXN and Oasis, followed by LC-ESI-MS in the positive ionization mode.
EXPERIMENTAL
Reagent and Chemicals. The ten sulfonylurea herbicides standards were purchased from ICAMA of Ministry of Agriculture, China. Their chemical structure is characterized by aryl group (R1), a sulfonylureic bridge and a nitrogen-containing heterocyclic portion (R2), as shown in Fig. 1.
Acetonitrile (ACN) and methanol (MeOH) of HPLC grade solvents were purchased from Honeywell, USA. Acetic acid, phosphoric acid and phosphate were purchased from VAS, China. Deionized water used for sample preparation and LC mobile phase was prepared by a Milli-Q water purification system (Millipore, Bedford, MA, USA). All chemicals used for buffer preparation were of analytical reagent grade.
Different types of sorbents were used to accomplish the solid-phase extraction: polymeric cartridges-Oasis
O O II H II H Ri—S—N-C-N—R2 1 II 2
O
Rx = substituted benzene or nitrogen-containing heterocycle
or sulfur-containing heterocycle R2 = substituted nitrogen-containing heterocycle
Fig. 1. General structure of sulfonylurea herbicides.
HLB (30 mg, Waters) and Cleanert HXN (100 mg, Agela Tec.), and silica-based bonded C18 cartridges (200 mg, Agela Tec.).
Procedures. Preparation of Standard Solutions. Individual stock solutions (1000 mg/L) of each pesticide standard were prepared by dissolving 100 mg in 100 mL of ACN. The analytical standard mixtures were prepared by diluting each herbicide stock solution with ACN to a final concentration of 100 mg/L. These stock solutions were stored at 4°C in glass bottles with PTFE-faced crew caps. Appropriate dilutions of this stock solution were made in ACN to obtain final concentrations of 0.001, 0.1, 0.1, 1 and 10 mg/L.
Water Sample Fortification. 500 mL tap water samples were fortified by adding a combined standard of the ten sulfonylureas dissolved in ACN to obtain a concentration of 0.2, 2, 20 |g/L of the ten compounds and pH of the fortified water was then adjusted to 2.5 with H3PO4.
SPE Procedure. Pre-concentration of the analytes from water samples were accomplished by solid-phase extraction. The SPE cartridges were conditioned with 5 mL of ACN and then 5 mL of water. The aqueous extract was passed through the cartridge at about 2 mL/min using a suction system. Cartridges were dried by passing an air stream through them for 10 min, and eluted with 3 mL of ACN (ACN: phosphate buffer = 9 : 1 v/v for soil extraction) by gravity. The organic solvent was evaporated and finally, the residue was dissolved in 500 |L of ACN by sonication and then filtered through 0.22 |m filter for chro-matographic analysis.
Apparatus and Chromatographic Conditions. The
LC/MS/MS experiments were performed on a liquid chro-matography-UV-electrospray-ion-trap mass spectrometry system (1100 Series LC/MSD, Agilent Technologies, USA) equipped with an automatic injector, a membrane degasser, a thermostated oven and a quaternary pump. The system was controlled by an Agilent Chemstation. The HPLC analytical column used was a Zorbax Eclipse® XDB-C18 (Agilent, USA) 250 mm x 4.6 mm packed with 5 |m particles. The mobile phase was ACN (solvent A)-MeOH (solvent B) -0.2% (v/v) acetic acid in water (solvent C). The elution was as follows: the mobile phase started with 10% of ACN and 10% of MeOH, which were all increased linearly to 45% in 14 min, and to 48% in next 2 min. Then, the percentage was returned to initial conditions in 2 min. The column was equilibrated for 5 min. The
column temperature was kept at 30°C and flow-rate was 1.0 mL/min, but the column effluent was split using a peek tee to allow only a flow of 0.4 mL/min into the ESI source. The injection volume was 10 |L.
Mass spectra were recorded in the range m/z 100-500 using positive ion electrospray ionization (ESI) mode. The operating conditions for ESI were as follows: Nebulizer gas (nitrogen) pressure 40 psi; drying gas (nitrogen) flow-rate 8.0 L/min; Dry temperature 350°C.
HV Capillary voltage was set at 3.5 kV and capillary exit voltage was set at 105 V. Skimmer 1 voltage was set at 32.7 V and skimmer 2 was set at 6 V. MS/MS ion trap parameters were the following: trap drive 80; maximum accumulation time 200 ms; fragmentation time 40 ms. The selected ions and MRM ions of ten sulfonylurea herbicides are shown in Table 1.
RESULTS AND DISCUSSION
Liquid chromatography method development. The
structures of these sulfonylureas were so similarly that the separation was difficult. In order to find suitable conditions for the separation of the target compounds, we programmed several mobile phases: Me0H-0.2% (v/v) acetic acid, ACN-0.2% acetic acid, ACN-Me0H-0.2% acetic acid. The results demonstrated the mobile phase of ACN-MeOH-0.2% acetic acid was the optimum condition. The injection of a standard solution prepared in ACN into a mobile phase based on an ACN-MeOH-0.2% acetic acid linear gradient afforded good chromatographic separation for ten analytes in 20 min (before it was 90 min [9]). The elution time of the ten compounds was at 8-15 min. So, the effluent of the column from 0 to 8 min and from 15 to 20 min was diverted to waste. Linear calibration was found between peak areas and analytes concentration in the whole range studied (0.1-10.0 mg/L). The detection limits of the method were calculated as a signal-to-noise ratio of 3.
Pre-concentration by SPE and recovery studies.
SPE prior to chromatographic determination was used in order to develop a more sensitive method for the quantification of these herbicides. A comparative study of efficiency of C18-bonded silica and polymeric sorbents, such as Oasis HLB and Cleanert HXN, was made in order to evaluate the feasibility of these sorbents in retaining the analytes from water. To accomplish this, water samples spiked with the herbicides at a concentration level of 0.2 |g/L were prepared. The three cartridges were used under the conditions described in SPE procedure. Higher recoveries were obtained when the three sorbents were used. Oasis HLB was the best of the three sorbents (Fig. 2). Only tribenuron showed low recovery as it was unstable in an acid medium (pH 2.5).
Liquid chromatography-mass spectrometry detection. The LC/MS technique is proposed in this work to be a viable alternative when several compounds of the sam
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