Oasis WCX: A Novel Mixed-Mode SPE Sorbent for LC-MS Determination of Paraquat and Other Quaternary Ammonium Compounds, MS Young, KM Jenkins

Tags: Injection volume, mobile phase, mL water, Column temperature, phosphate buffer, Sample Preparation, ammonium phosphate, river water, LC, Kevin M. Jenkins Waters Corporation Oasis, SPE, Michael S. Young, cation-exchange, HPLC, Quaternary Ammonium Compounds
Content: 2 Environmental
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THE APPLICATION NOTEBOOK -- September 2004
Oasis® WCX: A Novel Mixed-Mode SPE Sorbent for LC­MS Determination of Paraquat and Other Quaternary Ammonium Compounds Michael S. Young and Kevin M. Jenkins Waters Corporation
Oasis® WCX, a mixed-mode weak cation-exchange SPE cartridge, provides unique retention of paraquat and related compounds. An Atlantis® HILIC column was utilized for HPLC using no ion-pairing reagents. P araquat and diquat, quaternary ammonium compounds (quats), are high-use agricultural chemicals. The 1997 USGS estimated use of the herbicide paraquat was over 3,000,000 lb, and the estimated use of diquat was about 200,000 lb (see Figure 1). Figure 1: The structures of paraquat and diquat. Because these substances are ionic, solid-phase extraction (SPE) and HPLC usually have been accomplished with the aid of an ion-pairing reagent such as an alkyl sulphonic acid. liquid chromatography (LC)­mass spectrometry (MS) provides the analyst with a more sensitive and highly selective analytical method for these compounds. Strong catIon Exchange-based SPE methods (Oasis® MCX) that do not require ion-pairing reagents have been employed successfully for quats SPE using LC­UV, but the strong salts or strong acids used for elution are difficult to remove and are serious impediments to optimal LC­MS analysis. In order to overcome these problems, a new type of sorbent has been developed for the retention of quaternary ammonium compounds and strongly basic organic compounds. The Oasis® WCX sorbent incorporates a weak cation-exchanger bound to a polymeric reversed-phase particle. Using the new sorbent and HILIC-based tandem LC­MS, the quantitation limit for paraquat is below 100 ng/L (ppt) using only 20 mL of sample. Prior methods required larger sample volumes and were better suited for LC­UV. experimental conditions Instruments: Waters Alliance® 2695 separations module equipped with column heater and sample chiller, Waters/Micro-
mass Quattro MicroTM mass spectrometer, Waters 2487 dual wavelength UV detector. Sample Preparation A 20-mL river water sample was adjusted to pH 7 by dropwise addition of 1 M ammonium phosphate buffer. SPE Procedure 1. Condition a 60 mg, 3 cc Oasis® WCX SPE cartridge with 1 mL methanol and 1 mL water. 2. Load 20-mL river water sample. 3. Wash the cartridge with 1 mL of 10 mM pH 7 phosphate buffer, 1 mL water, and 1 mL of methanol. 4. Elute with 1.5 mL ACN/water/TFA 84:14:2. 5. Reconstitute in 0.5-mL mobile phase. LC Conditions Column: Atlantis® HILIC, 2.1 150 mm, 3.5 m Mobile phase: 40% acetonitrile, 60% aqueous buffer pH 3.7 (250 mM ammonium formate) flow rate: 0.4 mL/min Injection volume: 20: L column temperature: 30 °C Alternate column for LC­UV: Atlantis dC18, 2.1 150, 3.5 m Mobile phase: 25:75 acetonitrile/0.1% heptafluorobutyric acid in Water flow rate: 0.3 mL/min Injection volume: 20: L Column temperature: 30 °C Monitored MS Transitions
Table I: MS transitions for paraquat and diquat
Compound Paraquat
MW MRM Cone Voltage Collision Energy
261 171 77
40
35
171 155
40
35
Diquat
319 183 157
40
30
183 168
40
35
THE APPLICATION NOTEBOOK -- September 2004
ADVERTISING SUPPLEMENT
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Results and Discussion A typical matrix-matched calibration curve was linear in the range from 0.1 to 10 g/L using 20-mL samples of river water. Limit of quantitation is below 0.2 g/L. Figure 2 shows a chromatogram obtained from a 0.5-g/L spiked sample using HILIC chromatography. A series of 1-g/L (1 ppb) spiked samples was analyzed over a five-day period, five replicates/day. The results are summarized in Table II.
Although LC­MS might be preferable for this analysis, the Oasis® WCX SPE protocol is certainly compatible with ion-pair LC­UV analysis. Figure 3 shows a typical separation for a 0.5-g/L spiked sample using ion-pairing chromatography on an Atlantis® dC18 column. However, the ion-pairing method should not be used for high sensitivity LC­MS analysis. The inset (Figure 2) shows the relative LC­MS response obtained for the same standard using the HILIC method compared with the ion-pairing method.
Table II: Intraday results obtained from spiked river water samples (spike level 1.0 g/L)
Paraquat
Diquat
Day 1 1.08 g/L (8.1% RSD) Day 4 1.10 g/L (8.0% RSD) Day 5 0.95 g/L (7.1% RSD) Overall (n 15) 1.04 mg/L (9.8% RSD)
Day 1 1.05 g/L (2.9% RSD) Day 4 1.09 g/L (5.9% RSD) Day 5 1.08 g/L (4.4% RSD) Overall (n 15) 1.08 mg/L (4.8% RSD)
Figure 2: LC­MS chromatogram showing the separation of paraquat and diquat at 0.5 g/L using the Atlantis HILIC analytical column. Inset shows the relative LC­MS response for HILIC compared with ion-pair reversed-phase chromatography.
Figure 3: LC­UV chromatogram showing the separation of paraquat and diquat at 0.5 g/L using the Atlantis dC18 analytical column. The aqueous mobile phase contained 0.1% heptafluorobutyric acid as ionpairing reagent.
Waters Corporation 34 Maple Street, Milford, MA 01757 tel. (508) 478-2000, fax (508) 478-1990 www.waters.com For More Information Circle xxx

MS Young, KM Jenkins

File: oasis-wcx-a-novel-mixed-mode-spe-sorbent-for-lc-ms-determination.pdf
Author: MS Young, KM Jenkins
Author: Michael S. Young and Kevin M. Jenkins
Keywords: oasis WCX; SPE Sorbent; LC-MS; Paraquat; Diquat; quaternary ammonium compound
Published: Wed Jul 28 20:16:14 2004
Pages: 2
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