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
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
171 155
319 183 157
183 168
Environmental 3
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)
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
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