Multi-residue analysis of pesticides using on-line two dimension liquid chromatography-tandem mass spectrometry

Abstract

of thesis entitled Multi-Residue Analysis of Pesticides using On-Line Two Dimensional Liquid ChromatographyTandem Mass Spectrometry Submitted by Fiona Lo Fu Kiu for the degree of Master of Philosophy at The University of Hong Kong in August 2011 This thesis begins with an examination of the feasibility of using the Quick, Easy, Cheap, Effective, Rugged, and Safe (QuEChERS) method, and its variants, for the co-extraction of nonpolar and polar pesticides, including highly polar quaternary ammonium salts (QA) and organophosphates (OP). QuEChERS was performed on a mixture of nine different chemical classes of pesticides, with triphenyl phosphate (TPP) as the internal standard, on aqueous Chinese lettuce samples, at 0.1 and 1.0 μg/mL levels. QuEChERS provided superior sample recovery and precision (RSD) for nonpolar pesticides than it did for its polar variants, including most of the polar pesticides and several carbamates, as judged from quintuplicate analyses. A modification of the extraction buffer and cleanup reagent significantly enhanced the recovery of highly polar QA and carbamates at sub–microgram-per-milliliter levels, while maintaining the performance of the other nonpolar pesticides. Thus, QuEChERS is a versatile and convenient methodology for multi-residue analyses, although unambiguous confirmation and quantification of some polar pesticides, namely chlormequat, mepiquat, and glyphosate, required separate analyses using tedious and time-consuming single-residue methods. Liquid chromatography/tandem mass spectrometry (LC-MS/MS) provides the potential advantages of low detection limits, simple sample preparation procedures, and high selectivities for multi-residue analyses of organic toxics in food. In this study, a novel methodology based on LC-MS/MS was developed for the simultaneous analyses of both polar and nonpolar pesticides; an online-coupled hydrophilic-interaction chromatography/reversed-phase (HILIC-RP) separation platform, featuring a HILIC column for online sample enrichment and a downstream high-flow injector for solvent mixing and reconditioning, was constructed to allow efficient, concomitant analyses of both hydrophilic and hydrophobic pesticides from a single sample injection event. The performance of the HILIC-RP system, verified using a standard pesticide mixture, was satisfactory. 6 out the 7 polar pesticides were successfully retained on the HILIC column and 11 out of the 15 nonpolar pesticides were transferred to and separated by the RP column; the remaining compounds were determined as flow-through prior to the HILIC separation. No sensitivity loss of the polar pesticides was observed when using this hyphenated system. The calibration curves for 20 out of the 22 standard pesticides exhibited excellent linear responses (R > 0.995) over a typical working concentration range. The performance of the HILIC-RP platform was comparable with those of individual one-dimensional HILIC and RP analyses, thereby potentially eliminating redundant quantification procedures and allowing highthroughput multi-residue analyses of toxic organic compounds.