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

Abstract

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 (R2 > 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.