	The selection of analytical methods for gas chromatography (GC)-amenable pesticides is often based on requirements for sensitivity and selectivity for regulatory needs or other monitoring requirements. Methods with both electron ionization (EI) and negative chemical ionization (NCI) are often required to cover the full range of GC–amenable pesticides at trace levels. Pesticides fragment easily in EI and CI sources such that the molecular ion is often low in abundance. NCI can provide added selectivity and sensitivity over EI methods. NCI is most commonly used in selected-ion monitoring mode. The lack of availability of parent ions for collision-induced dissociation for tandem mass spectrometry (MS) can limit the feasibility of GC–MS-MS for pesticides that significantly fragment in the ion source. Options for improving sensitivity by using of large-volume cold on column or programmable temperature vaporizer injections are presented.	 	Read more 

Articles in this Issue

Improving Sensitivity and Selectivity in Pesticide Analysis with GC–MS and GC–MS-MS

The main objective of this study was to evaluate the capabilities of gas chromatography (GC) with time–of–flight mass spectrometry (MS) for screening pesticides in fruits and vegetables using a purpose-built accurate-mass database. Analytical performance was tested on four matrices: potato, tomato, spring onion, and orange. Two resolution modes, 7000 FWHM and 12,000 FWHM, were tested to establish the concentration range within which automatic identification was possible, considering the retention time window of 0.2 min and at least two ions with a mass error lower than 10 ppm as the identification criteria. The effects of the matrix on identification and quantification were also studied for the four selected matrices. The developed method was applied to real samples and the qualitative and quantitative results were compared with those obtained using GC with triplequadrupole MS.

An Accurate-Mass Database for Screening Pesticide Residues in Fruits and Vegetables by Gas Chromatography–Time-of-Flight Mass Spectrometry

A rapid, high-throughput analytical method was developed and evaluated for the simultaneous determination of pesticides and environmental contaminants in fish. The compounds included polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs), and flame retardants. The method was based on a QuEChERS (quick, easy, cheap, effective, rugged, and safe) technique with acetonitrile extraction, and a dispersive solid-phase extraction (dSPE) cleanup. Three sorbent combinations were compared for cleanup efficiency and recoveries of the contaminants: C18+PSA, traditionally used for lipid removal in dSPE, and two novel sorbents, based on silica coated with zirconium dioxide (ZrO2 ) and ZrO2 /C18, designed for phospholipid removal. The dSPE cleanup with ZrO2 sorbent provided the highest efficiency with the lowest baseline, as well as satisfactory recoveries (70–120% calculated based on isotope-labeled internal standards) for the most analytes. The method allows for quick sample preparation of fish samples for the analysis of almost 200 targeted contaminants using fast, lowpressure gas chromatography with tandem mass spectrometry (GC–MS-MS), thus providing a wide scope of analysis.
	 

Spectroscopy E-Books-05-01-2014