Comparison of Gas Chromatography-Mass Spectrometry and Gas Chromatography-Tandem Mass Spectrometry with Electron Ionization and Negative-Ion Chemical Ionization for Analyses of Pesticides at Trace Levels in Atmospheric Samples

Abstract

A comparison of detection limits of gas chromatography-mass spectrometry (GC-MS) in selected ion monitoring (SIM) with gas chromatography-tandem mass spectrometry (GC-MS/MS) in selected reaction monitoring (SRM) mode with both electron ionization (EI) and negative-ion chemical ionization (NCI) are presented for over 50 pesticides ranging from organochlorines (OCs), organophosphorus pesticides (OPs) and pre-emergent herbicides used in the Canadian prairies (triallate, trifl uralin, ethalfl uralin). The developed GC-EI/SIM, GC-NCI/SIM, and GC-NCI/SRM are suitable for the determination of pesticides in air sample extracts at concentrations Ͻ100 pg µL −1 (Ͻ100 pg m −3 in air). No one method could be used to analyze the range of pre-emergent herbicides, OPs, and OCs investigated. In general GC-NCI/SIM provided the lowest method detection limits (MDLs commonly 2.5-10 pg µL −1 ) along with best confi rmation (Ͻ25% RSD of ion ratio), while GC-NCI/SRM is recommended for use where added selectivity or confi rmation is required (such as parathion-ethyl, tokuthion, carbofenothion). GC-EI/SRM at concentration Ͻ100 pg µL −1 was not suitable for most pesticides. GC-EI/SIM was more prone to interference issues than NCI methods, but gave good sensitivity (MDLs 1-10 pg µL −1 ) for pesticides with poor NCI response (OPs: sulfotep, phorate, aspon, ethion, and OCs: alachlor, aldrin, perthane, and DDE, DDD, DDT).

Chemical Analysis of Pesticides Using GC/MS, GC/MS/MS, and LC/MS/MS

“…In general NCI-SIM provided the lowest method detection limits (MDLs) for the largest number of pesticides along with confirmation at these low levels. When confirmation by NCI-SIM was not sufficient, NCI-SRM could be used and gave additional sensitivity and confirmation ability to ~14% of pesticides studied (Raina and Hall, 2009). Others have also found that GC-MS/MS can provide added selectivity (Zhang and Lee, 2006).…”

“…OC degradation products have been routinely included in GC/MS methods either with EI or NCI and include OCs such as endosulfan sulphate, DDD, DDE, HCH isomers, endrin ketone, endrin aldehyde, heptachlor epoxide, methoxychlor. Chloroacetanilides are more sensitive with EI than CI modes with GC/MS (Raina and Hall, 2009;Dagnac et al, 2005;Gabaldon et al, 2002) but can be done with similar detection limits with LC/ ESI+ or APCI+ MS/MS (Dagnac et al, 2005). The transformation products of chloroacetanilides are not analyzed by GC/MS, however chloroacetanilide (eg alachlor, propachlor, metalochlor) analysis is frequently included with analysis of OCs by GC/MS.…”

“…There is another unique feature of pesticide analysis with mass spectrometry that is often not discussed in detail. Relative to other contaminants, many pesticides including OCs, OPs, pyrethroids, and chloroacetanilides exhibit low intensity for the molecular ion regardless of whether EI or CI is used (Raina and Hall, 2009;Húšková et al, 2009;Yoshida, 2009;Feo et al, 2010;Dagnac et al, 2005). Consequently in SIM mode the quantitative or qualifier ion is rarely selected as the molecular ion.…”

“…Sample matrix and sample preparation procedures including clean-up also dictate selection of the ionization method due to presence of co-eluting pesticides or matrix components which can interfere in analysis if they can not be distinguish in the mass spectra. If pesticides are electron-capturing such as those pesticides which contain halogen, NO 2 , or P ester groups then they will generally give an enhanced response (up to two or three orders of magnitude) with negative chemical ionization (NCI) in comparison to electron impact (EI) or positive chemical ionization (PCI) (Raina and Hall, 2009;Liapis et al, 2003;Bailey and Belzer, 2007;Húšková et al, 2009). The selection of ionization mode often depends upon whether the analysis is targeted for specific chemical classes or is a multiresidue analysis methods for determination of hundreds of pesticides in a sample extract.…”

“…The clear advantage of EI is the availability of extensive libraries in full scan mode for confirmation of compound identify by library search matching, however sufficient sample concentration must be available. Most quantitative analysis is completed in selected ion monitoring (SIM) mode with peak area of the most abundant ion in the MS spectra used for the quantitative analysis, and the peak area obtained from an additional one or two ions used for confirmation along with the ratio of ion responses and retention time match (Raina and Hall, 2009). At the concentration levels of routine analysis particularly for environmental sample analysis there is insufficient concentration to obtain full scan MS spectra of sufficient abundance for library matching when quadrupole or ion trap systems are used.…”