series: [Analysis of Pesticides in Food and Environmental Samples, edited by Jose L. Tadeo]

GC with Selected Detectors #

Flame photometric detector (FPD) in phosphorus or sulfur mode for organophosphorus pesticides;
Electron capture detector (ECD) for organochlorine pesticides; and
Nitrogen phosphorus detector (NPD) for nitrogen and phosphorus compounds with a total coverage of about 300 compounds.
Most of the pesticides that are not easily analyzed by GC can be separated using LC.

GC-MS #

GC-MS became an essential tool for pesticide residue analysis. A mixture of compounds to be analyzed is injected into the GC where the mixture is vaporized. The gas mixture travels through a GC column, where the compounds are separated as they interact with the column and then enter the mass spectrometer for ionization and mass analysis.

A number of problematic pesticides are prone to degradation in the GC injector, including phthalimide fungicides (e.g., captan), organochlorines (e.g., DDT and chlorothalonil), organophosphorus pesticides (e.g., dimethoate), and pyrethroids.

The major source of inaccuracy in pesticide residue analysis by GC-MS, especially with food, is related to the injection of coextractives from the sample, the so-called “matrix effect.” A buildup of coextractives in a GC inlet may lead to successive adverse changes in the performance of the chromatographic system such as the loss of analytes and peak tailing due to undesired interactions with active sites in the inlet and column. Analytes that give poor peak shapes or degrade have higher detection limits, are more difficult to identify and integrate, and are more prone to interferences than stable analytes that give narrow peaks. For susceptible analytes, significant improvements in peak quality are obtained when matrix components are present because they fill active sites, thus reducing analyte interactions. However, this can lead to problems with quantification. These matrix effects can produce an overestimation of the analyte concentration if calibration has been performed with standards in solvent. The presence of matrix effects should be evaluated for all tested analytes. There are a number of approaches for preventing, reducing, or compensating for the occurrence of matrix effects including the use of matrix-matched calibrants, which is recommended for the monitoring of pesticide residues within the European Union.

Chemical ionization (CI) gives better selectivity than EI for a limited number of specific compounds, which provides improvements in reporting limits. Negative chemical ionization (NCI) exhibits a high selectivity for “electrontrapping” compounds (e.g., halogen-containing and other heteroatomic compounds) and electron-deficient aromatic compounds. The sensitivity can be improved by two orders of magnitude compared with EI.

EI typically produces large number of characteristic fragments, when acquired under standardized condition (70 eV).

LC with Selected Detectors #

Many pesticides do not contain strong UV chromophores.

Analysis of Pesticides in Food and Environmental Samples, CRC Press, 2008, edited by José L. Tadeo