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

Tandem mass spectrometry (MS/MS), in which mass-selected ions are subjected to a second mass spectrometric analysis, can provide increased selectivity, which reduces the contribution to the analyte signal from isobaric interference leading to improvements in sensitivity. 68 Hence, lower limits of detection become achievable when using GC-MS/MS for pesticide residue analysis in complex matrices. The same selectivity, achieved by monitoring the transition from one parent ion to a characteristic product ion, provides a greater degree of confidence for confirmation of identity than SIM, which can suffer from isobaric interferences.

Based on the current EU quality control procedures for pesticide residue analysis, if using GC-MS, four ions have to be detected and all ion ratios have to be within the specified tolerance intervals for identity to be confirmed. Additional legislation directed at residues of substances in live animals and animal products introduced an identification point (IP) system that was weighted to the selectivity of the method used. When using the more selective MS/MS technique, monitoring and detection of two transitions exhibiting a ratio within tolerance is sufficient, as the precursor earns 1 point and each product ion earns 1.5 points, 4 points in total. The IP system has been applied to the determination of pesticide residues in animal products and may find wider usage. ¹

The analyzer of a triple quadrupole instrument consists in two quadrupoles, separated by a collision cell. The first quadrupole is used in SIM mode to select a first ion (precursor), which is fragmented in the collision cell. This is typically achieved in the collision cell by accelerating the ions in the presence of a collision gas. The energy of the collision with the gas can be varied to allow different degrees of fragmentation. The resulting fragments are analyzed by the second quadrupole and also typically used in SIM mode to monitor a specific fragment (product), the process known as multiple reaction monitoring (MRM) (also called selected reaction monitoring, SRM). As two analyzers increase the selectivity, the ion signal is reduced during the transmission, but the chemical noise, which is a major limitation for complex samples, is also largely decreased, leading to an improvement of the signal-to-noise ratio.