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

Single Quadrupole Analyzer ¹ #

The quadrupole consists of four parallel metal rods. Each opposing rod pair is connected together electrically and a radio-frequency (RF) voltage is applied between one pair of rods and the other. A direct current (DC) voltage is then superimposed on the RF voltage. Ions travel down the quadrupole in between the rods. Only ions of a certain m/z will reach the detector for a given ratio of voltages, other ions have unstable trajectories and will collide with the rods. This allows selection of a particular ion, or scanning by varying the voltages.

The quadrupole can be used in two modes: scan or single ion monitoring (SIM), also called single ion recording (SIR). In scan mode, the amplitude of the DC and RF voltages are ramped (while keeping a constant RF/DC ratio), to obtain the mass spectrum over the required mass range. Sensitivity is a function of the scanned mass range, the scan speed, and resolution. In SIM mode, the parameters (amplitude of the DC and RF voltages) are set to observe only a specific mass, or to “jump” between a selection of specific masses. The SIM mode provides the highest sensitivity but users are restricted to acquiring specific ions, typically EI fragments, since more time, the dwell time, can be spent on each mass. A longer dwell time would result in better sensitivity but, the number of data points acquired across a single peak, and the total number of pesticides that could be analyzed in a single run, are reduced. Due to insufficient sensitivity in scan mode, historically, quadrupoles have typically been operated in SIM for optimum sensitivity, limiting the amount of structural information that could be recorded.

SIM can suffer from isobaric interference.

Single Quadrupole vs. Tandem Mass Spectrometry #

As both single quadrupole and 3D QIT work at unit mass resolution, selectivity is limited, so these instruments can suffer from reduced sensitivity due to the contribution to the analyte signal from chemical noise. Although low reporting limits might be possible for simple matrices using GC-MS, these instruments can provide insufficient selectivity for complex food matrices. 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.