Selection of Instruments and Ionization Techniques for Pesticide Analysis

The choice of the most appropriate instruments to handle the majority of samples and analytes is one of the most important decisions on investments in residue analytical laboratories. The same decision was necessary for the comparison presented here.

A. GC–MS

Ionization of pesticides in GC–MS can be done by EI, and positive or negative chemical ionization (PCI, NCI). For ion separation, single quad instruments are used most frequently. Additionally, GC–MS systems with quadrupole ion traps, time‐of‐flight (TOF) mass spectrometers or tandem mass spectrometers are available.

Most of the published studies on residue analysis by GC–MS report on results obtained by single quadrupole instruments and EI ionization. Advantages of EI ionization are a low influence of molecular structure on response, and a large number of characteristic fragments. Extensive studies describe the simultaneous determination of 245–400 pesticides by GC–EI–MS with single quadrupole mass filters (Cairns et al., 1993; Fillion, Sauve, & Selwyn, 2000; Stan, 2000; Chu, Hu, & Yao, 2005). The use of ion traps in scan mode is more simple because no selection of characteristic ions is necessary during data acquisition. In full scan mode these instruments are quite sensitive, and confirmation by library search is possible at lower concentrations. But, compared to single quad instruments running in selected ion monitoring mode (SIM), identical pesticides are covered and the sensitivity do not differ significantly (Cairns et al., 1993).

Chemical ionization is used more rarely. Positive or negative CI–MS give better selectivity for several pesticides compared to EI. This results in chromatograms with reduced matrix interference (Hernando et al., 2001). But the signal intensity of different pesticides (if identical amounts are injected) varies much more compared to EI ionization. Preferentially, GC–MS with chemical ionization is focused on special substance classes only, for example, organohalogen pesticides (Artigas, Martinez, & Gelpi, 1988; Chaler et al., 1998), pyrethroids (Ramesh & Ravi, 2004), and organophosphates (Russo, Campanella, & Avino, 2002). It is rarely used in multi‐residue methods, because it is not a universal ionization technique. Finally, mass spectra produced by chemical ionization usually contain a smaller number of fragments, thus offering less information.

Available GC–TOF instruments can be operated in two different modes. One type offers very high scan rates, allowing the separation of overlapping peaks by automated mass spectral deconvolution of overlapping signals (de Koning et al., 2003; Patel et al., 2004). This can result in up to 30,000 peaks from cigarette smoke (Dalluge et al., 2002). Another type of GC–TOF instruments offers high mass resolution, allowing data evaluation with a narrow mass window of 0.02 Da (Cajka & Hajslova, 2004). However, most TOF instruments suffer from a reduced dynamic range (Dalluge, Roose, & Brinkman, 2002). For this review, no sufficient information on multi‐analyte GC–TOF was available.

In analogy to CI–MS and GC–TOF, a good suppression of matrix background is obtained by GC–MS/MS systems (Goncalves & Alpendurada, 2004). Even with extracts of tobacco, excellent selectivity and sensitivity were observed (Haib, Hofer, & Renaud, 2003). MS/MS experiments can be performed using ion trap (Gamon et al., 2001; Aguera et al., 2002; Martinez Vidal, Arrebola, & Mateu‐Sanchez, 2002) and triple quadrupole mass analyzers (Leandro, Fussell, & Keely, 2005). Some limitations in GC–MS/MS arise from the absence of a universal soft ionization mode, which could be used for the efficient production of molecular ions of most pesticide classes. Chemical ionization generates high‐intensity ions of only some pesticides classes. EI ionization is more universal, but often the total ion current is spread on many fragments, resulting in a low intensity of parent ions of MS/MS experiments. Up to now, the prospects of GC–MS/MS are not totally clear. GC–MS/MS acquisition parameters are published for a small percentage of selected pesticides. Therefore, it is too early to choose GC–MS/MS instead of GC—MS for a comparison with the most appropriate LC–MS(/MS) approach.

B. LC–MS

If pesticides are not amenable to GC, the application of LC is the best alternative. Likewise, LC may be combined with single quadrupole instruments, quadrupole ion traps, triple quadrupole (tandem) mass spectrometers, TOF spectrometers, or hybrid quadrupole TOF instruments.

In contrast to GC–MS, single quadrupole mass spectrometers are not used in the majority of recent studies dealing with LC–MS. A disadvantage of single quadrupole instruments (and ion traps operated in the SIM mode) is the high intensity of background signals produced from sample matrix and HPLC solvent clusters. Due to this chemical noise in real samples very low limits of quantification cannot be achieved, even if the sensitivity of these instruments is high (Hernandez, Sancho, & Pozo, 2005).

The chemical background can be reduced significantly if tandem MS in combination with selected reaction monitoring (SRM) is applied. Even if a co‐extracted matrix component has the molecular mass of a pesticide, usually both isobaric ions can be separated in SRM experiments, because their fragmentation in the collision cell most often results in different product ions. Therefore, tandem mass spectrometers offer excellent sensitivity and unsurpassed selectivity. For this reason, triple quadrupole mass analyzers have been the most often applied MS detectors until now (Pico, Blasco, & Font, 2004). Quadrupole ion traps may also be operated in the MS/MS mode, which reduces the background to a level known from tandem mass spectrometers. However, ion collection, fragmentation, and mass analysis of fragments is a step by step process in traps and requires much more time than in triple quadrupole instruments, which do this in parallel. Furthermore, ion traps suffer from a limited dynamic range, a smaller potential to fragment very stable ions and the inefficiency to trap low mass fragments (Pico, Blasco, & Font, 2004).

Time‐of‐flight mass spectrometers in combination with LC are more often used in high‐resolution mode (typical mass error <2 mDa), which provides better discrimination of background (Hogenboom et al., 1999; Ferrer et al., 2005). The main advantage of this type of instrument is the identification of unknown peaks in a sample even if analytical standards are not available (Garcia‐Reyes et al., 2005; Thurman, Ferrer, & Fernandez‐Alba, 2005). But, this advantage is usually not needed in the enforcement of maximum residue levels. Furthermore, identification of pesticides in samples is less certain by LC–TOF–MS than identification of pesticides by GC–EI–MS (Maizels & Budde, 2001).

The use of a hybrid quadrupole time‐of‐flight instrument (Q–TOF) allows the most certain confirmation. This confidence is based on the combination of retention time, mass of the quasi molecular ion selected by the quadrupole mass filter, and the complete collision induced mass spectrum obtained by the TOF analyzer (Hernandez et al., 2004). Unfortunately, the sensitivity of Q–TOF instruments in relation to triple quadrupole analyzers is one order of magnitude lower (Hernandez et al., 2004; Nunez, Moyano, & Galceran, 2004). Additionally to this drawback, a smaller linear range restricts the use of Q–TOF for the quantification of residues.

All LC–MS instruments can be equipped with at least three types of soft ionization techniques, that is, ESI, APCI, and photoionization. Up to now, articles on photoionization of pesticides have been rarely published (Takino, Yamaguchi, & Nakahara, 2004). ESI and APCI are applied more often. Comparing the suitability of ESI versus APCI for the ionization of many pesticides, electrospray was identified as more universal technique (Thurman, Ferrer, & Barcelo, 2001; Klein & Alder, 2003; Jansson et al., 2004; Hernandez, Sancho, & Pozo, 2005).

C. Final Decision

Any of the instruments discussed above have special merits, but none of them can detect the full range of all pesticides. However, if the selection of the most appropriate techniques is focused on the enforcement of maximum residue levels, simultaneous identification, and quantification of a very large number of target analytes will be more important than the detection, identification, and quantification of non‐regulated (non‐target) pesticides and/or metabolites. Under these conditions, EI ionization and single quadrupole MS was identified as the preferred GC detection system. If LC is used, most benefits should be obtained from tandem mass spectrometers operating in the electrospray mode. Therefore, in the next section scope and sensitivity of GC‐EI–MS will be compared to pesticide detection by LC–ESI–tandem MS.

References:

1. Mass Spectrometry Reviews Volume25, Issue6 November/December 2006 Pages 838-865