Pesticide Residues in Surface Water and Groundwater - Pesticides: Evaluation of Environmental Pollution - page 284

the years, going from persistent, not easily degraded pesticides (e.g., organochlorine
pesticides) to more polar, readily degradable pesticides (e.g., N-methylcarbamates). Polar
pesticides are not directly amenable for GC analysis. Since the introduction of reversed-
phase LC equipped with a UV or FLD detector, which occurred around 1980, LC became
adopted as a viable technique complementary to GC for the determination of polar pesti-
cides. The wide application range, easy use, low cost, and improved selectivity (by the use
of PDA detection) made UV detectors very popular and most widely used in LC. However,
confirmation was difficult for pesticides of the same class due to the high degree of simi-
larity between UV spectra. FLDs are distinctly more selective and sensitive than UV detec-
tors, but without a derivatization step, their applicability is limited to a small number of
pesticides having fluorophores.
Until two decades ago, when analyzing pesticide residues, methods based on LC were
applied less frequently than GC, because traditional LC detectors are less sensitive than
the various GC detection methods (Alder et al. 2006). The breakthrough came in the early
1990s, with the development of atmospheric pressure ionization (API) methods, which
made possible the coupling of LC with MS. Compared with traditional detectors, MS
detectors have increased the sensitivity of LC detection by several orders of magnitude
(Alder et al. 2006). Furthermore, MS has an advantage over conventional detectors because
it can provide information for unambiguous analyte identification even with poor LC sep-
aration. Otherwise, when using conventional detectors, optimization of LC separation is
tedious and time consuming, even with the support of the computer-assisted retention
modeling (Onjia et al. 2002; Vasiljevic et al. 2004a).
The most commonly used ionization techniques in LC-MS instruments are electrospray
ionization (ESI) and atmospheric pressure chemical ionization (APCI). The selection of the
most appropriate ionization source and ionization mode for pesticide analysis, for exam-
ple, positive ionization (PI) or negative ionization (NI) mode, depends on the pesticide
classes investigated. According to Thurman et al. (2001) and Baglio et al. (1999), who evalu-
ated the response of a number of pesticides using either APCI or ESI in LC-MS, neutral and
basic pesticides (carbamates, phenylureas, triazines) are more sensitive using APCI (espe-
cially in PI mode), whereas cationic and anionic herbicides (bipyridylium ions, sulfonic
acids, phenoxy acids, nitrophenols, bentazone) are best ionized with ESI (especially in NI
mode). However, as seen in Table 9.8 , ESI has found much wider application than APCI,
probably due to a wider range (in terms of polarity and molecular weight) of compounds
than can be measured with this interface (Thurman et al. 2001). Another general observa-
tion is that the NI mode is more selective and less prone to adduct formation than the PI
mode (Reemtsma 2003).
Single quadrupole was the predominant configuration of LC-MS in the early 1990s.
A disadvantage of single-quadrupole LC-MS is the high intensity of background signals
produced from sample matrix and LC solvent clusters (Alder et al. 2006). The chemical
background can be significantly reduced if tandem MS in combination with selected reac-
tion monitoring (SRM) is applied (Alder et al. 2006). LC coupled with MS/MS (LC-MS/
MS) is capable of differentiation between analyte and matrix signal, as well as between
the analytes that coelute, therefore permitting quantification of pesticide traces in very
complex matrices. In addition, the use of MS/MS detection allows analysis without the
complete chromatographic separation between the analytes, thus shortening the chro-
matographic run time. Nowadays, LC-MS/MS is one of the most powerful techniques for
the analysis of pesticides in a variety of complex matrices (Radišić et al. 2009). Basically, all
recently published LC-MS methods for pesticide determination in environmental samples
rely on the use of MS/MS detection (Baugros et al. 2008; Petrovic et al. 2010).