Pesticide Degradation in Water - Pesticides: Evaluation of Environmental Pollution - page 115

Detomaso and coworkers (2005a,b) reported the structural elucidation of polar degrada-
tion products of carbofuran photodegradation employing a QTOF. This technique provided
the exact mass measurement of the [M+H] + ions of the by-products and of the products
ions, obtaining the elemental formula and related structures of seven photodegradation
by-products. In addition, fragmentation patterns were proposed. Ibáñez and coworkers
(2006) identified, by means of LC-QTOF-MS, the transformation product of the insecticide
diazinon. Hydrolysis, hydroxylation, and oxidation were the most important processes
found to occur in the degradation of diazinon, and the degradation pathway could be pro-
posed. In the same way, LC-QTOF-MS was employed to study the transformation products
of the pesticides terbuthylazine, simazine, terbutryn, and terbumeton in the environment
(Ibáñez et al. 2004). The high sensitivity in full scan mode allowed the identification of
minor metabolites below 2% of the total peak area. The mass errors were below 2 mDa,
allowing the assignment of a highly probable empirical formula for each degradation
product. Identification of the photodegradation products of cyclohexanedione oxime her-
bicides by employing this technique has been reported recently (Sevilla-Morán et al. 2008,
2010a). Clethodim photodegradation gave rise to the formation of nine by-products, some
of them described for the first time. Main reactions observed were photoisomerization,
sulfur-oxidation of isomers leading to the formation of sulfoxide diastereoisomers, and
reduction of the oxime moiety. Table 4.2 shows the chemical structure of the identified
transformation products and the accurate mass measurements and elemental composition
of clethodim and its photodegradation products by LC-ESI-QTOF.
However, it is important to highlight that when there is no available analytical stan-
dard, the employment of high resolution analyzers for the complete and unequivocal
structural characterization of metabolites and transformation products is not enough.
The use of complementary and more powerful techniques able to unequivocally confirm
the identity of organic compounds is required. The analytical techniques applied for the
elucidation of organic compounds in addition to MS are UV, IR elemental analysis and
the monodimensional 1 H and 13 C-NMR and the bidimensional techniques such as COSY
(correlation spectroscopy [homonuclear chemical shift]), HMQC (heteronuclear multiple
bond coherence), and HMBC (heteronuclear multiple quantum coherence) spectroscopy
(Boschin et al. 2007; Kodama et al. 1999; Vialaton et al. 1998, 2001; Werres et al. 1996).
Pesticides and their transformation products are generally present in environmental
waters in trace amounts at and often below the ppb levels. As a result, pesticide residues
cannot be analyzed without some previous sample preparation. The sample preparation
is, nowadays, a key factor in the analysis of organic compounds and therefore, there is a
considerable interest in the development of new selective methods for extracting pesti-
cide residues from environmental matrices (Barceló and Hennion 1997; Pichon et al. 1996).
Sample preparation, which includes extraction, concentration, and isolation of analytes,
influences the accuracy of the methods. Different solid-phase extraction (SPE) techniques
can be employed by sample preparation as an alternative to the classical liquid-liquid
extraction that presented multiple disadvantages such as low recovery of polar pesticides
and transformation products and use of large volumes of solvents. These include SPE,
solid-phase microextraction (SPME) (Aulakh et al. 2005; Beltran et al. 2000), matrix solid-
phase dispersion (MSPD) (Kristenson et al. 2006), and stir-bar sortive extraction (SBSE)
(Sandra et al. 2001, 2003). SPE is the technique most employed for the monitoring of pes-
ticides and their transformation products in water (see Table 4.1 ; Carabias-Martínez et al.
2004; De la Pena et al. 2003; Sabik et al. 2000).
Nowadays, a large number of SPE materials are available for the analysis of pesticide
residues (Rodríguez-Mozaz et al. 2007; Soriano et al. 2001). The most common SPE sorbents