soils [114]. However, in the last two decades, MS has become the main detection system in environmental monitoring, as can be observed in Tables 1.1–1.4. Most of the methodologies involving GC make use of electron ionization (EI) [99], while electrospray (ESI) or heated ESI (HESI) in positive and/or negative ionization modes are mainly used in the case of LC. As can be seen from Tables 1.1–1.4, triple quadrupole (QqQ) analyzer is the most widely used in combination with both GC and LC.
QqQ, operating in selected reaction monitoring (SRM) or multiple reaction monitoring modes (MRM), provides high selectivity and sensitivity for target analytes. Thus, QqQ and ESI ± was used to determine 218 pesticides in soils, or to analyze propamocarb and fenamidone in soils [100]. Zhao et al. [65] determined LOQ lower than the previous works (0.22 to 1.54 µg kg−1) for the analysis of eight chiral pesticides (diniconazole, metalaxyl, paclobutrazol, epoxiconazole, myclobutanil, hexaconazole, napropamide and isocarbophos) in soils and river sediments, employing QqQ and ESI source.
Other types of MS analyzers coupled to LC have also been used. The single quadrupole MS was employed by Belmonte Vega et al. [107] to determine pesticides in environmental matrices, or the triple quadrupole-linear ion trap mass spectrometer (QTRAP), which was used to detect herbicides, insecticides and fungicides [93], achieving LOQ of 1–10 µg kg−1.
However, other types of analyzers coupled to LC have been employed. In GC, ion trap mass spectrometer detector was employed to detect endosulfan, chlorpyrifos and their metabolites in soil matrices [29] and isotope ratio mass spectrometer (IRMS) was used to determine OCPs and metabolites [115]. Additionally, the double-focusing magnetic sector high-resolution mass spectrometer is extremely sensitive when multiple ion detection (MID) is used as the acquisition mode. It has been utilized in combination with SPME and GC for the fully automated determination of priority substances at ultra-trace levels, including pesticides, in surface water [84], as well as more complex matrices such as treated and non-treated wastewaters [85].
Although QqQ was still used to monitor the presence of pesticides in environmental samples, when a large number of compounds should be monitored, the sensitivity of these analyzers decreased by the simultaneous monitoring of a huge number of transitions. Therefore, HRMS analyzers are also used in this field, because the number of compounds to be simultaneously analyzed is theoretically unlimited and other strategies can be applied [56]. Additionally, good chromatographic peaks and resolution are still important. The use of HRMS allows that customized database can be built, including CUPs, TPs, banished pesticides and other pollutants, increasing the scope of the analysis.
HRMS enables the analysis of target, post-target (or suspected) and non-target analytes. The high sensitivity of certain HRMS analyzers, when operating in full scan mode, combined with high resolving power (>30 000 full width at half maximum, FWHM) and accurate mass measurement (1–5 ppm), allows for retrospective analysis.
While most of the HRMS applications have been aimed at targeted pesticide analysis, there are already some studies aimed at the identification of non-target analytes including new contaminants such as metabolites or TPS for a more exhaustive monitoring of water quality.
Some examples of the successful application of HRMS analyzers, specifically Q-Time of Flight (TOF) systems, in combination with LC, in target screenings allowed for the monitoring of 474 emerging contaminants, including 296 pesticides, in coastal waters [70] and ca. 500 pesticides and TPs in surface and groundwater [23]. Likewise, and as shown in Table 1.1, the potential of Orbitrap analyzers has also been employed in methods involving emerging contaminants belonging to different families and classes.
Most of the non-target screenings have been applied to wastewater samples [116], which are considered an important source of contamination of natural waters when discharged into rivers and coastal compartments. This approach was also applied in soils, detecting suspected metabolites of famoxadone in samples [28].
Nevertheless, a recent study, making use of UHPLC-Q-Orbitrap MS and software Compound Discoverer 3.0 to perform non-target analysis of pollutants in surface water, has revealed that pesticides and plastic additives were the two main types of pollutants found in Dianshan Lake in China. Once the pollutants were identified, a target screening allowed for their confirmation and quantification [117].
Moreover, HRMS is a powerful tool for the identification of unknown TPs and metabolites. In fact, novel TPs resulting from the chlorination of clothianidin, imidacloprid, desnitro-imidacloprid, imidacloprid-urea and hydrolysis products of thiamethoxam were identified in drinking water by using a HPLC-TOF MS system [18].
1.3.2 Figures of Merit
The use of the analytical methods described in previous sections provides sensitive and reliable methods that allow for low LOQs and suitable recoveries, as it can be observed in Tables 1.1–1.4. Thus, despite the lower EQS set by EU legislation for some pesticides, lower LOQs can be achieved and, for instance, concentrations down to 0.0001 µg l−1 can be quantified (see Table 1.1), combining preconcentration techniques, such as SPE or SPME and sensitive analyzers. For instance, 51 pesticides, covering highly polar compounds, were determined in surface and groundwaters by using on-line SPE in combination with (U)HPLC and tandem mass spectrometry, providing LOQ values in the range of 0.005–0.025 µg l−1 [78]. Higher limits were obtained in soils, allowing the quantification at µg kg−1 or even lower (see Table 1.2), as well as in biota, where concentrations below µg kg−1 (ng g−1) can be detected (Table 1.3). Additionally, the use of passive sampling, such as in air, allows for achieving adequate recovery and precision values (Table 1.4) and low LOQs (< 6.5 pg m−3) for most of the compounds [56].
In terms of recovery, most of the current analytical methods provided recoveries between 70% and 120%, although when multiresidue methods are developed, recoveries < 70% or > 120% are sometimes achieved. In the case of lower recoveries, if precision is < 20%, correction factors can be used for quantification purposes. In this sense, the use of generic methods, such as QuEChERS, which has been widely used in soils (see Table 1.2), provides acceptable recoveries (70–120%) for the simultaneous environmental monitoring of 218 pesticide residues in clay loam soil [99], or the quantitative extraction of parent pesticides, such as afidopyropen, and metabolites (recovery values: 85–100%) [98].
1.3.3 Hints and Tips
There are several hints and tips that should be considered in order to fulfill current legislation as well as to ensure reliable results:
Sample collection: Apply a statistical-based sampling procedure to ensure spatial and/or time variation and collect representative samples. Avoid compound degradation, selecting suitable storage and transport conditions. For air or water sampling, in addition to active sampling procedures, passive sampling is an interesting alternative, which is easy to deploy and minimum maintenance is required.
Extraction: Use sensitive and/or generic extraction procedures. Thus, for aqueous samples, SPE or SPME, selecting suitable sorbents, are the most widely extraction techniques to ensure lower LOQs as well as suitable recovery. For solid samples, such as biota and soils, although PLE, combining different sorbents depending on the target compounds, is a good option,