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also been analyzed in this matter. Thus, 40 CUPs were determined in PM10 of Valencia Region (Spain) observing that omethoate was detected at the highest average level (141 pg m⁻³) [57]. A similar study was performed in the “Todos los Santos Bay” region (Brazil), where 13 pesticides were analyzed in PM2.5 samples, and concentrations ranged from 20 to 315 pg m⁻³, being carbofuran, malathion and permethrin, the compound most widely detected [58].

1.2 Sample Preparation and Collection

      In order to ensure accurate pesticide determination in environmental samples, sample collection and preparation should be carefully designed to minimize potential errors during these steps and therefore error propagation will be limited.

1.2.1 Protocols for Collecting and Preparing Samples

      Sample collection is critical to get representative samples as well as to avoid any modification of the initial chemical composition of the sample.

      For instance, a guidance on sampling water techniques can be found in the ISO Standard on Water Quality – Sampling 5667. The selection of sampling point, which should cover the area of surveillance, must be subject to local conditions, such as water homogeneity and vertical and lateral mixing.

The sample containers as well as transport and storage arrangements should not lead to changes in the relevant chemical status. Therefore, according to sampling for synthetic organic compounds, water samples must be stored in glass, polytetrafluoroethylene (PTFE) or stainless-steel containers, and they should be analyzed within 24 hours and stored in the dark at 1–5°C.

On the other hand, the development and application of passive sampling techniques are highly recommended [59]. Those techniques allow for the accumulation of pesticides by passive diffusion onto a liquid or solid absorbent showing affinity for a certain type of substance. The semipermeable membrane device (SPMD) and the polar organic chemical integrative sampler (POCIS) are the most common passive samplers for organic pollutants. SPMD based on triolein sorbent can be applied for neutral organic chemicals with a log K_ow > 3 (lipophilic pesticides), while POCIS, which uses Oasis HLB phase, is intended for the sorption of more water-soluble organic chemicals with K_ow < 3 (polar pesticides). To ensure the monitoring of a high number of pollutants, different types of passive samplers could be used together [6]. In this sense, an interlaboratory study on passive sampling of emerging water pollutants showed low interlaboratory variability in the analysis of replicate samplers when POCIS was used for the monitoring of 7 polar pesticides. The same study established a series of recommendations to take into consideration, especially when passive samplers are combined with liquid chromatographic-mass spectrometric (LC-MS) methods [60].

In the same way, soil samples should be collected from growing fields using a grid pattern uniformly distributed. For instance, a 3 × 3 grid is commonly used for smaller fields, whereas 5 × 5 or even larger grids are used for very large fields and a “W” or “Z” pattern is commonly used. Each sample site represents one portion of the total sample, and then a composite sample can be formed, ensuring homogeneity. Normally, samples are collected at a 15 cm depth. Moreover, additional steps such as removing litter, plant roots and big stones from the soil samples could be needed [61] and sometimes soils should be dried with [62] or without heat [63], homogenized and finally stored at −18°C until analysis [64]. Finally, an exhaustive characterization of the soils according to several parameters, such as pH, percentage of organic matter, carbon monoxide, sand, silt, clay and grit [28, 65], is advisable to provide useful information and set a relationship between pesticide presence and physico-chemical characteristics of the soil.

      In relation to air sampling, other important criteria should be considered as materials do not react with target compounds; it must be located in a place where free air masses can reach; the sampler should be protected from rainfall, dust or other sources of contamination as well as requiring low maintenance [33]. Furthermore, it should be planned bearing in mind whether only gas phase, particulate matter or both of them are going to be collected, among other factors.

Finally, for biota sampling, special care should be taken with small biota samples, which are more sensitive to contamination, and degradation of loss of analytes. Long-term storage should be performed in darkness and low temperature, and before sample treatment, dry or wet homogenization is needed [66].

1.2.2 Sample Extraction and Clean-up

For the monitoring of ultra-trace levels of pesticides in water, extraction and concentration steps are required prior to the analytical determination. Liquid-liquid extraction (LLE) allows for the detection of a large range of non-polar pesticides while requiring minimal instrumentation, being at the same time both a simple and a precise technique. Although less and less, this technique is still used for the extraction of pesticides from water samples prior to gas chromatography (GC) analysis [67, 68]. However, one major drawback is the large solvent volumes, usually dichloromethane, required in LLE. Therefore, solid phase extraction (SPE) has become the most common extraction technique [69]. Indeed, it is the most powerful sampling and enrichment approach for complex mixtures of known and unknown contaminants, and different sorbent phases can be used, allowing for the extraction of a wide range of pesticides with different physico-chemical properties.

As shown in Table 1.1, the polymeric reversed phase sorbent, Oasis HLB, is commonly used for the extraction of pesticides in water samples providing quantitative recoveries in most cases [18, 23, 70–72]. Furthermore, other sorbents have been successfully applied for the analysis of pesticides and their TPs in natural waters, such as a mixture of hydrophilic–lipophilic balance, weak anion and cation exchange sorbents (2 : 1 : 1, w/w/w) [73], and Strata-X reversed in combination with the mixture of Strata-X mixed-mode (AW and CW) and Isolute ENV + [74].

On-site integrated large-volume SPE has also been proven to be a promising tool for the monitoring of pollutants, including pesticides, in water sources [75]. On the other hand, the possibility of using on-line SPE systems allows for minimizing sample manipulation and cross-contaminations as well as improving sample throughput [76]. Hence the development of fully automated methods, based on the combination of on-line SPE and LC-MS, has been given much attention in the last few years, being applied for the monitoring of ca. 100 pesticides in natural and drinking waters [24, 77], as well as 51 pesticides, covering highly polar compounds, in surface and groundwaters [78].

Simple and miniaturized sample preparation techniques have been considered in recent years as optimal alternatives [79]. Among them, solid phase microextraction (SPME) is the most used technique, although the application of QuEChERS (Quick, Easy, Cheap, Effective, Rugged and Safe)-based protocols [21, 80], stir bar sorptive extraction (SBSE) [77], as well as liquid-phase microextraction (LPME) [81, 82], has also been suitable for the extraction of pesticides from water matrices.

SPME is a simple, sensitive, rapid and solvent-free technique in which the organic compounds are adsorbed/absorbed (depending on fiber coating) directly from the aqueous sample into the fiber and then thermally desorbed at the injection port of the GC, considerably simplifying the analysis procedure. In this sense, the availability of SPME devices in latest GC equipment leads to the complete automatization of the analytical process, allowing for improving data quality, the productivity of staff and instruments, and increasing the sample throughput [83]. This has been demonstrated in recent methodologies involving the on-line combination of SPME and GC coupled to high-resolution mass spectrometry (HRMS) allowing for the determination of priority substances, including pesticides, in surface and wastewaters [84, 85] providing limits of quantification (LOQs) at ng l⁻¹ levels. Novel SPME sorbents, such as magnetic deep eutectic solvent (DES)-based polymeric hydrogel [86] and carbon nanomaterials [87, 88], have been successfully applied for the monitoring of pesticides in different water resources as can be seen in Table 1.1.

On the other hand, vacuum-assisted evaporative

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