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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].