SAMPLE PRETREATMENT Generally, sampling techniques provide amounts of sample much higher (2 –10 L of

2.2 SAMPLE PRETREATMENT Generally, sampling techniques provide amounts of sample much higher (2 –10 L of

liquid samples and 1 –2 kg of solid samples) than those needed for the final analysis (just few milligrams). Thus, it is always necessary to carry out some pretreatments to get a homogeneous and representative subsample. Even if the sample is apparently homogeneous, that is, an aqueous sample, it will be at least necessary to perform a filtration step to remove suspended particles, which could affect the final determin- ation of target analytes. However, some hydrophobic analytes (i.e., organochlorine pesticides) could be adsorbed onto particles surface and thus, depending on the objective of the analysis, might be necessary to analyze such particles. This simple example demonstrates the necessity of establishing clearly the objective of the ana- lysis, since it will determine the sample pretreatments to be carried out, and high- lights the importance of this typically underrated analytical step.

Usually, environmental water samples just require filtration, whereas liquid food samples might be subjected to other kinds of pretreatments depending on the objective of the analysis. However, solid samples (both environmental and food samples) need to be more extensively pretreated to get a homogeneous subsample. The wide variety of solid samples prevents an exhaustive description of the different procedures in this chapter; however, some general common procedures will be described later.

Sample Handling of Pesticides in Food and Environmental Samples 37

2.2.1 D RYING The presence of water or moisture in solid samples has to be taken into account

since it might produce alterations (i.e., hydrolysis) of the matrix and=or analytes, which will obviously affect the final analytical results. Besides, water content varies depending on atmospheric conditions and thus, it is recommended to refer the content of target analytes to the mass of dry sample.

Sample drying uses to be carried out before crushing and sieving steps, although it is recommended drying again before final determination since rehydration process might occur. Typically, sample is dried inside an oven at temperatures about 1008C. It is important to stress that higher temperatures can be used to decrease the time devoted to this step but losses of volatile analytes might occur. In this sense, it is important to know a priori the physicochemical properties of target analytes to preserve the integrity of the sample. A more conservative approach, using low temperatures, can be followed but it will unnecessarily increase the drying time. Alternatively, lyophilization is recommended if a high risk of analytes losses exists and it is an appropriate procedure for food, biological material, and plant samples drying. However, even following this procedure, losses of analytes might occur depending on their physical properties (i.e., solubility, volatility).

The results are evident that it is not possible to establish a general rule on how to perform sample drying. Thus, studies on stability of target analytes in spiked samples should be carried out to guarantee the integrity of the sample before final determin- ation of the analytes.

2.2.2 H OMOGENIZATION As mentioned earlier, samples are heterogeneous in nature and thus, they must be

treated to get a homogeneous distribution of target analytes. Generally, soil samples are crushed, grinded, and sieved through 2 mm mesh. Grinding can be done manually or automatically using specially designed equip- ments (i.e., ball mills). It is important to stress that this procedure might provoke the local heating of the sample and thus, thermolabile or volatile compounds might be affected. In this sense, it is recommended to grind the sample at short time intervals to minimize sample heating. In addition, due to heating, water content may vary making necessary to recalculate sample moisture.

Food samples use to be cut down to small pieces with a laboratory knife before further homogenization with automatic instruments (i.e., blender). Sample freezing is

a general practice to ease blending, especially recommended for samples with high fat content (i.e., cheese) and for soft samples with high risk of phase separation during blending (i.e., liver, citrus fruits).

Apart from these general guidelines, especially in food analysis, the determin- ation of pesticides might be restricted to the edible part of the sample or to samples previously cooked and thus, sample pretreatments will vary depending on the objective of the analysis.

Finally, it is important to point out that, in most of the cases, samples need to be stored for certain periods of time before performing the analysis. In this sense, although sample storage cannot be considered a sample pretreatment, the addition

38 Analysis of Pesticides in Food and Environmental Samples of preservatives as well as the establishment of the right conditions of storage (i.e., at

room temperature or in the fridge) to minimize analyte=sample degradation are typical procedures carried out at this stage of the analytical process and need to be taken into account to guarantee the accuracy of the final result.