Introduction

In the analysis of contaminants and residues in food samples, enrichment is of vital importance because samples are too dilute (e.g., beverages) or too complex (e.g., meat) for direct analysis and need to undergo a chain of specific treatments to make them compatible with the analytical techniques. While the dictum ''the

Comprehensive Analytical Chemistry, Volume 51 © 2008 Elsevier B.V.

ISSN: 0166-526X, DOI 10.1016/S0166-526X(08)00005-6 All rights reserved.

best sample preparation is no sample preparation" is also true for food analysis, for the determination of traces of contaminants and residues, direct analysis is only applicable in some exceptional cases, e.g., the direct analysis of polycyclic aromatic hydrocarbons (PAHs) in drinking water by liquid chromatography (LC) with fluorescence detection and the determination of haloforms in drinking water with capillary gas chromatography with electron capture detection (CGC-ECD). Extraction, fractionation/clean-up, concentration and/or derivatization steps preceding the analysis, mostly by chromatographic techniques, are mandatory. A typical flow diagram for the determination of contaminants and residues in foodstuffs is shown in Figure 1.

Sampling and sample preparation remain among the more time-consuming, error-prone and contamination-prone aspects of the flow diagram. Obtaining a representative sample and proper storage of the sample are important parts of any analysis. Both are often overlooked by analytical chemists, who regard them as self-evident secondary problems, with the chromatographic analysis being of primary interest. However, errors or faults in the sampling protocol cannot be corrected at any point in the analysis. For the analytical data to be meaningful, a plan for acquiring samples and storing samples should be implemented and, if possible, validated by statistical techniques.

A discussion on sampling is beyond the scope of this chapter. On the other hand, problems of storage mostly occur in the analytical laboratory and are the responsibility of the analyst. Processes that occur in the sample between the time of sample collection and analysis such as adsorption on the container walls, vaporization loss, photoreactions, and microbial action, can invalidate the data. Samples should be properly stored in the dark in brown glass vials or containers and maintained at 4°C or lower. Especially for liquid samples adsorption on the glass wall should be controlled and, if needed, suppressed by adding a polar modifier like methanol, e.g., in beverages with no or low alcohol content.

Sampling

Sampling

Sample preparation <

Extraction

Fractionation/Clean-up

Concentration

Derivatisation

Chromatographic analysis Separation

Data Handling

Data Handling

Reporting

Figure 1 Flow diagram for the analysis of contaminants and residues.

In recent years, great efforts have been made to develop sample preparation techniques that guarantee high recovery and reproducibility and that moreover are faster, cheaper, greener and easier to automate than older methods. A recent trend is also the development of multi-residue methods (MRMs) for a great variety of matrices such as the QuEChERS method [1,2] and sorptive extraction [3,4].

In this chapter the currently used sample preparation methods for chromatographic analysis of contaminants and residues in foodstuff are reviewed and the new methods are discussed in depth and illustrated with examples from the laboratories of the authors. The subdivision of the chapter is, in first instance, based on the target compounds to be determined which can be divided into three main classes: (1) volatile organic compounds (VOCs) that can be analysed via headspace techniques eventually after derivatization, (2) semi-volatile compounds (SVOCs) that are GC amenable (thermally stable) like polychlorobiphenyls (PCBs), PAHs, most of the pesticides, etc. that require an extraction step and finally (3) non-volatile or thermally labile compounds (NVOCs) the analysis of which should be performed by LC after extraction. Also the different matrixes from which the target solutes are enriched should be taken into account and differentiated. In general, matrixes can be divided based on the absence or presence of fatty material. Non-fatty foodstuffs include non-alcoholic beverages, alcoholic beverages (ethanol content from 3% to 50% ethanol), fruits and vegetables and herbs, while the fatty food includes milk, vegetable oils and diary products.

It has to be noted that in recent years the sample preparation strategies have changed drastically. On the one hand, analysts are confronted with the continuous decrease of legislative limits for food contaminants and residues accompanied with higher requirements for more precise and accurate results and, on the other hand, the performance of analytical tools in terms of qualitative and quantitative analysis, has increased tremendously. Mass spectrometers are a must in trace analysis and because of their "recognition" features they have partly taken over the "selectivity" of classical sample preparation methods. Notwithstanding this, quantification is still challenging and both in target and multi-residue methods, isotope dilution and the method of standard addition provide the most accurate results by compensating for matrix effects (sample characteristic) and ion suppression in mass spectrometry (MS) (sample preparation characteristic). The cleaner the extract, the better are the quantitative data, stressing the importance of sample preparation.

Several sample preparation techniques will only be covered briefly in this chapter. For more details, refer to Refs. [5,6] and two recent special issues of J. Chromatogr. A [7,8] including a review on Sample Preparation Techniques for the Determination of Trace Residues and Contaminants in Foods by Ridgway et al. [9].

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