Masked Contaminants

A masked contaminant is defined as a contaminant present in such a form that it will escape from rapid screening or instrumental analysis and remain undetected. A contaminant might be conjugated with carbohydrates, sulfates, amino acids, fatty acids or bound to proteins or nucleic acids in the sample of interest. Rapid screening tests such as immuno or receptor assays are based on molecular recognition and will fail recognition when the binding sites of the molecule become less accessible due to modification or steric hindrance elsewhere in the molecule. Chromatographic and mass spectrometric methods will fail as well due to retention time shifts and changes in m/z. A typical example is the occurrence of glycosylated Fusarium mycotoxins in wheat and maize, as recently shown for deoxynivalenol (DON) and zearalenone by Berthiller et al. [44,45]. In addition to that it was shown that the ratio between free and glycosylated mycotoxins can change during fermentation processes, for example, in beer breweries [46]. It can be assumed that all biotoxins are vulnerable to some degree of conjugation and escape at least partly from detection, unless appropriate modifications to sample preparation schemes are being implemented. Risk assessments and intake data of biotoxins should be reconsidered taking this phenomenon into account, but first of all appropriate analytical methods should become available.

Another example is residues of the nitrofuran antibiotics, the use of which has been prohibited within the EU because of their potentially harmful effects on human health. Former analysis of the parent drugs turned out to be completely inappropriate since they do not persist in edible tissues due to rapid metabolization. The nitrofuran metabolites on the other hand form persisting protein-bound residues which will remain undetected. Conneely et al. [47] described a method for the determination of the total content of nitrofuran metabolites in tissues incorporating an acidic hydrolysis combined with a nitrobenzaldehyde derivatization step. The method was validated for porcine liver at the 5-ng/g level. Subsequent application to poultry had a global impact: high percentages of non-compliant samples were detected and an MRPL was assessed by the EC for harmonization of residue control in importing and exporting countries.

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