Knowing your sample is of paramount importance and guides you towards making the right choices in the analytical process. This knowledge also helps to avoid certain errors related to the heterogeneity of the sample.
Knowing the composition of your sample is very important, whether it concerns the major or minor compounds of the matrix or also the interactions of the contaminant with the matrix and its possible localization (free molecules of contaminant or bound to certain components of the matrix?). Based on the composition, you can also suggest ways to isolate the contaminant from the other components of the matrix (selectivity of the extraction), or to anticipate a possible purification step to eliminate certain components of the matrix that would be co-extracted with the contaminant.
Read the sheet “How to choose and develop an extraction method”
It is very rare that the sample taken for analysis is homogeneous. However, the heterogeneity of the sample can be an important source of error, because the analysis itself will be conducted on a subsample (called the test sample).
In the laboratory, homogenization is carried out thanks to a step of pre-treatment of the sample (liquid samples are simply agitated, while solid samples are ground and possibly dried). It is also advisable to perform duplicate or triplicate analyses (2 or 3 test samples are taken from the same homogenized sample, and each test sample is then analyzed independently).
The amount of sample available is sometimes restricted. In this case, the chosen analytical method must be sensitive enough to be able to determine the traces of the contaminant.
Having an idea of the expected contaminant concentration in the sample to be analyzed is very helpful: it can guide the orientation of the analytical method, especially in terms of its limits of detection and quantification (LOD, LOQ). If the contaminant is regulated in the matrix of the sample, this regulatory value also gives a useful indication.
In your view, what aspects of coffee can falsify an analysis of ochratoxin A?
Coffee beans can be heterogeneous and consist of different components (e.g. lipids) that could interfere with the contaminant analysis. Furthermore, as the ochratoxin A concentrations are expected to be low, a very sensitive analytical method will be required.