Homogeneity of Food Samples for Analysis of Trace Contaminants

Whenever food samples are analysed for trace contaminants, be it honey, cereals, teas, herbs, spices, nuts, seeds, etc, or products containing said ingredients, we are faced with the issue of ensuring that our sampling is representative of the entire sample. Each matrix provides differing obstacles to homogeneity and the solution isn’t always as simple as you think. In the most basic method, you would either measure all the sample or mix the sample so that any contaminants are thoroughly spread equally through the sample vessel.

Take Honey for example, which often forms crystals of sugar, the easiest way to ensure that the sample is mixed would be to heat the honey until it becomes a runny fluid. However, this isn’t always the most appropriate method because excessive heat can degrade the bonds between the sugars, and as the ratios of sugars are measured to determine adulteration with low-cost sugars, we cannot risk disrupting the sugar ratios. However, as honey is liquid, vigorous stirring with a spatula with often adequately mix the sample.

For a solid matrix like cereals which we often receive as powder (or is ground from grains to a powder), there will be pockets or ‘hot-spots’ of varying levels of analytes of interest. During our development of a method of digesting cereals for heavy metals analysis we found that the cereal subsampling would often result in significant variations in metal concentrations. This is likely due to the powder particulate holding an elevated level of metal within the particulate and as the powder is not a liquid the metal ions could not be spread across the entire sample.

To ensure that we could measure a representative sample we resorted to acid digesting around quadruple the amount of sample that was required and subsampling our digestion slurry. This acid digestion removed the contaminant from the sample and spread it thoroughly through the slurry resulting in a far more stable and confident result from our analysis.

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This article was written by one of our experts at Minerva Scientific – A Tentamus Company, Dr Benjamin J Fryer.