Chemistry and instrumental analysis

The microarray technique is a new analytical method in which molecules that can capture the analytes (e.g. complementary DNA strands, antigen-antibody reaction or the shape of the molecule as aptamers) are spotted onto a glass slide. The slide is immersed in the sample solution and the analyte bound to the spot is detected with a fluorescent dye in the microarray scanner. This enables rapid detection of many analytes in the smallest space of the slide in one step.

In an internship experiment, a commercially available microarray ("biochip") for animal species differentiation in food products with meat is carried out with the students. Simultaneous detection of the 8 animal species donkey, horse, cow, pig, chicken, turkey, sheep and goat on a biochip with a low detection limit (0.5 - 1%) allows the identification of the smallest animal components in food.

As a measuring principle, the fragment of the cytochrome b gene, which is specific for the corresponding animal species, is amplified on the DNA extracted from the samples by means of PCR (polymerase chain reaction) and fluorescently labelled with primers. After binding (hybridisation) of the amplified, labelled PCR products to the complementary DNA spots on the biochip, positively glowing spots can be assigned to a specific animal species by scanning at two wavelengths 532 (Cy3) and 635 nm (Cy 5).

The ETHOS.lab microwave system from MWS Vertriebs GmbH (= MLS GmbH) available in the chemistry laboratory is used, among other things, for the semi-automatic digestion and determination of fat from e.g. quark, cheese, yoghurt and other complicated matrices for which the established reference fat extraction methods according to Weibull-Stoldt and Schmid-Bondzynski-Ratzlaff or the reference method according to Röse-Gottlieb would otherwise be used for the determination of the fat content of milk. The simultaneous acid digestion and fat extraction into the solvent phase in one step under the influence of microwaves and strong stirring, as well as the evaporation of the solvent, ensure a high time saving and increase in work safety compared to the complex reference methods.

The Dumas combustion method is a traditional, safe, environmentally friendly and officially recognised routine method for determining the total nitrogen content of a wide variety of sample matrices (in our case mainly foodstuffs, animal feed and other natural substances) and the protein content of the sample to be calculated from this (multiplication by a specific factor).

The sample is completely burnt in a crucible at high temperatures (900 °C to 1200 °C) under oxygen supply and the combustion gases are freed from disturbing components (e.g. moisture, foreign elements) by various reduction and drying steps. The molecular nitrogen present at the end is quantitatively recorded with the help of a thermal conductivity detector, the nitrogen content is calculated in relation to the initial weight and output on the PC directly after the analysis. The protein content of the sample is calculated from the result by multiplication with a specific factor as in the reference method according to Kjeldahl.

Compared to the classical, wet-chemical and time-consuming reference method for nitrogen determination according to Kjeldahl, in which the sample is digested by boiling in high-percentage sulphuric acid, among other things, the Dumas method is characterised by a considerably lower workload, higher automation, time savings and, in particular, higher work safety (no aggressive chemicals or special laboratory equipment required).