High reproducibility, long membrane life, and very low detection limits for sugars make this dialysis system a powerful tool for overcoming difficult application problems
In food chemistry the determination of sugars in dairy products is of the utmost importance.
When carrying out direct ion chromatographic determinations the precipitation of protein constituents on the column is a recurring problem.
Instead of demanding precipitation procedures, the dialysis technique can be used to separate the interfering protein matrix.
The use of dialysis as an inline sample preparation step results in a rapid automated analysis system.
The sugar constituents are determined by means of pulsed amperometric detection.
The problem.
Sugar analysis plays an important role in food control.
In many cases, as the low-molecular sugar constituents are readily soluble, sample preparation is straightforward and rapid.
However, the analysis of protein-containing samples can pose severe problems. Precipitation of the proteins affects the column and ultimately destroys it.
This can be prevented if demanding precipitation procedures (eg, Carrez precipitation) are carried out before the analysis.
However, such sample preparation steps may lead to co-precipitation, inclusion or even decomposition of the sugars to be determined.
Solution outline.
Dialysis can solve this problem.
Dialysis is neither filtration, nor extraction, nor digestion.
In dialysis the analytes pass from the sample solution into the acceptor solution by diffusion through a dialysis membrane.
After only a short time a concentration equilibrium is established between the sample solution and the acceptor solution.
The number of analytes in the sample solution is not reduced as these are constantly being resupplied.
Whereas the mono-, di- and oligosaccharides can pass through the membrane almost without any hindrance, polysaccharides or larger particles - the matrix - are simply transported past the dialysis membrane. Matrix effects are therefore reduced to an absolute minimum.
The separation of the low-molecular sugars from the high-molecular proteins is determined exclusively by the osmotic pressure at the membrane.
This means that, due to the concentration difference existing across the cellulose membrane, the sugars migrate from the sample stream into the acceptor solution (eg, ultrapure water) while the high-molecular protein constituents stay on the sample side of the membrane.
This sample preparation step occurs inline and can therefore be completely automated.
The time needed is negligible as the dialysis is performed during the recording of the previous sample's chromatogram.
The instrumental setup is as follows.
MIC-8 + Dialysis.
Carbohydrate system with 817 Bioscan and dialysis.
Automated sample preparation is becoming more and more an integral part of a high-performance IC system.
With the introduction of the patented dialysis method, Metrohm says it has placed a new milestone on the way towards this goal.
The Metrohm dialysis method works according to the stopped-flow principle.
About 10ml of sample is passed along the dialysis membrane by the peristaltic pump built into the 788 IC dialysis sample processor, while the acceptor solution remains at rest.
The acceptor solution is then transferred to the sample loop of the 812 valve unit and the sample is analysed in the normal manner.
