MULTI MICRO HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY

The purpose of the study was to develop a micro HPLC system with physically changeable selectivity. This was done using two series-coupled beds of chemically differing stationary phases in a packed fused silica column. The selectivity change, in fine repeatable steps, was possible by the stepwise moving of a tube oven, covering only 50 % of this ''tandem column''. Each step of selectivity started with a position change of the oven, thus heating up a fresh position of the more polar and the non-polar column bed. As the chromatogram is thus repeated under these new selectivity conditions, multiple chromatograms result. For this reason we call the technique ''Multi Micro HPLC''. This very simple mode of operation provides an analytical tool for developing separation conditions and which allows optimizing for a given special sample and checking for one of the most critical problems in HPLC: unidentified peak overlapping. The selectivity changes are non-linearly correlated with the oven position with regards to temperature field distribution and are greater than is possible by simple change of column temperature. Micropacked columns are ideal for quick temperature changes. Elevated temperatures in HPLC help to shorten analysis time. The concepts discussed above provide an explanation of how a normal, formerly chemically homogeneous, column can become very temperature sensitive after detoriation at the inlet. In fact the column has become an unintentional ''tandem column'' and peak positions tend to move erratically as its selectivity changes drastically by inhomogeneous temperature changes along the separation path.