A novel technique for unsupervised feature selection is disclosed. This method is particularly suitable for polypeptides, which are known to be difficult to display on phages, and for proteins of cDNA libraries and other combinatorial libraries, in particular when derived from very fast folding, stable protein scaffolds. The sensor assembly further has a pair of magnetic field sensors arranged at a spatial distance from each other on the second part. Additionally, calibration may be performed by comparing a calibration pattern with a crease. Preferred serviceable filter cartridges are shown. The pair of capture and release sections include a concentration gradient of at least one of the target gas and the fluid.