In order to avoid problems caused by baseline drift, it is expedient in a method of an embodiment of the present application not to measure a signal rise in a detection space, but to allow a certain time period to elapse in order to enrich a detectable product (enrichment phase), then to measure a first detection signal, and to measure the baseline signal as second detection signal only after rinsing out the detection space and removing the enriched product. In at least one embodiment, the enriched product is not detected from a signal rise with reference to a baseline, but from a signal difference of first and second detection signals.