New heliXcyto chips have a raw fluorescence below 3 x 105 cps in the measurement channel when performing a chip test (LED = 0.5). Stable adherence of fluorescently labeled analytes to the chip material (glass, gold, polymer) might lead to an increase of this background fluorescence which can be noted by a difference of raw fluorescence counts before and after the measurement, as well as increasing signals in repeated chip tests. As the single photon counters are damaged by too high fluorescence, the shutter might close for protective reasons if the analyte signal on top of a high chip background reaches more than 2 x 106 cps. In this case you can lower the LED power in your next run, or reduce the analyte concentration. Make sure to clean the chip before storage with the Chip cleaning kit to prevent excessive fluorophore build-up.