In order to have a stable check cell over time, manufacturers prepare samples more stable, but at the same time these samples are predicted by a model, and normally this model is not a robust one, so it is sensible to temperature, stabilization of the instrument, laboratory and instrument conditions.
A new generation of instruments like the DS2500 and DA1650 from Foss comes with an artificial Check Sample which contains a cake of Melamine and Talc. The distance between the cake and the window can vary slightly, so when scanning the Check Sample, you don´t have to look to the value of the subsamples for the parameter called Distance, if not to the average value.
The Distance value uses a formula, which contains 5 different properties of the average spectra.
The distance value will change depending of the temperature, so it is important to scan it when the instrument is fully warm up, and also the lab conditions. It is not recommended to keep in the Check Sample History, scans at different conditions.
Check Sample Definition must be redefined after Instrument calibration or lamp replacement.
“Be aware that the artificial check sample cell is just for instrument monitoring. The can indicate that something is going on that needs investigating but seldom alone justify a corrective action” (FOSS advice).