30 jul. 2015

Artificial Check Cells (DS2500 and DA1650)

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).

22 jul. 2015

About using the Check Cell

NIR Systems Instruments, use to come in some cases with a Check Cell, with some files (.eqa, .std, master.nir,...). Once configure the Check Cell must be used carefully:

  • Keep it in the box until use at room temperature.
  • Handle it carefully (don´t drop it)
  • Keep the quartz glass clean.
  • Instrument must be warmed-up.
Starting using the Check Cell:
  • Scan the check cell three times every day (during two weeks) looking for a reasonable range of variation conditions.
  • After this, scan the check cell once a day.
Check Cell interpretation:
  • We will keep a file with just the 64 last scans of the check cell, where we have some important statistics:
    • Mean for each constituent of previous predictions
    • Std of previous predictions
  • When analyzed we get the predictions and the T statistic (must be between +/- 3.00).
  • If the Check Sample is out of tolerance, suspect of the instrument:
    • Is it warmed up?
    • Does it pass the diagnostics?.
    • Are the lab conditions fine? (temperature,.....)
See the plots:
  • Is the instrument drifting continuously overtime?
  • Is the instrument drifting dramatically from one day to the other?
  • Are all the parameters affected or some of them?
  • ..................
Check Cell can be used as a tool to understand better how our instrument is performing and also the variations you have in the lab.
Is important that they are correctly sealed, in order that the variations of the product inside be minimal during those 64 days, if not reduce it to lower number.
You can export the spectra of check cell to a Chemometric Software as Win ISI in order to understand the variation.
"Write comments with your experience with the Check Cell. Many users does not use it, others love it,......"
There are new check cells more artificial, or more stable to changes, which come with the new instruments.
To have the check cell scanned every day will help the technicians a lot, after repairing an instrument to check if the instrument performs like in the past when it was fine.