We have an FTIR instrument which was calibrated in transmission/absorption mode using a set of cells for different ranges of D2O concentration. One of the cells at high range broke and the new one is giving higher concentrations values using the same calibration curve. How can we account for the difference in path length without having to redo the calibration?
Denise, since no one else has yet offered an answer, I'll give you a couple. First, not an expert in infrared spectroscopy, so take any advice only if it applies.
1. Most sensitive instruments are designed to need recalibration. When the instrument and cell are calibrated together, they typically can give a higher rate of accuracy. The downside is that when the cell is replaced, as in your case, it can give out of tolerance readings. A recalibration may be required.
2. Presuming the instrument is designed to be plug and play, it was calibrated separately from the cell, then the cell added and instrument had the exact (optimal) value of the cell placed into the unit, allowing it to perform as it did. If this location is NOT in the calibration portion of the software, you can go there and input this value. This I've done a number of times, on various instruments, with mostly success. Said, I'll typically redo the calibration at this point as well.
Your cell, has a value you expect to get. Check the software to see if there's a location to input this value. Keep in mind, if this is in the calibration portion of the software, you're voiding not only the calibration, but also any results you receive with it. You cannot use uncalibrated equipment to get a calibrated answer.
Hope this helps and wish I had a better answer for you. Please keep us updated with how it turns out.
Hello Paul. Thanks for the comment. That is exactly what I had thought. The calibration was done via PLS but unfortunately the path length selection was embedded in the calibration as a constant value so I can't think of how to ensure precise calculation unless doing a recalibration...
Appreciate the input.
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