BUCHI NIRCal - Manual (Page 12)

NIRCal 5.5 Software Manual
12 NIRCal 5.5 Manual, Version A
1.1.3 Cluster calibration
Qualitative calibrations are used for identification of different chemical substances and for separation
of different qualities of the same substances. The possible applications include identification of:

substances with very different chemical characteristics;

chemically similar substances;

acceptable and rejectable qualities of a given substance.
The possible methods are Cluster or SIMCA. Both methods use PCA with the difference that Cluster
method is always used for a group of similar substances, while for each substance a calculation is
performed with SIMCA.
Choosing the Calibration Samples
Verifying substances in the laboratory often means, ascertaining if a sample can be assigned to a
specific category (property); e.g., when checking incoming raw materials in a pharmaceutical company
to see if the incoming raw material is the product that was expected or not. The question can be
answered efficiently by recording an NIR-spectrum of the raw material and analyzing the spectrum
with a qualitative calibration. The raw material can be correctly identified that it belongs to the
expected category or falsely identified that it does not belong to the expected category.
To obtain a useful, qualitative calibration, first, calibration samples should be measured that cover all
expected allowable variations of the quality of the product. For each property, several samples must
be collected to cover variations such as different particle size, temperature, moisture or supplier. To
obtain a representative set of spectra, we recommend measuring samples from at least five to fifteen
different batches of each product that have been collected over a period of at least 6 months. This
will ensure that all variations within a product will be represented in a calibration. Collecting samples
can speed up the building of a proper calibration. But only stable and unchanged substances can be
used.
Only samples that have been tested with reference analysis should be used for the calibration.
The combinations will be chosen randomly or with an adequate experimental design. Two thirds of
these samples are composed to be the basic calculation data for the calibration. The remaining third is
used for testing the calibration.
NOTE
When selecting the spectra in the calibration- and validation-set, it is important to assign all spectra of
one sample either to the calibration or to the validation set.
Calculating the Qualitative Calibration
The spectra of different substances show the physical and chemical characteristics of each substance.
Not all spectral differences are associated with the searched differences. “False“ differences arise, for
example through a varying presentation of the sample because of different particle size or other non-
interesting but allowable variations of the substance. Such “false“ differences can be reduced or partly
eliminated with the help of appropriate data pretreatments during the calculation.