Construction of spectral libraries

A NIR identification library shonld encompass all the raw materials used by the manufacturer in order to be able to identify all possible snbstances and avoid or minimize errors. The method to be nsed should allow the unequivocal identification of each componnd present in the library and the exclnsion of those not present. It should also be able to distinguish between very similar compounds nsed in different applications (e.g. products with different particle sizes, polymorphs, products in different grades or from different suppliers). 

The correlation coefficient (in wavelength space) is especially suitable for constructing a general library as it has the advantage that it is independent of library size, uses only a few spectra to define each product and is not sensitive to slight instrnmental oscillations. This parameter allows the library to be developed and validated more rapidly than others. Correlation libraries can also be expanded with new prodncts or additional spectra for an existing prodnct in order to incorporate new sources of variability in an expeditious manner. 

One of the crncial factors to ensnre adeqnate selectivity in constructing a spectral library is the choice of an appropriate threshold value or acceptance criteria. Such a threshold is the lowest value (for correlation) required to unequivocally assign a given spectrnm to a specific class. Too low a threshold can lead to ambiguities between substances with similar spectra, whereas too high a threshold can resnlt in spectra belonging 

Selection of catibraUon range, sample origin, ihreshoids, etc. 

This procedure can consist of different stages with one sublibrary being constructed at each stage. The first one enables the identification of the product within an ample library, a second defines a characteristic that produces an important variability in the spectra (e.g. particle size) and a third defines a smaller spectral variability (difference in the content of impurities, origin of manufacture, etc.). 

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