Construction of NIR libraries

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

Selection of calibration range, sample origin, thresholds, eta, 

The correlation coefficient (in wavelength space) is especially suitable for constructing the 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 instrumental oscillations. This parameter allows the library to be developed and validated more expeditiously than others. Correlation libraries can also be expanded with new products or additional spectra for an existing product in order to incorporate new sources of variability in an expeditious manner. 

This procedure can consist of different stages and in each one a sub-library is constructed. 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.). 

Once the general library has been constructed, those products requiring the second identification step are pinpointed and the most suitable method for constructing each sub-library required is chosen. Sub-libraries can be constructed using various algorithms including the Mahalanobis distance or the residual variance. The two are complementary,22 so which is better for the intended purpose should be determined on an individual basis. 

---