BUCHI NIRCal - Manual (Page 11)

Introduction
NIRCal 5.5 Manual, Version A 11
Beer Law:
has just limited validity in the NIR spectroscopy. The application of chemometrics for the evaluation of
NIR spectra is a must.
Advantages of NIR spectroscopy
The relative low degree of absorption coefficient of overtones and combination vibrations causes a low
degree of absorbance in the NIR region. Solids have a high degree of reflected light and liquids can be
measured for path lengths of several mm.

no sample preparation is necessary.
The materials for the optics can be quartz, glass or sapphire, which are

cheap and robust.
1.1.2 Chemometrics
NIR spectra are generally characterised by very broad peaks and a multitude of oscillation
superpositions. Visual evaluation is therefore all but impossible.
Differences in the spectra of similar substances often consist merely of a slight shift or small change in
shape of the wide absorption bands. For this reason, NIR spectra are basically evaluated with the aid
of mathematical methods, which is why such significance is attached to the chemometric software.
Chemometric is the application of mathematical, statistical procedures for processing, evaluating
and interpreting large amounts of chemical data (e.g. NIR spectra). The function of the chemometric
software in NIR spectroscopy is to find a statistical correlation between the spectral data and the
known (e.g. by laboratory analysis) property values of the samples used for the calibration.
If this correlation is systematic, it is possible to predict desired parameters of unknown samples
(e.g. identity, quality, quantity) by recording the spectrum and subsequent evaluation by calculation.
The Büchi NIR spectrometer systems allow several hundred intensity values (reflectance /
transmittance) of the measured NIR spectral region to be included in the calibration. In order to be
able to draw maximum benefit from the measured region, the Principal Component Analysis (PCA)
is applied.
The Principal Component Analysis for qualitative calibration allows the identification of different
substances or similar product qualities. There are 2 ways to use PCA:

Cluster calibrations and the

SIMCA method
For quantitative analyses, three different calculation procedures have been implemented:

Multiple Linear Regression ( MLR ),

Principal Component Regression ( PCR ) and

Partial Least Squares Regression ( PLS ).
These methods can be tested with a user made selection of independent validation samples or by
using each sample for the calibration in cross validation .
Chemometrics is the science of relating measurements made on a chemical system or process to the
state of the system via application of mathematical or statistical methods. [International Chemometrics
Society (ICS)]