BUCHI NIRCal - Manual (Page 15)

Introduction
NIRCal 5.5 Manual, Version A 15
Important:
For the choice of the reference samples, all components of the mixture (and eventually
varying parameters like temperature) must be considered, even components not being
calibrated. For complex compositions, like foodstuffs, the requested amount of samples might
be quite high.
The choice of calibration samples must be carried out very carefully. Spot test samples taken
from the production process might not meet the demands, because the reference values
typically show small variations around a nominal value. Suitable calibration samples should
whenever possible be chosen with the aid of an experimental design or specially produced. The
consistent and independent distribution has to be checked in any case.
Suggestions:
Figure 1: The distribution of the reference values is checked with the help of 2D plots.
The NIR calibration range should cover at least 20 times the error range of the reference method.
For example for H
2
O determination, the range of error of ± 0.2% leads to a minimum NIR-calibration
range of 4% around an expected mean value. Otherwise the error of the reference method has a high
influence on a calibration. In order to avoid extrapolation for the NIR calibration, select the calibration
range slightly wider than the working range.

Working range: concentration range of a parameter that usually is measured in a product;

Calibration range: for the calibration, the concentration range must be set broader than the
working range.
As a rule of thumb; 60 samples should ideally cover the calibration range homogeneously. 2/3 of
all samples are used for calibration, 1/3 of all samples are used for validation. Spectra of the same
sample must remain together and be put either in the calibration set or the validation set. For the
calibration set it is recommended to use at least 10 samples per parameter to be determined.
Spectra with the minimum and maximum concentration values (extreme values) must always be
selected as calibration spectra.