The influence of the reference method on the results of the secondary method via calibration

Indirect methods have often the advantage to be non-invasive and very rapid. They can sometimes even be applied in-line or at-line. The disadvantage is that they do not measure the target property of the product directly but either another property that depends on the target property or the response of the product to a physical influence. In all these cases it is necessary to know the relationship of the target property and the measured property or the response, respectively. This relationship is often very complex and needs to be established empirically for samples with known properties which have been measured with a reference method. In other words, a calibration must be established. The future predictions depend completely on the calibration. This means that the calibration must be as correct as ever possible. Inadequate calibration leads to erroneous results, although the user of the method may not be aware of this fact. This is shown at the example of near infrared spectroscopy (NIRS) for water content determination. The Karl Fischer titration as a selective method and drying techniques as non-selective methods were used as reference methods.