Multiple direct calibration method

For inttoductory purposes multiple linear regression (MLR) is used to relate the experimental response to the conditions, as is common to most texts in this area, but it is important to realise that odter regression methods such as partial least squares (PLS) are applicable in many cases, as discussed in Chapter 5. Certain designs, such as dtose of Section 2.3.4, have direct relevance to multivariate calibration. In some cases multivariate methods such as PLS can be modified by inclusion of squared and interaction terms as described below for MLR. It is important to remember, however, diat in many areas of chemistry a lot of information is available about a dataset, and conceptually simple approaches based on MLR are often adequate. 

The ultimate development in the field of sample preparation is to eliminate it completely, that is, to make a chemical measurement directly without any sample pretreatment. This has been achieved with the application of chemometric near-infrared methods to direct analysis of pharmaceutical tablets and other pharmaceutical solids (74-77). Chemometrics is the use of mathematical and statistical correlation techniques to process instrumental data. Using these techniques, relatively raw analytical data can be converted to specific quantitative information. These methods have been most often used to treat near-infrared (NIR) data, but they can be applied to any instrumental measurement. Multiple linear regression or principal-component analysis is applied to direct absorbance spectra or to the mathematical derivatives of the spectra to define a calibration curve. These methods are considered secondary methods and must be calibrated using data from a primary method such as HPLC, and the calibration material must be manufactured using an equivalent process to the subject test material. However, once the calibration is done, it does not need to be repeated before each analysis. 

Tung et al. (1988) calibrated their probe directly in a 90mm diameter fast fluidized bed with FCC catalyst. A multiple regression method was developed for calibrating the probe by traversing it through the bed and... 

Standard Addition(s) and Isotope Dilution. The traditional (single/multiple) standard addition(s) method, which can also be used for dealing with (strong) matrix effects, needs some adaptations (especially for data treatment) if direct solid sampling is intended, as taking exactly the same amount of solid material is not (always) possible. In spite of that, successful analyses relying on this calibration technique have been reported for a number of different polymers [21,27,32],... 

Although it is more complex than the direct method (Equation 48.50), this procedure provides the following advantages (1) multiple alloys can be measured in a single experiment (depending on the number of effusion cells in the furnace) (2) T measurements are calibrated with the Au melting in each experiment (Tmp(Au) = 1337.33 K is a fixed point defining ITS-90)... 

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