Читать книгу Chemical Analysis - Francis Rouessac - Страница 51

To calculate the composition of a sample or to assay an analyte responsible for a peak on a chromatogram, two basic conditions must be met. Firstly, an authentic sample of the analyte to be measured must be available, as a reference, to determine the detector’s sensitivity to this compound. Secondly, software calculating the areas (or failing that, the heights) of the different eluting peaks of interest is also required. All of the quantitative methods in chromatography are comparative methods (which is very often the case in quantitative analysis).

For a given tuning of the instrument, it is assumed that a linear relationship (linearization) exists for each peak of the chromatogram, over the entire concentration range, between its area (or failing that, its height) and the quantity of the analyte responsible for this peak in the injected sample. This relationship can be used for concentration ranges bounded by the detectable quantity limit for lower concentrations and the linearity limit for higher concentrations. This hypothesis is translated into the following equation:

(1.42)

where m_i is the mass of compound i injected on the column, K_i is the absolute response factor for compound i, and A_i is the area of the eluting peak for compound i.

The absolute response factor K_i (not to be confused with the partition coefficient) is not an intrinsic parameter of the compound, as it depends upon the tuning of the chromatograph. To calculate the response factor K_i of an analyte, according to the above expression, it is essential that both the area A_i and the mass m_i, of compound i injected on the column, are known. However, this injected mass is difficult to determine with precision. This is why most chromatographic methods used for quantitative analysis, preprogrammed into the software, do not make use of the absolute response factor, K_i.