Quantitative Mid-infrared Spectroscopy The Basics

The traditional means of measuring the mid-IR spectrum of a sample using an IR spectrometer has been by a transmission measurement, in which the intensity of mid-IR radiation passing through the sample, presented in some appropriate form, and reaching the detector is compared to the intensity of mid-IR radiation reaching the detector in the absence of the sample. If these two values are I and /q, respectively, then the % of radiation transmitted, %T, by the sample at a given wavenumber, v, can be represented as  

The more a sample absorbs the mid-IR radiation, he lower the value of T. The transmittance of a pure sample of pathlength (thickness) /, where I is in cm, is expressed as  

The amount of mid-IR radiation absorbed, A, by the pure sample at v is expressed as  

Equation 2.6 is a form of the basic law underlying quantitative analysis by mid-IR spectroscopy it is known as the Lambert-Beer Law and commonly referred to simply as Beer s Law. Providing this linear relationship holds for all molecules within the mid-IR beam, that is there are no intermolecular interactions between molecules, then for a mixture Beer s Law can be applied in an additive form. For an n-component mixture, then the measured absorbance A for all the i components present is given by  

Because of the linear relationship between the absorbance A and concentration c, quantitative analyses are always carried out on absorbance spectra (not transmission spectra), whether they are simple univariate determinations or involve multivariate analyses. Multivariate analyses, used to statistically analyse or classify data that involve more than one component e.g., different states of cancer aggressiveness) over a wide spectral range i.e., many wavenumbers), is that which is commonly practised in samples for medical diagnostics, such as those considered in this book. 

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