B eer's Law

Abstract

Abstract Beer's law is the fundamental law of quantitative absorption spectroscopy because it relates the absorbance to the product of the concentration of each analyte and the pathlength of the cell. In this article, the theoretical requirements for Beer's law to be obeyed are given; these include the need for a collimated beam of monochromatic radiation and the need for a dilute homogeneous solution of the analyte. In practice, these requirements can be relaxed but in order for Beer's law to be obeyed, interaction between the analyte molecules that can cause the absorptivity to change as a function of concentration must be constant. The optimum condition for quantitative spectroscopy is found when the transmittance is equal to 1/ e . Baseline correction is sometimes needed to obtain the optimum result, but this may at times be a subjective operation. Instrumental effects on Beer's law linearity are discussed in some detail. For measurements of concentrated solutions made with a monochromator, the effect of stray radiation may be significant. The effect of instrumental resolution on Beer's law linearity is discussed in terms of the resolution parameter, which is the ratio of the width of the instrument line shape function to the full width of the band. Measuring the spectrum at low resolution only leads to nonlinear Beer's law behavior when the true peak absorbance is greater than about 0.5. Deviations from Beer's law for measurements made with a Fourier transform spectrometer are strongly dependent on the apodization function that is used. The Norton‐Beer medium apodization is recommended for achieving Beer's law linearity over very wide concentration ranges.