2017-11-01
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There are two principal instrumental limitations to Beer’s law. The first limitation is that Beer’s law is strictly valid for purely monochromatic radiation; that is, for radiation consisting of only one wavelength.
However, even the best wavelength selector passes radiation with a small, but finite effective bandwidth. Using polychromatic radiation always gives a negative deviation from Beer’s law, but is minimized if the value of is essentially constant over the wavelength range passed by the wavelength selector.
For this reason, as shown in Figure 10.23, it is preferable to make absorbance measurements at a broad absorption peak.
Effect of wavelength on the linearity of a Beer’s law calibration curve
In addition, deviations from Beer’s law are less serious if the effective bandwidth from the source is less than one tenth of the natural bandwidth of the absorbing species. When measurements must be made on a slope, linearity is improved by using a narrower effective bandwidth.
Stray radiation is the second contribution to instrumental deviations from Beer’s law. Stray radiation arises from imperfections within the wavelength selector that allows extraneous light to “leak” into the instrument. Stray radiation adds an additional contribution, P stray, to the radiant power reaching the detector; thus 
For small concentrations of analyte, P stray is significantly smaller than P 0 and P T, and the absorbance is unaffected by the stray radiation. At higher concentrations of analyte, however, P stray is no longer significantly smaller than P T and the absorbance is smaller than expected. The result is a negative deviation from Beer’s law.
