A discussion about the significance of Absorbance and sample optical thickness in conventional absorption spectrometry and wavelength-modulated laser absorption spectrometry

One of the most frequently used concepts in atomic absorption spectrometry (AAS) is that of Absorbance, an easily measurable quantity that is linear with the concentration of the analyte over a large range of concentrations whenever Beer's law is valid. Laser-based modulation techniques, however, in particular wavelength modulated diode laser absorption spectrometry (WMAS), do not measure exactly the same physical quantities as conventional AAS techniques do, since they do not rely upon separate measurements of the intensities in the presence and in the absence of the sample, but rather (a certain fraction of) the difference between the signals ‘on’ and ‘off’ resonance. Hence, Absorbance is not as natural for the modulated laser-based absorption techniques as it is for the conventional AAS techniques. The entity called sample optical thickness (SOT) appears to be a natural entity as well as a natural unit (denoted SOT units) for the quantification of signals in WMAS. The present paper discusses the concept of measurable quantities and their units in WMAS in some detail and compares them (theoretical and practical considerations) with those of conventional AAS. In particular, it makes a distinction between the ‘observed’ sample optical thickness and the ‘true’ sample optical thickness and shows how these two entities are related to the Absorbance entity. Finally, this work also introduces a dimensionless sensitivity of the WMAS technique, and shows this quantity to be equal to the nth Fourier coefficient of the wavelength modulated, peak-normalized line shape function.