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- Axner, Ove, et al.
(författare)
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A discussion about the significance of absorbance and sample optical thickness in conventional absorption spectrometry and wavelength-modulated laser absorption spectrometry
- 2003
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Ingår i: Spectrochimica Acta Part B - Atomic Spectroscopy. - : Elsevier. - 0584-8547 .- 1873-3565. ; 58:11, s. 1997-2014
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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- Axner, Ove, et al.
(författare)
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Absorption spectrometry by narrowband light in optically saturated and optically pumped collision and doppler broadened gaseous media under arbitrary optical thickness conditions
- 2006
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Ingår i: Applied Spectroscopy. - New York : Society for applied spectroscopy. - 0003-7028 .- 1943-3530. ; 60:11, s. 1217-1240
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Tidskriftsartikel (refereegranskat)abstract
- This work examines absorption spectrometry by narrowband light in gaseous media with arbitrary optical thickness when the light induces optical saturation or optical pumping. Two quantities are defined: the observed absorbance, Aobs, and the true absorbance, Atrue. The former is the absorbance that is measured under the existing conditions, whereas the latter represents the absorbance one would measure if the light acted solely as a probe of the populations of the various levels, and it is therefore directly proportional to the concentration or density of absorbers. A general integral equation for the propagation of light in media of arbitrary optical thickness in which the light influences the populations of the levels involved is derived. This expression is transcendental in the observed absorbance and cannot be solved analytically. It is shown that an analytical expression can be derived by investigating the inverse relationship, i.e., Atrue = f(Aobs). Inasmuch as collision and Doppler broadened media react differently to optical saturation, they are considered separately. It is shown that a nonlinear response results if the medium is optically saturated (or pumped) and not optically thin. Expressions for the error introduced if the technique of standard additions is uncritically applied to such a system are derived.
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- Axner, Ove, et al.
(författare)
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Authors’ reply
- 2004
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Ingår i: Spectrochimica Acta, Part B. ; 59:3, s. 390-392
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Tidskriftsartikel (refereegranskat)
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