Absorbance or Absorption? A personal view "Following David Soderstrom's comments on whether absorbance or absorption was the correct term to use, I've consulted the USP (because I work in the pharmaceutical industry) on the matter. In it are the following descriptions that apply to optical spectroscopy that I think are relevant to answering this question. Absorbance [A]: The logarithm, to the base 10, of the reciprocal of the Transmittance [T]. Absorptivity [a]: The quotient of the absorbance (a) divided by the product of the concentration, expressed in g per litre, of the substance and the absorption path length in cm. Absorption Spectrum: A graphical representation of absorbance, or any function of absorbance, plotted against wavelength or function of wavelength. Transmittance [T]: The quotient of the radiant power transmitted by a specimen divided by the radiant power incident upon the specimen. My interpretation of these definitions is that all UV; IR and fluorescence spectra are absorption spectra. However, for UV etc., where the Y-axis is generally in absorbance units, these data are usually absorbance spectra. Applying this to IR data, if the Y-axis is in %T, the spectrum is an absorption spectrum. If the Y-axis is in A, the spectrum is also an absorption spectrum, but is more commonly called an absorbance spectrum. I conclude that all IR spectra should be described as absorption spectra. Please send any further comment to IJVS" - Don Clark, Pfizer Central Research
Division, Pfizer Limited Life is not meaningless - it just has a very low signal-to-noise ratio. "On
another topic, do you remember a paper published late 80's/early 90's in the vibrational
spectroscopy of nylons? I seem to remember that it contained spectra of lots of different
types of nylon and basically showed that, for example, nylon 6,6 had a different spectrum
to other nylons. I remember reading it on many occasions, but can't for the life of me
think where it was published. I'm sure it was one of yours! It might help my understanding
of the differences in wheat protein spectra. Is it to much to ask if you can remember this
off the top of your head? It's pretty difficult to do literature searches for polymers in
a wheat research unit! " "We have got recently an FT-Raman Perkin Elmer spectrometer for studies of heterogeneous catalytic reactions. At the moment, we don't have any "in situ" Raman cell for this application. Our studies will be performed at flow conditions, gas pressure close to 1 bar, temperatures from the room to the maximum one which can be reached with the thermal background filter. At least, we need a possibility to treat catalysts at different atmospheres and temperatures higher than 400°C and then to take a spectrum at a lower temperature. We would appreciate if you could provide us an
information about the manufacturer who can produce such a cell and approximate
price". "I'm really very glad and I've to
confess that it was a great surprise to find such helpful publication in the web that is
really helping us to solve problems or difficulties in spectroscopy particularly with
Raman techniques. I'm facing lots of problems with solid samples specially with metal
supported catalysts".
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