- #1
christianjb
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OK chem-wizards...
I'm looking at a paper which states
"...the integrated absorptivity of the stretching vibrations of a water molecule is 4.89 cm micro mol^-1"
(I'm transcribing a 'mu' character as micro in the above. If mu doesn't stand for micro, then let me know!)
OK, so some questions:
1) Is the paper quoting an integrated 'molar absorptivity'?
2) The integration is over the frequency axis, but is this frequency in wave-numbers?
3) Is this defn. for molar absorptivity correct?
Molar Absorptivity,? = A/ c l
( where A= absorbance, c = sample concentration in moles/liter
& l = length of light path through the cuvette in cm.)
taken from http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/UV-Vis/uvspec.htm
4) According to this Wikipedia article, absorbance is calculated as a base 10 logarithm of I/I0. Is that defn. universally used?
http://en.wikipedia.org/wiki/Absorbance
5) I can't understand the units. If A=absorption and alpha = absorption coefficient and L=sample length, then A=alpha L, so that means that alpha is in units of inverse length.
If e=absorptivity, and c is the concentration in mols per liter and e=A/cL=alpha/c, then e is in units of mols^-1 liters ^-1 cm^-1. Thus, I'd expect the integrated absorptivity to be in mols^-1 liters^ -1 cm ^-2, given that the integral is over wavenumbers which have units of cm^-1.
6) Basically what I want is to convert the number in the paper into a value for the absorption coefficient.
Thanks in advance for any help!
I'm looking at a paper which states
"...the integrated absorptivity of the stretching vibrations of a water molecule is 4.89 cm micro mol^-1"
(I'm transcribing a 'mu' character as micro in the above. If mu doesn't stand for micro, then let me know!)
OK, so some questions:
1) Is the paper quoting an integrated 'molar absorptivity'?
2) The integration is over the frequency axis, but is this frequency in wave-numbers?
3) Is this defn. for molar absorptivity correct?
Molar Absorptivity,? = A/ c l
( where A= absorbance, c = sample concentration in moles/liter
& l = length of light path through the cuvette in cm.)
taken from http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/UV-Vis/uvspec.htm
4) According to this Wikipedia article, absorbance is calculated as a base 10 logarithm of I/I0. Is that defn. universally used?
http://en.wikipedia.org/wiki/Absorbance
5) I can't understand the units. If A=absorption and alpha = absorption coefficient and L=sample length, then A=alpha L, so that means that alpha is in units of inverse length.
If e=absorptivity, and c is the concentration in mols per liter and e=A/cL=alpha/c, then e is in units of mols^-1 liters ^-1 cm^-1. Thus, I'd expect the integrated absorptivity to be in mols^-1 liters^ -1 cm ^-2, given that the integral is over wavenumbers which have units of cm^-1.
6) Basically what I want is to convert the number in the paper into a value for the absorption coefficient.
Thanks in advance for any help!
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