By craig on Thursday, March 25, 2004 - 01:48 pm:
I have determined the response factor of compound A in relation to an internal standard ("compound C"). I would like to know the response factor of compound B for which I do not have an authentic standard. One paper I reviewed estimated the response factor of compound B by determining its molecular mass ratio vis-a-vis the compound A. No other details given. This is for UV detection.
How might this have been done, and why would it work? Is absorbance that closely related to molecular mass?
By bill tindall on Saturday, March 27, 2004 - 06:03 am:
Quick answer, NO! Must have been a biochem paper, (chuckle). Any field that describes formation of an amide as "conjugation" can't be expected to be too precise.
If the rest of the molecule is in no way "communicating" with the part of the molecule that is the chromophore then the electronically isolated mass has no affect on molar absorbtivity, but absorbance will be diluted on a mass basis(slope of mass calibration curve lessened by mass MW ratio). So for example the molar absorbance of butryic acid and valeric acid will be similar, but the absorbance on a weight basis will be smaller for valeric acid.
But add some mass that electronically interacts and adding molecular mass can have a huge affect. I suspect that the molar absorbtivity of phenylacetic acid is 50 times that of acetic acid.
for compounds that have molecular weights of say about 400 I have seen molar absorbances of up to 60,000 (azo dyes), 2000 (simple aromatic molecule), 100( simple carbonyl compound) or near zero (hydrocarbon). So, as you can see, MW can enable a large molar absorbtivity (it takes some molecular weight to build an electronically highly conjugated system), but the structure not the weight determines absorbance.
By Anonymous on Tuesday, March 30, 2004 - 06:46 am:
Hey Bill, do You remember a biochemist named M.S. Tswett?
By DR on Tuesday, March 30, 2004 - 09:58 am:
He was a Botanist, I think.
And I agree with the above. MW has nothing much to do with absorbtivity. Thas being said, when doing stability studies, new peaks are assumed to have the same absorbtivity as the main peak as a function of being 'related' but this appropriately goes out the window once someone IDs the new peak and can then either look up or determine the absorbance of it relative to whatever is being used as a standard.