[smellor]

this may seem a silly question, but after poring through all my books and searching on the internet i have been unable to find any information relating to this. my question is, does IR spectroscopy in any way quantify concentrations or relative amounts of a compound in a sample? In different IR spectra slight variations can be seen in terms of baseline transmission, and I was wondering if this reflects the quantity/concentration of sample used
My personal assumption would be that IR provides no such information, but as i'm still new to the technique I would like to hear opinions of people more knowledgeable than me!
Looking forward to getting this cleared, thanks for reading!

[Borek]

In all spectroscopic methods signal intensity is directly proportional to substance concentration (at least for some range of concentrations). IR is no exclusion.

[ARGOS++]

Dear Smellor;

IR is quite common to measure Gas Concentrations whereat Cuvettes of several times one Meter are not rare.
In (special) solutions (liquids) most time you have more or less a problem with the pathlength of the “cuvette” you (should) use.
That’s why most prefer for quantitative measurements more the NIR region, but there you have the problem of calibration, which is mostly done with “Chemometrics”.

I hope it gives you at least an idea.

Good Luck!
                   ARGOS⁺⁺

[smellor]

Thank you for your answers, i'd like to venture into a specific example as i am unsure how to interpret a given IR spectrum in terms of quantities of the compound analysed.
The compound is cyclohexene, and assigning the peaks is not a problem, however we have been given no information as to sample preparation (i assume liquid sandwiched) nor have we been given clues as to pathlength of sample. Now, a feature of the spectrum is a baseline at around 80% transmission and all peaks are in the range of strong to medium (the c-h bend is the weakest one).

[ARGOS++]

Dear Smellor;

The method will be all time for Spectroscopy the same, so in IR too.

First you need an adequate “Blind-sample” and a “Reference”.
That’s even more important, as conditions have a very big influence in this Region.

Quintessence: 
Only with a Sample-Spectrum you will be lost for ever!

If you will have all done correctly the final “Base-line” will be very close to 100.0%.
For more explanation you may search for "Blank Experiment” I wrote a little time ago.
Good Luck!
                   ARGOS⁺⁺

[smellor]

once again thanks for the clarification - the spectrum in question was not run by myself but is an example given to us. Have i understood correctly when i say that without a reference for this sample, it is impossible to determine concentrations from the spectrum given?

[ARGOS++]

Dear Smellor;

Yes! – You are right!
It is because in this region “nobody” is publishing “Extinction Coefficients” too, so a “Reference” is indispensable!
The procedure with one or better several “References” (different Concentration) is called “Calibration”

Good Luck!
                   ARGOS⁺⁺

[smellor]

thank you for clearing that up, much appreciated!