A question about Intensities and Spectrometers.

[nordmoon]

When you work with spectra, recorded with a detector or a Fourier Transfer Spectrometer, I hear that quantitative intensities is hard thing to measure and that you deal with relative intensities or relations. A few questions about this:

• Why is it hard to measure quantitative intensities and why deal with relative? Don't you lose information?
• Can you do concentration and temperature measurements even if you 'normalize' the spectrum with for example the highest intensity in the spectra interval?
• Can you compare simulated spectra (that you calculate) and measured spectra, for example by normalizing the spectra as mention above? Are they comparable then? What if you don't know the unit of the measured spectra, can you still extract this information?
• Does there exist any procedure of how to compare spectra's (measured and simulated?)

Reading report about FTS I have encountered the term, instrumental line shape (ILS). What is this? Is this the same as the term, a spectrometers response function?

If a spectra has already been processed by using a measured BB (blackbody) and calculated BB to obtain the systems response function and then use this to obtain the measured *true* spectra, does the ILS need to be accounted for again?

 

[Rajini]

Hi,
Yes i agree in some applications it is not interesting to put unit for y-axis, i.e. many people just mention intensities as arbitrary units. However it has proper unit (for e.g., x-ray scattering experiments intensities has a accurate proper unit). Generally any spectroscopy has intensity units as something per [unit of x-axis], for e.g, per cm^-1
Also i think the spectrum won't change with temperature...the peak positions would be same but intensities may change...provided the sample being measure won't change its molecular structure.
Yes one can compare calculated and measured spectrum..only in relative scale..The most important thing is to compare the frequency instead of intensity...
Instrumental line shape: It defines the resolution of instrument..that means the minimum width of the peak that a spectrometer can find...usually after measuring something..you convoluted the spectrum with instrumental function (like smoothing the spectrum)..
what spectroscopy you are looking for!?
hope this helps.

 

[Modey3]

Why is it hard to measure quantitative intensities and why deal with relative? Don't you lose information?

When analyzing the chemical composition or structure of a sample the absolute intensities are meaningless and only relative intensities are meaningful. For a given sample, absolute intensities will vary from experiment to experiment, but relative intensities won't.

Can you do concentration and temperature measurements even if you 'normalize' the spectrum with for example the highest intensity in the spectra interval?

Yes and Yes. For a given temperature, the absolute intensity of an IR spectra will only change with sample size.

Can you compare simulated spectra (that you calculate) and measured spectra, for example by normalizing the spectra as mention above? Are they comparable then? What if you don't know the unit of the measured spectra, can you still extract this information?

In theory yes, but calculating an accurate spectra is far from trivial. You would need to use Configuration Interaction or Coupled Cluster methods to include electronic correlation. These calculations are very costly! You are better off measuring the spectra first and if some peaks don't make sense run a calculation. You also have to remember that the calculations won't predict the symmetry forbidden transitions which occur in real samples due to the symmetry breaking bond vibrations.

Does there exist any procedure of how to compare spectra's (measured and simulated?)

You can only compare the major peaks: the peaks not forbidden by symmetry.

Hope this helps

modey3