Both methods aren't exactly a great choice for elemental analysis. Better choices would be XPS, AES, EDX, SIMS*, combustion analysis* or ICP-MS*.

The methods marked with * are at least partially destructive.

Dear Jürgen Weippert,

Thank you for your prompt reply to my query.

But I need to know the usefulness of Raman and IR spectroscopy in detecting molecules present in the sample.

And which one to prefer?

Thanking you.

Short answer is : what works well with FTIR does not with Raman, and inversely.

It then all depends on which molecules/chemical moieties you're interested in.

If your question is about molecules instead of elements, as your original question suggested, you don't have any workaround and you have to check what the characteristic vibrations of these molecules are and the usefulness of IR and Raman may, as Pierre Caulet said correctly, behave inverse.

Dear Jürgen Weippert,

Sorry for the error in the question initially. Yes, I meant to know the usefulness and significance of the Raman vs IR.

Dear Pierre Caulet has put it nicely.

But if we get to choose one between the two, which one it should be?

PS: The sample has a mixture of unknown molecules.

If you've got to choose and can have access to only one of the techniques, go for FTIR first. Its scope is still broader, has more spectral libraries for unknowns and it is more accessible than Raman, as a general rule (but there are exceptions, as with all rules...).

Dear Pierre Caulet and Jürgen Weippert ,

Thank you for your valuable answers. This discussion has really helped me to clear the uncertainty in deciding between the two techniques.

Thank you all.

Warm regards

Arvind Kaushik

Dear Arvind Kaushik , both FTIR and Raman are good characterization tools you can use to identify different molecules in a sample, but as stated by Pierre Caulet and Jürgen Weippert , the applicability of these techniques would depend on the kind of molecules you want to identify and also the possible negative effects of the sample´s matrix, which could make difficult the identification process. This last issue would be more relevant in the case of Raman, since the laser source used to exciting the sample could also excite luminiscence effects from some sample´s components which could cause a strong background that would make identification more complex. These effects can affect more the Raman signal because it is usually orders of magnitude weaker than the FTIR signal.

Bear on mind, however, that new Raman instruments and appropriate set-up designs or the use of Surface Enhanced Raman Scattering (SERS) sensors can overcome most of these problems.

Hope this helps. Good luck with your experiments.

Dear Manuel Gómez,

Thank you for adding important points to the discussion. Yes, the point of luminescence interfering with the Raman signal is crucial in deciding between the two techniques, which is nicely explained in your answer.

However, not to forget the sensitivity and application of SERS and other techniques in enhancing the Raman signal adds a lot more value to the Raman spectroscopy.

Thanking you.