[DeanEdwards]

Question:
The DS Company used methyl salicylate in its wintergreen flavoured chewing gum. Quality control is done on the methyl salicylate by FTIR spectroscopy. Methyl salicylate has two close resonances at 1618 cm-1 and 1680 cm-1.

(a) What is the minimum distance that the moving mirror in the FTIR instrument must transit to resolve these two resonances?

(b) What is the minimum sampling rate required at this resolution for a scan from 700 to 4000 cm-1?

Attempts:

I am pulling a blank on this one. Any pointers as to how to tackle this question would be much appreciated.

[Polytriazole]

I think a good reference on how FTIR works will point you in the right direction.  I would recommend searching your library for a copy of Spectrometric Identification of Organic Compounds by Silverstein, Webster, and Kiemle.  It has a really nice chapter on FTIR, breaking it down into the theory behind the technique, how the instruments that use it work, and how to interpret FTIR data.  Pay attention especially to how the instruments work, you should be able to find the theory in there to get you started.

[DeanEdwards]

I think a good reference on how FTIR works will point you in the right direction.  I would recommend searching your library for a copy of Spectrometric Identification of Organic Compounds by Silverstein, Webster, and Kiemle.  It has a really nice chapter on FTIR, breaking it down into the theory behind the technique, how the instruments that use it work, and how to interpret FTIR data.  Pay attention especially to how the instruments work, you should be able to find the theory in there to get you started.

today in the lecture our professor explained to us how FTIR spec works. Correct me if I am wrong but you use the sin and cosine functions to map out the wavelength you are measuring and essentially, it is able to detect via mathematical treatment the peaks (which are not the noise, they are the signals).

I know that the more times you replicate the data, the lower the noise since the only thing amplified is your signal.

[marquis]

You have the basic idea.

The increase in signal to noise ratio is not linear.  It's the law of diminishing returns. You might think running a spectrum 100 times as compared to 1 time would give you a 100 times better signal to noise (or just a 100 times better spectrum).  It might be 10 times better, but not 100 times better.

At times, with extremely weak spectrum, we've tried going to the extreme of running 1000 or even 10000 spectrum.  Besides taking a very long time, it didn't really help much.  There was enough drift in the baseline signal to mess up the spectrum.