I am designing a mass spectrometer and the last task asks to figure out a certai

Question

I am designing a mass spectrometer and the last task asks to figure out a certain compound based on the ions the spectrometer has found. It gives an ion ratio of 108:218:432:540 for m/z ratio of 16:14:12:1. I know the compound is supposed to be caffiene which has a formula of c8h10n4o2. Dividing the number of ions by 54 gives me the appropriate chemical formula, but I do not know the reasoning behind why I would divide by 54. I was wondering if someone could explain to me, based on the number of ions detected, how I would arrive to the chemical formula for caffeine?

Explanation / Answer

Dividing the number of ions by 54 gives me the appropriate chemical formula:

The reason is Subtract the total mass of lowest mass isotopes of carbon, hydrogen, and halogens from M m/z to get a leftover m/z number.So It can be seen that molecular ions which are indicative of caffeine are m/z 54, 67, 82, 109, 137 and 165. Subtraction spectra are very useful in this type of analysis.So the least number is 54.

how I would arrive to the chemical formula for caffeine?
             If it is divisible by 16, then all of the leftovers are oxygen. If it is divisible by 14, then all of the

leftovers are nitrogen. If it is divisible by neither 14 nor 16, then it has both nitrogen and oxygen. the mass spectrum and it gives you a molecular weight

of 194. Dividing 194 by 13 gives you 14.9231.

Multiplying 0.9231 by 13 gives you 12. Therefore, th

e molecular formula of the alkane is

C14H26. Caffeine has four nitrogen atoms and two oxygen atoms which equals 88.

Subtract 6 carbons and 16 hydrogens to get 88. Therefore, the molecular formula for

caffeine is C8H10N4O2

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