Mass Spectrometry Homework: Relative Abundance of S-32 & S-34

[lkh1986]

Homework Statement

An organic compound contains some C and H atoms and 1 S atom. Element sulfur, S has 2 isotopes, S-32 ans S-34. Their relative isotopic abundance is 100:4 (or 25:1). On the mass spectrometry graph, how would the peak of M+2 compared to that of M+'s peak?

Homework Equations

The Attempt at a Solution

This is a question in Wade Organic Chemistry book. The answer given in the book is that the peak at M+2 is 4% of that of M+'s peak. However, my lecturer points out that the book is wrong. He says that the peak of M+2 should be 104% of that of M+'s peak. Who is correct?

I think the book is correct. Since the ratio of compound with S-32 and S-34 is 25:1. So I don't see any reason why we need to add 100 to that 4.

 

[chemisttree]

You are right. Are you sure you heard the lecturer correctly?

 

[Blueshift5]

it is important to always question and critically evaluate information presented to us. In this case, there seems to be a discrepancy between the answer given in the textbook and that of your lecturer. To determine who is correct, we need to understand the principles behind mass spectrometry and how it relates to the relative abundance of isotopes.

In mass spectrometry, ions are separated based on their mass-to-charge ratio (m/z). The peak at M+2 corresponds to the molecular ion (M+) with an additional two mass units, which indicates the presence of the heavier isotope, S-34. The peak at M+ corresponds to the molecular ion with only the lighter isotope, S-32.

In this case, the ratio of S-32 to S-34 is 25:1, which means that for every 25 molecules with S-32, there is only 1 molecule with S-34. This translates to a relative abundance of 96% for S-32 and 4% for S-34. Therefore, the peak at M+2 should be 4% of the peak at M+.

To further support this, we can use the equation for relative isotopic abundance, which is the ratio of the intensity of the peak for one isotope to the sum of the intensities of all peaks for that element. In this case, the relative isotopic abundance for S-32 is 25/26, which is approximately 96%, and for S-34 it is 1/26, which is approximately 4%. This further confirms that the peak at M+2 should be 4% of the peak at M+.

In conclusion, based on the principles of mass spectrometry and the given information, the book is correct and the peak at M+2 should be 4% of the peak at M+. It is important to always question and critically evaluate information, and in this case, the principles of mass spectrometry support the answer given in the book.