TBR: Mass Spec

[justadream]

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TBR Ochem Book II page 266 Example 8.25

Which implies CHLORINE presence?

Choice C is the answer.

I get that choice C has peaks in a 3:1 ratio (apparently indicating Cl presence).



But doesn’t that 3:1 ratio apply to the M and M+2 peaks? TBR talks about the 3:1 ratio between 131/133 and 132/134.

So even if only 132/134 had the ratio, you'd still conclude cholrine?

 

[JMMTB]

So the molecular mass of the molecule would be 131 (tallest peak in group of heaviest signals). A normal M+1 peak is due to C13 and then the M+2 peak is due to Cl37. An M+3 peak is thus due to Cl37 and C13 in the same molecule.

131 is the molecule with the most common isotopes. 133 is the molecule with a Cl37 isotope.
132 would be the molecule with C13 and Cl35 isotopes. 134 would be C13 and Cl37 isotopes.
135 would probably be due to 2 C13 in the molecule along with a Cl37 isotope.

The reason why Cl gives a 3:1 ratio is due to the isotope abundance between Cl35 and Cl37. The same thing is seen with Br except 1:1 because the two isotopes (79 and 81) are in nearly equal proportions.