Mass Spectrometry

Question 1

What is the peak furthest to the right on a mass spectrum and what does it tell us?

Answer 1

Parent Peak or Molecular Ion (M+); it tells us the molecular weight of the molecule.

Question 2

What is the tallest peak on a mass spectrum and what does it tell us?

Answer 2

Base Peak; it tells us the most stable ion created by the fragmented molecule.

Question 3

What causes the molecule to ionize and fragment?

Answer 3

A beam of electrons

Question 4

What does m/z stand for?

Answer 4

Mass to charge ratio of an ion

Question 5

What determines the curvature of an ion’s path as it is passed through the varying magnetic field?

Answer 5

Officially, the m/z; however, since the majority of ions have a +1 charge, we can assume that the curvature of their paths are dependent only on mass.

Question 6

What is the ration of carbon 12 to carbon 13? What will this cause on a mass spectrum?

Answer 6

98.9 : 1.1. It will cause a small M+1 peak.

Question 7

What is the ratio of chlorine 35 to chlorine 37? What will this cause on the mass spectrum?

Answer 7

75.5 : 24.5. So molecules with chlorine will have an M+2 peak that is 1/3 as tall as the M peak.

Question 8

What is the ratio of bromine 79 to bromine 81? What will this cause on a mass spectrum?

Answer 8

50.5 : 49.5. So molecules with bromine will have an M=2 peak that is the same height as the M peak.

Question 9

How does a radical cation typically fragment in a mass spectrometer?

Answer 9

Fragments to give the most stable cation and a radical.

Question 10

Where do alkanes most often fragment? Why?

Answer 10

At branched points, because this creates inductively stabilized secondary and tertiary cations.

Question 11

What does a peak at M-15 usually symbolize?

Answer 11

Loss of a methyl radical.

Question 12

What should you suspect if you see a molecule that has an even molecular weight fragment and an even MW?

Answer 12

Loss of a neutral molecule

Question 13

What should you suspect if you have a spectrum with a peak at M+2 that is 1/3 the height of M and peaks at M-36 and M-38?

Answer 13

A molecule containing Cl and loss of HCl

Question 14

What sort of molecule do you have if you observe an M-15 peak and peaks at increments of additional losses of 14?

Answer 14

Straight chain alkane

Question 15

What is the fragmentation mechanism for an alkene?

Answer 15

Radical mechanism (single-hooked arrows).

Question 16

What do alkenes tend to fragment to yield?

Answer 16

An allylic cation (very stable, should be the base peak).

Question 17

What does a peak for a tropylium ion indicate?

Answer 17

A benzyl group

Question 18

What does a peak for a phenyl cation indicate?

Answer 18

A benzene ring

Question 19

What is the m/z for a tropylium ion?

Answer 19

91

Question 20

What is the m/z for a phenyl cation?

Answer 20

77

Question 21

For what functional groups will cleavage be observed adjacent to the alpha carbon?

Answer 21

Ethers, alcohols, and amines

Question 22

Where will cleavage be observed if there are two alpha carbons on a hetero atom?

Answer 22

Adjacent to both carbons

Question 23

What is diagnostic of an alcohol?

Answer 23

Loss of H20 (M-18)

Question 24

What is diagnostic of a compound containing nitrogen?

Answer 24

Odd MW (parent peak) and even molecular weight fragments.

Question 25

What very stable fragment does alpha cleavage of a carbonyl compound result in?

Answer 25

An acyllium ion

Question 26

What will you see in a compound that contains only C, H, and O?

Answer 26

MW is even and fragments are odd

Question 27

In what molecules can you see a McLafferty Rearrangement?

Answer 27

Carbonyl compounds (acids, ketones, aldehydes, and esters) with one or two three-carbon chains that have a H on the gamma carbon.

Question 28

What does the McLafferty Rearrangement represent?

Answer 28

Formation of a new radical cation and loss of a neutral molecule

Question 29

Why is alpha cleavage so stable?

Answer 29

It is resonance stabilized by the lone pair on the adjacent oxygen or nitrogen.