Mass Spectrometry

2023/03/02 07:47 AM posted in  MS Notes   comments
Tags:  #MS Basics

Introduction to MS

For an unknown compound, the first thing for us is to determine what is the molecular weight of the compound.

We gave the C-H an electon beam, which will knock out a core electron out. Then we will create a molecular ion after we dislodged an electron.

One specie gets the positive charge and the other one gets the electron.

Let the charged ion pass a magnet area, then it will hit at a certain location on the detector.

The case dislodging the core electron

\[r = \frac{mv}{qB} \]

The case dislodging the bonding electron

We get a carboncation and the hydrogen with extra electron there.

We generate a fragment. But the fragments can be multiple species.


Molecular ion peak + fragment ion peak

The toppest peak is called base peak.

The base peak in the spectrum is usually due to the most stable fragment

Sample Question

Interpreting M+ Peaks in MS

Key points

  1. The relative intensities on the mass spectrum correspond to the relative abundances of isotopes
\[\begin{equation} \text { 2. \# of carbon atoms }=\frac{\text { relative intensity of } M+I \text { peak }}{.011 \times(\text { relative intensity of } M \text { peak) }} \end{equation} \]
  1. M+ peaks can indicate if a compound contains an atom(s) pther than carbon

The relatively abundance of C12 and C13 is 98.89% and 1.11%.

This formula works also to other atoms like Br, Cl


Sample problem: The spectrum of a corresponding molecule containing a halogen has the following data:

Relative Abundances

MS Fragmentation

Key Points

  1. Three general types:
    1. Heterolytic cleavage
    2. Homolytic cleavage(alpha)
    3. McLaferty Rearrangement
  2. Halogens, ethers and alcohols can cleave heterolytically and homolytically
  3. Alcohols can fragment by loss of water
  4. Ketones can fragment homolytically and via McLafferty Rearrangement

Heterolytic cleavage

Alcohol, halogon, ethers

Homolytic Cleavage(alpha cleavage)


MS Gamma Cleavage Sample Problem