The magnetic sector mass analysis: The kinetic energy of an accelerated ion imparted by the voltage V is:

Where m is the mass of the ion, v is its velocity, and z is the charge. When the ions pass through the two poles of a magnet, the charged particles take a curve path. The radius of curvature (r) of this path is:

Where B is the magnetic field strength. Combining the above two equations:

Greater the value of m/z value larger is the radius of the curved path. The analyzer tube has a fixed radius of curvature, and the magnetic field strength is varied such that all the ions reach the detector.

The mass spectrometer consist of five important components:

a. Sample inlet: Here, the sample under study is introduced into the chamber, and the pressure inside the sample inlet is much lower than the outside pressure. A stream of molecules is produced here.

b. Ion Source: The stream of molecules undergoes ionization by various methods. Different compounds are ionized by different methods. This includes: Electron Ionisation(EI), Chemical Ionisation (CI), Secondary ion mass spectrometry (SIMS), Fast Atom Bombardment (FAB), and Matrix Desorption Ionisation (MALDI)

c. Mass Analyser: Separates the compound based on their mass to charge ratio (m/z).

d. Detector: The ions that are deflected under the magnetic field are detected by the detector.

e. Output: The signal is then recorded and processed by the data system. The output is the mass spectrum.

Picture source:

Spectroscopic Analysis

Electron Ionisation method:

In all the mass spectrometers, regardless of the method used to determine the mass-to-charge ratio of a molecule/compound, first, the sample has to enter the mass spectrometer, then converted to charge particle by ion source before there are deflected by the magnetic field and detected. In EI-MS, a beam of high-energy electrons strikes the incoming molecules. The electron and the molecules collide, and the molecules form cations. A plate that carries a positive electrical potential directs the newly created ions towards a series of acceleration plates. A large potential difference (1 to 10 kV) is applied across these plates to produce a beam of positive ions. There is a repeller plate that absorbs any negative ions. Molecules that are not ionized are drawn off by the vacuum. Most of the molecules or atoms require 8 to 15 electron volts to get ionized. But usually, the beam of electrons strikes the molecules at 50 to 70 eV. For recording, simple proton NMR spectra deuterated solvents, especially for the use of NMR, are used.

Salient features of the EI method:

1. Basic and well-understood ionization method

2. Ideal for all volatile compounds with masses less than 1000.

3. Leads to molecular ion fragmentation; fragmentation provides structural information.

4. Mass spectra are reproducible.