Mass Spectrometry: Mass analyzer and Detection

 After the ionization in ionization chamber, the ions get accelerated due to electric field and reaches to the mass analyzer where they get separated on the basis of their m/z (mass to charge) ratio.

There are basically 4 types of mass analyzers as mentioned below:

• Magnetic sector mass analyzer
• Double focusing mass analyzer
• Quadruple mass analyzer
• Time of four (TOF) mass analyzer

In magnetic sector mass analyzer, the ions formed in earlier step are passed between the magnet poles.This cause the ions to ascribe a curved path which depends upon their m/z ratio. 

Below equation represents the effect of m/z ratio on the radius of curvature.

 

 Where , r = radius of the curvature of ions

                V= potential difference of ion accelerating plates

                B= magnetic field

The more is the mass to charge ratio of an ion, the more will be the radius of curvature. Generally, the varying magnetic field is used to detect the all ions.

Double focusing mass analyzer contains electrostatic analyzer in addition to simple magnetic mass analyzer. This balances the velocity of all of ions to the same level and hence increases the resolution. TOF analyzers are best suited for the MALDI technique ionized biomolecules.

Detection of the ions

During the process of ionization, a lot of ions remains unionized or fragments into smaller fragments. Also in mass analyzer, the large number of ions strike to the wall of analyzer tube and not able to reach to the detector. Only a few of several hundred are able to reach to the detector. So it is very important to detect the ions reaching to the detector.

Multiplier circuits are very important in this regard as even a single ion striking to it can be detected by it. The surface of electron multiplier is basically a lead doped glass coated with lead oxide. Around 2kV potential difference is applied across the two ends of the detector. Striking of an ion to the surface to multiplier ejects two electrons from it. The electrons move further due to the potential difference.

As the electrons move further, they further strikes to the surface and each electron ejects two electrons. In this way the electrons get multiplied and an electric current is analyzed and recorded. Use of a curved electron multiplier can result in result with less noise as the path of ions shortens.

The output of detector is finally fed to the computer interface in modern instruments.

Mass Spectrum

 

Mass spectrum is plot between the m/z ratio of an ion (on x-axis) and its relative abundance( on y-axis) which is obtained after the mass analysis of a chemical compound in the mass spectrometer. Generally, all compounds have different mass spectrum. It can be used for the structure elucidation and identification of compounds.

Electron- Ionization Mass spectrum of toluene is given below.

 

Molecular ion peak and base peaks are the two important things to consider while studying a mass spectra. The basic information related to mass spectra is covered here.

During the ionization step in mass spectrometer, the molecules get converted to ions (cations) due to the loss of one electron on being exposed to high energy electron beam as in case of electric ionization. The ions so formed can further fragments into smaller ions due to the high energy beam or the resulting ion being unstable if any.

The resulted after acceleration and passing through mass analyzer, finally reaches to the detector after which the mass spectrum is formed. As in above mass spectrum of toluene, the peaks corresponds to the various ions reached to the detector and their intensity represents their amount which reached to the detector.

Base peak in any typical mass spectrum is the tallest peak. It corresponds to the ion present in abundance among all. The peak at m/z ratio 91 is the base peak in the given figure.

Molecular ion (usually represented by M+) is the ion formed when one electron gets ejected from it. The peak corresponding to molecular ion is termed as molecular ion peak. Radical cation is he another name of molecular ion as after the removal of one electron from molecule, the cation formed possesses unpaired electron.

Generally, molecular ion peak corresponds to the largest mass except in case isotopes of higher mass are present in sample.

Metastable ion peak is the peak generated in the mass spectrum due to the ion fragmented in the mass analyzer. These peaks are generally broad.