List of Different Types of Mass Spectometric Techniques

Mass spectrometry (MS) is a method for determining the mass-to-charge ratio of charged molecules. The findings are shown as a mass spectrum, which is a plot of intensity vs mass-to-charge ratio. Mass spectrometry is utilised in various disciplines and may be used for both pure samples and complicated mixtures.

A mass spectrum is a plot of an ion signal in terms of the mass-to-charge ratio. These spectra are used to identify a sample’s elemental or isotopic signature, particle and molecule masses, and the chemical identification or composition of macromolecules and other chemical substances.

The mass-to-charge ratio is a parameter that mass spectrometers use to distinguish molecules. The material to be recognised is ionised first, then passed through a magnetic field. A mass for the ion may be determined based on criteria such as how long it would take the molecule to traverse a specific distance or the degree of deflection generated by the field. There are several approaches for ionising and identifying chemicals, as will be detailed later.

MS is often limited by substances that are not readily ionizable or do not break down following ionisation. Geometric isomers are usually easy to discern, while chirality discrepancies are difficult to resolve. Samples that are difficult to dissolve in standard solvents might also cause complications.

A sample, which might be solid, liquid, or gas, is ionised in a conventional MS method, for example, by blasting it with an electron beam. Some of the molecules in the sample may break up into charged particles due to this, or they may just become positive ions without disintegrating. These particles (fragments) are then sorted based on their mass-to-charge proportion, for example, by propelling them and exposing them to an electrostatic field: ions with the same mass-to-charge proportion will deflect in the same amount. A machine capable of detecting positive ions, as in an electron multiplier, detects the ions. There are different types of mass spectrometry detectors. The results are shown as spectra of detected ion signal strength as a function of the mass-to-charge ratio. The atoms and molecules in the specimen may be recognised by comparing known masses (e.g., a whole molecule) to the determined masses or by observing a distinctive fragmentation pattern.

Mass Spectrometry Techniques

A mass spectrometer produces various ions from the material under examination, separates them based on their unique mass-to-charge ratio (m/z), and records the quantity of each ion type.

The initial stage in the mass spectrometric examination of substances is the generation of gas-phase ions of the chemical, which is accomplished mainly by electron ionisation. This molecular ion becomes fragmented. Each primary ion formed from the molecular ion is fragmented in turn, and so on. In the mass spectrometer, ions are separated based on their mass-to-charge ratio and detected in proportion to their quantity. As a result, the molecule’s mass spectrum is generated. It shows the outcome as a plot of ion frequency vs mass-to-charge ratio. Ions provide information about the type and structure of their progenitor molecule. The molecule ion, if existent, shows at the greatest value of m/z in the spectrum of a pure molecule (followed by ions carrying heavier isotopes) and provides the molecular mass of the chemical.

There are various types of mass spectrometry, which are listed down below.

Electron Impact Ionisation (EI)

Electron Impact Ionization (EI) – EI is performed by immediately volatilizing a sample in the source, which is housed in a vacuum system directly connected to the analyzer. A stream of electrons created by warming a filament’s biases at a low polarity relative to the source bombards the gas-phase molecules. The charge is typically set at -70 volts. The beam of electrons dislodges ions from the gaseous phase molecule, resulting in the formation of a radical ion. Since it leads the ions to fragment, this process is classified as hard ionisation. EI is by far the most often used technique for GC-MS.

Fast Atom Bombardment (FAB)

FAB is a technology that was prominent in the 1980s and early 1990s, as it was the first approach that permitted simple ionisation of non-volatile chemicals. It was accomplished by blasting a specimen in a condition with a stream of particles accelerated to Kilovolt energy, generally Ar or Xe. Typically, the sample was combined in a matrix. Glycerol and 3 Nitro-benzoic acids were the two most frequent matrixes. The sample was able to refresh itself thanks to the matrix. FAB ions were added to the molecules, which may be ions, sodium ions, potassium ions, or ammonium ions. FAB was modified by replacing the atom beam with an ion beam, commonly caesium ions, which was known as secondary ion mass spectrometry (SIMS). SIMS spectra were often similar to FAB spectra, and the words were used interchangeably.

Electrospray Ionisation (ESI)

Electrospray ionisation (ESI) is a method for producing ions for spectrometry by delivering a high volt to a liquid to create an aerosol. Because biomacromolecules are very fragile, their structures are quickly damaged during the dissociation and ionisation processes. ESI overcomes these compounds’ tendency to fragment upon ionisation. ESI differs from conventional atmospheric pressure ionisation techniques in that it may generate multiple charge ions, substantially increasing the mass range of the analyzer to fit the magnitude of kDa-mDa reported by proteins and their related peptides.

ESI puts a high voltage to the capillary’s exit, and the resulting strong electric field annihilates the fluid flowing out from the capillary into small charged droplets. As the liquid passes, the intensity of the charge on the droplet’s surface steadily rises, and the droplet eventually divides into one or more charged ions, enabling the sample to reach the gas phase as a single charge or many charges and be a gas phase ion.

Other mass spectrometric techniques include Atmospheric Pressure Chemical Ionisation (APCI), a technique that uses a similar source to ESI. Rather than placing a volt on the spraying directly, the volt is placed on a tip that causes a pressure drop at elevated pressure. This discharge generates ions, primarily H3O+ or water groups and Matrix-Assisted Laser Desorption Ionisation (MALDI).

Conclusion

Different ionisation procedures may be required depending on the information sought from mass spectrometry analysis. For a complicated molecule, for example, a hard ionisation approach such as electron impact may well be employed to identify the component components via fragmentation. A high-molecular-weight polymer or protein sample, on the other hand, may need an ionisation process such as MALDI to be volatilized. Frequently, samples may be readily examined using numerous ionisation techniques, and the decision is reduced to selecting the most convenient approach. Electrospray ionisation, for example, is conveniently connected to liquid chromatography technology since no further sample preparation is necessary.