Forensic Physics
Mass Spectrometer: Principle, Working & Applications

Mass Spectrometer: Principle, Working & Applications

Mass spectrometry(MS) is a quantitative analytical technique used to determine the components present in a compound with their structure and chemical properties. It is an instrumental method that uses the mass-to-charge ratio of a substance for analysis. The unique feature of the technique is that it can elucidate the molecular mass of a compound, hence the name “mass spectrometry”.

The modern mass spectrometers were devised by Arthur Jeffry Dempster(1918) and F.W. Aston(1919).

Basic Principle of Mass Spectrometer

The principle of Mass Spectrometry states that, “The ions with different mass-to-charge ratios are deflected by different angles in an electric or magnetic field.”

When a beam of electrons is bombarded in the analyte compound, that leads to the removal of one electron from the analyte. Due to the removal of electrons, the molecule becomes positively charged which is known as a molecular ion. Molecular ions then split into fragmented ions which are detected by the spectrometer

The fragmented ions are separated in space or time, based on their mass-to-charge ratio, and are quantified. All the ions are accelerated across the same distance by the same force, therefore they have the same kinetic energy, determined by the acceleration voltage of the instrument and the charge of the ion. This is given by:

KE = zV = ½ mv²
Where,
KE = kinetic energy of the ions
z = charge to mass ratio
V = voltage of the instrument
m = mass of ions
v = velocity of ions

As charged ions move through the magnetic field, they are bent in an arc to follow a circular path of a certain radius in the direction perpendicular to the applied magnetic field.

Hzv = mv²/r
Where,
H = magnetic field
r = radius of the path
mv²/r = centripetal force

The magnetic force on the ions is balanced by the centripetal force.

The ions of a certain mass-to-charge ratio value have a unique path radius which can be determined if both, H and V are held constant.

All ions having the same mass-to-charge ratio value are deflected to the same degree and follow the same trajectory.

This deflection produces a mass spectrum of the analyte in the form of a plot of ion abundance versus mass-to-charge ratio. Ions provide information concerning the nature and structure of their precursor molecule. 

Instrumentation of Mass Spectrometer

Basic Instrumentation of Mass Spectrometrty

The instrumentation of Mass SpectrometryA Detailed Note on Electrocution is composed of:

  • Sample inlet
  • High vacuum system
  • Ionization source
  • Mass analyzer
  • Ion detector
  • Amplifier and detector

Sample Inlet

Since the Mass Spectrometry instrument only detects the ions so the analyte should be in gaseous form. But not all the analytes are in a gaseous state (solid and liquid samples) so they are first converted into gaseous form and then injected into the instrument.

Sample inlet commences the introduction of the analyte into the instrument. The most common method used is a direct vapor inlet, in which a high vapor-pressure gas phase sample is introduced directly into the source region through a needle valve. The sample can also be introduced to the ionization source through a chromatographic system such as coupled HPLC or GC.

High Vacuum System

A vacuum system is required to maintain the smooth flow of the ions throughout the instrument. The ionization source, mass analyzer, and ion detector are enclosed in the vacuum system. The high vacuum also reduces the collision of ions with other molecules to prevent scattering and fragmentation. 

Ionization Source

Mass Spectrometry is significantly dependent upon the ionization method. The sample molecules are excited so that they eject an electron to form a radical cation(M⁺) or are forced to undergo ion-molecule reactions to produce adduct ions(MH⁺) mostly by the addition of a proton(H⁺) to the molecule(M⁺).

M + H⁺ = MH⁺

The amount of fragmentation is controlled by substantial ionization energy observed in the mass spectrum. Some ionization methods are easy and only produce molecular ions. While some are hard ionization techniques that are very active and cause ions to undergo extensive fragmentation. 

Variation in the spectrum is introduced in terms of the number and intensity of peaks. 

The ionization can be categorized as soft ionization and hard ionization. Soft ionization exhibits low energy that decreases fragmentation. On the other hand, hard ionization possesses high energy which leads to increased fragmentation.

There are various kinds of ionization methods that can be carried out and are classified as:

  • Gas phase ionization
  • Desorption ionization
  • Evaporation ionization

Mass Analyzer

Once the electrons are ionized, they are sorted by the mass analyzer according to their mass-to-charge ratio. They can be continuous or pulsed mass analyzers. Continuous analyzers consist of specific quadrupole filters and magnetic sectors which work on single ion monitoring that improves the signal-to-noise ratio.

The pulsed analyzers consist of time-of-flight, ion cyclotron resonance, and quadrupole ion trap mass spectrometers which collect an entire spectrum from a single pulse of ions. And this results in high transmission efficiency which increases the signal-to-noise ratio.

Ion Detector

The sorted ions from the analyzer strike the detector which amplifies the signal and transfers it to the recorder. The detector detects the mass-to-charge ratio of the ions and records the relative abundance of each of the resolved ionic species. The detectors used are:

  • Photomultiplier tubes
  • Electron multiplier tube  
  • Micro-channel plate detectors  
  • Faraday cup

Recorder

The recorder is a computer system that records the signal in the form of a graph called the mass spectrum which is plotted between the mass-to-charge ratio and ion signals which may represent the molecular mass, number of components, and their relative abundance in the compound.

Advantages of Mass Spectrometer

  • It is a qualitative as well as quantitative analytical method
  • The purity of a compound can be determined easily
  • Highly sensitive, even can detect a trace of parts per million

Disadvantages of Mass Spectrometer

  • The instrument is expensive and needs high maintenance
  • It cannot differentiate between the optical and geometric isomers
  • Hydrocarbon-fragmented ions cannot be distinguished

Applications of Mass Spectrometer

Mass Spectrometry has a wide application in Physical sciences, Radiochemistry, Geochemistry, medicine, biochemistry, and material sciences.

Now it has made its place in the forensic sciences also. It is a widely used analytical method in toxicological analysis (metallic or non-metallic poisons), explosives analysis (constituents of the explosive), trace evidence analysis (fibers, paint, glass, etc.), arson evidence analysis (an accelerant), etc.

Conclusion

Mass Spectrometry is a qualitative and quantitative analytical method used in various scientific disciplines. It elucidated the structures and chemical properties of a substance based on its mass-to-charge ratio which is a unique feature of every substance.

Therefore it is a highly recommended instrumental technique in forensics. In fact, it can be coupled with other analytical techniques such as HPLC or Gas Chromatography to give quantitative results.

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