Mass Spectrometry (MS) - Principle, Protocol, Applications − BiologyGate.com

Mass spectrometry (MS) is a widely used instrumental technique, with the first such instrument, known as a parabola spectrograph, being reported in 1912. Since then, numerous advances and improvements in MS have made this technique a mainstay, first in physics laboratories and now in analytical chemistry and forensic science laboratories.

MS is based on ionization and fragmentation of sample molecules in the gas phase. Since molecules fragment in a unique manner, the resulting ion fragmentation pattern can be used to obtain structural information for a given molecule. In forensic science, MS has become the technique of choice for the definitive identification of a wide variety of evidence including controlled substances and fire debris. In these cases, the mass spectrometer is normally coupled with either a gas chromatography (GC) or liquid chromatography (LC) system and is used as the detector; that is, separation of the sample is achieved via chromatography and the separated compounds enter the mass spectrometer sequentially for ionization, separation, and detection of the generated ions. Coupling the chromatography system to the mass spectrometer in this way yields two pieces of information for each analysis: retention time and mass spectral information for each separated compound, both of which can be used for comparison with suitable reference standards.

Mass spectrometry (MS) is a mainstream chemical analysis technique in the twenty-first century. It has contributed to numerous discoveries in chemistry, physics and biochemistry. Hundreds of research laboratories scattered all over the world use MS every day to investigate fundamental phenomena on the molecular level. MS is also widely used by industry—especially in drug discovery, quality control and food safety protocols. In some cases, mass spectrometers are indispensable and irreplaceable by any other metrological tools. The uniqueness of MS is due to the fact that it enables direct identification of molecules based on the mass-to-charge ratios as well as fragmentation patterns. Thus, for several decades now, MS has been used in qualitative chemical analysis. To address the pressing need for quantitative molecular measurements, a number of laboratories focused on technological and methodological improvements that could render MS a fully quantitative metrological platform. In this theme issue, the experts working for some of those laboratories share their knowledge and enthusiasm about quantitative MS. I hope this theme issue will benefit readers, and foster fundamental and applied research based on quantitative MS measurements.

Mass Spectrometry (MS) is an analytical chemistry technique that helps identify the amount and type of chemicals present in a sample by measuring the mass-to-charge ratio and abundance of gas-phase ions. In this instrumental technique, the sample is converted to rapidly moving positive ions by electron bombardment and charged particles are separated according to their masses. A mass spectrum is a plot of relative abundance against the ratio of mass/charge (m/e). These spectra are used to determine the elemental or isotopic signature of a sample, the masses of particles and of molecules, and to elucidate the chemical structures of molecules and other chemical compounds.

Principle of Mass Spectrometry (MS)

In this technique, molecules are bombarded with a beam of energetic electrons.

The molecules are ionized and broken up into many fragments, some of which are positive ions. Each kind of ion has a particular ratio of mass to charge, i.e. m/e ratio (value).

For most ions, the charge is one, and thus, the m/e ratio is simply the molecular mass of the ion.

The ions pass through magnetic and electric fields to reach the detector where they are detected and signals are recorded to give mass spectra.

Working of Mass Spectrometry (MS)

In a typical procedure, a sample, which may be solid, liquid, or gas, is ionized, for example by bombarding it with electrons.

This may cause some of the sample’s molecules to break into charged fragments. These ions are then separated according to their mass-to-charge ratio, typically by accelerating them and subjecting them to an electric or magnetic field:

Ions of the same mass-to-charge ratio will undergo the same amount of deflection.

The ions are detected by a mechanism capable of detecting charged particles, such as an electron multiplier. Results are displayed as spectra of the relative abundance of detected ions as a function of the mass-to-charge ratio.

The atoms or molecules in the sample can be identified by correlating known masses (e.g. an entire molecule) to the identified masses or through a characteristic fragmentation pattern.

Instrumentation and Steps of Mass Spectrometry (MS)

A. Sample Inlet

A sample stored in the large reservoir from which molecules reach the ionization chamber at low pressure in a steady stream by a pinhole called “Molecular leak”.

B. Ionization

Atoms are ionized by knocking one or more electrons off to give positive ions by bombardment with a stream of electrons. Most of the positive ions formed will carry a charge of +1.

Ionization can be achieved by :

Electron Ionization (EI-MS)
Chemical Ionization (CI-MS)
Desorption Technique (FAB)

C. Acceleration

Ions are accelerated so that they all have the same kinetic energy.

Positive ions pass through 3 slits with voltage in decreasing order.

Middle slit carries intermediate and finals at zero volts.

D. Deflection

Ions are deflected by a magnetic field due to differences in their masses.

The lighter the mass, the more they are deflected.

It also depends upon the no. of +ve charge an ion is carrying; the more +ve charge, the more it will be deflected.

E. Detection

The beam of ions passing through the mass analyzer is detected by a detector on the basis of the m/e ratio.

When an ion hits the metal box, the charge is neutralized by an electron jumping from the metal onto the ion.

Types of analyzers:

Magnetic sector mass analyzers
Double focussing analyzers
Quadrupole mass analysers
Time of Flight analyzers (TOF)
Ion trap analyzer
Ion cyclotron analyser

Applications of Mass Spectrometry (MS)

Environmental monitoring and analysis (soil, water, and air pollutants, water quality, etc.)

Geochemistry – age determination, soil, and rock composition, oil and gas surveying

Chemical and Petrochemical industry – Quality control

Identify structures of biomolecules, such as carbohydrates, nucleic acids

Sequence biopolymers such as proteins and oligosaccharides

Determination of the molecular mass of peptides, proteins, and oligonucleotides.

Monitoring gases in patients’ breath during surgery.

Identification of drug abuse and metabolites of drugs of abuse in blood, urine, and saliva.

Analyses of aerosol particles.

Determination of pesticides residues in food.

References

https://www.thermofisher.com/au/en/home/life-science/protein-biology/protein-biology-learning-center/protein-biology-resource-library/pierce-protein-methods/overview-mass-spectrometry.html

https://www.slideshare.net/akshukumarsharma/mass-spectroscopy 55382941

http://www.chem.ucalgary.ca/courses/350/Carey5th/Ch13/ch13-ms.html

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