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Mass Spectrometry (MS)- Principle, Working, Instrumentation, Steps, Applications
- 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, sample is converted to rapidly moving positive ions by electron bombardment and charged particles are separated according to their masses.
- 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, m/e ratio is simply the molecular mass of the ion.
- The ions pass through magnetic and electric fields to reach detector where they are detected and signals are recorded to give a 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
- Sample stored in large reservoir from which molecules reaches ionization chamber at low pressure in steady stream by a pinhole called “Molecular leak”.
- 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 charge of +1.
- Ionization can be achieved by :
- Electron Ionization (EI-MS)
- Chemical Ionization (CI-MS)
- Desorption Technique (FAB)
- Ions are accelerated so that they all have same kinetic energy.
- Positive ions pass through 3 slits with voltage in decreasing order.
- Middle slit carries intermediate and finals at zero volts.
- Ions are deflected by a magnetic field due to difference in their masses.
- The lighter the mass, more they are deflected.
- It also depends upon the no. of +ve charge an ion is carrying; the more +ve charge, more it will be deflected.
- The beam of ions passing through the mass analyzer is detected by detector on the basis of m/e ratio.
- When an ion hit the metal box, charge is neutralized by an electron jumping from metal on to the ion.
- Types of analyzers:
- Magnetic sector mass analysers
- Double focussing analysers
- Quadrupole mass analysers
- Time of Flight analysers (TOF)
- Ion trap analyser
- 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 molecular mass of peptides, proteins, and oligonucleotides.
- Monitoring gases in patients breath during surgery.
- Identification of drugs abuse and metabolites of drugs of abuse in blood, urine, and saliva.
- Analyses of aerosol particles.
- Determination of pesticides residues in food
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