Introduction of mass spectrometer:
- In the technique of mass spectrometry, the compound under investigation is bombarded with a beam of electron which produces an ionic molecule or ionic fragments of the original species.
- Then, the separation of the resulting assortment of charged particles is done according to their masses.
- The spectrum produced, known as mass spectrum is a record of information regarding various masses produced and their relative abundances.
- Mass spectrum is an analytical technique which can provide information concerning the molecular structure of organic and inorganic compounds. It can be used to determine directly molecular weight as high as 4000.
- It is one of the few methods that can be used as a qualitative analytical tool to characterize different organic substances.
- With it, one can do analysis of mixture (gases, or liquids, and in some cases solids) quantitatively.
- A mass spectrometer is also useful to investigate reaction mixtures and in tracer work.
- It is also used in understanding kinetics and mechanisms of unimolecular decomposition reaction.
- Although the mass spectrometer is based on simple principle, yet it is a very complex and very expensive instrument.
- Nevertheless, it is becoming a common tool because it gives the largest amount of specific information about the substances to be analysed.
Principle of Mass Spectrometer:
- The mass spectrometer is an instrument which helps in separating the individual atoms or molecules because of the difference in their masses.
- Let us consider, a molecule M, which is bombarded with a beam of electrons.
- Suppose this is ionized as follows:
- M +e–à M++ 2e–
- Where M+ is an ionized molecule and e– is an electron. The ions are now accelerated in an electric field at voltage V.
- If this is the condition, the energy given to each particle is eV and this is equal to the kinetic energy which is equal to ½ mv2.
- This can be expressed as:
- ½ mv2=eV———————– equation (1)
- Or, v2= 2eV/m
- Or, v=
m—————-equation (2)
- Where v is the velocity of the particle of mass m, e is the charge on an electron and V is the accelerating voltage.
- The same energy eV is possessed by all the particles.
- Also, all particles have the same kinetic energy ½ mv2. As the value of m varies from the particle to particle, the velocity v also changes such that ½ mv2 remains a constant.
- For a particle of mass m1 and velocity v1, equation 1 becomes as:
- ½ m1v12=eV———————– equation (3)
- Similarly, for a particle of mass m2 and velocity v2, equation becomes as:
- ½ m2v22=eV———————– equation (4)
- And for particles m3, m4……. Of velocities v3, v4…..respectively, the equation (1) becomes as:
- ½ m3v32=eV———————– equation (5)
- ½ m4v42=eV———————– equation (6)
- ½ m5v52=eV———————– equation (7)
- From equation 3,4,5,6 and 7, we have
- ½ m1v12= ½ m2v22= ½ m3v32= …….. and so on….. equation (8)
- From relation (8), it shows that the velocity of different particles will vary, depending on the mass of the particles.
- After the charged particles have been accelerated by an applied voltage, they enter a magnetic field H.
- This field attracts the particles and they move in a circle around it.
- This attractive force, due to magnet is HeV, whereas the balancing centrifugal force of the particle is mv2/r.
- As the particle starts moving uniformly around the circular path, the two forces become equal, i.e.
- mv2/r= HeV
- Or, 1/r = HeV/mv2
- Or, r = mv/eH ————-equation (9)
- Where ‘r’, is the radius of the circular path of the particle
- On substitution equation (9) in (2), we get
- r= meH
- r= meH
- On squaring both sides,
- r2= m2/e2H2 2eV/m ————equation (10)
- Or, r =
2e) ————-equation (11)
- Equation (10) can be written as m/e= H2r2/ 2V———equation (12)
- From equation (11), it follows that the radius of the circular path of particles depends on the accelerating voltage V, the magnetic field H, and the ratio, m/e.
- As e, V and H are constant, it means that the radius of the ionized molecule depends on m, its mass.
- The relation between m, the radius of the circular path of the ion, V and H i.e., equation (11) is the basis of separation of particles according to their masses.
- Thus, the radius of the path may be changed by varying either the magnetic field (H) or the accelerating voltage (V).
- By, either method, ions of the different mass to charge ratio (m/e) can be made to impinge upon the collector in turn thus giving rise to a spectrum.
Instrumentation of mass spectrometer:
- The mass spectrometers used for the investigation of any compound may vary in their types but generally all contain the following components:
- The inlet system (or sample handling system):
- As a mass spectrometer should have a vapor sample and to ensure that the sample enters the ionization chamber at a constant rate, the sample is converted into the gaseous state in the inlet system.
- To achieve this, the system is usually heated.
- To handle different types of materials, different inlet systems are employed.
- In order to transfer gases, these involve merely transfer of the sample from a gas bulb into the metering reservoir and then expand into an expansion reservoir, having volume of three liters.
- In the metering reservoir, the pressure ranges from 30-50 torr and after expansion the pressure ranges from 10-3 to 10-1 torr in expansion reservoir.
- Liquid samples are handled by hypodermic needle injection through silicone rubber dam.
- As the reservoir has low pressure, it draws the liquid immediately and vaporizes it instantly.
- From the sample reservoir, the gaseous samples are leaked into the ionization chamber or ion source through a pinhole restriction of about 0.013 to 0.050mm in diameter in a gold foil.
- For MS, a sample size of about 1 µmole is required.
- The ion source or ionization chamber:
- From the inlet system, the sample is introduced into the ionization chamber where the beam of the electrons is put across the molecules of the samples.
- The molecules become ionized. The electron beam is accelerated by a high voltage up to 100V on the collector, and then the collision between electrons and molecules results in the production of well-defined fragments, carrying a definitive positive charge.
- The electrostatic accelerating system: The positive ions formed in the ionization chamber are withdrawn by the electric field.
- A strong electrostatic field of 400-4000V accelerates the ions of masses m1, m2, m3…to their final velocities.
- Whenever the mass spectrometer is started to record the spectrum, the second accelerator is charged to an initial potential of 4000volts.
- Then, this charge is permitted to leak off to ground at a controlled rate over a period of 25minutes.
- Magnetic field:
- As the accelerated particles from the electrical field enter the magnetic field, the force of the magnetic field requires of them to move in a curved path.
- The radius of this curvature, r , relies upon the mass m, the accelerating voltage V, the electron charge, e, and the strength of the magnetic field, H.
- It is the two properties m/e and r upon which mass spectrometry is based.
- The mass to charge ratio and the radius to the curvature are interdependent, whereas a change in either the accelerating potential or the magnetic field will change m/e and r.
- In the magnetic deflection, mass spectrometer the radius, r is fixed and all the particles of a single mass to charge ration m/e, are focused on the detector at any given applied potential.
- The ion separator (analyzer):
- This is that part of MS which separates ions according to their masses.
- An analyzer must possess the following characteristics:
- It should have a high resolution.
- It must have a high rate of transmission of ions.
- The ion collectors:
- The ion beam currents are of the order of 10-15 to 10-19 ampere.
- This small current has to be detected in mass spectrometers.
- Photographic plates, Faraday cylinders, electron multipliers and electrometers are generally employed.
- The readout display usually possesses a direct writing recording oscillograph which has 3-5 galvanometers with relative sensitivities of 1, 3, 10, 30 and 100.
- This method is cheap, easy to operate and reliable.
- Vacuum system:
- A high vacuum is to be maintained in the instrument.
- The inlet system is generally maintained at 0.015 torr, the ion source at 10-3 torr and analyzer tube at 10-7 torr or as low as possible.
- The oil diffusion and mercury diffusion and mercury diffusion pumps are commonly used in different types of combinations.
Applications of mass spectrometer:
- For Molecular mass determination
- The isotopic abundance of easily vaporizable elements can be determined
- Mass spectrometry is used to determine the amount of component of a complicated mixture from which it cannot be separated quantitatively. This determination is often made possible by the determination of isotope ratios.
- For Quantitative analysis of mixtures
- Mass spectrometry is successful in making distinction between cis- and trans- isomers.
- For Evaluation of heat of sublimation
- MS spectrometer can be used to determine the bond-dissociation energies of molecules by employing the concept of appearance potential of a fragment ion.
- MS is very frequently used in the kinetic and mechanistic reaction studies.
- MS can be used for the determination of latent heat of vaporization of liquid using the Clausius-Clapeyron equation which expresses the saturated vapor pressure of a liquid as a function of the absolute temperature.
- MS is one of the best methods to detect impurities.
- MS is used for characterization of polymers.