Mass Spectrometry: Basics

Mass Spectrometry

Spectrometry = use of no radiations.

Compound under investigation is bombarded with a beam of electrons to produce the ionic fragments or an ionic molecule of the original species.

Resulting charged particles are then separated according to their masses.

The spectrum produced shows information abound various masses produced and their relative abundance.

PRINCIPLE OF MASS SPECTROMETRY

This technique is based on the principle of separation of individual atom or molecule according to difference in their masses.

When molecule M is bombarded with a beam of electrons,

M + e^- à M⁺ + 2e^-

Resulting ionized molecule M⁺ is then accelerated in an electric field at voltage V. In this condition, the energy given to the particle is eV and this is equal to the kinetic energy (1/2mv²).

When there are multiple particles, all the particles will possess the same energy eV and also the same kinetic energy.

As the value of ‘m’ in kinetics equation that is mass is different for different molecules/species, the value ‘v’ that is velocity of particle changes so that total value of equation, 1/2mv² = eV remains constant for every species.

Therefore, velocity of different particles will vary depending on the mass of the particle.

After the charged particles are accelerated by applying voltage, they enter in magnetic field ‘H’. this field attracts the particles and they move in a circle around it.

Attractive force (Magnetic energy) = Centrifugal force

                                       HeV = mv²/r

                                        m/e = H²r²/2V

where, V is an accelerating voltage, H is magnetic field, m is mass, r is radius, m/e ratio is known as mass to charge ratio.

INSTRUMENTATION

INLET SYSTEM (SAMPLE HANDLING SYSTEM)

- Mass spectrometer needs vaporized sample. So every sample is converted into gaseous state to ensure constant injection rate.

- This system is usually heated.

- Gaseous sample is added in the metering reservoir and then carried to expansion chamber. (Initial pressure= 30 to 50 torr, After expansion= 10^-3 to 10^-1 torr).

- Liquids are handled by hypodermic needle.

- Sample size of about 1 μmol is required. (only about 0.1% ionizes and tested).

ION SOURCE (IONISATION CHAMBER)

An electrically heated filament produces thermal electrons which are accelerated by an anode. The stream of molecules collides with this beam of electrons to produce ionized species. These are then withdrawn by an electrical field and are accelerated.

- Knudsen cell: used for thermodynamic studies and for the analysis of solids and low vapor pressure liquids.

- Surface ionization: Solid sample is coated on a ribbon filament. That filament is heated to produce ionized species of coated material. This technique is useful for inorganic compounds with low ionization potential.

- Spark source ionization: the inorganic sample to be analyzed is formed into electrodes and placed on movable wires. After application of potential, positively charged ions are discharged.

- Chemical ionization: Reaction gas like methane is introduced in the system along with the sample to be analysed. This reaction gas produces ions which react with neutral molecules to produce products. These chemically reactive products then interact with sample molecules to produce positively charged ions.

Mass spectrometer Schematics 丨Source- Wikimedia Commons

ELECTROSTATIC ACCELERATING SYSTEM

Positive ions produced in the ionization chamber are accelerated using a strong electric field (400-4000V) to reach their final velocities.

MAGNETIC FIELD

The accelerated ions enter into magnetic field from electric field and it requires them to move in a curved path. Mass to charge ratio (m/e) and the radius of curvature (r ) are interdependent. That ratio is focused on detector. Particles with greater or lesser m/e ratio than the focused one get bombarded on the walls of curvature and are neutralized.

ION SEPARATOR (ANALYZER)

It separates the ions according to their masses.

Single focusing magnetic deflection: this is the most common type of analyser which collects the ions travelling in particular radius.

Double focusing: here ion currents (transmission) are slow and resolution is very high. This double beam causes electrostatic field to select ions with particular velocity or kinetic energy. Precise molecular weight is detected with double focusing.

Standard used: perfluorokerosene (as a mass marker)

Resolving power: 30000

Cycloid focusing: Ions employ cycloid path.

Quadrupole mass spectrometer: Initially used for separation of uranium isotopes.

After acceleration of ions from ion source, these are focused by quadrupole mass filter and are separated be masses and detected by electron multiplier.

Structure: four parallel quadrant circular/hyperbolic tungsten rods.

Method of focusing: by oscillation and variable radiofrequency field.

Time of flight system: Ions are allowed to travel in a straight line through a magnetic field free region. The ions take different time to travel at a specific distance. Here, ions are produced in pulses (0.25 μ sec, frequency 10000 times/ sec).

ION COLLECTOR (DETECTOR AND READOUT SYSTEM)

Ion beam currents are of 10^-15 to 10^-19 ampere.

Ion current below this range: ion multiplier is required (Be-Cu e multiplier).

VACUUM SYSTEM

Oil diffusion pumps, mercury diffusion pumps.

TYPES OF IONS

Molecular ion/ parent peak:

Formed by loss of a single electron (at 9 to 15 eV energy).

Aromatics> conjugated olefins > alicyclics > sulphates > unbranched > hydrocarbons> ketones> amines> esters> ethers> carboxylic acids> branched hydrocarbons and alcohols. (height of parent peak)

Base peak:

Formed by loss of a single electron (at energy 70 eV) to form a molecular ion which splits further into many fragments.

Dissociation process:

Formation of molecular ions retains excess energy in the system. That energy further causes fragmentation or cleavage of molecular ions with or without rearrangements. Branched carbons and high molecular weight species undergo cleavage easily.