Basic components of the mass spectrograph
Mass spectrograph has the following basic components
(1) Ion source ; (2) Filter and focusing chamber ; (3) Recording chamber
(1) Ion source
Singly or multiple ionized ions of those elements whose atomic mass is to be determined, are produced in the chamber having a very small hole in it for an exit of ions. The method of producing ions depends, in general on the element of a compound selected for automatic mass determination, Such as
(a) Alkali metals such as Li, Na, K, Cs whose ions can be produced easily by heating their minerals.
(b) To produce ions of the elements whose melting points are very high, such as gold, silver, copper, uranium etc, oscillating spark discharge is passed between two electrodes made from the same elements.
(c) The gas whose ions are to be produced in filled up in a glass tube and a discharge is passed through the gas with the help of two electrodes. Positive ions of this gas are collected near the cathode. For enhancing the number of positive ions near the cathode small amount of neon gas is also introduced into the discharge tube. To obtain a beam of positive ions a small hole is drilled in the tube along the cathode axis.
(d) If a substance is handy in the form of a gas or vapour, then a beam of ions of these substances are produced by passing low voltage discharge through that gas.
The ions so produced have a different charge, mass, energy, and momentum and do not move in the form of the narrow beam. The apparatus used for separating the ions of the specific charge, mass, energy or momentum from the beam is called filter.
(a) Energy filter:
The apparatus by which ions of the same energy are obtained is called energy filter. A simple energy filter is shown in fig a low-velocity ion source enters from left through very small orfice. An into the electric field acting between plates A and B. These ions are accelerated by the voltage V between the plates. If the charge of each positive ion is ne, then the kinetic energy of all ions coming out to the orfice B will be
1/2mv²= neV …(1)
If the charge of all ions is the same, then the kinetic energy of all ions coming out through the orfice B will be the same. Therefore this simple apparatus is called an accelerated energy filter. The main disadvantage of this apparatus is that whatever the velocity spread of ions will be orfice A, the same velocity spread would be at the orfice B, therefore poor focusing of this energy filter will be.
The cylindrical electrostatic energy filter
There is another type of energy filter, called a cylindrical electrostatic energy filter shown in fig. This energy filter is more sensitive than the accelerated energy filter. It consists of two cylindrical plates C1 and C2 and the distance between them is d. A radial electric field E=V/d is produced by applying voltage V between the plates. The positive ions of charge ne are passed through the slit P in between the cylindrical plates and they are allowed to come out from the slit Q tangentially.
Due to the radial filed electric E between the plates, the electric force F= neE acts on the positive ions towards the center O., As a result, the positive ions moving tangentially at the entrance slit P will be forced to move in a circular path. These positive ions would move in a circular path without touching the plates only when their velocities satisfy the following equation,
Mv²/R= F = neE …(2)
where M is the mass of positive ion and R is a radius of the arc. In this arrangement, the energy of the positive ions coming out from the slit Q will be,
1/2Mv²= 1/2 neER (3)
In the arrangement shown in fig, the positive ions enter from slit P and exit from slit Q and the electric potential V remains the same on the motion of the ions between the slits. Therefore the kinetic energy of the ions does not charge. out of the ions entering the slit with different velocities, only those ions which satisfy the equation
(2), would come out from slit Q without touching the plates.
It is observed for the cylindrical condenser sector in fig that when the sector angle is θ= π√2 or 127 °, then its directional focusing is optimum. In this case, if a beam of positive ions of the same energy entering the slit P has a divergence of 1°, even then a focused beam of ions would emerge from the slit Q. Due to this reason, the intensity of the beam of positive ions increases and resolving power of this energy filter does not vary.
(b) Momentum filter:
The apparatus used for producing a narrow beam of positive ions of the same momentum from a beam having ions of different momenta is called momentum filter. This apparatus as shown in fig. In which a sectorial magnetic field B is applied perpendicular to the axis of the cylindrical tube. When a positive ion (mass M and charge ne) enters in the magnetic field from the slit P, a magnetic force F= nevB acts on this ion towards the center O of the cylindrical tube and due to this reason ion is deflected in a circular path. The same ion emerges from the slit Q whose velocity satisfies the following equation.
F= Mv²/R = nevB …(4)
Where Ris the radius of the sectorial arc. Thus the momentum of the ion emerging from the exit slit would be –
Mv= neBR …(5)
A focused beam of positive ions having the same momentum can be obtained from this filter. The focusing of ion beam due to sector magnet as shown in fig. for optimum focusing slits P and Q, center O and magnetic force F should lie in the plane.
- The beam of positive rays (1) having ions of the same momentum entering at slit A within an angular spread of ± with the central beam, is focused at point B.
- The central beam of positive rays (2) entering at slit A and incident at 90° to the magnetic plane OE. And emerging from the magnetic plane of at 90° is focused at C.
The beam (3) whose momentum is greater then Mv, is focused at point D. For this type of sector magnet, the lateral spread or deviation is given by- S = aα²/2(sin²θ/sinΦ+sin²Φ/sinθ)
Where distance a, angle α, angle θ and angle Φ are defined according to the fig. If the sector magnet is symmetrical, then θ=Φ.
∴ S = aα² sinθ
or S = Rα² …(6)
From, R = a sin θ. Thus is this filter the lateral spread S does not depend on the angle 2θ. It is directly proportional to R.
(3.) Velocity filter:
The apparatus used to select the velocity of the particles is called a velocity selector or a velocity filter. As shown in fig Crossed uniform electric and magnetic fields are applied on an incoming beam of positive ions. If electric force neE acting on positive ions moving with a velocity v balances the magnetic force nevB. In that case, the ions would move undeviated. I.e. those ions entering through the slit S1 of the velocity filter would emerge from slit S2 undeviated and will satisfy the following equation.
neE = nevB
or v = E/B …(7)
Ions having velocities other than giving in equation (7) get deviated after passing through the slit S1. Due to the unbalanced electric and magnetic forces acting on them and do not emerge from the slit S2. The monovelocity ion beam can be obtained from the velocity filter.