c) MASS SPECTROMETER
An instrument which is used to measure the exact masses of different isotopes of an element together with their relative abundances is called mass spectrometer.
Types of mass spectrometer
• Aston's mass spectrometer
it was designed to identify isotopes of an element on the basis of atomic masses.
• Dempster's mass spectrometer
it was designed to identify isotopes of an element which are in the solid state.
Parts of modern mass spectrometer • Vaporization chamber
Parts of modern mass spectrometer • Vaporization chamber
• Ionization chamber
• Electrical field acceleration
• Magnetic field deflection
• Ion collector detection
• Recorder
• Amplifier
The first five parts are present in gan
Dempster's mass spectrometer while last two parts are additional present in modern spectroscope.
Instrumentation of Dempster's mass spectrometer
It has following five main parts.
Vaporization chamber
In this Chamber the sample of the element is vaporized and the vapour pressure is reduced to 10^-7 to 10^- 6 torr.
Ionization chamber
In this chamber sample of the element is ionized either by alpha rays or by electron beam.
e- + X ------> X+ + e- + e-
Electrical field
Electrical field is applied to accelerate the positive ion.The applied potential difference is of 500-2000 volts.
Magnetic field
On passing through magnetic field,deflection of positive ions take place according to their mass/charge ratio.
Formula
m/e = Hr/E
Electrometer or ion collector
Ion collector receives positive ions according to their mass/charge ratio related to isotopes.
1. The strength of current measured by ion collector gives the relative abundance of ions of a definite m/e value.
2. The to the same experiment is performed with C-12 isotope and the current strength is compared.
Other techniques for the separation of isotopes
• Gaseous diffusion
• Thermal diffusion
• Distillation
• Ultracentrifuge
• Electromagnetic separation
• Laser separation
Examples
i) Mass spectrum of Neon
• The number of isotopes
The three peaks in the mass spectrum show that neon contain three isotopes with the relative isotopic masses 20,21 and 22 respectively
• The abundance of isotopes
The relative heights of of the peaks give a direct measure of the relative abundances of the isotopes.
ii) Mass spectrum for Boron
• Number of isotopes
The two peaks in the mass spectrograph shows that there are two isotopes of Boron with relative isotopic masses of 10 amu and 11 amu on the carbon-12 scale.
• Abundance of the isotopes
The relative height of the peaks give a direct meaure of the relative abundance of the isotopes.
• Working out the relative atomic mass
Suppose there are 123 typical atoms of Boron. 23 of these would be be B-10 and 100 would be be B-11.The total mass of these would be
= 23 × 10 + 100 × 11 = 1330
The average mass of these 123 atoms would be
= 1330/123 = 10.8 (to a significant figures)
• 10.8 is the relative atomic mass of Boron. A simple average of 10 and 11 is of course 10.5.Our answer of 10.8 allows for the fact that there are a lot of more of the heavier isotope of Boron and so the weighted average out to be closer to that.
iii) Mass spectrum of chlorine
The ions formed from chlorine elements are Cl2+ in the mass spectrum.These ions do not be particularly stable, and some will fall apart to give a chlorine atom and a Cl+ ion. This is term as fragmentation.
Cl2+ --------> Cl + Cl+
• If the chlorine atom formed is not then ionized in the ionization chamber it's simply gets lost in the machine neither accelerated nor the deflected.
• The Cl+ ions will pass through the machine and will give lines at 35 and 37 depending on the isotope and you would get exactly the pattern in the last diagram. • The problem is that we also observe lines for the unfragmented Cl2+ commented Cl2+ ions.
• The possible combinations of chlorine 35 and chlorine-37 atoms in the Cl2+ ions as
i) Both atoms in Cl2+ could be Cl-35 with relative mass of 70.
ii) Both atoms in Cl2+ could be Cl-37 with relative mass of 74.
iii) One atom in Cl2+ could be Cl-35 and second atom could be Cl- 37 with the relative mass of 72.
Total masses of the three possible Cl2+ could be of:
35 + 35 = 70
35 + 37 = 72
37 + 37 = 74
Conclusion
We will have two sets of lines.
1. Two lines for monatomic ions at m/z= 35 and 37 respectively,indicating that it has two isotopes.The realtive heights of the peaks at m/z = 35 and 37 are in the ratio 3:1.
2. Three lines for molecular ions at m/z = 70,72 and 74 respectively.The relative heights of the peaks at m/z = 70,72 and 74 are in the ratio 9:6:1.
The overall mass spectrum is as follows
• Number of isotopes
The two peaks in the mass spectrograph shows that there are two isotopes of Boron with relative isotopic masses of 10 amu and 11 amu on the carbon-12 scale.
• Abundance of the isotopes
The relative height of the peaks give a direct meaure of the relative abundance of the isotopes.
Suppose there are 123 typical atoms of Boron. 23 of these would be be B-10 and 100 would be be B-11.The total mass of these would be
= 23 × 10 + 100 × 11 = 1330
The average mass of these 123 atoms would be
= 1330/123 = 10.8 (to a significant figures)
• 10.8 is the relative atomic mass of Boron. A simple average of 10 and 11 is of course 10.5.Our answer of 10.8 allows for the fact that there are a lot of more of the heavier isotope of Boron and so the weighted average out to be closer to that.
iii) Mass spectrum of chlorine
The ions formed from chlorine elements are Cl2+ in the mass spectrum.These ions do not be particularly stable, and some will fall apart to give a chlorine atom and a Cl+ ion. This is term as fragmentation.
Cl2+ --------> Cl + Cl+
• If the chlorine atom formed is not then ionized in the ionization chamber it's simply gets lost in the machine neither accelerated nor the deflected.
• The Cl+ ions will pass through the machine and will give lines at 35 and 37 depending on the isotope and you would get exactly the pattern in the last diagram. • The problem is that we also observe lines for the unfragmented Cl2+ commented Cl2+ ions.
• The possible combinations of chlorine 35 and chlorine-37 atoms in the Cl2+ ions as
i) Both atoms in Cl2+ could be Cl-35 with relative mass of 70.
ii) Both atoms in Cl2+ could be Cl-37 with relative mass of 74.
iii) One atom in Cl2+ could be Cl-35 and second atom could be Cl- 37 with the relative mass of 72.
Total masses of the three possible Cl2+ could be of:
35 + 35 = 70
35 + 37 = 72
37 + 37 = 74
Conclusion
We will have two sets of lines.
1. Two lines for monatomic ions at m/z= 35 and 37 respectively,indicating that it has two isotopes.The realtive heights of the peaks at m/z = 35 and 37 are in the ratio 3:1.
2. Three lines for molecular ions at m/z = 70,72 and 74 respectively.The relative heights of the peaks at m/z = 70,72 and 74 are in the ratio 9:6:1.
The overall mass spectrum is as follows
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