Mass spectroscopy is an instrumental technique that involves four main stages: 1) Ionization, where a sample is bombarded with electrons to form positively charged ions, 2) Acceleration, where the ions are accelerated to the same kinetic energy, 3) Deflection, where ions are separated by a magnetic field based on their mass-to-charge ratio, and 4) Detection, where the ions are detected electrically to produce a current. Mass spectroscopy is widely used to determine molecular masses and identify elements in samples.
3. Mass Spectroscopy
?Definition: Mass spectroscopy is an instrumental technique in
which sample is converted into rapidly moving positive ions by
exposure to ionizing conditions which are separated and
characterized according to their mass to charge ratio.
? Mass spectroscopy is widely used to determine and identify the
elements present in samples and to determine their
concentrations.
? Mass spectroscopy is the most accurate method for determining
the molecular mass of the compound and its elemental
composition
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5. INSTRUMENTATION-STAGES
? There are 4 main key stages in the instrumentation process for mass
spectroscopy
1. Ionisation
2. Acceleration
3. Deflection
4. Detection
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6. STAGE-1 : IONISATION
?The electron ionisation is the most common type ionisation
?The atom is ionised by knocking one or more electrons off to give a
positive ion. (Mass spectrophotometer always work with positive
ions)
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7. ?The particles in the sample (atoms or molecules) are bombarded with
a stream of electrons which come from a heated filament and to
knock one or more electrons out of the sample particles to make
positive ions
?Most of the positive ions formed will carry a charge of +1
?These positive ions are persuaded out into the rest of the machine by
the ion repeller which is another metal plate carrying a slight positive
charge.
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9. ?The positive ions are repelled away from the positive ionisation
chamber and pass through three slits with voltage in the decreasing
order.
?The middle slit carries some intermediate voltage and the final at ¡®0¡¯
volts.
?All the ions are accelerated into a finely focused beam.
?The speed of acceleration can be adjusted by changing the charge on
the plate
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10. STAGE-3 : DEFLECTION
? The ions are then deflected by a magnetic field according to their
masses.
? The amount of deflection depends on the number of positive charges
on the ion
? The heavier ions and ions with two or more positive charges are
deflected the least (Ion stream C)
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11. ? The lightest ions and ions with 1 positive charge are deflected most
(Ion stream A)
? The ions at the correct mass and charge travel to the detector (Ion
stream B)
? The amount of deflection depends on:
?The mass of the ion: Lighter ions are deflected more than heavier
ions.
?The charge on the ion: Ions with 2 (or more) positive charges are
deflected more than ones with only 1 positive charge
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12. STAGE-4 : DETECTION
? The beam of ions passing through the machine is detected
electrically.
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13. ?The beam of ions passing through the mass analyser are detected by
detector on the basis of m/z ratio.
?When an ions hits the metal box, its charge is neutralised by an
electron jumping from the metal on to the ion.
?That leaves a space amongst the electrons in the metal, and the
electrons in the wire shuffle along to fill it.
?A flow of electrons in the wire is detected as an electric current which
can be amplified and recorded.
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14. References
?Chatwal R Gurdeep, Sham K Anand; Text Book Of Instrumental
Method of Chemical Analysis; Himalaya Publishing House,Delhi; Mass
Spectroscopy; 2007, Fifth Revised Ed,2.272-2.302.
?Sharma Y.R, Text Book Of Elementary Organic Spectroscopy, S Chand
& Company Ltd, New Delhi; Mass Spectroscopy; 2007, Multicolour Ed,
291-353.
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