Mass Spectrometry was discovered by “Denpsten” in 1918.
“It is an analytical technique in which a sample is subjected to electrons from a source, as a result the sample is ionized and these ions have specific m/e ratios which are analyzed through a mass spectrum”
In mass spectrometry sample is passed through a vaccum, where electrons from source, leads to the ejection of electrons from the sample, such that:
M à M+ + e-
Construction of Mass Spectrometer:
Mass Spectrometer consist of following instrumentations.
1. Inlet System:
In inlet system heating coils are present. Here samples, whose mass spectrum is measured, are evaporated by heating them.
2. Ionization Chambers:
In this chamber ionization of sample is done, mostly by electron gun.
3. Accelerating Systems:
Accelerating system is in the form of linear acceletor , which provides electric field and accelerates the sample towards analyzer.
In analyzer cations are deflected by magnet. Analyzer may be:
· Single focusing magnetic analyzer.
· Double focusing magnetic analyzer.
5. Ion Collector:
Finally cations are reached towards ion collector which is Be-Cu electron multiplier system or photographic films.
Detectors are used for detection of electric signals. Photographic plate is commonly used detectors.
The current from detector is fed into amplifier, which amplifies the current.
Working of mass spectrometry:
The sample is injected as a gas in ionization chamber.
Electrons come from the electron gun collide with the sample atoms or molecules. As a result of these collisions electrons from the sample atoms are knocked out and they become positive ions.
The positive ions are accelerated by the electric field and finally reach a region where an electromagnet is applied.
Separation of ions:
Different ions separate on the basis of m/e ratio. In the magnetic field the ions deflect in circular path. Lighter ions (low m/e) deflect to large extent than heavier ions.
Ions strikes the detector and electric signals detected and amplified.
In recorder the electric current produce peaks on a chart. Higher is the peak of a species higher is its abundance.
The intensity of the electric signal is directly proportional to the number of ions striking the detector.
“Mass spectrum is a chart in which m/e is taken on x-axis and relative abundance is taken on y-axis”.
When a molecule is introduced into mass spectrometer, the molecular ion (M+) is produced due to loss of an electron. They are produced when energy of ion source is 9-15ev.
At energy above 70ev the sample is converted into various fragments.
Applications of Mass Spectrometry:
Mass spectrometry is a useful technique for quantitative and qualitative analysis.
Mass spectrum of neon:
Neon spectrum obtain by its mass spectrometery shows three peaks with m/e equal to 20,21, and 22 which correspond to 20Ne,21Ne and 22Ne. Neon -20 is the most abundant and Neon -21 is the least abundant isotope as clear from the heights of their peaks.
Mass spectrum of Cl:
Mass spectrum of Cl shows 2 isotopes Cl-35 and Cl-37.
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