Mass Spectrometry

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History:

Mass Spectrometry was discovered by “Denpsten” in 1918.

Introduction:

“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”

Basic Principle:

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.

4. 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.

 6. Detector:

Detectors are used for detection of electric signals. Photographic plate is commonly used detectors.

7. Amplifier:

The current from detector is fed into amplifier, which amplifies the current.

 

Working of  mass spectrometry:

Ionization chamber:

The sample is injected as a gas in ionization chamber.

Electron gun:

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.

Accelerated System:

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.

Detection:

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.

Signal intensity:

 The intensity of the electric signal is directly proportional to the number of ions striking the detector.

Mass spectrum:

“Mass spectrum is a chart in which m/e is taken on x-axis and relative abundance is taken on y-axis”.

Molecular ions:

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.

Fragmentation ions:

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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(Arsal Khan)

Writer: Muhammad Arsalan.



About the author

arsal-khan

I am student of CHEMISTRY doing Bs hons in Chemistry from GPGC college Haripur.I love to write blogs.

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