Mass Spectrometry
Mass spectrometry is a powerful analytical technique used to quantify known materials, to identify unknown compounds within a sample, and to elucidate the structure and chemical properties of different molecules. The complete process involves the conversion of the sample into gaseous ions, with or without fragmentation, which are then characterized by their mass to charge ratios (m/z) and relative abundances.
Basic Principle:
The mass spectroscopy is based on the positive ion generation.
For its most popular
model, the electron impact ionization with magnetic sector analyzer, the sample
under investigation is converted into vapor phase and bombarded with electrons
having energy sufficient to knock out one electron from it (>10 eV) to
produce a positively charged ion called molecular ion or parent ion which is
denoted by M+. Positively charged molecule M+ is often
unstable, and with increase in energy (10–70 eV) according to bond strength,
they break into fragments called fragment or daughter ion which is denoted by M+1.
Ions formed are separated in analyzer under the influence of electric and
magnetic field and are recorded by the detector to give rise a mass spectrum
Components:
The instrument consists of three
major components:
1.
Ion
Source: For producing gaseous ions from
the substance being studied.
2.
Analyzer:
For resolving the ions into their characteristics mass components according to
their mass-to-charge ratio.
3.
Detector
System:
For detecting the ions and recording the relative abundance of each of the
resolved ionic species.
In addition, a sample introduction
system is necessary to admit the samples to be studied to the ion source while
maintaining the high vacuum requirements (~10-6 to 10-8 mm of mercury) of the
technique; and a computer is required to control the instrument, acquire and
manipulate data, and compare spectra to reference libraries.
Working:
With all the above components, a
mass spectrometer should always perform the following processes:
1.
Produce ions from the sample in
the ionization source.
2.
Separate these ions according to
their mass-to-charge ratio in the mass analyzer.
3.
Eventually, fragment the selected
ions and analyze the fragments in a second analyzer.
4.
Detect the ions emerging from the
last analyzer and measure their abundance with the detector that converts the
ions into electrical signals.
5.
Process the signals from the
detector that are transmitted to the computer and control the instrument using
feedback
APPLICATION:
Ø
Mass spectrometers are sensitive
detectors of isotopes based on their masses.
Ø
They are used in carbon dating and
other radioactive dating processes.
Ø
The combination of mass
spectrometer and a gas chromatograph makes a powerful tool for the detection of
trace of trace quantities of toxins or contaminants.
Ø
A number of satellite and
spacecraft have mass spectrometer for the identification of small number of
particles intercepted in space.
Ø
For example: SOHO satellite uses
mass spectrometer to analyze solar wind.
Ø
Mass spectrometer are used for the
analysis of residual gases in high vacuum system.
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