Mass Spectrometry (MS)

in Mass

Introduction

Mass Spectrometry devices can identify the type of materials in the sample by electrically charging the specimen molecules. These molecules will be accelerated via a magnetic field, consequently, the molecules will be broken into charged fragments and then the different charges will be detected. The mass of each fragment will be displayed in the form of a spectral plot, and then the compound's mass spectrum can be used for qualitative identification.

The process here is that the fragment masses of the molecules used to piece together 'the mass' of the 'original molecule'.

The analyzing work, therefore, is that from the 'molecular mass' and the 'mass of the fragments', reference data is compared to find out the identity of the sample. It is possible to do that because each substance's mass spectrum is unique, as long as the parent mass correctly fit the output, or visa versa.

Process Description

The basic description of MS is that it contains a sample inlet, an ionization source, a molecule accelerator, and a detector. Of course there are many variations of MS but for the lack of space here, a look at the conventional and basic MS will be sufficient.

To make sure that the sample need to be analyzed will stay as a gas, the sample inlet kept at 400º C mainly because in MS analysis there is the need for a pure gaseous sample.

The following steps illustrate:

1. Sample enters the ionization chamber
2. With a high voltage, a beam of electrons is accelerated
3. With high voltage electrons, sample molecules are shattered (well-defined fragments)
4. Every fragment is charged then travels to the accelerator as 'an individual Particle'
5. Under the influence of an accelerating voltage, the charged particles velocity increase in the acceleration chamber
6. One mass accelerates sufficiently to reach the detector, under one value of voltage only
7. In order to cover different masses, so that all fragments reach the detector, the accelerating voltage varies
8. Several electrons plus charged particles will be emitted from the detector surface when an individual charged particle collides with the detector surface
9. Step 8 is repeated but this time with more electrons colliding with the surface and consequently generating more of them

More multiple collisions create thousands of electrons which emit from the last part of the curved surface. That means an amplification of the first charge reaching the collector.

As the mass is considered proportional to the detected charge, the Mass Spectrometry (MS) device will measure the charge and register the fragment mass.

The output is produced in the shape of array of peaks on a chart; it is called the 'mass spectrum'. Every one 'peak' is equivalent to the value of a fragment mass. The more fragments detected with one particular mass, the highest the peak' will be.

Output Analysis

Under certain controlled condition, each substance has a characteristic mass spectrum. That means it is possible to identify a specimen by comparing the specimen's mass spectrum with known compounds. In measuring relative intensities of the mass spectra, only then quantitative analysis can be possible.

For the unfragmented molecule sample, a mass spectrum usually displays a peak.

As has been mentioned above, it is called 'parent mass' and it is the biggest detected mass.

The mass of the molecule is revealed by the parent mass. However, various other peaks reveal the molecule's structure. Probably the hardest part during MS analysis is finding the parent peak, and the finding of the molecular mass of the sample.

Obviously, computer hardware and software in various spectral analyses play vital role in obtaining the final results.

Najib Altawell
Altawell © 2009

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Mass Spectrometry (MS)

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This article was published on 2010/03/26