Sunday, 22 November 2015

How do mass spectrometry works?

How do mass spectrometry works?
A mass spectrometer can split atoms and molecules based on the mass. It can also give us a series of data about the compounds and elements present in the sample. With that series of data about the atoms or molecules, that would be represent on a graph in a computer from which we can identify the elements and compounds present in the sample.
The mass spectrometry is an ideal device for measuring relative mass of  an element/ compound  in a given sample since it can measure very accurately.


In order to move through the mass spectrometer, sample must be
a) first vaporised,
b) secondly ionised.
The air is first pumped out of the mass spectrometer  to avoid ionisation of air.

Diagram:












Vaporisation :
The sample which is needed to analyze must be in gaseous state in order to move  easily through the mass spectroscopy. There is a high vacuum area in the 1st section of the instrument where the given sample is vaporised. 
The air particles are first pumped out of the vacuum chamber from the mass spectroscopy in order to prevent air particles get ionised . This is because we want only the sample to be ionised. If there is any air molecules present, then that will also get ionised. So, it would be pretty difficult for us to analyse the actual ions present in the sample.
Then,  the desired sample is injected into the mass spectroscopy and is first vaporized. Here, the given sample is vaporised at a given temperature , if the sample is not in the gaseous state.   
                                                                                   
Ionisation :
Then in the next section , the vaporised sample is bombarded with high energy electrons. These high energy electrons knock one or more electrons in the valence making ions , molecular ions. It doesn’t make any significant differences in mass since the mass of electrons are negligible. Now the cations are formed which can move to the electric field.
X (g) + e - → X+ + 2e-
Two types of ions and free radicals are formed in the ionisation:
1) Molecular ions    2) Fragmented ions

Diagram:

** Note : Later we will study the fragmentation pattern of molecular ions .





Acceleration :
Then the cat ions passes through the electric field to get accelerated. The positive ions pass through the slits and comes out like stream of  beams. The cat ions get accelerated but negative ions don’t get accelerated in the electric field.

Velocity selector :
Then the positive ions pass through the velocity selector where a fixed velocity is set for all the ions . The velocity selector makes sure that all the positive ions are travelling at constant speed .
This means that affect of the magnetic field in the next section would be due to the differing mass and charge / mass/ charge ratio (m/z) but not for the speed since the speed is constant.     

Uniform Magnetic field :
Then the ions passes through the uniform magnetic field where deflection of ions occurs. Deflection depends on both mass and charge. The ions with large mass and small charge would deflect least . On the other hand, the ions with small mass and large charge would deflect most .
The strength of magnetic field is gradually increased,  only ions with specific mass/charge ratio can pass through the passage at a selected settings of the magnetic field. Others would strike the wall by deflecting high or low and failed to move through the pathway to the detector.



Detector :
Then the detector detects the no. of positive ions pass through and transform them as a tiny currents and transmit as electric signal to the computer.

Display:
Then the Mass spectrum is obtained as a result. The computer would produce a graph of abundance against mass/ charge ratio (m/z) where you would have relative peaks and different m/z ratio values.  The relative height shows relative abundance cations . The m/z ratio gives us the information about relative mass of the particles present in the compound. Most of the charge of the ion is +1. So mass of the ions = m/z ratio of the ions.

***Note: We will later study the mass spectroscopy graph in later chapter.


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