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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