GCMS (Gas chromatography mass spectrometer)

GCMS (Gas chromatography mass spectrometer)

History:

In gas chromatography, the mass spectrometer was used as a detector.It was discovered by Roland gohlke and Fred Mclafferty in 1950.

In 1996, a high-powered & high-speed GCMS passed the fire acceterants test in less than 90 seconds, while the first generation of the GCMS did the task in 16 minutes.

After some time it was simplified with the help of cheap and small computers and together it has saved a lot of time.

The GCMS is made up of two parts, GC the first part and the second part is MS. The injector, inlet, column, oven in the GC part while the MS acts like a detector. The MS part consists of a heater chamber, ionization chamber, quarter pole and a detector.

Relative abundance of isotopes:

The percentage of a particular isotope of an element which exits naturally is called the relative abundance of isotopes.

Mass Spectrometer:

The technique which atom or molecules are converted into ions and are separated on the basis of mass to charge (M/Z)  ratio.

The mass spectrum represents how a molecule fragments. When a molecule is bombarded by a stream of electrons, it displays a characteristic fragmentation pattern. Under the same conditions, this pattern will be the same from laboratory to laboratory. There are two modes of generating mass spectra, electron ionization and chemical ionization.

Electron Ionization (EI):

This is the most common mode of operation for GC/MS, accounting for over 90% of all work In this form of ionization, a molecule emerging from the GC column is fragmented by interaction with a

streams of electron.

The conditions used result in considerable fragmentation of the molecule.

The spectrum represents the abundance at various mass / charge ratios.

Its spectral pattern is used to identify the compound.

Chemical Ionization (CI):

In chemical ionization, molecules are introduced into an abundance of a reagent gas, typically Methane or Ammonia.

A stream of electrons ionizes the reagent gas which, in turn, reacts with the analyte. In the case of ammonia, you can obtain [M+H]+ and [M+NH4]+ as the products.

Aston’s Spectrometer: This spectrometer is used to identify the isotopes of elements on the basis of their atomic mass.

Dempster’s Spectrometer: This spectrometer has been designed for the identification of elements which are found in solid state.

According to Dempster’s we are determine as:

·         Number of isotopes of an elements

·      Relative isotopes masses and relative abundance of isotopes

·         Relative molecular masses and structure of organic compound.

1) Ionization: The sample first enters the ionization chamber through the prop. Here, with the help of electronic beam and heater filament, the sample is captivated and ionized.The molecules that remain neutral are expelled through the vacuum pump.

The ionization chamber consists of an electronic gun that produces high electron energy.

When the electrons in the outer orbit of the atom collide with the electron of the electronic gun, the electrons of the outer orbit of atom ejected. Due to which the atom is converted in to positive charge.

Electron ionization work is generally carried out at a standard setting of 70eV. Reference library data is collected at this setting. However, if this value is decreased, less fragmentation will occur and the ion abundance pattern will change.

2) Accelerator: The Accelerator consists of two slits with different charge on the first slits having positive charge and the second having negative charge. The ions move forward due to the negative charge of the second slits.
It has an accelerating grid where all the ions gather and move together with a force.

3) Electronic field and Magnetic field: Here a magnetic field or electronic field is generated so heavy and light ions slide move their way and change their path. After this, ions arrive at the detector.
I) Electronic field: It consists of two electrical plates. There is a positive charge on one and a negative charge on the other, as well as two magnetic plates that are perpendicular to the electrical plate. Magnetic field and electronics field produce same force of velocity.

So:

             Fm=FE

             qvB=qE        (Fm=qvB and FE=qE)

Where,

           q= Charge

           V= velocity

           B= Magnetic field

           E=electronic field

So that,

           Fm= qvm

If an ion has more Velocity then it will have more magnetic force on it, due to which it will turn towards the magnetic field. If the velocity of an ion is low, the magnetic force will be less on it and it will turn towards the electronic field.

Note: Here, magnetic force and electronic force are equal (Fm=FE), in this case if the magnetic force is less, then it will be more electronic force which the ion will move towards the electronic field.

The velocity of the ions was higher its turned towards the magnetic field and the ions whose velocity was less. They turned to the electric field.

II) Magnetic field: It consists of two large magnets, due to which the magnetic field is created. When ions enter the magnetic field, a centrifugal force is applied on it. Here, magnetic force and electrical force are equal. 

Due to centrifugal force, ions arrive at the detector, forming a curve.The ion which is less mass is more effective on the magnetic field and the mass which is more is less effective on the magnetic field. Because of which an angular structure is formed. Therefore small mass ions make small curve and big mass ions make big ones.

                 Fm=Fc

                       mv2

              qvB=------

                          r

                     mv

             qB=-----

                      r

                    mv

              r =-------

                  qB

Radios is directly proposal to mass

Where's

           c=centrifugal force

           q=charge

           V=velocity

           B=magnetic

           M=mass

           r =radios

4) Detector: The detector detects the radius of the ions. Radios is directly proposal to mass thus the mass to charge ratio is calculated by the detector.
The detector detects the ion and sends the signal to the amplifier.
5) Graphic interpretation: 
It has two axis, One is X-axis on which the mass to charge ratio and the other is the y-axis. At which relative absence occurs.

Mass spectrometer used for: 

* For isolation of isotopes

* For study of moliqueler fragmentation

* For analysis of unknown atom of moliques

Understand the capabilities and limitations of gas chromatography / mass spectrometry

1. GC/MS can provide general or specific information

2. GC/MS can provide data to identify unknown peaks

3. GC/MS can provide data to recognize co eluting peaks

4. GC/MS can provide quantitative information in complex matrices

5. A CI mass spectrum can provide molecular weight information

Limitation of GCMS

1. GC/MS cannot always provide unique identification for isomers. Co eluting isomers will not be distinguished if they have identical spectra.

2. GC/MS cannot be used with compounds that are not suitable for GC analysis

Single Quadrupole:

1. All of the ions (+, -, and neutrals) are formed in the Ion source.

2. Ion optics guides the ions to the quadrupole mass analyzer.

3. In the analyzer, only ions of a particular m/z value, represented are allowed to pass through to the detector.

4. The detector completes the analysis.
MS1= Quadrupole One 

Tripal Quadrupole:

1^st hot quartz hyperbolic quadrupole precursor isolation
Remove all ion except those with m/z of your analyte
2^nd Hexapole collision cell with helium quenching noise reduction

Ion undergoes fragmentation in the collision cell creating unique
3^nd hot quartz hyperbolic quadrupole product ion mass analysis
Isolate analyte fragments and goes to dynode/multiplier 
MS1= Quadrupole One
Collision cell= Quadrupole two
MS2= Quadrupole three