Fundamental of GC-MS
What is GC-MS ?
GC + MS = GC-MS
(Separation ) ( Identification ) (Separation Identification )
Instrumentation of GC-MS
Overview of the major hardware components of the mass Spectrometer.
- GC inlet
- Ion source
- Mass Analyzer
- Vacuum System
1. GC intel
The GC inlet should the three purpose.
- Place of entry
- Vaporization of sample
- Transports the sample to the column
Interfacing GC and MS
3. Ion Source
electrons to interact.
- Electron Focus
in the ionization process.
- Electron Bombardment
- Energized Molecules
- Repeated Process
As a result of the collision with the
electron, the sample molecule loses an
electron. The resulting species is called a
molecular ion. The molecular ion has a
- First form ions from a “reagent gas ” by bombardment with electrons.
- Reagent gas ions under go subsequent reactions with sample molecules to form sample ions (“Bronsted acid”).
- Cl ion information is much more “gentle” than electron ionization (EI) three fore less fragmentation.
- most common reagent gas is methane, produces ions with almost any sample molecule.
- Other reagent gases (isobutane, ammonia) are more selective and even less fragmentation.
- Source pressure – 0.2 Torr
- Detection limits are generally high because of background from the reagent gas (methane)
- most often used to determine the molecular weight of a compound.
- a.k.a “electron capture negative ion chemical ionization “.
- First form a “Cloud” of electrons with little excess energy (“thermal electrons”)
- “Thermal electrons” are captured by sample molecules
- Buffer gas required (removes energy from electrons/ions)
- Methane is by far the most often used buffer gas.
- Source pressure – 0.4 torr ( higher than for PCI mode)
- only certain types of molecules are capable of capturing thermal electrons (selectivity)
- Extremely efficient for some molecules (selectivity).
- Detection limits are generally very low due to lack of response from contaminants or matrix.
- most often used for selective high sensitivity analysis.
each pair of rods. Electrically, the pairs of rods have
opposite potentials. Although the rod pairs have opposite
charges, the four rods work together to move the ions of
selected mass through the quadrupole.
mass will make it through the mass filter. The higher the RF and DC amplitude, the higher the
mass selected. The lower the RF and DC amplitudes, the lower the mass selected.
6. Vacuum System
- Provide adequate mean free path
- Provide collision-free ion trajectories
- Reduce ion-molecular reactions
- Reduce background interference
- increase filament life time.
- Avoid electrical discharge
- increase sensitivity