Factors In Gas Chromatography

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Gas chromatography (GC) is a chromatography technique where the separation of individual components (called analytes) from a sample relies on their differing distribution between the mobile phase and a stationary phase. The mobile phase is what carries the analyte (components being analysed) through the stationary phase and in GC, it is an inert gas, (usually helium or nitrogen). The gas must be inert so it will not react with the samples to give a false reading. The stationary phase is the substance that is fixed in place during chromatography to which the sample adsorbs. GC can have two types of columns, capillary or stacked. In capillary columns, the stationary phase is a thin layer of non-volatile liquid or polymer with a high boiling point …show more content…
This is due to the intermolecular forces that exists between the molecules, the molecular weight of the molecule, the geometry of the molecule, and the polar/non polar nature of the molecule. The three forces that exist between molecules (intermolecular forces) are dispersion forces, dipole-dipole forces, and hydrogen bonds. Dispersion forces arise from the random motions of electrons within a molecule. The fluctuations in electron density causes a temporary partial negative charge (dipole) and can repel electrons from a nearby molecule, inducing a partial positive charge. These two temporary dipoles are electrostatically attracted to each other but these dipoles are only temporary and are the weakest of the the intermolecular forces. These forces occur in every atom and molecule because of the random motions of electrons. Dipole-dipole forces is when a molecule has a permanent partial charge on its poles because of an atom covalently bonding to another atom with a different electronegativity, causing electrons to spend more time in one area than the atom with the lower electronegativity. These are stronger than dispersion forces because of the permanent dipoles. There is also asymmetry in the molecule that makes the molecule polar. Hydrogen bonding is an extreme form of dipole-dipole forces where …show more content…
GC is used over other techniques such as TLC or high performance liquid chromatography (HPLC) because:
• It does not require much of a sample to be used (micrograms/microliters can be used)
• It’s a non-destructive technique (frequently coupled with MS),
• It’s more sensitive and more accurate than HPLC when detecting volatile compounds
• It’s highly sensitive when used with thermal detectors (measures ppm and ppb)
• The temperature can be altered and be maintained
• It has good accuracy and precision
• GC instruments are generally less expensive than HPLC
• GC has a higher efficiency (resolves more compounds per unit of time)
• Easier to operate than HPLC so highly skilled personnel are not required, making it suitable for routine

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