Gas Chromatography: GC MS enables products to be identified- by separating the components
in a mixture and identify them by comparing with a database.
GC MS used in forensics, environmental analysis and airport security.
Number of products = number of peaks. Proportions of product given from relative peak areas.
Stationary Phase: Stationary phase is a liquid adsorbed onto a solid support in the capillary
column.
Mobile Phase: Mobile carrier, an inert gas, carries sample through capillary column. E.g. helium
or neon.
Process of Gas Chromatography: Volatile mixture injected into gas chromatograph. Separation
by relative solubility in stationary phase and NOT by adsorption. The more soluble, longer the
retention time- the interaction since the slower it moves. Alcohols have short retention time.
Alkane long retention time because alkane more soluble in the alkane liquid stationary phase.
Retention Time: Retention time is the time taken from injection of each component to when
leaves column. Can compare retention time of unknown components to retention times for
known components.
Similar compound means similar retention time as similar solubilities.
Disadvantages of TLC/ GC/ Uncertainties: Similar compounds have similar retention times/Rf
values due to similar structures. Unknown compounds have no reference retention times/Rf
values.
Advantages of GC-MS: Gas chromatography separates but cannot conclusively identify
components. Gas Chromatography can give the relative proportions from the relative peak
areas. Mass Spectroscopy identifies structures by comparing fragment ions with a spectral
database.
Determining Concentration of Compound: Prepare standard solution of known concentrations
of compounds investigated. Obtain their gas chromatograms. Plot calibration curve of peak area
against concentration – called external calibration. Obtain gas chromatogram of compound being
investigated under same conditions. Use calibration curve to convert peak area to concentration.