Separation Science Differences in IMFs can be used to separate chemical substances for further analysis. Differences in IMFs can be used to separate chemical substances for further analysis. Applications: forensics, atmospheric chemistry, research, environmental chemistry, food science, etc.

Separation Science How CAN we separate and identify a mixture of alkanes and their isomers? Which of these isomers of pentane has the highest boiling point?

Gas Chromatography Uses differences in IMF to separate compounds and isomers. Used to determine the composition of a mixture: qualitatively (what is present) and qualitatively (what is present) and quantitatively (how much of each component is present). Gas chromatography using modern instrumentation and columns requires only a trace of sample dissolved in a volatile solvent.

Sample: mixture of volatile liquids (~1L) Gas Chromatography Gas Chromatogram Time (minutes) Abundance A B C D E Gas Chromatograph Each substance will elute from the chromatograph after a characteristic time – and recorded on a chromatogram.

Gas Chromatograph Data System or Recorder Carrier Gas Supply Oven Capillary Column Injection Port Detector

Gas Chromatography (GC) A mixture (liquid or gas) is injected into a thin, heated column packed with a solid coated with a viscous liquid. The mixture vaporizes and is carried through the column by a carrier gas (generally helium). detector column Carrier gas + mixture Packing material covered with a viscous liquid

Compounds A and B interact with the stationary phase through intermolecular forces: (van der Waals or dipole-dipole forces, including hydrogen bonding). A interacts more strongly with the stationary liquid phase and is retained relative to B, which interacts weakly with the stationary phase. Thus B spends more time in the gas phase and advances more rapidly through the column and has a shorter retention time than A. Gas Chromatography (GC)

Typically, components with similar polarity elute in order of volatility. Thus alkanes elute in order of increasing boiling points; lower boiling alkanes will have shorter retention times than higher boiling alkanes. Gas Chromatography (GC) out first out last

Concept Test Which hydrocarbon has the highest boiling point?

Concept Test Which hydrocarbon will come out of the GC column first?

Gas Chromatograms Air peak Peak due to hexane t R = “retention time”

ConcepTest Which chromatogram on the next slide is due to heptane, and which to octane?

A B heptane octane octane heptane