Separation ScienceSeparation Science

Differences in IMFs can be used to Differences in IMFs can be used to separate chemical substances for separate chemical substances for further analysis.further analysis.

Applications: forensics, atmospheric Applications: forensics, atmospheric chemistry, research, environmental chemistry, research, environmental chemistry, food science, etc.chemistry, food science, etc.

Separation ScienceSeparation Science

How CAN we separate and identify a How CAN we separate and identify a mixture of mixture of alkanesalkanes and their isomers? and their isomers?

Which of these isomers of pentane has the Which of these isomers of pentane has the highest boiling point?highest boiling point?

Gas ChromatographyGas ChromatographyUses differences in IMF to separate compounds Uses differences in IMF to separate compounds

and isomers.and isomers.

Used to determine the composition of a mixture:Used to determine the composition of a mixture:

qualitativelyqualitatively ( (whatwhat is present) and is present) and quantitativelyquantitatively ( (how muchhow much of each component is of each component is present). present).

Gas chromatography using modern instrumentation and Gas chromatography using modern instrumentation and columnscolumns requires only a trace of sample dissolved in a requires only a trace of sample dissolved in a volatile solvent.volatile solvent.

A

BC

D

E

Sample: mixture ofvolatile liquids (~1L)

Gas ChromatographyGas Chromatography

Gas Chromatogram

0 5 10 15 20

Time (minutes)

Abundanc

e

A

B

C

D

E

Gas Chromatograph

Each substance will elute from the chromatograph after a characteristic time – and recorded on a chromatogram.

Gas ChromatographGas Chromatograph

Data Systemor Recorder

Carrier GasSupply

Oven

Capillary Column

Injection Port

Detector

Gas Chromatography (GC)Gas Chromatography (GC)

A mixture (liquid or gas) is injected into a thin, A mixture (liquid or gas) is injected into a thin, heated column packed with a solid coated with a heated column packed with a solid coated with a viscous liquid. The mixture vaporizes and is viscous liquid. The mixture vaporizes and is carried through the column by a carried through the column by a carrier gas carrier gas (generally helium). (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 TestConcept Test

Which hydrocarbon has the highest boiling Which hydrocarbon has the highest boiling point?point?

Concept TestConcept Test

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

Gas ChromatogramsGas Chromatograms

Air peak

Peak due to hexane

tR = “retention time”

ConcepTestConcepTest

Which chromatogram on the Which chromatogram on the next slide is due to heptane, next slide is due to heptane,

and which to octane?and which to octane?

A B

heptane octane

octane heptane