Chromatography - HPLC and GC

Question 1

HPLC stands for

Answer 1

HPLC: High Performance Liquid Chromatography

Question 2

HPLC is a refinement of column chromatography, but has the following differences to column chromatography

Answer 2

• the solvent is forced through a metal tube under high pressure, rather than being allowed to pass through the stationary phase by gravity
• the particle size of the stationary phase is much smaller, which leads to better separation of the components
• the sample if injected into the column
• the components are detected after passing through the column, usually by their absorption of ultraviolet radiation
• the whole process is automated and the results are quickly available on a computer display

Question 3

in HPLC, instead of an Rf value, the ………… value is used

Answer 3

Retention time, Rt value

Question 4

the Rt value of a component is

Answer 4

the time taken from injection to detection

Question 5

retention times depend on the following variables:

Answer 5

• the nature of the solvent
• the pressure used
• the temperature inside the column

Question 6

Gas chromatography (GC) is another refinement of column chromatography, but has he following differences:

Answer 6

• the metal tube can be several metres long, and s coiled to save space
• the stationary phase is a solid or liquid coated on the inside of the tube
• the mobile phase is an inert carrier gas (often nitrogen, helium or argon)
• the sample is injected into the column, like in HPLC
• the components passing through the column are detected
• the whole process is automated and the results are quickly available on a computer display

Question 7

in GC, the components separate by:

Answer 7

• the components vaporise and move through the coiled tube with the gas carrier
• they move at different speeds depending on how strongly they are attracted to the stationary phase
• those with weaker attractions more more quickly and have shorter retention times