Lecture 7: Gas Chromatography

Question 1

What is gas chromatography?

Answer 1

Separation technique using an inert gaseous mobile phase (e.g. helium, hydrogen, nitrogen) to transport sample through column containing a stationary phase.

Question 2

What must samples be like in GC?

Answer 2

• Samples must be volatile (<~250-300 degrees C) e.g. low MW organic analytes
  o Otherwise require chemical modification (derivatisation) to increase volatility

Question 3

How does GC work?

Answer 3

o Sample components ‘partition’ between mobile and stationary phases with stationary phase interaction dependent on volatility (vapour p.) and affinity

Question 4

Describe the elution order in GC

Answer 4

weaker interaction/more volatile to stronger interaction, less volatile

Question 5

Main difference between LC and GC

Answer 5

• Temperature gradient rather than solvent gradient

Question 6

What are the practical considerations for GC?

Answer 6

inlet projector
- split/splitless
- thermal desorption
- headspace

column
- length
- stationary film thickness

column oven

detector
- derivitisation

Question 7

Role of the injector/inlet

Answer 7

o Sample (0.5-2ul) injected into heated inlet, volatising sample which is carried by mobile phase through the column.

Question 7

what is important about the ion source in the inlet/injector?

Answer 7

must be adequately and evenly heated to avoid cold spots and sample condensation

Question 8

Explain split/splitless as a practical consideration of the inlet/injector

Answer 8

o Limits amount of sample injected – de-risks injection of unknown samples (to protect the detector).
o Sample gas enters column (splitless) or column and split outlet (split); % sample on-column depends on ratio of flow rate on column and split flow line
o Higher split, less on column

Question 9

Explain thermal desorption as a practical consideration of the inlet/injector

Answer 9

o Inlet accommodate a TD sampling tube or SPME fiber containing sorbent used to capture very volatile analytes.
o Sorbent is heated in presence of inert gas (mobile phase) extracting the analyte for transfer on column

Question 10

what is thermal desorption?

Answer 10

removing with temperature

Question 11

Explain headspace as a practical consideration of the inlet/injector

Answer 11

o Samples volatile species in vapour above solid/liquid (often following application of heat)
o Sample is introduced to column using a heated injector to minimise analyte condensation

Question 12

what is the column in GC?

Answer 12

o Long, thin open silica tubes containing layer of polymeric liquid (stationary phase) = WCOT
o Typically, high resolution and efficiency
o Stationary phases can be different chemistries = separation by differences in interaction on stationary phase (‘like dissolves like’) and volatility

Question 13

Explain column length as a practical consideration

Answer 13

o <15m for ‘simple’ mixtures, >15m if complex.

Affects:
 Separation efficiency – proportional to length, shorter column = poorer separation efficiency and resolution
 Analyte retention and elution time = increases with column length, used if very complex mixtures.
 Cost = longer columns are more expensive

Question 14

Explain stationary film thickness as a practical consideration

Answer 14

Affects:
 Analyte retention and elution time (thicker films required for retaining more volatile analytes – reduce film thickness to elute strongly interacting analytes.
 Column bleed (typically increases with film thickness)
 Resolution (improves for volatiles but limited for strongly retained with increasing thickness)

Question 15

Explain column oven as a practical consideration

Answer 15

o Isothermal (constant temperature) – for analytes with similar chemistries and bpt.
o Gradient or temperature programme – for chemically different analytes to avoid long retention/analysis times and poor separation (gradually heat column suspended in cage and selectively elute analytes).

Question 16

role of detector in GC

Answer 16

o Analyte enters detector, generates a response based on a physiochemical property relevant to the detector.
o Response is amplified and presented as a chromatogram.

Question 18

Why is derivitisation used in GC?

Answer 18

o Modify analyte using derivitisation reagent e.g. for increased volatility
o Common reactions include acylation, silylation and alkylation:
 Trimethylsilylation:

Question 19

Advantages of GC

Answer 19

fast analysis times as high resolution resulting in efficient separation

sensitive detection

accurate

rugged and reliable

small sample volumes

Question 20

Disadvantages of GC

Answer 20

low sample volumes

limited sample chemistries - volatile (unless derivatisation) and low MW

unsuitable for thermally liable species

upper-temperature limit of operation limited by the stationary phase