W12 Gas Chromatography

Question 1

properties of gas chromatography

Answer 1

mobile phase: gas (usually He, N2 or H2) > inert, low viscosity and compatible with detector

stationary phase: thermally stable, nonvolatile, chemical inert liquid or solid with characteristics such that retention factor and selectivity factor all fall within suitable range

analyte: gas or volatile liquid stable at the temp used

Question 2

what are the different open tubular columns

Answer 2

wall coated (WCOT): thick film of stationary liquid phase on inner wall of column

porous-layer (PLOT): solid particles as active stationary phase > separate compounds like O2, N2 or CO (too volatile for WCOT)

support-coated (SCOT): solid particles coated with stationary phase liquid phase attached to inner wall

Question 3

properties of packed columns in GC

Answer 3

contain fine particles of solid support coated with nonvolatile liquid stationary phase (or the solid itself may be the stationary phase)

solid support is often silica that is silanized to reduce hydrogen bonding to polar solutes

Question 4

comparison between open and packed columns

Answer 4

packed columns provide greater sample capacity BUT give broader peaks, longer retention times and less resolution

Question 5

what does bleeding of stationary phase mean

Answer 5

as column ages > surface silanol groups are exposed > interact more strongly with solute > increase tailing

exposure to O2 from air at high temp also degrades column

Question 6

solution to reduce tendency of stationary phase to bleed from column

Answer 6

stationary phase usually covalently bonded to silica surface or covalently cross linked to itself

thick films of stationary phase can shield analyses from the silica surface and reduce tailing but can also increase bleed of stationary phase at elevated temperature

Question 7

what is derivatization

Answer 7

chemical reaction to convert analyte into a form that is more convenient to separate or easier to detect

Question 8

order of optimal flow rate for N2, He and H2

Answer 8

increases from N2 < He < H2

fast diffusion of analyte in 2, He > lower mass transfer resistance > higher resolution than N2 at high flow rate

Question 9

drawbacks of using H2 as mobile phase

Answer 9

cannot be used with mass spectrometry detector as it breaks down the vacuum pump

explosive gas

Question 10

what is temperature programming

Answer 10

at low temp of column > more volatile compounds move through column quickly > separated well but less volatile compounds take very long time

solution: temp programming where temp of column is gradually increased

Question 11

advantages of temp programming

Answer 11

compounds with mixture of low and high boiling points of similar polarity can all have sufficient retention resolution yet all be eluted in reasonable amount of time

sharper peaks to improve sensitivity

low temp allows used of splitless or on-column injection

Question 12

disadvantages of temp programming

Answer 12

may cause sloping baseline if there is stationary phase bleed due to decomposition of stationary phase at high temp

time is needed to cool the column oven between separations

Question 13

advantages of pressure programming

Answer 13

high pressure gives higher flow rate

increased pressure reduce retention times of late eluting peaks

same effect as high temp programming but high temp not required

reduces likelihood of decomposing thermally sensitive compounds

Question 14

disadvantage of pressure programming

Answer 14

costly to purchase and maintain high pressure pump