Gas chromatography

Question 1

gas chromatography principles

Answer 1

separation of nanogram to microgram quantities of vaporised sample in a gas steam passing through a column containing a stationary liquid or solid phase. Components migrate at different rates due to differences in b.p., solubility and adsorption

Question 2

gas chromatography applications

Answer 2

very widespread use, almost entirely organic.
rapid, simple
can cope with separations of complex mixtures (100 + )
copes with v. small amounts of analyte ( nanogram)
qualitative and quantitative analysis
2-5% relative precision

Question 3

gas chromatography disadvantages

Answer 3

must be volatile and thermally stable below 400 degrees C

Question 4

gas-solid chromatography (GSC)

Answer 4

stationary phase is a solid and adsorption is the major sorption process

Question 5

gas-liquid chromatography (GLC)

Answer 5

stationary phase is aa liquid and the sorption process is mainly partition

Question 6

carrier gas.

Answer 6

flow rates typically 0.5-50 ml min-1

commonly N2, He or H2 - high purity

Question 7

flash-vaporisation injector

Answer 7

used with packed and capillary columns. diluted analytes injected into heated zone where the are flash volatilised and carried onto the column by the carrier gas flow. Can be used with analyses up to 500 amu. 250 degrees

Question 8

Split/splitless injector

Answer 8

Type of flash-vaporisation injector
in splitless mode the waste trap is closed and all of the sample is transferred to the column
In split mode there is an adjustable flow through the waste trap and only a proportion of the flow (e.g. 1 in 50) is directed onto the column. Good for concentrated solutions of analyte - higher cones, using a split instead of diluting

Question 9

On-column injector

Answer 9

Used with capillary columns where analyte concentrations are dilute (10-100 ng micro litre -1). Solution injected directly into column at low temp, 50C, using a very fine needled syringe.
Solvent evaporates GC oven temp raised to bring analytes to vapour phase
Heating avoided, thermal degradations and molecular weight discrimination reduced . Thermally stable compounds of weights up to 1000 amu

Question 10

GC column

Answer 10

coil of stainless steel, glass or fused silica tubing between 1 and 100 m long, internal diameter 0.1-3 mm
thermostatically controlled to within 0.1C

Question 11

Packed column

Answer 11

2-3 m long by 2-3 mm internal diameter
packed with inert porous granular soil support which is coated with a thin film of stationary phase (1-5 % w/w of the support)
Particle size of the solid support crucial - smaller means lower mass transfer term, C.u, more theoretical plates generated by the column - usually 125 -150 micro m

Question 12

Capillary columns

Answer 12

superior resolving power
100,000 plates for a 30 m capillary column compared with a few thousand for a packed column
usually: fused silica 15-50 m long, 0.1-0.6 mm internal diameter
externally coated with polyamide - mechanical strength
internal wall coated with 0.1 -5 micro m thick stationary phase

Question 13

Column selection

Answer 13

Polarities of analyte and stationary phase somewhat alike. as column polarity increases the retention of non-polar compounds decreases and the retention of polar compounds increases
low molecular weight - long column, high stationary phase loading (thick)
high molecular weight - shorter column, low stationary phase loading (thin)

Question 14

Temperature programming

Answer 14

beginning analysis at low temp and gradually raising at constant rate. achieves optimal resolution and detection limits for high and low molecular weight components
Disadvantages - lengthening analysis times due to cooling between runs

Question 15

isothermal analyses disadvantages

Answer 15

highly complex mixtures with varying boiling points - inadequate results
early eluting components may not be resolved
too low temp, later eluting components unacceptably long retention times - peaks broaden to point where theyre undetectable

Question 16

detector needs

Answer 16

respond rapidly (ideally instantly) to composition changes
possess a wide linear operating range
exhibit high sensitivity and stability

Question 17

flame ionisation detector (FID)

Answer 17

effluent gas from the column is mixed with H2 and air and burned at a small jet. the jet forms the -ve electrode of an electrolytic cell. The +ve collector electrode is positioned above the flame
the potential across the two electrodes being ca. 200 V
organic compounds burn in flame creating ions which create a current between the electrodes. Responds to all organic compounds except formic acid and low response to water
v. high sensitivity and widest linear range (E7) of any detector

Question 18

electron capture detector (ECD)

Answer 18

based on use of a beta-ray ionising source
carrier gas flows through the source, 63N and 3H ionise the gas forming slow electrons which migrate towards the anode - standing current when only the carrier gas is present
analyte with high electron affinity elutes some electrons captured -reducing the current in proportion to its conc.
V. sensitive to halogens, S, anhydrides, peroxides, conjugated carbonyl, nitrites, nitrates and organometallics
V. insensitive to hydrocarbons, alcohols, ketones and amines
linear range - 10^2 to 10^3

Question 19

derivatisation

Answer 19

compounds too in volatile or thermally unstable chemically modified i.e. protic or hydrogen bonding sites converted to apolar groups
also used to enhance separations or increase sensitivity of specific analyses towards selective detectors
high yield reactions, one step and mild conditions to avoid degrading other moieties

Question 20

derivatisation: alcohol or amine

Answer 20

BSA = N,O-bistrimethylsilylacetamide
Trimethylsilylation
polyfunctional - fine

Question 21

derivatisation: acylation

Answer 21

Ac2O / pyridine

alcohol to acetate

Question 22

derivatisation : esterification

Answer 22

H+ / CH3OH

carboxylic acid to ester