Gas Chromatography

?
  • Created by: Rscottqub
  • Created on: 03-03-20 14:05
The mobile phase is always
a gas
1 of 72
the stationary phase is normally a
liquid, but can be a solid
2 of 72
what is GC used for ?
to separate volatile compounds
3 of 72
what controls the flow rate ?
flow controller
4 of 72
the sample is inserted by the
sample injector
5 of 72
KT - Inflammable
easily set on fire
6 of 72
Carrier gas used
Helium, Hydrogen, Nitrogen, Argon , CO2
7 of 72
all of these are
Inert - except Hydrogen
8 of 72
Hydrogen as a carrier gas
low density, inflammable, reacts with unsaturated compounds
9 of 72
Helium as a carrier gas
v. good thermal conductivity, expensive
10 of 72
Nitrogen
cheaper , reduced sensitivity
11 of 72
what do we want our carrier gas to be
inert, easily available, suitable to the detector , less risk of explosion
12 of 72
Carrier gas choice depends on
the type of detector
13 of 72
Flow rate is controlled by
1. a 2 stage regulator on the cyclinder. 2. controls within the instrument , also FR depends on type of collumns
14 of 72
Inlet pressure of 10-50 psi will result in a FR of ___ in packed collumsn
25-150ml/min
15 of 72
10-50 psi to capilary will result in FR of
1-25ml/min
16 of 72
If the inlet pressure is constant
the FR will be constant
17 of 72
Collumn efficiency depends on
the injection port
18 of 72
samples need to be suitable
size
19 of 72
large samples
result in band spreading + poor resolution
20 of 72
we want to inject
the narrowest band possible
21 of 72
Most common injection method
microflash vaporizer (cheaper)
22 of 72
other method
rotary sample valve - more precise/advance, reproducible results , solid samples require crushing for this
23 of 72
what is used to inject the sample
syringe - to inject into the hot stream of the mobile phase
24 of 72
the sample injected needs to be
High temp - at least 50C above BP to ensure vapor. Small sample
25 of 72
How much do we inject into packed columns
1-5 micro litres
26 of 72
how much do we inject into capiallry
has a split valve to introduce small amounts
27 of 72
Oven temp
so important - needs to be controlled to within 1/10 of degree.
28 of 72
Why is temp control so important
to ensure well separated peaks
29 of 72
How is temp increased
in steps, or continuosly
30 of 72
the Minium temp
will give good resolution but a longer elution time
31 of 72
Types of detectors
Hot wire, Flame ionization, electron capture
32 of 72
T/F - GC detectors are x4/5 more sensitive than liquid Chrom
TRUE - common MCq
33 of 72
Hot wire aka TCD - thermal conductivty
2 wires heated electrically, we assume temp is in equilibrium with resistance , compares TC of 2 gases - the carrier gas and the sample
34 of 72
advantages of hot wire
non destructive, simple, easy maintenace
35 of 72
disadvantages
low sensitivity , cant analyse biosamples, affected by fluctuations in temp
36 of 72
Flame ionisation (FID)
Based on electrical conductivity of carrier gas. Normally gases are insulators , but if ions are present can be conductors . the flame generates electrons - which is measured
37 of 72
advantages -
very sensitive , low background
38 of 72
disadvantages
destructive , insensitive to water,carbon dioxide and sulphur dioxide
39 of 72
insensitive to water
can be used for samples which have been contaminated by water
40 of 72
Electron Capture (ECD)
radio isotope decays and emits electrons. this produces secondary electrons which are collected at annode. if a 20V is applied. molecules containing e. neg can capture these electrons - resulting in a decrease in current
41 of 72
ECD is not as sensitive as FID
FALSE - they are equally sensitive
42 of 72
disadvanatges
can only analyse compounds with electron affinity / e neg groups
43 of 72
types of GC coolum
packed, capillary ( open tubular)
44 of 72
packed mobile phase
inert carrier gas
45 of 72
packed stationary phase
attached to the solid material
46 of 72
Capillary mobile
inert gas
47 of 72
capillary stationary
thin silica coat on inside of tube
48 of 72
packed column properties
made of metal/glass. Greater sample affinity, broader peaks, longer RT, less resolution
49 of 72
how can we improve resolution for packed
inject small uniform particle size
50 of 72
Capillary column properties
normally quartz, but can be metal or glass.
51 of 72
Types of capillary collumn- in terms of stationary phase
WCOT, SCOT, PLOT
52 of 72
WCOT
Wall coated open tubular - walls coated by SP
53 of 72
SCOT
Support coated open tubular - SP coated on a solid support
54 of 72
PLOT
porous layer open tubular - SP coated on pourous absorbent
55 of 72
which column - packed or capillary - gives better separation
Capillary - but it is easily overloaded with too much sample
56 of 72
Resolution of narrower columns
increased resolution
57 of 72
resolution of longer collumns
increased resolution
58 of 72
Choice of liquid stationary phase
based on the " like dissolves like" rule
59 of 72
non polar columns for
non polar solutes
60 of 72
polar columns for
polar solutes
61 of 72
To reduce bleeding of SP
bind covalently to silica, covalently cross link to itself
62 of 72
polar compounds are
less volatile
63 of 72
Less volatile compounds have
higher RT
64 of 72
can all compounds be directly analysed by GC
no , about 25% can, the rest need to be derivitized
65 of 72
why ?
many are thermobile at temp needed for GC
66 of 72
Polar compounds
need derivitisation to obtain a good peak - else would be abosrbed by column
67 of 72
COOH
can form H bonds , hard to vaporise, and may bind to column - to get round this we form esters
68 of 72
OH and amines
Low MW OHs are ok - they can be analysed. High MW OHs - need derivit. - use TMS of chlorTMS--> this produces a compound which is thermally stable and easily vaporised
69 of 72
applications of GC
ID compound, purity testing, ID of mixtures
70 of 72
Advantages of GC
Fast, cheap, non destructive, small samples
71 of 72
disadvantages
only used for volatile samples, must not react with column, no for thermally labile samples
72 of 72

Other cards in this set

Card 2

Front

the stationary phase is normally a

Back

liquid, but can be a solid

Card 3

Front

what is GC used for ?

Back

Preview of the front of card 3

Card 4

Front

what controls the flow rate ?

Back

Preview of the front of card 4

Card 5

Front

the sample is inserted by the

Back

Preview of the front of card 5
View more cards

Comments

No comments have yet been made

Similar Pharmacy resources:

See all Pharmacy resources »See all Gas Chromatogtraphy resources »