gas chromatography Flashcards
what is chromatography
Separation process that is achieved by distributing the components of a mixture between 2 phases
what is the mobile phase
contains the analytes
what is the stationary phase
enables chemical partitioning
which phase drives separation
mobile
analytes will interact differently with the stationary phase
Analytes favouring the mobile phase will take short time to elute
Analytes favouring the stationary phase will take longer time to elut
separation fundamentals - stationary phase
The stationary phase is usually fixed in a column
how is separation achieved
2 component mixture
- onto stationary phase (polar)
- continuous flow of mobile phase (varying polarity)
- called ‘normal phase’ chromatography
components have difference affinities for the phases
•greater affinity for the stationary phase = more polar
- spends more time in that phase not moving
- less time moving in mobile phase
- moves slowly through system
•less affinity for stationary phase = less polar
- spend less time in that phase not moving
- more time moving in mobile phase
- moves through system quickly
Normal phase
polar stationary phase
less polar mobile phase
non-polar components travel further (faster) than polar
chromatographic parameters
resolution Rs
retention (capacity) factor, K
selectivity factor (a)
efficiency – number of theoretical plates (N)
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what is GC sometimes also referred to as
GLC
gas-liquid chromatography
what state is the mobile phase in GC
gas
usually nitrogen or helium (sometimes hydrogen)
what state is the stationary phase in GC
liquid
very high boiling point
what does GC do
separate volatile organic compounds
analyte in gas phase
-suitable for thermostable, non-polar compounds
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GC instrument
mobile phase in GC
nitrogen, hydrogen, helium
carrier gases drive the analyte forward through the column
must be chemically inert
what do you use to decide on a gas for GC
van Deemter plots
van Deemter plots
want a high linear flow in GC so that the separation is quick
the higher linear flow, the higher the back pressure so want a gas with low viscosity
there is less time for retention of analytes
-less efficient due to higher theoretical plate height
carrier gas choice and linear velocity significantly affect column separation efficiency
-illustrated using van Deemter plots
gas inlet
gas is fed from cylinders through supply piping to the instrument
it is usual to filter gases to ensure high gas purity
gas supply may be regulated at the bench to ensure an appropriate supply pressure
sample inlet - injector
heats injected liquid samples to gas phase
temperature of the sample inlet is usually about 50 degrees Celsius higher than the b.pt of the least volatile component of the sample
many inlet types exist including
-split/split less
split less injection
a proportion of the analyte/solvent gas passes onto the column, most exists through the split outlet
in split less mode, the split vent is closed during the first part of the injection
all sample goes into the column
-leads to much higher detection limits
split less is used for low concentration samples
must be careful with solvent peak
split injection
all analyte/solvent gas enters the column
in split mode, the split vent is opened during the first part of the injection
a fraction of the sample enters the column
- usually ratios such as 1:10, 1:20, 1:50, 1:100
- leads to average detection limits
primarily used for non-trace analysis of volatile samples
what are the two types of sample inlet
split
splitless
what are the two types of GC column
packed
capillary
packed GC columns
finely divided, solid support materials coated with liquid stationary phase
made of glass or stainless steel
1.5 – 10m length
Internal diameter of 2-4mm
Large sample capacity – used for preparative work
capillary/open tubular column
Internal diameter of a few tenths of a mm
10-80m in length
Film thickness of 0.1-5um
High efficiency
Sample size sample
Used for analytical application
WCOT stationary phases include unreactive silicones, saturated hydrocarbons, esters and amines
what does WCOT stand for
wall coated open tubular
what does PLOT stand for
porous layer open tubular
what does SCOT stand for
support coated open tubular
GC oven settings
The column is contained in a thermostatic oven controllable to 0.1 degrees Celsius
-Separation of the analytes depends on vapour pressure which depends on temperature
Separation can be improved by adjusting column temperature
temperature control
When a single temperature is used – isothermal (isocratic)
A changing temp profile is called a temperature gradient
Temp changes can be finely controlled
Can be increased or decreased either quickly or slowly
-Useful when compounds have a wide range of boiling points
Adjustments are made to
- Increase separation
- Increase resolution
- Decrease run time
GC temperatures
for a range of compounds of the same type but with a wide range of boiling points:
at a low temp are all separated but higher boiling ones are broad and take longer to elute
at a higher temp, above boiling point, all will be vapour but lower boiling ones will not be separated
a temp gradient allows all to be separated and resolved, as T rises higher boiling components will vaporise and be separated.
name 5 types of GC detector
flame ionisation detector
thermal conductivity detector
electron capture detector
nitrogen-phosphorus detector
mass spectrometers
choice of detector will depend on the analyte and what the purpose of the analysis is
flamed ionisation detector
Ignition of effluent from column
Organic compounds burning the flame produce ions and electrons
Conduct electricity through the flame
A large electrical potential is applied at the burner tip and a collector is located above the flame
Quantities of the analyte down to ug-level can be detected
Good linearity
Signal is proportionate to concentration
analysis of BAC using GC-FID
Blood alcohol concentration (BAC) corresponds directly to the level of impairment of an intoxicated driver
Breathalyser and field sobriety test provide subjective indication of impairment
Any court requires quantitation of ethanol content
Due to the large number of samples and their relative short hold times there is a need for rapid and accurate tests
Headspace GC is widely used
This combined with a FID is the most common set-up
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Gc detector comparison
GC positives
fast analysis
high efficiency - leading to high resolution
sensitive detectors
high quantitative accuracy
requires small samples
rugged and reliable techniques
GC negatives
limited to volatile samples
not suitable for samples that degrade at elevated temp
not suited to preparative chromatography
requires MS detector for analyte structural elucidation
most non-MS detectors are destructive
separation fundamentals -mobile phase
GC – inert gas such as N2, He or H2
LC – water mixed with organic solvent
