Gas Chromatography Flashcards
what is chromatography
- Separation process that is achieved by distributing the components of a mixture between 2 phases
what are the two phases
o A stationary phase – chemical partitioning
o A mobile phase – contains the analytes
- Mobile phase flow will drive separation
- Analytes interact different with the stationary phase
o Analytes favouring the mobile phase will take short time to elute
o Analytes favouring the stationary phase will take longer time to elute
separation fundamentals
- The stationary phase is usually fixed in a column
- Mobile phase
GC- inert gas such as N2, He or H2
LC- water mixed with organic solvent
how is separation achieved
2 component mixture
- Onto stationary phase (polar)
- Continuous flow of mobile phase (varying polarity)
- Called norm phase chromatography
- More polar the component the slow it will travel through the column
Components have different affinities for the phases
Greater affinity for stationary phase (more polar)
- Spends more time in that phase not moving
- Less time moving in mobile phase
- Moves through system slowly
Components have different affinities for the phases
Less affinity for stationary phase (less polar)
- Spends less time in that phase
- More time moving in mobile phase
- Moves through system quickly
chromatographic parameters
- Resolution Rs
- Retention (capacity) factor, K
- Selectivity factor (α)
- Efficiency- number of theoretical plates (N)
resolution
see powerpoint for resolution equation
Gas chromatography
Sometimes referred as GLC – Gas-Liquid-Chromatography
- Mobile phase is a gas (usually nitrogen or helium)
- Stationary phase is a liquid (very high boiling point)
Separates volatile organic compounds (VOC)
Analyte in gas phase (suitable for thermostable, non polar compounds
Gas chromatography instrument
see powerpoint
mobile phase
- Three main gases (nitrogen, hydrogen and helium, helium most frequently used)
- Carrier gases drives the analyte forward through the column
- Must be chemically inert
gas choice- van deter plots
- You want a high linear flow in GC so that the separation is quick
- The higher the linear flow, the higher the back pressure so want a gas with low viscosity
- Also there is less time for retention of analytes (less efficient due to high theoretical plate height)
- Carrier gas choice and linear velocity significantly af fect column separation efficiency (illustrate using van Deemter plots)
SEE POWERPOINT FOR GRAPH
The optimum linear velocity for each gas is at the lowest point on the curve, where plate height H is minimised and efficiency is maximised
- Nitrogen provides the best efficiency
Steepness of its van Deemter plot on each side of optimum means that small changes in linear velocity can result in large negative changes in efficiency - Helium has a wider range for optimal linear velocity, but offers slightly less efficiency
Only a small decrease in efficiency when velocity changes slightly - Hydrogen has the shortest analysis times and the widest range of average linear velocity over which high efficiency is obtained
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 500C higher than the b.pt of the least volatile component of the sample
- Many inlet types exist including
Split/splitless
two sample inlet modes
- Split: proportion of the analyte/solvent gas passes onto the column, most exits through the split outlet
- Splitless: all analyte/solvent gas enters the column
splitless injection
- In splitless mode the split vent is closed during the first part of injections
- All sample goes into the column (much higher detection limits)
- Splitless is used for low concentration samples
- Must be careful with solvent peak
split injection
- 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 and leads to average detection limits
- Primarily used for non-trace analysis of volatile samples
two types of GC columns
packed
capillary
packed GC columns
- Finley divided, solid support materials coated with liquid stationary phase
- Made of glass or stainless steel
- 1.5 – 10m in 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- 5 micrometers
- High efficiency
- Small sample size
- Used for analytical application
- WCOT stationary phase include unreactive silicones, saturated hydrocarbons, esters and amines
WCOT
wall coated open tubular
PLOT
porous layer open tubular
SCOT
support coated open tubular
GC oven settings
- The column is contained in a thermostatic oven controllable to 0.1 degrees C (separation of the analytes depends on vapour pressure which depends on temperature)
- Separation can be improved by adjusting column temperature