GC Flashcards
In GC lowering the temperature will do what to retention times
Will increase
Two methods to control the temperature in GC
Isothermally, temperature programming
Isothermally
Constant column temperature
When would you use Isothermally programming
Only useful if components have a similar boiling point. Otherwise will produce overlapping peaks which means the mixture wasn’t well separated. This method would also have long run times
Temperature programming
Changing the temperature during the separation.
When to use temperature programming
If components have a similar boiling point
Improving separation conditions will have what affect
Provide better separation
reduce the runtimes
Allow complex mixtures to be separated
Types of columns used in GC
Packed or capillary
Examples of capillary columns in GC
SCOT and WCOT (both examples of fused silica glass capillary columns)
SCOT
Extra layer of support
Support costed open tubular column
there is an extra layer of support material inside the column which the stationary phase is coated to
WCOT
Wall coated open tubular column
The stationary phase is bound to the capillary coating.
GC:
Mobile phase
Columns
Stationary phase
Temperature control
Detectors
Mode
MP: inert gas eg helium
Packed or capillary columns
High boiling point liquid eg PEG
Via oven (temperature programming or Isothermally)and injector port
FID
Partition
Benefits of using an FID
Similar response to different kinds of compounds, detects a wide concentration range, linear response, good resolution and is non destructive
Packed column
Material
Dimensions
Contents
Efficiency
Diagram
Glass or steel
5mm diameter 1-10m long
Crushed fire brick support w liquid coating SP ie PEG
300-3000 plates per metre
Packed (not open)
Uneven SP coating
Capillary column
Material
Dimensions
Contents
Efficiency
Diagram
Silica glass
0.5mm diameter, 10-100m long
1um SP layer
SP has high MW, heat stable polymer
High efficiency More than 10,000 plates per metre
polyimide used as coating to strengthen silica glass
Why are capillary columns more efficient?
The even coating of SP on silica ensures even amounts of sample hitting the detector
SCOT
Has a solid support coated with SP on the capillary column
WCOT
a thin layer of stationary phase, is coated on the capillary’s inner wall.
Split injection
Controls the fraction of the sample that enters the colimn
Splitless injector
Loads the entire sample into the column which is useful when completing trace analysis
SP in GC
High MW
Heat stable
Polymer eg silica
Things that affect quality of separation in GC
Flow rate and temperature
How an FID works
The eluent mixes with H and is burned in air, this produces a flame that contains electrons and the cation. Applying a potential between the flame’s tip and the collector gives a current that is proportional to the concentration of cations in the flame.
Schematic GC diagram
MP and ideal characteristics
Injector split or split-less
Oven with temperature regulation
Columns (packed or capillary) holds SP
SP ideal characteristics
Detector FID/mass spec
State one advantage and one disadvantage of metal columns used in GLC.
Are robust but may react with acids eg steel
State one advantage and one disadvantage of glass columns used in GLC
are less reactive and see-through but they are fragile
Isothermal chromatogram
constant temperature can lead tooverlapping peaks ie not as good separation and long run times e.g. 60 minutes
Temperature programming chromatogram
provides a good baseline, good separation of peaks, and shorter runtimes e.g. 20 minutes rather than 60 minutes .
Benefit of using temperature programming
Better separation of complex mixtures.
If you have a complex sample which will take too long to elute, what is wrong with simply raising the temperature? Why is temperature programming more useful?
If you just increase the temperature this will provide a shorter run time. However, it will not provide better separation as the peaks will be overlapping. Temperature programming is better in this example. If you are using temperature program between 30 and 90° does a provide better separation in the beginning and delays the later analytes.
Packed columns v capillary column characteristics:
Packed columns: are made of glass or steel, are large and short and provide low efficiency
Capillary columns: are made of silica glass, are thin and long. With a layer of stationary phase coated on the capillary and provide high efficiency
Which column provides better efficiency
a capillary column is better then a packed column. Capillary columns are more efficient i.e. they provide thin narrow peaks on the chromatogram.
Ideal characteristics of GC stationary phase
Dissolves analytes
Forms a thin film
Inert (does not react)
Thermally stable (doesn’t degrade)
Lower volatility (does not evaporate )
Able to form a thin film
Mobile phase ideal characteristics GC
Gas is the mobile phase.
Pure
Free from oxygen
Compatible with detector
If you believe a peak is component a. What what should happen if you add some to the sample and re-run? What is this called?
If it is a: the peak should get bigger
If it is not a: a new peak at a different retention time should appear and no change to the original sample
This is called “spiking “
What does the flame do in FID. Explain how flames and ionisation are involved in the working of FID use a diagram
the analytes are mixed with hydrogen, they come into contact with oxygen and are burned in the flame. This creates ions and they are counted. The ions are detected across the anode and cathode.
Advantages FID:
Good for linear response
Good for a wide range of concentrations
Good at detecting low concentrations
Disadvantages of a FID:
I had of the components have a low molecular weight
Not good at detecting halogens
Destroys the sample
The characteristics of a good detector are the same for GLC and HPLC. Learn top six
Similar response to all kinds of compounds Wide concentration range
Low detection limit
Linear response
Good resolution
Fast response to change
Stability
Reliability
Ease of use
Working temp. range ambient – 400oC
And be non-destructive
Why are capillary columns more efficient and packed columns:
Capillaries columns have an even layer of stationary phase does allows analytes to hit the detector more evenly
Partition chromatography
SP
MP
Interactions
Diagram
Uses
Support material eg C18
Gas/liquid eg methanol and water
Solution/ polarity
Diagram sample dissolved in liquid phase coated on surface of solid support)
HPLC paper
Ion Exchange
SP
MP
Interactions
Diagram
Uses
Polymer SO3- N(CH3)3+
Ionic solution
Electrostatic
Diagram + molecule bonds to - beads and - molecule elute quickly
HPLC/ column
Size exclusion
SP
MP
Interactions
Diagram
Uses
Porous gel or beads
Liquid
Exclusion based on size
Diagram large molecules eluding faster as they push through the beads)
What does the flame do in FID
Counts the number of carbon atoms, once the hydrogen in an organic compound is burned, the FID can tell us about the analyses concentration
ideal characteristics of gases to be used in GLC.
Volatile
Different molecular weights
What is the effect on retention time of a polar sample in GC if you..’
halving column length
doubling solvent flow rate
decreasing stationary phase particle size increasing density of mobile phase increasing the temperature
Increasing the polarity of the mobile phase
Shorter rt :halving column length
Shorter rt: doubling solvent flow rate
Longer rt: decreasing stationary phase particle size
Longer rt: increasing density of mobile phase increasing the temperature
Longer rt: Increasing the polarity of the mobile phase
Benefit of using temperature programming
Good baseline
Good peak separation
Short run times eg 20 v 60 minutes
Packed v capillary column
Both fused silica glass columns
PACKED
large and short
Low efficiency
CAPILLARY
thin and long
Layer of SP coated on capillary
High efficiency
What does good efficiency mean?
Thin narrow peaks
GLC
Mode
sp
mp
Partition mode
sp: liquid eg peg in a column either packed or capillary
Packed: liquid coated on a solid support and pa led into column
Capillary column - liquid bonded to wall of glass silica capillary
Restrictions on mobile phase in GLC
The mobile phase carrier has must be:
Pure
Compatible with the detector
Oxygen free
Injector in GC
Heated
1ul
Splitter used with capillary columns
Types of capillary column
(SCOT)
Core is empty, stationary phase on particles attached to wall. Less common, can give problems.
OR
Core is empty, stationary phase is covalently bonded to wall. More common
GC sp desirable properties
Low volatility (high boiling point)
Thermally stable
Chemically inert
Dissolves analyte
Able to form a thin film
Dissolves the analytes to be separated
Oven in GC
How to operate it
Oven temperature very important - must be controlled within 0.1oC
Isothermally or temperature programmed
Oven can operate from 30 – 400oC and can heat up or cool down at 0.5 – 50oC/min
Care – packed columns must not be heated fast – lag in temperature
Isothermally
one temperature only
Suitable for components which have similar boiling points
Temperature Programmed
(Ramped) – temperature increases during the separation
Used for mixtures with a broad range of boiling points, allows much better chromatograms
An ideal detector for GLC would have:
Similar response to all kinds of compounds
Wide concentration range
Low detection limit
Linear response
Good resolution
Fast response to change
Flame ionisation detector
How are analytes identified in GC
The analytes are identified by comparing Retention Times with standards under the same conditions
Spiking
adding the standard. If the peak you think is the standard is, it will increase
Advantages of GC
Linear response
Wide concentration range
Can detect as low as 1 ng
Suitable for use with capillary columns
Easier to use than Mass Spectrometer (see later) Robust
Disadvantages of gc
Can not detect low molecular weight compounds such as CO2,
Poor detection of halogens
Destructive, so can not collect eluent after detection
Applications of GC
Pollutants in water and air samples Volatiles in wine samples
Urine and blood samples for drug residues
Restrictions on analytes in GC
Volatile
fairly non-polar
thermally stable
Split v splitless injector
The most common inlet for capillary GC
can be operated in two modes, spilt or splitless.
during theinjection process, the liquid sample is vapourised into the gas
phase prior to transfer onto the capillar column.
Fid
It’s
high sensitivity and linear range for carbon-containing
compounds make it very popular in organic analysis.
& volage of
horun 200 and 300V across these components
The flame ionisation detector
produces a proportional response to the number of carbon
ators in a molecule.
