Gas Chromatography Flashcards
In gas chromatography what things a special
The mobile phase and the analytes are in the gas phase
The stationary phase is a solid or an immobilized liquid
The machine is run at high temps (50-250)
What are the types of interactions the solutes can have with the stationary and mobile phase
Stationary:
Solubility and polarity
Mobile:
Condensation and vaporization
How does an analyte move through the GC
the analyte is pushed across the column by the gas flow of the mobile phase
It can then leave the gas phase by condensing/adsorbing
Once this happens it goes to the surface of the liquid layer where it then diffuses into the layer
The solute then diffuses back to the surface and the time it takes to do this depends on its solubility and intermolecular interactions with the stationary liquid phase
It then goes back to the gas phase
In GC if things have a higher bp (less likely to boil) what phase do they stay in
They evaporate and turn back into gas slower so they stay in the stationary liquid phase longer
What does the retention factor K depend on in gas chromatography
The Enthalpy of boiling of the solute and the Enthalpy of mixing with the stationary phase
The retention factor k increase when
Decrease when
Increase:
The boiling point is higher
The Enthalpy of mixing with the stationary phase is higher
Decrease:
When the columns temp is increased (more boiling, more evaporation back to mobile phase)
What is the retention index and why do we use it
It’s when values of K and Tr vary depending on diff things so it’s different during each run
To account for this we use the index that gives values relative to the straight chain hydrocarbon ladder
What are the conditions needed to do a retention index
The ladder needs to be on the same stationary phase as the unknown with isothermal conditions
How to calc index slide 6
In tablet put on sheet
Slide 8 explain why the K values are what they are
I’n tablet
What are the charactertic of the columns for GC
The columns are long (diff than liquid chromatography) because the flow rates are higher (5-50cM/sec)
The diameters are smaller than in liquid chromatography (0.1mm-5mm)
What are the two types of columns in gas chromatography
Open tubular (the middle is free flowing)
Packed (the particles are inside the column)
What are open tubular columns in gc
The analyte is within a hollow tube lined with the stationary phase of fused silica (or it’s a coiled polymer tube)
The internal diameter is 0.1-1mm
The analyte hits the edge of the tube (the stationary phase) then comes back in to the mobile phase
The a term in the van Demeter equation is 0
What are packed columns in GC
Their larger and longer (10-100 of CN in length) meaning they can take a larger sample size
The tube is made of stainless steel or glass
The tube is packed fully with silica based material that can also be coated with liquid
Effect of column length on tesolution slide 11
In pics put on sheet
Effect of column length on resolution what happens if the length is longer
The peaks will still be wider but they will be better spaced out so the resolution is better
N=L/H
If L increases N increases
If the stationary phase is thicker how does this affect the peaks
The separation of peaks improves because the analyte spends more time in the stationary phase , still broader peaks though
But if the stationary phase gets too thick, the peaks get TOO broad an the resolution worsens
What can happen to stationary phases to make them decompose
Column bleed: Some stationary phase evaporates off due to oxidation and hydrolysis damaging the coating
What happens if column bleed occurs
What can increase the bleed
What can decrease it
The peaks in the chromatogram come off at an angle
To reduce bleed you can use the thinnest amount of stationary phase and the shortest column to get separation
Overheating of the column increases bleed and damages the coating the columns have a max safe temp that they can run at so we can’t go over this limit
What is the general elution problem
There’s a lot of solutes on the column and each thing has its own condition it needs to separate
The condition that gives good results for one don’t work so well with others
there no one single condition that give good results for all
How do we fix the general elution problem
If we start at a cold temp for gc, the low boiling point solutes elute first and have better separation
Then we slowly increase the temp to the max (or 10 degree above the analyte highest bp) and let the high boiling point solutes elute
What types of carrier gasses are used for the mobile phase in GC
Inert gasses that don’t react with the analyte such as:
n2 which is heavier so it gives less efficient diffusion, it runs slower, best at lower flow rate)
H2 and He : light and small, diffusion in Them works very well
Chemical interaction with the mobile phase are ____ important than with the stationary phase/temp
Less
What is used to inject samples into the gc
An auto sampler (same type of thing as a regular injection)
There is a heated injection port which is hotter than the column. It turns the injected liquid to gas
50 degrees above the highe bp sample
What is a SPLIT injection port
What happens if it’s not used
It mixes the injection with carrier gas and rejects a percent of the injection to avoid overloading the column
Allows for better detection and resolution
Tailing peaks occur and they’re not as good resolution (too much sample in)
What are the types of detectors for GC
Universal
Selective
Mass spec
What are universal detectors
These respond to almost all analytes, can tell that it’s there but not exact what it is
Might not respond equally to all
Might have limitations on concentration or distinguishing between compounds
What are selective detectors
Mainly used for gc
They only respond to certain analytes
Ex. Only heteratoms (cl,F) and not carbons
What is the mass spec detector
Is can measure and detect almost all molecules
We can choose to monitor only a specific m/z so that we get a selective response
Flame ionization is more sensitive to ______
What does this mean
More sensitive to more carbons
If more carbons it elutes slower and would have a larger signal
