Gas Chromatography

Question 1

In gas chromatography what things a special

Answer 1

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)

Question 2

What are the types of interactions the solutes can have with the stationary and mobile phase

Answer 2

Stationary:
Solubility and polarity

Mobile:
Condensation and vaporization

Question 3

How does an analyte move through the GC

Answer 3

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

Question 4

In GC if things have a higher bp (less likely to boil) what phase do they stay in

Answer 4

They evaporate and turn back into gas slower so they stay in the stationary liquid phase longer

Question 5

What does the retention factor K depend on in gas chromatography

Answer 5

The Enthalpy of boiling of the solute and the Enthalpy of mixing with the stationary phase

Question 6

The retention factor k increase when

Decrease when

Answer 6

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)

Question 7

What is the retention index and why do we use it

Answer 7

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

Question 8

What are the conditions needed to do a retention index

Answer 8

The ladder needs to be on the same stationary phase as the unknown with isothermal conditions

Question 9

How to calc index slide 6

Answer 9

In tablet put on sheet

Question 10

Slide 8 explain why the K values are what they are

Answer 10

I’n tablet

Question 11

What are the charactertic of the columns for GC

Answer 11

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)

Question 12

What are the two types of columns in gas chromatography

Answer 12

Open tubular (the middle is free flowing)

Packed (the particles are inside the column)

Question 13

What are open tubular columns in gc

Answer 13

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

Question 14

What are packed columns in GC

Answer 14

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

Question 15

Effect of column length on tesolution slide 11

Answer 15

In pics put on sheet

Question 16

Effect of column length on resolution what happens if the length is longer

Answer 16

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

Question 17

If the stationary phase is thicker how does this affect the peaks

Answer 17

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

Question 18

What can happen to stationary phases to make them decompose

Answer 18

Column bleed: Some stationary phase evaporates off due to oxidation and hydrolysis damaging the coating

Question 19

What happens if column bleed occurs

What can increase the bleed

What can decrease it

Answer 19

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

Question 20

What is the general elution problem

Answer 20

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

Question 21

How do we fix the general elution problem

Answer 21

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

Question 22

What types of carrier gasses are used for the mobile phase in GC

Answer 22

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

Question 23

Chemical interaction with the mobile phase are ____ important than with the stationary phase/temp

Answer 23

Less

Question 24

What is used to inject samples into the gc

Answer 24

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

Question 25

What is a SPLIT injection port

What happens if it’s not used

Answer 25

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)

Question 26

What are the types of detectors for GC

Answer 26

Universal

Selective

Mass spec

Question 27

What are universal detectors

Answer 27

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

Question 28

What are selective detectors

Answer 28

Mainly used for gc

They only respond to certain analytes

Ex. Only heteratoms (cl,F) and not carbons

Question 29

What is the mass spec detector

Answer 29

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

Question 30

Flame ionization is more sensitive to ______

What does this mean

Answer 30

More sensitive to more carbons

If more carbons it elutes slower and would have a larger signal