Lecture 2 - GC

Question 1

Which factors effect rate of migration in GC?

Answer 1

• Type and properties of the SP
• Intermolecular forces between components and the SP
• Temperature (doesnt effect order of elution)
• Carrier gas flow rate (doesnt effect order of elution)

Question 2

What does the order of elution depend on for a non specific SP?

Answer 2

• The boiling point of the compounds
• Compounds with larger boiling points will be retained for longer.
• For low boiling points, there is more of the compound in the carrier gas and less in the SP so can move through column faster.

Question 3

If there are compounds with similar boiling points how are these separated?

Answer 3

By using a specific SP

Question 4

What is the Kovat plot?

Answer 4

This allows the retention index of a certain chemical compound to be calculated, this is done by adjusting the retention index based of of standards from n-alkanes.

When compounds from a homologous series are separated, there is similarities in there retention index.

It is plotted via no of carbon atoms vs log of adjusted retention time.

Known as Kovats retention index, I.

Question 5

What is Kovats retention index used for?

Answer 5

To quantify the retention of an analyte by comparing it with the retention time of a pair of n-alkanes from a homologous series.

I = 100z + 100 (log(alpha)z,u/log(alpha)z,z +1

z is the number of carbons in the SMALLEST alkene
z + 1 is the number of C atoms in the HIGHER alkene
(a)zu is the selectivity constant for the first alkane and unknown
- (a)z,z+1 is selectivity for the 2 alkanes

Question 6

How are the selectivity constants calculated?

Answer 6

• Using adjusted retention times (retention time - void time)
  alpha = adjusted RT of later eluting peak/adjusted RT for first eluted peak

Should always be > 1

Question 7

What are the alpha values for increasing alkanes?

Answer 7

Methane - 100
Ethane - 200 etc

As carbons increase the number + 100

Question 8

What does the kovats index allow?

Answer 8

• Allows an appropriate SP to be selected for a particular application
• The I value indicates how strongly the analyte interacts with the SP
• A large value of I indicates that the analyte of interest interacts with the SP strongly

Question 9

What is the second classification?

Answer 9

• McReynolds constant

Question 10

What is the McReynold constant?

Answer 10

• Known as DeltaI, and is the difference in a compounds Kovat index on a standard non-polar liquid phase with that of the compounds Kovat index of the SP of interest.

Question 11

What are the 5 standards for McReynolds constant? and what are they used for?

Answer 11

Benzene, n-butanol, pentanone, nitropropane, pyridine.

• These five compounds are used to examine the retention characteristics of different analytes on a particular stationary phase

Question 12

How would you use the McReynolds graph of numbers to choose which SP should be used to separate 2 compounds?

Answer 12

• If the compounds are polar you would pick a polar solvent (squalene being the most non polar)
• But the one with the biggest I value difference between the 2 compounds would be chosen
• Example, if a non-polar SP was used for polar molecules, the I values wouldn’t have a big difference.