Gas chromatography

Question 1

How is separation achieved in GC?

Answer 1

◾ separation based on distribution of analytes in two-phase system
◾ moblie phase inert gas
◾ analytes differentially retained (based on distribution constant)
◾ distribution partially determined by:
- boiling point
- selective interactions

Question 2

Name the function and types of mobile phases used in GC

Answer 2

function:
◾mobilise analytes through column

types:
◾hydrogen gas
◾helium
◾nitrogen

Question 3

Name the types of stationary phases used in GC as well as the type of separation

Answer 3

Apolar: PDMS
boiling point based separation

Polar: PEG (wax)
separation based on a number of interactions

Question 4

How is resolution optimised?

Answer 4

◾ Retention factor 
       - decrease temp
       - change film thickness
       - change phase
◾ Efficiency
      - increase L
      - decrease internal diameter
      - work at optimum velocity
◾ Selectivity
      - change phase

Question 5

Name Golay’s conclusions on open tubular columns

Answer 5

◾ Hmin = dc
smaller internal diameter columns will be more efficient and faster
◾ Uopt = 2.1 (Dm/rc)
hydrogen or helium should be used for faster analysis (higher Dm)

Question 6

What is the general elution problem and how is it solved?

Answer 6

◾Problem encountered when analysing molecules with large variation in k using isothermal analysis
◾weakly resolved peaks and late eluting peaks very broad
SOLUTION: temperature programming

Question 7

Name the factors that can lead to band broadening related to the columns used in GC

Answer 7

◾ Eddy diffusion (packed columns)
◾ Longitudinal diffusion
◾ Resistance to mass transfer