Question

[30 Marks] Q 1(b) When comparing column performance between a capillary and a packed GC column, one is struck by how similar the plate height is on the two columns. And yet as shown in Table 1, we see that we get almost 10x better resolution on the capillary column Discuss why this difference in resolution arises, in terms of the van Deemter equation Tabie 1 Comparison of packed and wall coated open tubuiar column performance Open Tubular Packed 0.73 mm 1.5 Property Piate height for methy oleate Resolution of methy stearate and 034 mm 10.6 methyf oleate

Answer 1

A Capillary column for GC is basically a very narrow tube with the stationary phase coating the interior surface .In packed columns the stationary phase is coated onto the packing particles.In all chromatography the seperation of different compounds occurs because of their interaction with the stationary and mobile phases.For example in simple paper chromatography a solvent (water,alcoholetc.)flows over the paper (stationary phase )moving the sample with it.The different compounds which make up the sample will move more or less slowly depending ,in simple terms ,on how much they stick to the paper.In gas chromatography the gas is the mobile phase ,the column coating is the stationary phase and the vapourised sample is seperated by how long the constituent compounds take to emerge from the other end of the column and pass into the detector .This is called the retention time .you can buy columns coated with different stationary phases depending on what type of compounds you want to analyse as the type of stationary phase will determine which compounds pass over it more quickly or slowly.

The van deemter equation- The efficiency of a column is measured by theoratical plates,N th and can be normalized with the length of the column to give the height equivalent theoratical plate ,called HETP or H. The van deemter equation describes the various factors influencing H,and is divided into eddy diffusion ,longitudinal diffusion ,and mass transfer terms..The relative importance of these factors varies with mobile phase velocity.particle size and morphology contribute to H , along with a variety of other factors .Understanding the van deemter equation allows the determination of of the optimum mobile phase velocity

factors affecting column efficiency(plate number)

1.column length

2.particle size

3.packing quality

4.linear velocity

5.instrument quality

6.retention factor

stationary phase particle size is one of the most important factors in the van deemter equation,for a given column length ,the plate number is inversly related to the particle size of the column packing.The smaller the particles ,the higher the plate number and the seperation power.