Question

Compare nitrogen and helium from the van deemter curve. Explain their appearance when it comes to bandwidth in GC. Why is plate height (H) higher when using helium instead of nitrogen at low flow rate and (plate height, H) low at high flow rates?

He Plate height (mm) 7 10 20 30 40 50 60 70 80 90 Linear velocity (cm/s)

Answer 1

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 the optimum mobile phase velocity.

Stationary phase particle size is one of the most important factors in the van Deemter equation. For a given column length, the plate number (N_th) is inversely related to the particle size of the column packing. The smaller the particles, the higher the plate number and the separation power.

The plate number is also dependent on the flow rate (F) of the mobile phase. There is a certain velocity, the so-called optimum flow, at which the plate number is highest (and H is lowest). A lower or higher flow rate provides fewer plates (higher H). In routine HPLC, columns are always operated at velocities above the optimum. The reduced column efficiency is less significant than the shorter analysis time at the higher than optimal flow rates.

The flow rate is inversely proportional to plate height.

He has a low flow rate as compared to H because of the lightness of hydrogen. Thus, He has a higher plate height but H has a lower plate height.