Structural Biochemistry/Chromatography/Vandeemter

Van Deemter Plot

General information

The plot

• 
  Van Deemter plot

The Equation

H= A+ (B/u)+ C_su +C_Mu

Variables	Meaning
H	Is the theoretical Plate Height (cm)
u	Velocity of mobile phase (cm/s)
A	Multipath effects ( also known as eddy diffusion)
B	Longitudinal Diffusion coefficient
Cs	Mass Transfers for stationary phase
Cm	Mass Transfers for Mobile phase

The point of the Van Deemter plot is to gauge the column efficiency under gas (GC) or liquid chromatography (LC).¹

Multipath Effect(A)

Zone broadening in the mobile phase is caused by the multiple pathways a molecule can go through a packed column. Diffusion can take place anywhere a molecule transfer from a stream following one pathway to another pathway. At lower velocities, the molecule transfers streams multiple times so the effects are minimal. However, band broadening is more obvious at lower velocities.¹

Longitudinal Diffusion(B/u)

Diffusion is the movement from a concentrated portion of the mobile phase to a dilute region. The rate of diffusion is proportional to the concentration difference as well as the diffusion coefficient. The diffusion coefficient varies from species to species as well as the medium. Longitudinal diffusion goes towards and against flow of the mobile phase. This is not as prevalent in LC as it is in GC¹

C_S and C_M

Not much is know about the complex nature of these two variables. However, theories have been formed.

How to Reduce Band Broadening

The diameter of the packing is important. If you refer to the Van Deemter plot, greater velocities equate to higher theoretical plate height (lower column efficiency). By lowering the temperature, it is diffusion will occur at a less prominent rate in gas chromatography. However, the effect is less pronounced in LC once again.¹

References

¹ Skoog, Holler, & Nieman. Principles of Instrumental Analysis, 6th Ed., 2007.