Six Factors Affecting Peak Diffusion in Liquid Chromatography

The difference in rate theoretical equations between gas chromatography and liquid chromatography is mainly due to the difference in the properties of gas and liquid.

The viscosity of the liquid is about 100 times that of the gas, the surface tension is about 10,000 times, and the density is about 1,000 times. Gases also have a high compressibility coefficient. The diffusion coefficient of solute in liquid is much smaller than that in gas, and the influence of liquid phase on the height of theoretical plate is especially great during the mass transfer process. Different from the theoretical rate equation of gas chromatography, liquid chromatography increases the retention flow mass transfer term in the fixed phase pore structure.

Therefore, we can know that there are mainly the following aspects that will affect the chromatographic peak expansion:

1. Eddy diffusion

Eddy diffusion is the eddy movement of mobile phase in chromatographic column, which is caused by the different particle size of column packing and the uneven filling.

2. Molecular diffusion

The concentration gradient of solute molecules on the longitudinal axis of the column caused the concentration diffusion and widened the spectral band after injection. Due to the slow mass transfer rate of liquid mobile phase, the molecular diffusion term B/u can be ignored.

3. Mass transfer resistance

There are interactions between solute molecules, stationary phase and mobile phase molecules, and the process of diffusion, distribution and transfer does not reach equilibrium instantaneously. The actual mass transfer speed is limited, and there are always leading and lagging phenomena. This causes the column to always work in a non-equilibrium state, resulting in peak broadening.

4. Flow rate

It is generally difficult to observe the optimal flow rate corresponding to the lowest H, because H always decreases as the flow rate decreases. When u>uopt, H increases with the increase of u, and the peak expansion caused by mass transfer is more obvious.

5. Stationary phase particle size

The vortex diffusion term A and the flow mass transfer resistance term Cm are proportional to the square of the column packing particle size dp, so the stationary phase particle size has a great relationship with the chromatographic peak expansion.

6. Column temperature

The column temperature directly affects the diffusion coefficient DS and Dm of molecules in the stationary phase and the mobile phase, thus affecting the molecular diffusion and mass transfer rate. DS and Dm increase with the increase of column temperature, and molecular diffusion results in the decrease of column efficiency. The improvement of mass transfer leads to the increase of column efficiency. Therefore, the influence of column temperature on chromatographic peak expansion is contradictory.