The main differences gas chromatography (GC) and liquid chromatography (LC) are primarily due to the differences in the properties of gas and liquid. The viscosity of a liquid is approximately 100 times greater than that of gas, surface tension is approximately 10,000 times greater, and density is approximately 1000 times greater. Gas also has a higher compressibility factor. The diffusion coefficient of a solute in a liquid is much smaller than it in gas. The effect of the liquid phase on the theoretical plate height in mass transfer processes is particularly significant.
Below are 6 Factors that mainly Cause Liquid Chromatography Peak Diffusion.
1.Eddy Diffusion. Eddy diffusion occurs when the mobile phase undergoes vortex-like movements within the chromatographic column due to factors such as variations in particle size and uneven packing of the stationary phase.
2.Molecular Diffusion. Molecular diffusion occurs along the axial direction of the column due to concentration gradients, leading to concentration gradient diffusion and broadening of the peaks. The contribution of molecular diffusion term B/u can be neglected since the mass transfer velocity of the liquid mobile phase is slow.
3.Mass Transfer Resistance. Interactions between solute molecules and the stationary and mobile phases cause diffusion, distribution, and transfer processes to take place with finite rates, resulting in a non-equilibrium state within the chromatographic column and peak diffusion.
4.Flow Rate. The optimal flow rate corresponding to the minimum H is generally difficult to observe. As the flow rate decreases, H also decreases. When u > uopt, H increases with increasing flow rate, and the chromatographic peak broadening caused by mass transfer becomes more pronounced.
5.Particle Size of the Stationary Phase. Both the eddy diffusion term (A) and the mass transfer resistance term (Cm) are directly proportional to the square of the particle diameter (dp) of the stationary phase. Therefore, the particle size of the stationary phase has a significant impact on peak diffusion.
6. Column Temperature. The column temperature directly affects the diffusion coefficients (DS and Dm) of molecules in the stationary and mobile phases, thereby influencing the rates of molecular diffusion and mass transfer. An increase in column temperature leads to an increase in DS and Dm, which results in increased molecular diffusion and reduced column efficiency. However, improved mass transfer leads to higher column efficiency.
