The transition from elution development to frontal analysis is shown experimentally in figure 5 where progressively larger charges are placed on the column starting with 10 ml and ending with 16 ml. Frontal analysis curves can also demonstrate the need to take certain precautions when employing an external loop valve for injecting the sample. As a result of the parabolic velocity profile of the sample and mobile phase as it is discharged from the sample loop onto the column, dispersion of the sample volume occurs resulting in the concentration profile of the sample entering the column taking the form of a tailing Poisson function as opposed to a rectangular slug. Thus, if the sample loop is left in line with the mobile phase flow subsequent to injection, the peaks will exhibit serious tailing. This tailing is shown in the lower frontal analysis curves in figure 6.
After, J. Chromatogr., Ref. (3).
Figure 6. Frontal Analysis Curves Demonstrating Dispersion Effects Resulting from Incorrect Sampling Techniques