Peak refocusing using subsequent retentive trapping and strong eluent remobilization in liquid chromatography: a...

The present paper presents the mathematical expressions for the concentration enhancement which can be expected when applying a subsequent retentive trapping and strong eluent remobilization process in a packed trap column connected to the end of an analytical separation column. The established expressions for the optimal loading times, trap dimensions and expected concentration enhancement are illustrated and confirmed using numerical simulations of the trapping and release process. These simulations also provide a direct insight in how the bands are deformed and sharpened during the different steps of the process. The simulations, as well as the established expressions, for example show that in the backward elution mode the loading time should exceed a minimum value to allow the strong eluent front to fully overtake the remobilized band as this is the necessary condition to get a completely sharpened peak (in both the backward and the forward elution modes). The simulations also show it is very critical that this occurs as close as possible to the trap exit, as the bands are most sensitive to band broadening once they have been sharpened. The refocusing traps should hence be filled with particles producing the smallest possible plate heights, and connected to the detector using very short and small i.d. connection tubing. It was also found that, if there would be no band broadening in the trap, the achievable concentration enhancement would be the same for either a strong or weakly retaining trap. The true advantage of a high retention in the trap is that it leads to small trap volumes, which in turn minimizes the distance the band has to travel to reach the detector. This then minimizes the band broadening (inevitable in practice) and helps keeping a high concentration enhancement.