Extension of the Gradient Downwards by Constant Slope

The earlier gradient start from %B = 45% down to 10% in steps of five causes a significant change in the separation. The two peaks for ethyl paraben and benzene even move past each other. Interesting is that the last peak is shifted toward the end only by almost a minute (differences between 0.97 and 1 are shaded gray), this means that butyl paraben in contrast to all earlier peaks remains stuck at the front of the column. The peak width of butyl paraben remains constant, although the retention times increase. The elimination concentration does not change either. 

The smaller the differences are, the further the substances move due to the respective gradient extension. Only when ln(/c) 5, that is, k 150 do the substances freeze, they do not move any further, and without forward movement no separation is possible. In contrast to propyl paraben, toluene and chlorobenzene do show a certain degree of mobility despite higher retention times, even in the 12-min chromatogram. This is explained by the higher ln(/c)-slope ofthe parabens, through which the ln(/r) limit is reached earlier. 

It should be noted that by reducing the gradient initial value %B more changes are to be expected in the front part of the chromatogram as in the rear section. 

The upshot of the gradient extension and the dwell time is when %B is reduced from its maximum value % o,max down to zero, then the farther forward that the peaks come in the chromatogram, the greater is the change in the separation. This can lead to a better separation, but also a change for the worse. In the rear section, only a slightly modified separation is to be expected. 

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