Boundary conditions gradient elution

Another example is the determination of bentazone in aqueous samples. Bentazone is a common medium-polar pesticide, and is an acidic compound which co-elutes with humic and/or fulvic acids. In this application, two additional boundary conditions are important. Eirst, the pH of the M-1 mobile phase should be as low as possible for processing large sample volumes, with a pH of 2.3 being about the best that one can achieve when working with alkyl-modified silicas. Secondly, modifier gradients should be avoided in order to prevent interferences caused by the continuous release of humic and/or fulvic acids from the column during the gradient (46). 

Figure 2.2 Classical boundary conditions of common chromatographic problems, (a) Elution of a rectangular pulse, (b) Multiple gradient elution of a rectangular pulse, (c) Staircase frontal analysis, (d) Displacement. Dotted line component 1. Shaded line component 2.

In gradient elution the boundary condition for the additive is (Figure 2.2b)... 

The simplest continuous injection profile is a linear profile, i.e., a linear concentration gradient [23]. So, let us assume as the boundary condition of the problem a trapezoidal injection profile with a linear ramp ending in a plateau of constant concentration (see Figure 7.5). This profile is the classical boundary condition for the strong solvent in linear gradient elution (see Chapter 2, Section 2.1.4). Its equation is... 

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