Gradient elution high-pressure systems

Moreover, low-pressure systems often use three or four solvents. This multiple-solvent blending might also be useful for the optimization of both isocratic and gradient elution methods. This is an advantage that the high-pressure system also has when not using a gradient system, the operator has two independent isocratic pumps. 

The main advantages of gradient over isocratic systems are provided in the analysis of unknown or multicomponent samples where a range of solvent strengths may be required both to ensure that aU the components loaded are eluted and also that the retention times of later eluting peaks are minimised. Gradient systems may be subdivided into two classes those where solvents are mixed prior to the pump (low pressure systems) and those where solvents are mixed after the pump (high pressure systems). 

Figure 32. Gradient elution technique A) Low-pressure system, B) High-pressure system...

An HPLC method using progressive electrochemical detection of SPA was described by McCabe and Acworth (128). Samples were mixed with hexane, and SPA were extracted with acetonitrile. An HPLC analysis of the extracts was performed, without an evaporation step, on a high-pressure Coul Array system in which analytes were detected on two coulometric array-cell modules, each containing four electrochemical sensors attached in series after the column. Analytes were separated on a Supelcosil LC-18, 5-/tm column using gradient elution and detected at potentials of —50, 0, 70, 250, 375, 500, 675, and 825 mV. To remove oxidative impurities to be coeluted with BHT, a guard cell with applied potential of 900 mV was also placed in the system. 

Various improvements in HPLC systems have been reported and include an inexpensive injection and pressure relief device [283], automatic gradient elution devices [284, 285], and a high precision sampling device [286]. Some idea of the potential sophistication possible in HPLC can be judged from the description of the use of an on-line computer coupled to a liquid-chromatograph [287]. 

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