Isocratic optimum range

In a sample containing many different solutes, with isocratic elution it is sometimes impossible to choose a suitable mobile phase that will result in all k values being within the optimum range. If this is the case, the chromatogram may appear as in Fig. 4.3a. 

The sample of figure 5.14 can be eluted under isocratic conditions, with all capacity factors in the optimum range. 

However, gradient elution is often employed, as an alternative isocratic development, to avoid the design and construction of the optimum column which is seen as a procedure which can be tedious and time consuming. Samples that contain solutes that cover a wide polarity range, when separated with a solvent mixture that elutes the last component in a... 

For our purposes, n is plotted as a function of [hiba]tot for all of the lanthanides, as shown in fig. 12. Vertical lines on this plot provide guidance for choosing the optimum concentration for an isocratic elution. Based on this calculation, the mutual separation of Ce, Pr, Nd is attained at about 0.03 M hiba at pH 4.5. However, under these conditions, Er, Tm, and Yb are relatively poorly differentiated. The dotted line spanning the hiba concentration range of 0.0003 M to 0.1 M represents a possible effective gradient for optimum separation of all of the lanthanides. Note that the predicted eluting position... 

Recendy several studies have reported the use of analytical columns packed with sub-2 pm size particles (11, 12, 14, 15). These columns are especially designed for use with ultrahigh-performance liquid chromatography (UHPLC). The smaller particles are more efficient because they can be used at higher linear velocities, which provides both better resolution and shorter analysis times. UHPLC methods have been reported using isocratic and gradient elutions, a variety of mobile phases, and aqueous buffers with pH ranging from 2.6 to 8.6 to ensure optimum ionization and intensities for specific mass spectrometer ionization sources (14). 

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