Isocratic separations of mixtures

Zhang et al. investigated two modes of step gradients for the separation of mixtures of ketones, aldehydes, and aromatic hydrocarbons. The first approach involved changing the inlet and outlet vials as described above. The second method involved dropping another mobile phase directly into the stirred inlet vial using a pipette. Both methods shortened the analysis time and improved the detection limits compared to isocratic elution. The separation of a mixture of 13 aromatic hydrocarbons was achieved in 14 min by changing the mobile phase from 80% methanol to 80% and 90% acetonitrile, respectively [36]. 

The present work demonstrates that there is linear relationship between the log k and both the log V (volume fraction of organic solvent in the mobile phase) and the reciprocal of the absolute temperature. Therefore, it is possible with a small number of initial experimental data of the capacity factor (k ) to predict k for each cephalosporin as a function of T and V, which reveals the optimal elution conditions for the isocratic separation of a mixture of cephalosporins. 

Figure 7.5 Extracted ion chromatogram that illustrates the isocratic separation of a peptide mixture on a Poros particle-filled LC chip GGK (261.1+), AAWGK (532.3+), VDEVGGEALGR (551.32+), YIPGTK (678.4+).

Several other isocratic separations of phospholipids on silica have been reported using mobile phases comprising mixtures of hexane-propanol-water (Geurts van Kessel et al., 1977 Abood et al.,... 

Organometallic systems incorporating either Fe or Si served as representative model mixtures. Figure 2 illustrates a simple isocratic separation of a mixture of six organo-iron compounds. 

A good example of the capability of MLC, in the separation of mixtures of drugs, is given by the screening of twelve sulfonamides in human urine and cow milk with an SDS-2-propanol mobile phase and a hydrophilic endcapped C18 column (Fig. 11.9) [30]. Using a 0.07 M SDS-6% 1-propanol eluent at pH 3.0, isocratic separation of the sulfonamides was achieved within 15 min. 

Figure 2 Isocratic separation of standard mixture of 14 lanthanides and yttrium on a 250 mm x 4 mm column, packed with 5 pm silica IDA. Eluent 1.6x 10" moll" HNO3 with 0.5moll KNO3 flow rate 1.0 ml min " k column temperature 65°C, sample volume 20 pi, sample concentration of each metal was 4 ppm in 0.2% HNO3. Detection, Arsenazo III postcolumn reaction at 658 nm. (Reprinted with permission from Nesterenko PN and Jones P (1998) Isocratic separation of lanthanides and yttrium by high performance chelation ion chromatography. Journal of Chromatography A 804 223-231 Elsevier.)...

Chromatography is essentially a method of separation based on two phases, one stationary and one mobile. If the composition of the mobile phase is not changed during the separation, the term isocratic elution is used. For separation of complex mixtures with wide ranges of polarities, the composition of the mobile phase can be changed during separation, a process known as gradient elution. 

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