Gradient capabilities, micellar

It is always recommended to use the same column with the same type of surfactant. A column should be dedicated to the anionic surfactants, a second one to the cationic surfactants, etc. The reproducibility of the results in MLC depends on the column equilibration. The adsorbed layer of surfactant should be done correctly. It was shown that the time to reach the equilibrium between the stationary phase and the mobile phase could be very long in ion-pair chromatography with sub-micellar mobile phases. Two days at 1 mL/min were necessary to equilibrate a 15 cm x 4.6 mm i.d. column of Hypersil ODS with a mobile phase containing 0.0003 M CTAB [19]. These low surfactant concentration solutions do not contain micelles. So, they are not used in MLC. With a micellar phase, the equilibration time is reduced. It is possible to use the rapid gradient capability just mentioned above. Typically, a mobile phase containing a high surfactant concentration (10 to 100 cmc) can be used to quickly saturate the column with surfactant. Then 5 to 10 column volumes are used to rinse the column with the mobile phase containing the desired amount of surfactant. 

Landy J S, Dorsey J G 1984 Rapid gradient capabilities of micellar liquid chromatography. J Chromatogr 22 68-70... 

In 1980, Armstrong moved to Georgetown University at Washington DC. He started to apply his ideas on the capabilities of micellar phases to HPLC. The first MLC article presented the separation of phenols and PAIfc. The first published MLC chromatogram is shown in Figure 3.4 [16]. The efficiency of the o-cresol peak (Peak 7) was only 800 plates for 30 cm ODS column (h.e.t.p = 375 pm or 38 particle diameters). This low efficiency was probably not pointed out because a flow gradient was used [16]. 

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