Multimodal separation methods

MulCifacior optimization system, 91-92 Multimodal separation methods, 41 Multiresidue mediods for analysis of pesticides, 793-799... 

An examination of technical literature and trade publications indicates that a wide variety of instmments are commercially available for PSD analysis.The classical methods are based on either electrical properties (e.g., the Coulter Counter principle) or optical properties (e.g., laser scattering) of the analyte. However, none of these techniques are separation methods. Because particulate dispersions are often highly complex in terms of the polydispersity index, multimodal size distribution, and density, it is hardly possible, without the use of separative methods, to obtain an accurate determination of their... 

Comparisons of LC and SFC have also been performed on naphthylethylcar-bamoylated-(3-cyclodextrin CSPs. These multimodal CSPs can be used in conjunction with normal phase, reversed phase, and polar organic eluents. Discrete sets of chiral compounds tend to be resolved in each of the three mobile phase modes in LC. As demonstrated by Williams et al., separations obtained in each of the different mobile phase modes in LC could be replicated with a simple CO,-methanol eluent in SFC [54]. Separation of tropicamide enantiomers on a Cyclobond I SN CSP with a modified CO, eluent is illustrated in Fig. 12-4. An aqueous-organic mobile phase was required for enantioresolution of the same compound on the Cyclobond I SN CSP in LC. In this case, SFC offered a means of simplifying method development for the derivatized cyclodextrin CSPs. Higher resolution was also achieved in SFC. 

Different kinds of heterogeneity can be imagined. In the most simple case only a few differing structural entities are found to coexist without correlation inside the volume irradiated by the primary beam. In this case it is the task of the scientist to identify, to separate and to quantify the components of such a multimodal structure. In an extreme case heterogeneity may even result in a fractal structure that can no longer be analyzed by the classical methods of materials science. 

Stasko et al. [8] elaborated two interesting procedures of multimodal TLC for the separation of taxol and related compounds from Taxus brevifolia. For the first procedure, a cyano-modified silica gel plate was developed, in the first dimension, in dichloromethane-hexane-acetic acid (9 10 1) and, in the second dimension, in water-acetonitrile-methanol-tetrahydrofuran (8 5 7 0.1). For the second procedure, a diphenyl-modified silica gel plate was developed, in the first dimension, in hexane-isopropanol-acetone (15 2 3) and, in the second dimension, in methanol-water (7 3). These two methods enabled paclitaxel resolution from cephalomannine, which is impossible on silica gel plates, and at least another 20 compounds. 

The term multimodal has been used in two ways in TLC, to designate layers such as bonded cyano sorbents that can operate with two or more mechanisms (see Section IV.C) or, in the context of this section, to specify multidimensional separations that are performed by coupling TLC, HPTLC, or OPLC (223) with another technique, such as gas chromatography, supercritical fluid chromatography (224), countercurrent chromatography (225), and, most commonly, HPLC (145, 226-228), in order to improve the separation capacity available from either of the individual methods. For example, the combination of adsorption AMD-HPTLC and partition HPLC for water analysis produced as many as 700 individual densitometric peaks (49). Multimodal TLC separations have been reviewed (229-231). 

Some methods of producing polymers can lead to distinct subpopulations, widely separated in mass. When there is a distribution with only a single maximum, the population is termed unimodal. When two or more identifiable maxima exist within a polymer population, it is termed multimodal that is, bimodal for two subpopulations, trimodal for three, and so on. 

There are many classes of CSPs applicable in different mobile-phase modes. In particular, CSPs based on derivatized polysaccharides, native and derivatized cyclodextrins, macrocyclic glycopeptides, and Pirkle-type chiral selectors operate quite well in four separation modes, i.e RP, polar organic phase, NP, and super- or subcritical fluid chromatography (SFC) conditions. It is common that a chiral compound can be separated on the same CSP in more than one separation mode [58, 160, 166, 170-176]. For example, Nutlin-3, a small molecule antagonist of MDM2, has been baseline resolved from its enantiomer in all four mobile-phase conditions (Fig. 16) [170]. Multimodal enantioseparation on the same CSP would be greatly beneflcial for chiral method development in pharmaceutical industry. 

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