Separator mixed-mode

Shaped) selectivity new and others Shape selectivity. .. old Saulen Low pH-value 2D- separation Mixed mode in general Orthogonality of column types... 

Separation by size can occur even on a nonporous material, as the flow in the center of a flow channel is faster than that near the walls. Since large molecules are excluded from the wall regions, they tend to travel in the faster flow down the flow channel center, a phenomenon known as hydrodynamic chromatography (HDC).5658 As discussed in Chapter 1, a mixed-mode form of HDC called "slalom chromatography" has found application in DNA analysis. HDC, however, is far less efficient as a separation process than GPC. 

In reality, many proteins demonstrate mixed mode interactions (e.g., additional hydrophobic or silanol interactions) with a column, or multiple structural conformations that differentially interact with the sorbent. These nonideal interactions may distribute a component over multiple gradient steps, or over a wide elution range with a linear gradient. These behaviors may be mitigated by the addition of mobile phase modifiers (e.g., organic solvent, surfactants, and denaturants), and optimization (temperature, salt, pH, sample load) of separation conditions. 

This section deals briefly with classical methods based on conventional mixing of the sample and reagents such as the batch mode and low-pressure flow mixing methods, as well as the use of CL detection in continuous separation techniques such as liquid chromatography and capillary electrophoresis for comparison with the unconventional mixing mode. 

Ferguson et al. [18] reported on the application of a mixed-mode HPLC separation, coupled with ESI-MS for the comprehensive analysis of NPEOs and nonylphenol (NP) concentrations and distributions in sediment and sewage samples. The mixed-mode separation, which operates with both size-exclusion and reversed-phase mechanisms, allows the resolution of NPEO ethoxymers prior to introduction to... 

Fig. 9. Basic principle of a mixed mode P-CAC the unwrapped cylinder shows the two chromatographic layers separated by an inert layer...

Fig. 11. Experimental chromatogram of a separation of a solution containing PGMs and base metals using a mixed mode P-CAC system 1...

The recent development of mixed-mode stationary phases may provide new approaches for the separation of neuroactive steroids from biological matrices. These stationary phases typically embed a charged functional group within the nonpolar carbon chains typically employed in RP separations, and as such separations based on both RP and ion-exchange characteristics can be designed. Examples of this column type are the Primesep line of columns from SIELC Technologies. This separation approach maybe quite useful for the separation of steroid sulfates from complex mixtures. 

Ohyama, K., Fujimoto, E., Wada, M., Kishikawa, N., Ohba, Y., Akiyama, S., Nakashima, K., and Kuroda, N. (2005). Investigation of a novel mixed-mode stationary phase for capillary electrochromatography. Part III separation of nucleosides and nucleic acid bases on sulfonated naphthalimido-modified silyl silica gel.. Sep. Sci. 28, 767—773. 

Fernandez et al. [27] applied HPLC with diode array detection (DAD) to the determination of heroin, methadone, cocaine and metabolites in plasma after mixed-mode SPE. Analytes were separated using a RP8 column (250 mm x 4.6 mm i.d., 5 jam particle size) and acetonitrile-phosphate buffer pH 6.53 as mobile phase with elution in the gradient mode. The method, which provides a LLOQ of 0.1 j,g/mL for all compounds, was successfully applied to 21 plasma samples from fatal overdoses. 

---