Mixed-mode columns

The combination of solid-phase extraction (SPE) with HPLC analysis or preparative HPLC can be a valuable tool in concentrating and identifying degradation products. SPE can be a useful technique for the isolation and concentration of analytes from a complex mixture. Selection of the appropriate column depends on the properties of the API and the suspected degradants (70,71). Mixed mode columns having both non-polar and ion... 

Reversed phase, normal phase, ion exchange, drug columns, mixed-mode columns, oil and grease vacuum manifold and accessories, 96-Well format. 

F. Mashige, Y. Matsushima, C. Miyata, R. Yamada, H. Kanazawa, I. Sakuma, N. Takai, N. Shinozuka, A. Ohkubo and K. Nakahara, Simultaneous determination of catecholamines, their basic metabolites and serotonin in urine by high-performance liquid chromatography using a mixed-mode column and an eight-channel electrochemical detector, Biomed. Chromatogr, 9, 221-225 (1995). 

Walshe, M. Kelly, M.T. Smyth, M.R. Ritchie, H. Retention studies on mixed-mode columns in high-performance liquid chromatography. J.Chromatogr.A, 1995, 708, 31-40... 

The most widely used sorbents with their characteristics are presented in Table 4. Mixed-mode columns containing mixtures of nonpolar (Cg, Cig, C4, etc.) and strong anion or/and strong cation exchange functional groups are available for applications for both acidic and basic compounds. These columns are designed to capitalize on multiple interactions of the analyte or different types of interactions from more than one type of analyte. Mixed-mode columns allow the development of more robust procedures, which are less dependent on the matrix, making cleaner extracts possible. 

In fact, every silica-based reversed phase represents a mixed-mode column, because besides the alkyl chains it has silanol groups, which are groups capable of weak cation exchange. Some C18 materials of the first generation, such as Partisil ODS from Whatman or Spherisorb ODSl from Waters, have only a very low carbon content (<5%) and a high number of free silanol groups. With these... 

Figure 4.16 Procedure with ion-pair chromatography or by the use of mixed-mode columns.

The antidepressant venlafaxine and its metabolite oxydesmethylvenlafaxine were extracted from plasma and separated on a C g/cyanopropyl mixed mode column (electrochemical detector at 0.65 V and 0.98 V). These compounds have UV chromophores as well (phenyl groups). A 10-min analysis was achieved with a 30/70 water (50mM KH2PO4 at pH 4.8)/methanol mobile phase. Note that the buffer has a very low capacity here (nearly equidistant from the first two phosphate pK values). A standard range from 0 to 200 ng/mL was developed and detection limits of 0.5 ng/mL (S/N = 3) were reported [566]. 

Polymer-based mixed-mode columns are commercialized under the trade name Thermo Scientific Dionex OmniPac (Thermo Fisher Scientific, Waltham, MA, USA). They allow separations based on ion-exchange, ion-pair, reversed-phase, and ion-suppression interactions, depending on the type of eluent. Any combinations of these interactions, at the same time or in succession, can also be employed to separate analyte species. 

The ability to apply a combination of UV- and conductivity detection to analyze inorganic cations and vitamins in the same run is advantageous for the determination of water-soluble vitamins with a mixed-mode column. 

Figure 6.75 Types of RP/IEX bimodal mixed-mode columns classified by the arrangement of functional groups.

Technologies, are zwitterionic type mixed-mode columns with either RP or normal-phase (NP) characteristics. Obelise R incorporates a cationic group close to the silica surfece separated from an anionic group by a hydrophobic chain, while in Obelise N the two functional groups are separated by a hydrophilic chain. 

More than 5 years ago, two type IV mixed-mode columns that are based on an alkyl chain with terminal ion-exchange functional groups were commercialized under the trade name Acclaim Mixed-Mode WAX-1 [67] and Acclaim... 

Reversed-phase/anion-exchange mixed-mode columns such as Acclaim Mixed-ModeWAX-1 can be used to separate basic, neutral, and acidic... 

---