Column surface chemistry

The basic requirements for an aqueous SEC column are (1) the beads must exhibit an extremely hydrophilic surface chemistry, (2) the beads should exhibit... 

If you move left one column in the periodic table from the halides, the chalcogenides need two electrons to complete their valence shell, and thus can bond to the surface and each other simultaneously. This appears to account for much of the interesting surface chemistry of chalcogenide atomic layers. Chalcogenides, including oxides (corrosion), are some of the most studied systems in surface chemistry. The oxides are clearly the most important, but significant amounts of work have been done with sulfur, selenium and tellurium. 

Obviously, the monolithic material may serve its purpose only if provided with a suitable surface chemistry, which depends on the desired application. For example, hydrophobic moieties are required for reversed phase chromatography, ionizable groups must be present for separation in the ion-exchange mode, and chiral functionalities are the prerequisite for enantioselective separations. Several methods can be used to prepare monolithic columns with a wide variety of surface chemistries. 

One of the important operational variables in CEC is the analyte—sorbent interaction. In reversed-phase separations (typical in CEC) the hydrophobicity of the stationary phase determines the selectivity of the separation, and retention can be controlled by adjusting the surface chemistry of the packing, composition of the mobile phase, and temperature. In contrast to HPEC, the CEC column plays a dual role in providing a flow driving force and separation unit at the same time hence electrophoretic and chromatographic processes are operational. The stationary phase chemistry is dealt with in detail in Section III on column technology. 

A continuous-bed or monolith is a capillary containing a wall-supported porous continuous bed that is formed in situ. These columns have been developed for CEC use in recent years.The surface chemistry can be functionalized to convert it into a phase with the desired chromatographic properties. Monolithic columns are stable and have shown great potential for CEC due to the absence of a requirement for retaining frits, thereby eliminating the drawbacks in OT-CEC and packed columns. 

However, silica monoliths and organic polymers both exhibit very advantageous chromatographic characteristics enhanced mass transfer characteristics, high reproducibility, and versatile surface chemistry, which make monolithic column attractive for a variety of forward-looking applications. 

From these nine basic quantities, numerous other SI units may be derived. Table B.2 lists a number of these derived units, particularly those relevant to colloid and surface chemistry. The table is arranged alphabetically according to the name of the physical quantity involved. Note that instructions for the use of the conversion factors —depending on the direction of the conversion —are given in the top and bottom headings of the columns. Table B.2 is by no means an exhaustive list of the various derived SI units Hopkins (1973) reports on many additional conversions, as do most handbooks and numerous other references. 

Name Type Surface chemistry Column dimensions Lifetime... 

The use of hydrothermally formed retaining frits in capillary columns packed with stationary phase particles is an accepted limitation in CEC. The introduction of the frit to hold the packed bed is vital, yet introduces problems such as EOF and flow non-uniformities, compromised frit permeability [87], capillary fragility, increased likelihood of bubble formation [88] and a thermally induced modified frit surface chemistry which can detrimentally alter the chromatography [23]. Practical aspects to be considered include the appreciable effort and skill of the analyst who is required to repeatably manufacture capillaries of a particular phase and redevelop the fritting and packing methodology for each different stationary phase type. 

Little has been done to date to tailor the surface chemistry of the stationary phase in CEC. Novotny prepared a series of monolithic columns incorporating a variety of monomers that differ in the length of their pendant alkyl chains [35]. Unfortunately, the performance of these monoliths containing butyl, hexyl, and dodecyl methacrylate... 

The importance of tailoring surface chemistry was first demonstrated by three different monolithic capillary columns that were prepared by directly incorporating of the chiral monomer 2-hydroxyethyl methacrylate ([Pg.239]

These examples clearly demonstrate the benefits of the facile tuning of surface chemistry afforded by the monolithic media. The wealth of commercially available monomers possessing a wide variety of functionalities, together with the extreme simplicity of the preparation of the monolithic columns, makes this approach an appealing option for the design of capillary columns with high selectivities. 

The need of column configurations and surface chemistries especially designed for CEC is now generally appreciated and novel approaches to improve the column technology for CEC—MS applications include the use of monolithic stationary phases [109,110], open-tubular capillary columns [86] and chip technology [111]. These configurations are currently under detailed investigation and the future will have to prove their applicability in routine analysis. 

The positioning of fibrils between the lithographic columns was made specific through the use of both noncovalent and covalent chemistry shown in Figure 16. Dinca first deposited a layer of photobiotin on the column surface, this layer was then exposed to ultraviolet light in order to activate... 

The NPD is similar in design to the FID (flame ionization detector), except that the hydrogen flow rate is reduced to about 3 mL/min, and an electrically heated thermionic bead (NPD bead) is positioned near the column orifice. Nitrogen or phosphorus containing molecules exiting the column collide with the hot bead and undergo a catalytic surface chemistry reaction. The resulting ions are attracted to a collector electrode, amplified, and output to the data system. The NPD is 10-100 times more sensitive than FID. 

Since the pioneering work of Knox et al. on CEC [9,10], porous silica particles have been used as the column packing material in the majority of research studies and applications. Porous silica has a number of characteristics that make it suitable for use in CEC. These are a large surface area, a high surface potential at moderate pH values, which allows the generation of a high EOF, and the commercial availability of materials with various surface chemistries. However, other support materials, such as polymeric phases [11] and alternative inorganic base materials [12], are also applicable in CEC. 

Figure 1-9 demonstrates that satisfactory separation could be obtained by optimization of either efficiency or selectivity or both at the same time. Efficiency is essentially the property of the column, but selectivity is the reflection at the nature of analytes and the surface chemistry of the packing material. Combination of these descriptors would allow the characterization of the overall separation power of a particular chromatographic system. 

Most geometry-related properties of packing materials are related to the column efficiency and fiow resistance particle size, particle shape, particle size distribution, packing density, and packing uniformity. Surface-chemistry-related properties are mainly responsible for the analyte retention and separation selectivity. 

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