Stationary phase Carbon-based columns

For the detection of mineral acids in the presence of an excessive amount of nitrate, the IonPac AS2 separator column was developed from which bromide and nitrate elute after sulfate. The selectivity of this stationary phase is based on the hydrophobic properties of the exchange groups bound to the latex beads (see Section 3.3.1.2). As shown in Fig. 3-47, small quantities of chloride, orthophosphate, and sulfate can be determined in the presence of high amounts of nitrate. The best separation is obtained with an eluent mixture of sodium carbonate and sodium hydroxide. 

What differentiates SFC from other separation techniques such as GC and HPLC is the use of this supercritical fluid as the mobile phase. Analytes that cannot be vaporised for analysis by GC and have no functional groups for sensitive detection with HPLC, can often be separated and detected using SFC. In SFC, the sample is carried by a supercritical fluid (typically carbon dioxide) through a separating column where the mixture is divided into unique bands based on the amount of interaction between the analytes and the stationary phase in the column. As these bands leave the column their identities and quantities are determined by a detector. 

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. 

Chromatographic approaches have been also used to separate nanoparticles from samples coupled to different detectors, such as ICP-MS, MS, DLS. The best known technique for size separation is size exclusion chromatography (SEC). A size exclusion column is packed with porous beads, as the stationary phase, which retain particles, depending on their size and shape. This method has been applied to the size characterization of quantum dots, single-walled carbon nanotubes, and polystyrene nanoparticles [168, 169]. Another approach is hydro-dynamic chromatography (HDC), which separates particles based on their hydro-dynamic radius. HDC has been connected to the most common UV-Vis detector for the size characterization of nanoparticles, colloidal suspensions, and biomolecules [170-172]. 

One disadvantage of all silica-based stationary phases is their instability against hydrolysis. At neutral pH and room temperature the saturation concentration of silicate in water amounts to lOOppm. Solubility increases with surface area, decreasing particle diameter drastically with pH above 7.5. This leads also to a reduction of the carbon content. Hydrolysis can be recognized during the use of columns by a loss in efficiency and/or loss of retention. Bulky silanes [32], polymer coating [33], or polymeric encapsulation [34] have been used in the preparation of bonded phases to reduce hydrolytic instability, but most of the RPs in use are prepared in the classical way, by surface silanization. Figure 2.3 schematically shows these different types of stationary phases. 

The suitability of a stationary phase for a specific separation depends upon the selectivity of the phase. This is a measure of the degree to which polar compounds are retarded relative to their elution on a nonpolar phase. A systematic method for expressing the retention data is based on retention indices. For this sytem, the retention indices of the n-paraffins are by definition equal to 100 times the number of carbon atoms in the molecule. For example, the retention index for n-hexane is 600 and for n-octane 800. These values are defined and apply regardless of the column used and regardless of the temperature. 

Both capillary and packed columns are available for use. Much of recent interest in SFC is based on the stability and high efficiencies capillary columns. Because these columns are deactivated, the elution analytes with unmodified carbon dioxide is possible. Some of the comm stationary phases available as capillary columns are as follows ... 

Ion exchange chromatography is based upon the differential affinity of ions for the stationary phase. The rate of migration of the ion through the column is directly dependent upon the type and concentration of ions that constitute the eluent. Ions with low or moderate affinities for the packing generally prove to be the best eluents. Examples are hydroxide and carbonate eluents for anion separations. 

Rohrschneider [205,210] has developed a scheme for the characterization of stationary phases for gas chromatography. The scheme is based on the retention index (/). The retention index is a dimensionless retention parameter, designed to be independent of flow rate, column dimensions and phase ratio. The retention index of a solute is defined as 100 times the number of carbon atoms in a hypothetical n-alkane, which shows the same net retention time as that solute. This definition is illustrated in figure 2.2. By plotting the logarithm of the net retention time against the number of carbon atoms in n-alkanes, a straight line is obtained. The net retention time for a solute may then be located on the vertical axis, and the retention index found on a horizontal scale, which represents 100 times the scale for na... 

Chemical stability of carbon over the entire pH range has led to considerable interest in the development of carbon-based stationary phases for RPC. Porous graphitised carbon with sufficient hardness, well-defined and stable pore structure without micropores, which ensures sufficient retention and fast mass transfer can be prepared by a complex approach consisting of impregnation of the silica gel with a mixture of phenol and formaldehyde followed by formation of phenol-formaldehyde resin in the pores of the silica gel, then thermal carbonisation and dissolution of the silica gel by hydrofluoric acid or a hot potassium hydroxide. solution [48. The retention and selectivity behaviour of carbon phases significantly differs from that of chemically bonded pha.ses for RPC. Carbon adsorbents have greater affinity for aromatic and polar substances so that compounds can be separated that are too hydrophilic for adequate retention on a Cix column. Fixed adsorption sites make these materials more selective for the separation of geometric isomers [49]. 

Polydivinylbenzene and polystyrene polymer based stationary phases also eliminate these effects. Porous graphitic carbon provides a highly non-polar surface with excellent chemical stability under acidic and basic conditions. However, they suffer from lower sample loading capacity and lower efficiency than conventional columns. 

Several other stationary phases made from different proportions of typical phases (methyl, phenyl, cyanopropyl), or from special compounds such as polytrifluropropyl-siloxane, or different columns such as PLOT (porous layer open tubular), columns coated with a modified graphitized carbon or with a silicone based polymer with chiral groups incorporated into the polymeric chain, columns coated with derivatized cyclodextrins (for the separation of chiral compounds), etc. are also utilized. 

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