Selection, stationary-phase

Several hundred types of Hquid phases are commercially available. These have been used individually or in combination with other Hquid phases, inorganic salts, acids, or bases. The selection of stationary phases for a particular appHcation is beyond the scope of this article, however, it is one of the most important chromatographic tasks. Stationary phase selection is discussed at length in books, journal articles, and catalogs from vendors. See General References for examples. 

In common with all multidimensional separations, two-dimensional GC has a requirement that target analytes are subjected to two or more mutually independent separation steps and that the components remain separated until completion of the overall procedure. Essentially, the effluent from a primary column is reanalysed by a second column of differing stationary phase selectivity. Since often enhancing the peak capacity of the analytical system is the main goal of the coupling, it is the relationship between the peak capacities of the individual dimensions that is crucial. Giddings (2) outlined the concepts of peak capacity product and it is this function that results in such powerful two-dimensional GC separations. 

In many respects, the coupling of GC columns is well suited since experimentally there are few limitations and all analytes may be considered miscible. There are, however, a very wide variety of modes in which columns may be utilized in what may be described as a two-dimensional manner. What is common to all processes is that segments or bands of eluent from a first separation are directed into a secondary column of differing stationary phase selectivity. The key differences of the method lie in the mechanisms by which the outflow from the primary column is interfaced to the secondary column or columns. 

Stationary Phases The best general purpose phases are dimethylsiloxanes (DB-1 or equivalent) and 5% phenyl/95% dimethylsiloxane (DB-5 or equivalent). These rather nonpolar phases are less prone to bleed than the more polar phases. The thickness of the stationary phase is an important variable to consider. In general, a thin stationary phase (0.3 /im) is best for high boilers and a thick stationary phase (1.0 /urn) provides better retention for low boilers. (For more detailed information, see Stationary Phase Selection in Appendix 2.)... 

The specific interactions that will produce the necessary retention and selectivitv must dominate in the stationary phase to achieve the separation. It follows that it is important that they are also as exclusive as possible to the stationary phase. It is equally important to ensure that the interactions taking place in the mobile phase differ to as great extent as possible to that in the stationary phase in order to maintain the stationary phase selectivity. 

The last tvo approaches represent promising beginnings for new methods to characterize stationary phase selectivity. The methods are evolutionary and not fully developed at present. Their future prospects are quite good and should eventually evolve into a standardized protocol for phase characterization. This is urgently required to make both the selection of stationary phases from those currently available and the rationale synthesis of new phases a logical process. 

Selecting proper sample solvent and stationary phase selectivity can prevent sample break-through. 

KayiUo, S., Deimis, G.R., and ShaUiker, R.A., An assessment of the retention behaviour of polycyclic aromatic hydrocarbons on reversed phase stationary phases selectivity and retention on C18 and phenyl-type surfaces, J. Chromatogr. A, 1126, 283, 2006. 

The relative importance of column efficiency, retention and selectivity is presented with applications to the analysis of complex, volatile samples. Recent trends in the analysis of aroma samples by high resolution gas chromatography include utilizing specific stationary phases to analyze particularly difficult samples. Important considerations in selecting the optimum column for an analysis include the overall efficiency generated by the column, the partition ratio of the solutes to be resolved and the selectivity of the stationary phase towards the compounds of interest. After comparing the relative contributions these three factors, methods of optimizing stationary phase selectivity will be described. 

To a first approximation [393] the selectivity (a) on a given stationary phase may be expected to be independent of the mobile phase density. Consequently, the problem of stationary phase selection is similar to that encountered in GC. In GC each stationary phase will require a given temperature at which the capacity factors are in the optimum range. In SFC, each stationary phase will require a given mobile phase density. Different phases may be compared at their individual optimum conditions. 

Figure 12.3. Stationary phase selection guide. Courtesy of EM Labs.

A general account of chromatographic theory was presented in volume 2 of Encyclopedia of Pharmaceutical Technology.Therefore, the following discussion will focus specifically on GC theory. The separation of the component of a mixture depends upon the column performance (efficacy) and the relative retention capability of the stationary phase (selectivity). The former determines the width of the peaks relative to the length of time a component spends in the column, while the latter determines the relative position of each emerging component (resolution). 

Another possibility is the use of alternative stationary phases. A strong trend of the last decade is the employment of spedalty phases in challenging and complex separations. Thus, newer Cg, NH2, CN, mixedmode phases (materials incorporating both ion exchange and reversed-phase moieties), new polymeric phases, and zirconia-based materials offer attractive stationary-phase selectivities. 

The second example is a hplc experiment in the reversed-phase mode involving the separation of nitrogen bases. Here the stationary phase selectivity is altered for some bases by the addition of a nickel complex to the mobile phase. As demonstrated by LochmUller and Hangac (6) these uncharged, coordinatively unsaturated complexes interact selectively with some bases but not others. Enhanced resolution is acheived at low concentrations (.0001 M) because some bases undergo a 10-fold Increase in retention. The result is... 

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