Stationary Phases for RPLC

To better distinguish the contributions of polar interactions to retention, the LEER model was transformed into the so-called hydrophobic subtraction model (HSM) for RPLC, where the hydrophobic contribution to retention is compensated for by relating the solute retention to a standard nonpolar reference compound. This approach was applied to characterize more than 300 stationary phases for RPLC, including silica gel supports with bonded alkyl-, cyanopropyl-, phenylalkyl-, and fluoro-substituted stationary phases and columns with embedded or end-capping polar groups. The QSRR models can be used to characterize and compare the suitabihty of columns not only for reversed-phase, but also for NP and HILIC systems. 

Stella, C., Galland, A., liu, X., Testa, B., Rudaz, S., Veuthey, J. L, Carrupt, P. A. Novel RPLC stationary phases for lipophilicity measurement solvatochromic analysis of retention mechanisms for neutral and basic compounds. /. Sep. Sci. 2005, 28, 2350-2362. 

Many of the possible column combinations that are useful in 2DLC are listed in Chapter 5. Besides the actual types of column stationary phases, for example, anion-exchange chromatography (AEC), size exclusion chromatography (SEC), and RPLC, many other column variables must be determined for the successful operation of a 2DLC instrument. The attributes that comprise the basic 2DLC experiment are listed in Table 6.1. 

There are two other phases indicated in figure 3.8. The first is a so-called pyrocarbon material. Such a stationary phase is formed by pyrolizing an organic layer on a silica substrate. The idea is to combine the mechanical strength of silica with the chemical inertness of carbon. The value of 14 used here can be thought of as typical for carbonaceous materials. These materials do not seem to behave like non-polar phases in the tradition of chemically bonded phases for RPLC, but rather like phases of intermediate polarity. Hence, as for silica, they may be most useful in the reversed phase mode for the separation of very polar molecules using aqueous mobile phases. 

Polar chemically bonded stationary phases (section 3.2.2.2) may be used as an alternative stationary phase for both RPLC and LSC, if variations in the mobile phase do not result in an adequate separation. If polar CBPs are used in combination with more polar mobile phases (reversed phase mode), then table 3.10c may be used to find the most appropriate optimization parameters. If operated in the normal phase mode, table 3.1 Od... 

Figure 5.33 Chromatograms obtained during the optimization of the composition of a ternary mobile phase for RPLC for the separation of five substituted diphenyl amines (DPAs). Solutes (1) N-nitroso-DPA, (2) 4-nitro-DPA, (3) 2,4 -dinitro-DPA, (4) DPA and (5) 2-nitro-DPA. Stationary phase Hypersil ODS. Figure taken from ref. [576]. Reprinted with permission.

The original work by Tswett was carried out with a solid stationary phase (LSC), and most of the early LC work used silica gel as the stationary phase. For this reason it has become the convention to label LSC methods using silica gel as normal phase LC (NPLC). To generalize, a normal LC system is one that has a polar stationary phase and a nonpolar mobile phase. The opposite situation is called reverse phase LC (RPLC). We noted in Table 1 that RPLC is now the most common type of LC, and consequently these two terms, NPLC and RPLC, do not reflect current usage and can be misleading. Unfortunately they are well established in the chromatographic literature and therefore will be used in this monograph as just defined. 

Park et al. reported a method to modify zirconia by adsorption with BSA and, subsequently, crosslinked by glutaraldehyde. The BSA-zirconia showed good enantios-electivity for some enantiomers and could be used for RPLC separation in mobile phases of alkaline pH. They also developed a carboxymethyl-p-cyclodextiin-coated zirconia stationary phase for the separation of racemic 2,4-dinitrophenyl amino acids. 

There are at least two main sources of resistance to mass transfer (Figure 5.4 [96]) external film mass transfer resistance and intrapartide diffusion that is composed of pore and surface diffusion. The latter diffusion is insignificant in numerous adsorbents but plays an important role in most adsorbents used in RPLC. For particles having micropores, there is an additional mass transfer resistance, the resistance to diffusion through micropores which is often important. This explains why considerable attention is paid in the preparation of stationary phases for FIPLC to avoid the formation of micropores. This explains also why graphi-tized carbon black, which tends to be plagued by a profusion of micropores, has not been a successful stationary phase for HPLC. 

A particular column can be used for different types of LC by changing the eluent components. For example, a column packed with RP-18 bonded silica gel can be used for SEC with THF, NPLC with n-hexane, and RPLC with aqueous acetonitrile. When separation cannot be achieved by improving the theoretical plate number of a column, it may be achieved by selection of an appropriate stationary phase material and/or eluent. 

Standardisation of RP columns is an issue of debate. RPLC stationary phases were reviewed and compared and the ideal properties were defined [543]. For the development of RP packing materials, see Dolan [544]. 

BHA, BHT, PG, TBHQ and tocopherols) a variety of stationary phases, mobile phases and detectors can be used [711]. Common antibacterials such as carba-dox, thiamphenicol, furazolidone, oxolinic acid, sul-fadimethoxine, sulfaquinoxaline, nalidixic and piromidic acid can be analysed by GE-RPLC-UV (at 254 nm). Collaborative studies have been reported for the HPLC determination of the antimicrobial sodium benzoate in aqueous solutions [712], Plastics devices used for field collection of water samples may contain polymer additives (such as resorcinol monobenzoate, 2,4-dihydroxybenzophenone or bisphenol A) or cyanobac-terial microcystins [713],... 

---