Stationary Phases in NP-HPLC

It is possible to apply gradients to silica gel see Fig. 6 for an example. There is no problem with re-eqmlibration if the solvent B is non-localizing, e.g. dichloro-methane. Other solvent systems have also been successfully used, although a 

Column 25 cm x 3.2 mm stationary phase LiChrosorb SI 60 5 pm eluent A n-hexane eluent B dichloromethane gradient 0-2 min 100% A, 2-12 min 0-80% B flow 1 mLmin temperature ambient detector UV 254 nm. 

Compounds (1) 2-phenylethylbromide, (2) 1,4-diphenylbutane, (3) phenetole, (4) nitrobenzene, (5) trans-chlorostilbene oxide, 6 = Sudan red 7B. 

Other polar phases, i.e., silicas with covalently bonded nitrile, nitro, amine, or diol groups, are valuable alternatives to silica. They are less polar (hence the mobile phase can be weaker) and have different selectivities. However, in most cases, they are used in the reversed-phase mode with aqueous mobile phases. Normal-phase applications of some importance are the separation of aromatics on nitro-derivatized phases, and of non-ionic surfactants on diol-derivatized phases. The theoretical plate numbers are usually lower than with non-derivatized sUica. Activity control (see Section 2.2.4 on troubleshooting of NP separations) is not necessary. The stability of the chemical bonding can be limited. 

A rarely used polar stationary phase is alumina (aluminum oxide alox). It has a different selectivity to silica, and its packings show higher theoretical plate heights (i.e., the number of theoretical plates per unit length is lower). In contrast to silica, irreversible adsorption of analytes can be a problem. 

---