Stationary Phases Column Packings 531 Overview

HPLC stationary phase materials have been reviewed and compared on several occasions, e.g., in [21, 22],

In order to obtain log £:w values closely resembling log P„a values, it has been suggested to coat the solid phase with 1-octanol [3, 4, 23], However, this appeared of limited practical use due to a rapid bleeding of the column. A more successful approach is used at Zeneca [24, 25]. 1-Octanol adsorbed onto a hyflo-supercel diatomaceous earth solid support is more stable. When such a system is calibrated with compounds of known log Poct it can be used to obtain satisfactorily log P values for new compounds (J. J. Morris, personal communication).

Very popular and common are lipophilicity measurements made on octadecylsilane packings. However, it has been observed that these types of stationary phase packing are not ideal, since they contain a rather high proportion of free acidic silanol groups (pK.d = 6.8 ± 0.2). This leads to difficulties with strong hydrogen-bonding compounds [25], For this reason a masking agent should be added to the mobile phase. More recently altenatives have become available on the market, namely, polymeric packings and end-capped silica materials (see Table 1). These latter columns are treated with a secondary silanization using short alkyl groups or zwitterionic fragments. Nevertheless, even these new packings must be cafully evaluated, e.g., by solvatochromic analysis (see below).

Table 1. Selection of column packings used for lipophilicity measurements Packing material Abbreviation Reference

Traditional phases (alkyl-bonded silica)

Table 1. Selection of column packings used for lipophilicity measurements Packing material Abbreviation Reference

Traditional phases (alkyl-bonded silica)

Octadecylsilane

ODS (RP-18)

[7-9]

Octyl-silane

OS (RP-8)

[29]

Glyceryl-coated controlled-pore glass

Gly-CPG

[30]

Suppressed silanophilic interactions (polymer-

coated silica and polymer based)

Deactivated (end-capped) octyl-silane

DOS

[31]

Octadecyl polyvinylalcohol copolymer

ODP

[29,31,32]

Deltabond C8

DB

[29]

Styrene-divinylbenzene copolymer

PS-DVB

[29, 33]

C18-derivatized PS-DVB

[6]

End-capped RP-18

ABZ

[26-28]

Polyethylene

PE

[34]

Octadecyl-bonded alumina

ODA

[35]

Immobilized artificial membranes (IAM)

Dipalmitoyl phosphatidylcholine or

IAM

[36-40]

diacylphosphatidylcholine coated silica diacylphosphatidylcholine coated silica

The ODP column has the advantage that one can measure log /cw values directly, i.e., at 100 % water as mobile phase; furthermore, no masking agent is required. A further advantage of polymer-based columns is that they can be used in the full pH range 1-14. The disadvantages of the PLRP-S (poly(styrene-divinylbenzene)) phase are of physical nature, since it undergoes considerable shrinking and welling. Ci8-derivatized PS-DVB columns can be used for alkane-water partition coefficient determinations [6]. End-capped RP-18 columns with maximally suppressed silanophilic interactions are of high interest for reliable lipophilicity measurements [26], Recently, a Supelcosil ABZ-LC column was tested, which is end-capped by a small zwitterionic fragment [27, 28]. Log kw values measured on this column give good correlations with log Pocl values (see below). It was also found that an ABZ column can be mimicked by adding a small amount of 1-octanol or LiCl to a normal ODS column [28]. Nevertheless, some presently unexplained artefacts may occur with an ABZ column. For instance, anions have a longer than expected retention on the ABZ packing [26].

0 0

Post a comment