Specialty Columns

Poly-OL: The Challenge of Aromatic Geometric Isomer Separation

Geometric isomers - particularly meta and para aromatic isomers - can present difficult challenges for analytical liquid chromatography. Because standard reversed phase and normal phase liquid chromatography is typically not useful, analysts have resorted to the use of chiral phases. Although this approach is effective, it is costly, column life is short, and mobile phases are limited for chiral columns. At the preparative scale, the challenges are even greater. Chiral phases may not be available or are cost prohibitive.

The para isomer of methylbromobenzoate is an important intermediate in some pharmaceutical syntheses. Quantitation of this isomer relative to its meta counterpart is necessary. Separation of these isomers is imperative if HPLC is to be used for analytical quality control.

Chiral phases are normally used for this difficult separation. Reversed phase HPLC columns fail to resolve the isomers. Several normal phase columns also failed to give adequate separation.

Separation of Geometric Isomers with the Diazem Poly-OL

The Diazem Poly-OL, a high load polyol phase on a high surface area silica, effectively separates meta and para methylbromobenzoate isomers (Figure 1).

Conditions are straightforward: Isooctane is used as the mobile phase, with detection at 230nm and normal flow rates. At 230nm the meta isomer gives stronger absorbance and the para isomer is greater at 254nm. The difference provides a convenient method of identification and quantitation of both isomers.

With the Diazem Poly-OL silane bonding technology, a wider range of mobile phase conditions can be used without the column degradation inherent with many chiral and normal phases. The Poly-OL bonding also gives a further advantage over standard normal phase columns: higher retentivity and better selectivity.

In Figure 2, the Poly-OL is compared with an SiO2 standard normal phase column on a test mixture containing isomeric nitroanilines. Poly-OL has a 26% increase in capacity factor in the retention of p-nitroaniline over an SiO2 phase with the same surface area.