Resolution

Since many real samples are complex mixtures of acids (and other components), high efficiency columns are essential for satisfactory resolution. This requirement has made packed columns effectively obsolete for such samples and use of capillary (or open tubular, OT) columns is becoming routine. The high efficiency of OT columns requires correspondingly less selectivity to gain the necessary separation. Therefore, relatively few different stationary phases in OT columns will adequately separate the majority of mixtures encountered.

Nonpolar stationary phases have the advantages of greater inertness, thermal stability and operation at lower temperatures. Since retention times increase with increasing polarities of the stationary phase and analyte, the least polar column affording the necessary resolution should be selected. Silyl derivatives are usually adequately separated on nonpolar polydimethylsiloxanes (e.g. DB-1, SE-30, OV-101); for greater selectivity somewhat more polar phases such as DB-5, SE-54, OV-17 or even OV-1701 may be used. On the other hand, stationary phases containing hydroxyl groups (such as the polyethylene glycols, PEGs) should be avoided because they react with silylation reagents.

Saturated and unsaturated FAMEs are generally separated on more polar columns because they tend to cluster together on nonpolar phases, with the un-saturated ones preceding the saturated. On polar phases such as PEGs, the unsaturated are eluted after the saturated with minimal overlap of different chain lengths. This shift in retention behaviour is further enhanced on very polar stationary phases such as the cyanosilicones (CP-Sil-88, OV-275, DB-23) which are used for resolving cis, trans isomers and very complex mixtures.

Relatively nonpolar columns are used for the separation of diastereomeric esters formed from optically active carboxylic and amino acids. As an alternative approach, amino acid enantiomers have been separated as their alkyl N-perfiuoroacyl derivatives on a chiral column, e.g. Chirasil-Val.

Solar Panel Basics

Solar Panel Basics

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