Selectivity differences for drugs of abuse when using Rxi-5ms and Rxi-5Sil MS GC columns: pendant diphenyl versus silarylene stationary phases.

Previously I had written about seeing differences in selectivity when analyzing pesticides on Rxi-5ms and Rxi-5Sil MS GC columns.  As you may remember (if you’re younger than me because I can’t remember almost anything these days!), the 5ms is a 5% pendant phenyl column and the 5Sil MS has phenyl content in the siloxane stationary phase backbone to reduce column bleed.  Well, when analyzing drugs of abuse recently, I noticed that morphine and diazepam were separated on the 5ms but coeluted on the 5Sil MS when chromatographed using exactly the same GC oven temperature program.  Other than that, the chromatograms were essentially identical (complete chromatograms not shown here; trust me).   

Restek makes it easy to remember GC columns that have the silarlyene modification by using “Sil” in the name (e.g. Rxi-5Sil MS, Rxi-35Sil MS, and Rxi-17Sil MS).  If you decide to change from your “standard” GC phase to an arylene phase to improve the thermal stability of the column and/or the maximum operating temperature, watch out for slight selectivity differences.