By Jack Silver, Teledyne LABS Application Chemist and Chromatography Expert
I recently was asked to help a customer purify a porphyrin compound on the Teledyne LABS ACCQPrep SFC system. They can be run with LC (liquid chromatography) with a hexane/toluene gradient on silica, and the customer wanted to see if they would run with SFC.
While studying the history of SFC (supercritical fluid chromatography), I was bemused to find the first compounds purified with SFC were porphyrins, as documented in a 1962 paper, “High Pressure Gas Chromatography Above Critical Temperatures."1 In fact, (SFC) actually began as a type of gas chromatography. For this reason, the nomenclature for SFC is different, despite it being considered a type of normal phase chromatography.
The strong solvent that increases the elution speed is called a “modifier" in SFC, while in liquid chromatography (LC), this term refers to something added to the mobile phase to alter the pH, provide ion-pairing, or otherwise change the mobile phase to improve the chromatography. The strong solvent in SFC is also referred to as the “co-solvent," which is the term we'll use to reduce confusion. Meanwhile, SFC has an “additive," which plays the role of a solvent “modifier" in LC.
The first compound run on SFC was a porphyrin, but in our current experiment, we used very different conditions compared to that first SFC run. A quick solvent/column screen demonstrated that 5,10,15,20-tetraphenyl-21H,23H-porphine elutes with SFC using toluene as a co-solvent, the same as LC. We used a PEI column, and carbon dioxide as the supercritical liquid.
Below: 5,10,15,20-tetraphenyl-21H,23H-porphine elutes from SFC with a toluene gradient on a PEI column (scout run, top graph, preparative gradient, bottom)
The original SFC experiment used very different gasses and columns. The gasses used were dichlorodifluoromethane, monochlorodifluoromethane, monochlorotrifluoromethane, or trifluoromethane and a stationary phase of 33% polyethyleneglycol (Carbowax 20 M) on Chromosorb W (a type of diatomaceous earth) with no co-solvent.
The technique has changed since 1962, but the compounds of interest are still the same!
1 Klesper, E.; Corwin, A. H.; Turner, D. A., Communications to the editor. The Journal of Organic Chemistry 1962, 27, (2), 700-706
