In a recent paper, Zheng and coworkers (J. Zheng, L. T. Taylor, J. D. Pinkston, and M. L. Mangels, J. Chromatogr. A, 2005, 1082, 220) discuss the elution of polar and ionic compounds in SFC.
(a) Why are highly polar or ionic compounds usually not eluted in SFC?
(b) What types of mobile-phase additives have been used to improve, the elution of highly polar or ionic compounds?
(c) Why is ion-pairing SFC not often used?
(d) Why arc ammonium salts sometimes added as mobile-phase modifiers in SFC?
(e) The authors describe an SFC system that uses mass spectrometry (MS) as a detector. Discuss the interfacing of an SFC unit to a mass spectrometer. Compare the compatibility of SFC with MS to that of HPLC and GC with MS.
(f) The authors studied the effect of column outlet pressure on the elution of sodium 4-dodecylbenzene sulfonate on three different stationary phases with five mobile-phase additives. What effect was observed, and what was the explanation for the effect?
(g) What elution mechanisms were considered by the authors?
(h) Which mobile-phase additive gave the fastest elution of the sulfonate salts? Which provided the longest retention times?
(i) Did a silica column give results similar to or different from a cyano bonded phase column?