Hold-and-flush, a novel fraction collection method in semi-preparative subcritical and supercritical fluid chromatography

In supercritical fluid chromatography, the partial phase separation that occurs in the dead volume between automatic backpressure regulator and collection vessels causes significant peak tailing and delayed arrival of compounds to the collectors. As a result, when two peaks are barely baseline separated, it becomes very difficult to correctly set fraction collection triggers, which in turn results in fractions being collected with lower purity and lower yield. The problem can be solved with a simple addition of a four-port, two-position switching valve between UV detector and automatic back pressure regulator. The valve acts as a timed gate to release each peak out of the back pressure regulator and into the collection vessel, while at the same time it holds later peaks inside the UV detector and the column in a closed loop.