MODIFIER ADDITION IN MICROCOLUMN SUPERCRITICAL-FLUID CHROMATOGRAPHY WITH A HIGH-PRESSURE PULSED VALVE

The need for mixed mobile phases at low flow rates suitable for capillary and packed capillary supercritical fluid chromatography (SFC) has led to the use of an air actuated high pressure prime/purge valve for on-line modifier introduction. This valve can consistently deliver small amounts of modifier into the mobile phase, and allows rapid changes of modifier concentrations. Methanol modified carbon dioxide(CO2), from 0.5 to 15 mole percent, has been generated using this valve at a flow rate compatible with a packed capillary column without the need for flow splitting. Two instrumental configurations were used to evaluate the valve. One configuration used separate pumps for the mobile phase and the modifier. The other configuration used only one pump with a pneumatic amplifier as the modifier reservoir. The valve provides reproducible modifier concentrations, percent relative standard deviation (%RSD) for retention times less than 2%, and dependable use, leak free after > 250,000 pulses. The use of this valve, with both the 2-pump and 1-pump/pneumatic amplifier configuration is demonstrated. The effects of modifier addition on the retention times of components in several samples are shown.