Variable flow control and collection device for use with supercritical fluids

Real world samples which contain high concentrations of water and/or extractable material frequently cause intermittent or irreversible plugging of the flow control device during off-line supercritical fluid extraction (SFE). A supercritical fluid (SF) flow control/collection device has been developed which can simultaneously maintain the extraction flow-rate of the SF (+/-0.1 ml/min) and quantitatively (>90%) collect analytes as volatile as n-octane directly into an organic solvent. With this device, the extract is partially depressurized through a heated capillary restrictor and into a pressurized collection solvent, so that both temperature and pressure are used to maintain the solubility of the extract in the SF. The pressurized mixture is finally depressurized to atmospheric conditions using a backpressure regulator, so the extract can be recovered in the collection vial. Depending on the sample matrix, a restrictor heater temperature of 200 degrees C and a backpressure regulator with a heated (>5 wt% water in sample) or unheated (<5 wt% water in sample) exit tube is required to eliminate restrictor plugging. By varying the pressure of the collection solvent, a range of reversible and reproducible flow-rates were obtained at both high (400 bar, 3.5 to 0.2 ml/min liquid CO2) and low (100 bar, 0.8 to 0.1 ml/min CO2) SFE pressures using a 50 mu m I.D. capillary restrictor. At low (0.2 ml/min) SF flow-rates the solubility of several metal complexes (e.g., ferrocene and Ni[C22H22N4] complex) was measured and reproducible solubility values and flow-rates (R.S.D.<8%) were obtained.