Supercritical Fluid Chromatograpy & extraction
First discovered in 1879, supercritical fluids have been used for
extraction applications since the 1950’s. The 1980’s saw an increase in
their use as mobile phase for analytical separations. The work was
mainly capillary scale (GC type) work, but some packed column (LC type)
applications were developed. Since then, the expected growth in the
technique has not taken place but the availability of new programmable
pumps and an electronic back pressure regulator opens the door for
future development.
The benefits of using supercritical fluids are their liquid-like
densities offering higher solubility and increased column loading. They
have low viscosity and are highly diffuse enabling faster separation
and extraction. The reduction in the use of organic solvents has cost,
health, and safety benefits as well as faster (cleaner) sample recovery
during experimental procedures.
The Benefits of Electronic Back Pressure Regulation
A supercritical fluid chromatography system differs from other
chromatographic systems (eg. Liquid-LC Gas-GC) by the inclusion of a
back pressure regulating device. This device maintains the system
pressure (from delivery pump through to detector) above the critical
pressure (Pc) of the fluid used. The main pressure drop in the system
takes place in the back pressure device. In HPLC and GC, the pressure
gradually drops to atmospheric pressure as the mobile phase passes
through the column. Thermostatting of the SFC system is also necessary
in order to keep the temperature above the critical temperature (Tc) of
the fluid being used as mobile phase. The supercritical fluid offers
increased solvating power and the maintenance of experimental
conditions ensures improvement in chromatographic separations. There
are two types of back pressure device currently used in SFC:
1. Simple restrictor
2. Mechanical or electronic feedback regulator
The simple restrictor consists of a length of capillary tubing having
an appropriate internal diameter for the required pressure and flow
rate used in the system. This device which is easy to make, is used
mainly for open tubular or packed capillary column work. However, in
order to change the system pressure the flow rate also has to be
changed, because the pressure is built up only by the flow resistance
of the restrictor.
The mechanical or electrical feedback device is used for conventional
size packed columns. This regulator consists of a pressure sensing
device and a needle valve. It controls the back pressure independently
of the mass flow rate of the fluid. This device is more convenient than
the simple restrictor for the precise examination of retention behavior
in SFC. However, conventional back-pressure regulators suffer the
disadvantage of a large dead volume, ranging from a few milliliters to
tens of milliliters. These large volumes do not allow changes in
pressure to take place fast enough for pressure programming to be
performed. Successful fractionation of solutes is also less likely due
to the large dead volume of the regulator. In order for the system to
control the pressure independently of the mass flow rate, the regulator
must have a low dead volume, less than 10 microliters.
SFC/SFE PU-1580-CO2 Pump and BP-1580-81 Back Pressure Regulator
JASCO has solved this problem by developing a pressure regulating system which operates on different principals from conventional regulators.The system is comprised of a regulative valve, pressure transducer, and control electronics. The regulating valve consists of a needle which is driven by a solenoid and a valve seat. In general this type of valve controls the flow resistance by changing the gap between the valve needle and seat. The new valve is based on high speed switching of the fluid flow by periodically opening and closing the flow path. It eliminates the possibility of blocking the valve flow path.
Precipitated solutes and dry ice originated from carbon dioxide are trapped and forced to pass through the valve. A glass collection reservoir connected to the outlet port of the regulating valve allows fractionation of system eluent. When a measured pressure is lower than a reference signal, the output of the comparator turns the solenoid drive current on to close the drive flow. When the measured pressure is higher than the control signal the output switches off the current allowing the fluid to pass through the valve. In this way the control
electronics and hydraulics cause the valve to periodically open and close.
This new back pressure regulator offers greater experimental flexibility by enabling super critical conditions to be maintained independently of mass flow rate.
Fractionation is made easier by utilizing the collection facility at
the outlet of the regulator, and simplifies preparative scale chromatography.
