SFC high-pressure flow cell. A BPR is mounted after

SFC instruments have made
remarkable advances in the past few years with the availability of high
performance commercial instruments, which are as reliable and robust as the
HPLC or UPLC/ UHPLC instruments. Currently available analytical SFC instruments
has pushed the speed versus efficiency envelop beyond that was achieved with
the UHPLC instruments. Van-deemter plots, comparing the performances of
commercially available instruments with 3.5 and 1.7 _m particles, published by Perrenoud et al.,(2012) provides a clear
view of the prowess of modern SFC performance vis-a-vis LC performance.

 A
binary pump is used to pump CO2 and modifier. The CO2
pump head and incoming CO2 are generally chilled in order for the
pump to pump liquid CO2. The two streams are mixed. An external loop
autosampler is used to make injections.The mobile phase is preheated in a heat
exchanger, HX, to the column initial temperature. After the column the fluid
passes through another HX to condition the fluid temperature to the temperature
of the detector flow cell. The most common detector is a diode array UV-Vis
detector with a high-pressure flow cell. A BPR is mounted after the detector to
main the mobile phase in a dense state. After the pressure is dropped, the
liquid waste is collected in an enclosed bottle and the CO2 vented
through a fume hood (Kirkland J., 2000)

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Figure3:
Generalized scheme of a supercritical fluid chromatography (SFC) instrument.

Some major changes that took
place in SFC instrument components are described below-Commercially available
SFC pumps, which tried to mimic LC pumps, could not deliver repeatable and
linear results because of the compressibility of the mobile phase ( Tarafder
and Guiochon, 2013). By employing appropriate reference conditions
(De Pauw et al., 2016) it is possible to design an SFC pump that delivers a
mass flow rate that is directly proportional to the volumetric flow rate.

   CO2  being
compressible, compressing and metering with the same pump stroke can be
challenging. Design changes have taken place in CO2 handling too – e.g. for handling CO2 in a
reciprocating pump, vapor from the cylinder needs to be liquefied through some
heat-exchanger. Traditionally chillers, supplied with anti-freeze coolants,
were employed. These are being replaced by Peltier cooling elements. Besides
pump, changes have taken place in the choice of detectors. Detectors currently
used with an SFC system are UV, DAD, ELSD, CAD, MS (ESI, APCI). Remarkable
changes have also taken place in the design of the ABPR.(Shreve et al., 2015).