Chromatographic Methods for Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 266

are analogous to conventional solvent. For example, the commonly used
supercritical CO 2 fluid can readily dissolve analyte, and the used solvent (i.e., the
innocuous CO 2 ) can be easily removed by reducing the pressure. Equally important
is its superior diffusivities that are orders of magnitude higher and viscosities that
are orders of magnitude lower than liquid solvents, which are critical for

chromatography.
The hardware components of a SFC instrument include (a) a fluid source
(a tank of CO 2 ), (b) a syringe pump that can sustain at least 400 atm pressure,
(c) a valve to control the flow into a heated extraction cell, (d) an exit valve leading
to the flow restrictor that depressurizes the fluid and transfers it to a collection
device, (e) a detector, and (f) a data acquisition system. SFC can use either packed
columns or capillary columns that are essentially the same as in HPLC or GC. Both
UV-VIS and FID detectors (Section 10.3) are common for SFC. If FID is used, the
restrictor must be placed before the detector to remove CO 2 . A brief comparison of
SFC with GC and HPLC is given in Table 10.4. It should be noted that although
SFC has some ideal features, this relatively new technique still has not gained
popularity and its wide analytical applications remain to be seen.

Table 10.4 Comparison of SFC with GC and HPLC

GC HPLC SFC

Common mobile Gas (N 2 , Helium) Liquid solvent (e.g., Supercritical

phase water: methanol) fluid (CO 2 )

Common column Capillary Packed (mm i.d.) Capillary/Packed

(50 mm i.d.)

Separation principle Partition, Adsorption, Partition, Adsorption Dissolution -

and Others Ion Exchange Size precipitation

Exclusion

Major operating Temperature Mobile phase Pressure,

parameter temperature

Common detector Vary but not UV-VIS Vary, UV-VIS FID and / or

most common UV-VIS

10.3 COMMON DETECTORS FOR CHROMATOGRAPHY

A chromatographic detector is a device that is able to recover chemical
information (e.g., concentration, mass, structure) from the column effluent and
convert it to a measurable form of signal. Coupled with a data-acquisition system,
such a signal may need further amplification before a chromatogram can be
generated. The process of converting chemical signals to electric or electronic
signals and then generating a chromatogram is much more complicated than the
separation process discussed previously. Fortunately, it is usually not necessary
for an analyst to have a detailed knowledge of the internal mechanical and