Detectors

SFC can be carried out with the widest range of detectors of any chromatographic technique: detectors from gas and liquid chromatography are employed, and SFC can be coupled to a number of information-rich spectroscopic detectors (Table 4). SFC detection can be carried out in the gas phase after decompression, or in both supercritical and liquid phases. The flame ionization detector (FID) is the most commonly used if unmodified CO2 is the mobile phase, as is the case for capillary columns, whereas the UV/visible detector is most often used if a mobile phase contains a modifier, e.g. in packed column work. Some limited use has been made of the FID with CO2 modified by water, formic acid or formam-ide which give no, or a very small, FID signal.

From 1981 the range of detectors for SFC showed a rapid expansion. The thermionic, electron-capture, photoionization and chemiluminescence detectors from GC, and the fluorimetric and light-scattering

Table 4 Detectors in SFC

Type Established Developmental

GC type Flame ionization Chemiluminescence

Thermionic Photoionization

Flame photometric Ion mobility

Electron capture Element-specific plasma

LC type UV absorption Fluorescence

Light scattering Electrochemical

Coupled Diode array UV Nuclear magnetic resonance Fourier transform IR Inductively coupled plasma Mass spectrometric spectrometry detectors from HPLC were all demonstrated in SFC. Element-specific plasma emission detectors were demonstrated in SFC from 1987, and since 1990 electrochemical detection has been explored; if the electrodes are sufficiently small, oxidative and reductive detection is possible in CO2 containing only small concentrations of water or organic modifier.

The detector providing the most structural information in chromatography is the mass spectrometer (MS); a packed-column SFC-MS interface was first reported in 1978, and a practical SFC interface was developed in 1982 for capillary SFC-MS where the low flow rates permit direct introduction into the ion source. A practical packed-column SFC-MS interface was developed in 1987 to cope with the higher flow rates. SFC with both electron impact and chemical ionization has been demonstrated. The interfacing of capillary SFC to Fourier-transform infrared spectroscopy (FTIR) was achieved as early as 1983. Both solvent-elimination (evaporation of CO2 to deposit separated compounds on a solid substrate or IR transparent plate) and online direct recording of FTIR spectra in a flow cell with IR transparent optics were developed between 1983 and 1990. A major problem with the latter approach is strong IR absorption by CO2; alternative supercritical mobile phases, especially xenon, have therefore been employed but xenon is vastly more expensive than CO2. Recently, NMR detection has been explored in SFC.

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