Detectors supercritical fluid chromatography

The use of separation techniques, such as gel permeation and high pressure Hquid chromatography interfaced with sensitive, silicon-specific aas or ICP detectors, has been particularly advantageous for the analysis of siUcones in environmental extracts (469,483—486). Supercritical fluid chromatography coupled with various detection devices is effective for the separation of siUcone oligomers that have molecular weights less than 3000 Da. Time-of-flight secondary ion mass spectrometry (TOF-sims) is appHcable up to 10,000 Da (487). 

Mixtures can be identified with the help of computer software that subtracts the spectra of pure compounds from that of the sample. For complex mixtures, fractionation may be needed as part of the analysis. Commercial instmments are available that combine ftir, as a detector, with a separation technique such as gas chromatography (gc), high performance Hquid chromatography (hplc), or supercritical fluid chromatography (96,97). Instmments such as gc/ftir are often termed hyphenated instmments (98). Pyrolyzer (99) and thermogravimetric analysis (tga) instmmentation can also be combined with ftir for monitoring pyrolysis and oxidation processes (100) (see Analytical methods, hyphenated instruments). 

Supercritical Fluid Chromatography. Supercritical fluid chromatography (sfc) combines the advantages of gc and hplc in that it allows the use of gc-type detectors when supercritical fluids are used instead of the solvents normally used in hplc. Carbon dioxide, -petane, and ammonia are common supercritical fluids (qv). For example, carbon dioxide (qv) employed at 7.38 MPa (72.9 atm) and 31.3°C has a density of 448 g/mL. 

T. L. Chester and J. D. Pinkston, Pressure-regulating fluid interface and phase behavior considerations in the coupling of packed-column supercritical fluid chromatography with low-pressure detectors , ]. Chromatogr. 807 265-273 (1998). 

A method which uses supercritical fluid/solid phase extraction/supercritical fluid chromatography (SE/SPE/SEC) has been developed for the analysis of trace constituents in complex matrices (67). By using this technique, extraction and clean-up are accomplished in one step using unmodified SC CO2. This step is monitored by a photodiode-array detector which allows fractionation. Eigure 10.14 shows a schematic representation of the SE/SPE/SEC set-up. This system allowed selective retention of the sample matrices while eluting and depositing the analytes of interest in the cryogenic trap. Application to the analysis of pesticides from lipid sample matrices have been reported. In this case, the lipids were completely separated from the pesticides. 

A number of analytical techniques such as FTIR spectroscopy,65-66 13C NMR,67,68 solid-state 13 C NMR,69 GPC or size exclusion chromatography (SEC),67-72 HPLC,73 mass spectrometric analysis,74 differential scanning calorimetry (DSC),67 75 76 and dynamic mechanical analysis (DMA)77 78 have been utilized to characterize resole syntheses and crosslinking reactions. Packed-column supercritical fluid chromatography with a negative-ion atmospheric pressure chemical ionization mass spectrometric detector has also been used to separate and characterize resoles resins.79 This section provides some examples of how these techniques are used in practical applications. 

Supercritical fluid chromatography (SEC) was first reported in 1962, and applications of the technique rapidly increased following the introduction of commercially available instrumentation in the early 1980s due to the ability to determine thermally labile compounds using detection systems more commonly employed with GC. However, few applications of SEC have been published with regard to the determination of triazines. Recently, a chemiluminescence nitrogen detector was used with packed-column SEC and a methanol-modified CO2 mobile phase for the determination of atrazine, simazine, and propazine. Pressure and mobile phase gradients were used to demonstrate the efficacy of fhe fechnique. 

In supercritical fluid chromatography (SFC) the mobile phase is a supercritical fluid, such as carbon dioxide [15]. A supercritical fluid can be created either by heating a gas above its critical temperature or compressing a liquid above its critical pressure. Generally, an SFC system typically has chromatographic equipment similar to a HPLC, but uses GC columns. Both GC and LC detectors are used, thus allowing analysis of samples that cannot be vaporized for analysis by GC, yet cannot be detected with the usual LC detectors, to be both separated and detected using SFC. SFC is also in other... 

Electrochemical detectors (ECD) gas chromatography, 4 615 liquid chromatography, 4 622 6 387, 449 supercritical fluid chromatography, 4 631... 

Supercritical fluid chromatography uses detectors from both liquid chromatography and gas chromatography. A summary of detection systems used in supercritical fluid chromatography has been documented [24]. 

One of the most commonly used detection systems in a gas chromatography laboratory is the electron capture detector. The first paper [25] to be published demonstrating the use of an electron capture detector with supercritical fluid chromatography showed that with supercritical fluid chromatography sensitivity to about 50pg minimum detection limit on column was obtainable. 

A paper has been published showing the use of the photoionization detector [26], Polyaromatic hydrocarbons are very sensitive using the photoionization detector and the levels detected did not break any new ground in terms of sensitivity. It did inspire HNS Systems (Newtown MA, USA), who market a photoionization detector, to try the detector with a capillary system, interfaced to a Lee Scientific 602 supercritical fluid chromatography (Lee Scientific, Salt Lake City, Utah, USA). 

The amount detected by this system (0.3pg on column) was below the level which could have been determined using a flame ionization detector. Initial indications show that the photoionization detector may be a very useful detector for people who wish to get to lower levels on the supercritical fluid chromatography and cannot concentrate their sample. 

Hadj-Mahammed, M., Badjah-Hadj-Ahmed, Y., and Meklati, B.Y., Behavior of pol5nnethoxy-lated and polyhydroxylated flavones by carbon dioxide supercritical fluid chromatography with flame ionization and Fourier transform infrared detectors, Phytochem. Anal, 4, 275, 1993. 

Artemisinin 9a has been extracted from Artemisia annua L. by supercritical fluid extraction and analyzed by supercritical fluid chromatography (SFC) using a capillary column coupled with a flame ionization detector <1997JCFI(A)353>. The SFC method has also been used for the determination of artemisinin in whole blood <1995JCH(B)183>. 

Supercritical fluid chromatography provides increased speed and resolution, relative to liquid chromatography, because of increased diffusion coefficients of solutes in supercritical fluids. (However, speed and resolution are slower than those of gas chromatography.) Unlike gases, supercritical fluids can dissolve nonvolatile solutes. When the pressure on the supercritical solution is released, the solvent turns to gas. leaving the solute in the gas phase for easy detection. Carbon dioxide is the supercritical fluid of choice for chromatography because it is compatible with flame ionization and ultraviolet detectors, it has a low critical temperature. and it is nontoxic. 

Another technique is supercritical fluid chromatography (SFC), which is a chromatographic technique that in many ways is a hybrid of GC and HPLC. It is recognized as a valuable technique for the analysis of thermolabile compounds, which would not be amenable to analysis by GC or HPLC. Few applications have been reported for SFC in the field of OCP and OPP determination (16). The advantages reported for SFC are versatility in separation (by the addition of modifier or the choice of stationary phase) and detection (with LC or GC detectors). However, SFC is a little-used technique because it still presents a wide range of instrumental problems (14-16). 

Several researchers have combined the separating power of supercritical fluid chromatography (SFC) with more informative spectroscopic detectors. For example, Pinkston et. al. combined SFC with a quadrupole mass spectrometer operated in the chemical ionization mode to analyze poly(dimethylsiloxanes) and derivatized oligosaccharides (7). Fourier Transform infrared spectroscopy (FTIR) provides a nondestructive universal detector and can be interfaced to SFC. Taylor has successfully employed supercritical fluid extraction (SFE)/SFC with FTIR dectection to examine propellants (8). SFC was shown to be superior over conventional gas or liquid chromatographic methods. Furthermore, SFE was reported to have several advantages over conventional liquid solvent extraction (8). Griffiths has published several... 

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