Supercritical fluid extraction with detectors

McNally and Wheeler [364] used supercritical fluid extraction coupled to supercritical fluid chromatography to determine sulfonylurea herbicides in soil. Klatterback et al. [365,366] used supercritical fluid extraction with methanol-modified carbon dioxide followed by high-performance liquid chromatography with UV detection to determine sulfonylurea herbicides obtained on a Cis solid-phase extraction disc. Alternatively the determination was carried out by gas chromatography of the dimethyl derivatives of the sulfonylurea herbicides, employing an electron capture or a NP detector on the gas chromatograph. 

Ca. 1 g sample was extracted by supercritical fluid extraction with CO2 and a mixture of acetic acid and 0.2% tropolone with hexane as solvent (pressure of 50 atm, temperature of 50 Q. Ethylation was carried out with 2 mol L ethylmagnesium chloride. TPeT was added as internal standard. Separation was by capillary GC (column of 30 m length, 0.25 mm internal diameter, DB-17 as stationary phase, 0.25 pm film thickness H2 as carrier gas at 5 mLmin N2 as make-up gas at 30 mLmin injector temperature of 250 °C column temperature ranging from 60 to 280 °C). Detection was by FPD (detector temperature of 300 °C). Recoveries were assessed by standard additions results were (82 4)% for DBT, (75 + 2)% for TBT and (65 + 3)% for TPhT. Calibration was by calibration graph and standard additions, using the calibrants provided by SM T. 

One example of normal-phase liquid chromatography coupled to gas chromatography is the determination of alkylated, oxygenated and nitrated polycyclic aromatic compounds (PACs) in urban air particulate extracts (97). Since such extracts are very complex, LC-GC is the best possible separation technique. A quartz microfibre filter retains the particulate material and supercritical fluid extraction (SPE) with CO2 and a toluene modifier extracts the organic components from the dust particles. The final extract is then dissolved in -hexane and analysed by NPLC. The transfer at 100 p.1 min of different fractions to the GC system by an on-column interface enabled many PACs to be detected by an ion-trap detector. A flame ionization detector (PID) and a 350 p.1 loop interface was used to quantify the identified compounds. The experimental conditions employed are shown in Table 13.2. 

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>. 

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... 

Of all the materials available for use as a supercritical fluid, CO2 has become the material of choice because of its chemical properties. Instruments have been developed to utilize the principles described to effect extractions of compounds from a variety of sample matrices including asphalt, plant material, and soils (Figure 25.1). The supercritical fluid is pumped through the sample, through a filter or column to a trap where the fluid vaporizes and solvent is added to transfer the analyses to a vial for analysis. More recent instruments combine the supercritical fluid extraction system with a variety of columns and detectors to acquire data from complex samples. 

To gain further information concerning the hexane fractions, on-line supercritical fluid extraction/SFC was coupled with on-line FTIR detection. FT1R should have an advantage in that it is more sensitive to aliphatic hydrocarbons than the UV detector. 

Because all three types of chromatography can be coupled to SFE, the choice in each case should be dictated by the characteristics of the analytes to be isolated and determined. If GC can be used, it is to be preferred on account of its consolidated status and the high sensitivity and flexibility of the detectors with which it is compatible. Analysing supercritical fluid extracts by HPLC provides one special advantage over GC as most... 

Many detectors have been used to detect pesticides and herbicides in SFC. Among these detectors, the flame ionization detector (FID) is most commonly used for detection of a wide range of pesticides and herbicides, with a detection limit ranging from 1 ppm (for carbonfuran) to 80 ppm (for Karmex, Harmony, Glean, and Oust herbicides). The UV detector has frequently been used for the detection of compounds with chromophores. The detection limit was as low as 10 ppt when solid-phase extraction (SPE) was on-line coupled to SFC. The mass spectrometric detector (MSD) has also been used in many applications as a universal detector. The MSD detection limit reached 10 ppb with on-line SFE (supercritical fluid extraction)-SFC. Selective detection of chlorinated pesticides and herbicides has been achieved by an electron-capture detector (ECD). The limit of detection for triazole fungicide metabolite was reported to be 35 ppb. Other detectors used for detection of pesticides and herbicides include thermoionic, infrared, photometric, and atomic emission detectors. 

Schneiderman MA, Sharma AK, Focke DC. Determination of menadione [menaphthone] in an animal feed using supercritical-fluid extraction and HPFC with electrochemical detector. J Chromatogr Sci 1988 26 458-462. 

When smokeless powder is suspected as having been used in a pipe bomb, the inside surface of the pipe is swabbed, rinsed, or, more effectively, the pipe fragments are sonicated with methylene chloride. The extract is filtered and concentrated by slow evaporation. The extract is examined by GC-TEA or LC-TEA or using an electrochemical detector to detect NG indicative of double-base powder. Similarly, GC-MS may be used for detection of NG with good sensitivity and excellent selectivity. The use of supercritical fluid extraction (SEE) has been used to extract smokeless powders and residues but the equipment cannot accommodate bulky fragments such as pipe fragments. 

The use of supercritical fluid extraction (SEE) as an extraction technique is related to the unique properties of the supercritical fluid. These fluids have a low viscosity, high diffusion coefficients, low toxicity, and low flammability, all clearly superior to the organic solvents used in SPE extraction. The most common fluid used is carbon dioxide. SEE extractions of sediment samples have shown recoveries of >95% for all the individual PCBs. The separation of PCDDs from PCBs and chlorinated benzenes is difficult because of their similar solubility. An interesting development is the use of fat retainers. Samples, mixed in different weight ratios with, e.g., silica/silver nitrate 10% or basic alumina, can be placed in 7 ml extraction cells. The analytes are recovered by elution with 1.5-1.8 ml of hexane. With the correct fat-silica ratios and SEE conditions, no additional cleanup procedure is necessary for GC with an electron-capture detector (ECD). One drawback of SEE may be that the methods developed are valid for a specific matrix, but as soon as, e.g., the fat content of a biota sample or the type of lipids changes, the method has to be adapted. SEE is relatively complicated compared to other extraction techniques. In addition, the cell volumes are small, which limits the sample intake, and, with that, the detection limits. Einally, some reliable types of SEE equipment have recently been withdrawn from the market. This will have a substantial negative effect on the use of SEE in the near future. 

Figure 2.5 Supercritical fluid chromatogram, with use of a miniaturized evaporative lightscattering detector, of an extract obtained from oat bran. Column 100 nun x 0.9 mm, packed with LiChrosorb Diol, 5 pm. Conditions temperature 22°C pressure 300 atm. Mobile phase carbon dioxide modified with 19 mol% methanol. Restrictor 45 mm x 9 pm at 90°C. Peaks TG = triacylglycerols MGDG = monogalactosyldiacylglycerols PC = phosphatidylcholines DGDG = digalactosyldiacylglycerols.

Figure 2.8 Supercritical fluid chromatography, with use of a miniaturized evaporative lightscattering detector, of seed extracts from (a) an autumn rapeseed (b) Camelina sativa (triacyl-glycerides containing long-chain monoenes have not been identified because reference substances are not available). SFE/SFC was off-line. Column as in Fig. 2.2. Separation conditions temperature 140°C pressure 160 atm, after 1 min programmed at -10°Cmin to 100°C and 25 atm min" to 260 atm, then programmed at -l°Cmin to 75°C and at 2 atm min to 260 atm. Extraction conditions temperature 90°C, pressure 160 atm. Abbreviations L = linoleate Ln = linolenate 0 = oleate P = palmitate S = stearate SFC = supercritical fluid chromatography SFE = supercritical fluid extraction.

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). 

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