Supercritical Fluid Inlets

The conditions used for extraction have a lot to say about the selection and optimization of the interface for on-line SFE-GC. Transfer times are typically 15-60 min with a fluid flow that generates about 50 - 1000 ml/min of gas after depressurization. Small extraction cells (0.1 - 1.0 ml) are generally preferred for online coupling. These cells can be operated at optimized flow rates compatible with most interfaces and provide sufficient extract for analysis in most cases. The main limitations on the smallest sample size that can be analyzed are the concentration of the target analytes and whether a representative sub-sample can be obtained. At the other end of the sample scale up to about 1 g can be handled with an on-coIumn interface and about 15 g with a split interface based on their ability to operate at typical fluid flow rates with extractors of different size. 

In the split mode the supercritical fluid is depressurized at the tip of the transfer capillary located inside the liner of the heated inlet. The analytes are continuously 

When on-column injection is used the end of the transfer capillary is inserted into the column inlet or retention gap where decompression of the supercritical fluid occurs. Carbon dioxide gas exits through the column and the seal made between the restrictor and septum (unless a closed injector is used). The analytes are focused by cold trapping in the stationary phase. The transfer line must be physically removed from the injector at the completion of the extraction to establish the normal carrier gas flow for the separation. Analyte transfer to the column is virtually quantitative but blockage of the restrictor is more conunon and involatile material accumulates in the injection zone eventually degrading chromatographic performance. The on-column interface is probably a better choice for trace analysis of relatively clean extracts with modest fluid flow rates than the split interface. When optimized both the on-column and split interfaces provide essentially identical peak shapes to those obtained using conventional solution injection. 

A number of important sample preparation techniques rely upon gas extraction or the analysis of samples in the gas phase. These samples usually contain low concentrations of volatile analytes and higher concentrations of water vapor in a low molecular weight gas or air. Direct sample introduction by syringe or valves is only suitable for small volumes of relatively concentrated samples (section 3.4.2). More common sample introduction methods involve analyte accumulation by sorbent or cryogenic trapping followed by vaporization in the presence of a flow of gas to transport the 

The standard apparatus used for thermal desorption is equally suitable for thermal extraction. Thermal extraction is used for the analysis of volatile compounds in solid samples of low moisture content (e.g. plant materials, soil, polymers, etc). In this case, a few milligrams of sample in place of the sorbent contained in a standard desorption tube is heated to a temperature below its melting or decomposition temperature. The volatile compounds released from the sample are accumulated and transferred to the column in the same way as for thermal desorption. 

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Fig. 7.17. High-pressure flow-cells. (A) Cell for voltammetric measurements in water-in-CO, microemulsions a working electrode, b reference electrode, c supercritical fluid inlet, d supercritical fluid outlet, e sample port, f Teflon O-ring. (B) Fibre optic-assisted cell for fluorimetric measurements. (Reproduced with permission of the American Chemical Society.)...

A Control electrode B Reference electrode C Supercritical fluid inlet D Supercritical fluid outlet E Sample port F Teflon 0-rtng. 

The mass spectra of mixtures are often too complex to be interpreted unambiguously, thus favouring the separation of the components of mixtures before examination by mass spectrometry. Nevertheless, direct polymer/additive mixture analysis has been reported [22,23], which is greatly aided by tandem MS. Coupling of mass spectrometry and a flowing liquid stream involves vaporisation and solvent stripping before introduction of the solute into an ion source for gas-phase ionisation (Section 1.33.2). Widespread LC-MS interfaces are thermospray (TSP), continuous-flow fast atom bombardment (CF-FAB), electrospray (ESP), etc. Also, supercritical fluids have been linked to mass spectrometry (SFE-MS, SFC-MS). A mass spectrometer may have more than one inlet (total inlet systems). 

In addition, solute focusing is possible by maintaining a low initial temperature (e.g. 40 °C) for a long period of time (8-12 min ) to allow the mixture of decompressed carbon dioxide, helium gas and the solutes to focus on the GC column. The optimization of the GC inlet temperature can also lead to increased solute focusing. After supercritical fluid analysis, the SF fluid effluent is decompressed through a heated capillary restrictor from a packed column (4.6 mm i.d.) directly into a hot GC split vaporization injector. 

The viscosity of a supercritical fluid is constant at constant density, regardless of the temperature and pressure. It increases with density, but it is still lower than liquid viscosities by a factor of ten or more. As a consequence, pressure drops across SFC columns are smaller than across LC columns, requiring less pressure for a given flow and making high velocities realistic. OT columns can be operated at pressures only slightly above the critical value of 7 MPa (73 atm or 1100 psi), and even packed columns show pressure drops as low as 1.5 MPa, resulting in inlet pres-... 

In addition to the previous three commercial extractors, some authors have developed custom models [78,79], adapted in most cases from a supercritical fluid extractor [25,58,80]. For example, Heemken et al. altered a Suprex SF extractor for use in ASE [81] they disconnected the syringe pump from the COj cylinder and filled it with a suitable ASE solvent. The restrictor was replaced with a stainless steel capillary tube leading into the trapping vial and an additional nitrogen pipe was installed at the inlet valve of the extraction vessel for purging after extraction. In the extraction of PAHs from soil [79], a custom extractor and commercially available equipment provided equivalent results on the other hand, in the extraction of benzene and toluene from soil [78], the former provided even better results than the latter. 

Sample inlets can also be interfaces that allow the effluents of a variety of powerful separation techniques to enter the ion soimce at a rate that allows for vaporisation to take place. Hence, acronyms such as GC-MS, LC-MS, SFC-MS, etc., relating to gas chromatography, hquid chromatography, supercritical fluid chromatography, etc. coupled to mass spectrometry are now well estabhshed in the jargon of the anal5d ical community as these techniques have been widely accepted and they can be interfaced to any t5q)e of modern mass analyser. 

Modem gas chromatography cannot be fnlly treated withont also discussing sample preparation. Unlike most other instrnmental techniqnes, gas chromatography reqnires many specialized sample preparation techniqnes, due to the requirement that samples for GC be vaporized in the inlet. Further, in most cases, the analytes mnst be distribnted in an organic liqnid or a vapor phase prior to injection. The myriad sample matrices and interferences that may be present further complicate this. As a result, there are a tremendous variety of sample preparation techniques available for gas chromatographers. These range in complexity from simple dilutions and injection of neat samples, to sophisticated fnlly on-line instruments such as supercritical fluid extractors. 

Now consider the rapid expansion of a supercritical fluid through the expansion nozzle depicted in Figure 3, which is in use in our laboratory and is similar to those typically reported in the literature (19,20). A tapered inlet (usually 120° angle) is followed by a cylindrical capillary section in which L/D typically ranges from 3 to 6000. Given the comparatively low viscosity of a supercritical fluid, the effects of acceleration and friction on the pressure are weak at the low flow speeds that exist upstream of the nozzle in the process tubing. Consequently, the pressure drop up to this point is small. However, when the fluid passes into the tapered inlet section of a typical RESS nozzle, the slow expansion gradually turns rapid. If, to simplify analysis, we subdivide the process into an isobaric part followed by a rapid expansion, the question of where the rapid expansion truly starts must be addressed. 

The extraction solvent is fed to the extractor and evenly distributed to the inlet of the fixed bed. The compound resulting from mixture of the solute and the solvent removed from the extractor and fed to the precipitator, where both are separated (Brunner, 1994). Table 2 shows the comparison of the properties of gas, liquid, and supercritical fluids. 

The cost of transporting wood chips by truck and by pipeline as a water slurry was determined. In a practical application of field delivery by truck of biomass to a pipeline inlet, the pipeline will only be economical at large capacity (>0.5 million dry t/yr for a one-way pipeline, and >1.25 million dry t/yr for a two-way pipeline that returns the carrier fluid to the pipeline inlet), and at medium to long distances (>75 km [one-way] and >470 km [two-way] at a capacity of 2 million dry t/yr). Mixed hardwood and softwood chips in western Canada rise in moisture level from about 50% to 67% when transported in water the loss in lower heating value (LHV) would preclude the use of water slurry pipelines for direct combustion applications. The same chips, when transported in a heavy gas oil, take up as much as 50% oil by weight and result in a fuel that is >30% oil on mass basis and is about two-thirds oil on a thermal basis. Uptake of water by straw during slurry transport is so extreme that it has effectively no LHV. Pipeline-delivered biomass could be used in processes that do not produce contained water as a vapor, such as supercritical water gasification. 

B. Shiralker and P. Griffith, The Effect of Swirl, Inlet Conditions, Flow Direction, and Tube Diameter on the Heat Transfer to Fluids at Supercritical Pressure, J. Heat Transfer (42) 465-474,1970,... 

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