Phase trapping

Interactions in the stationary phase employing a porous stationary phase or support must also involve the mobile phase trapped in a static form inside the pores. It follows that the diffusivity of the solute in the stationary phase (Ds) will be similar to that in the mobile phase (Dm). Thus, to a first approximation, it can be assumed that Ds = coDm, where (co) is a constant probably close to unity. Thus, equation... 

Where u, is the mobile phase velocity at the column outlet, Fg the column volumetric flow rate, and Ag the column cross-sectional area available to the mobile phase. In a packed bed only a fraction of the column geometric cross-sectional area is available to the mobile phase, the rest is occupied by the solid (support) particles. The flow of mobile phase in a packed bed occurs predominantly through the interstitial spaces the mobile phase trapped within the porous particles is largely stagnant (37-40). 

Miller and Hawthorne [416] have developed a chromatographic method that allows subcritical (hot/liquid) water to be used as a mobile phase for packed-column RPLC with solute detection by FID, UV or F also PHWE-LC-GC-FTD couplings are used. Before LC elution the extract is dried in a solid-phase trap to remove the water. In analogy to SFE-SFC, on-line coupled superheated water extraction-superheated water chromatography (SWE-SWC) has been proposed [417]. On-line sample extraction, clean-up and fractionation increases sensitivity, avoids contamination and minimises sources of error. 

Also, subcritical (hot/liquid) water can be used as a mobile phase for packed-column RPLC with solute detection by means of FID [942]. In the multidimensional on-line PHWE-LC-GC-FTD/MS scheme, the solid sample is extracted with hot pressurised water (without the need for sample pretreatment), and the analytes are trapped in a solid-phase trap [943]. The trap is eluted with a nitrogen flow, and the analytes are carried on to a LC column for cleanup, and separated on a GC column using the on-column interface. The closed PHWE-LC-GC system is suitable for many kinds of sample matrices and analytes. The main benefit of the system is that the concentration step is highly efficient, so that the sensitivity is about 800 times better than that obtained with traditional methods [944]. Because small sample amounts are required (10 mg), special attention has to be paid to the homogeneity of the sample. The system is... 

The IEX—Dual Trap RP system configuration (Fig. 13.2) directs first-dimension IEX effluent to two alternating reversed-phase trap columns (typically 2.1 x 10 mm... 

This contrasts with the results obtained by Yardley who, in gas phase trapping experiments with Cr(CO)6 (45) and Fe(CO)5 (46), showed that uv light of wavelength shorter than 355 nm could promote loss of two or more CO groups. 

There are three broad categories of materials that have been utilized in this endeavor. In the first, even in fully stoichiometric compounds, the ionic conductivity is high enough to be useful in devices because the cation or anion substructure is mobile and behaves rather like a liquid phase trapped in the solid matrix. A second group have structural features such as open channels that allow easy ion transport. In the third group the ionic conductivity is low and must be increased by the addition of defects, typically impurities. These defects are responsible for the enhancement of ionic transport. 

Hartonen K, Bowadt S, Hawthorne SB, et al. 1997. Supercritical fluid extraction with solid-phase trapping of chlorinated and brominated pollutants from sediment samples. J Chromatogr A 774(l-2) 229-242. 

Samples, even at moderate concentrations, injected into the HPLC column may precipitate in the mobile phase or at the column frit. In addition, the presence of other compounds (e.g., lipids) in the injection sample may drive the carotenoids out of solution or precipitate themselves in the mobile phase, trapping carotenoids. It is best to dissolve the sample in the mobile phase or a slightly weaker solvent to avoid these problems. Centrifugation or filtration of the samples prior to injection will prevent the introduction of particles that may block the frit, fouling the column and resulting in elevated column pressure. In addition to precipitation, other sources of on-column losses of carotenoids include nonspecific adsorption and oxidation. These can be minimized by incorporating modifiers into the mobile phase (Epler et al., 1993). Triethylamine or diisopropyl ethylamine at 0.1% (v/v) and ammonium acetate at 5 to 50 mM has been successful for this purpose. Since ammonium acetate is poorly soluble in acetonitrile, it should be dissolved in the alcoholic component of the mobile phase prior to mixing with other components. The ammonium acetate concentration in mobile phases composed primarily of acetonitrile must be mixed at lower concentration to avoid precipitation. In some cases, stainless steel frits have been reported to cause oxidative losses of carotenoids (Epler et al., 1992). When available, columns should be obtained with biocompatible frits such as titanium, Hastolloy C, or PEEK. 

The feasibility of extracting substituted phenols from an aqueous solution with supercritical CO2 is reported A special extraction vessel was used in order to overcome the mechanical difficulty in retaining the liquid matrix in the extraction vessel. Solid phase trapping was utilized with a diol silica bonded phase. Methanol was used to rinse the trap. Below 300 atm extraction recovery paralleled CO2 pressure at fixed temperature. Phenol was least extractable while, 2,4-dichlorophenol yielded the greatest percent recovery. Above 300 atm extraction yield declined with pressure. It is theorized that at high CO2 density there is less mixing with the aqueous phase because of increased fluid-fluid interaction. 

Chemical equilibria are frequently temperature dependent, and we should not expect that the organic phase trapped in the quenched catalyst will always be identical to that existing immediately prior to the quench. However, the organic material in the quenched catalyst should be sufficiently related to its antecedents that inferences about structures present at higher temperatures will be possible. Another advantage of the pulse-quench reactor that proved useful is the fact that products are removed from the catalyst bed in this experiment. This has proven useful for the observation of hydrolytically unstable species which form in reactions that also generate water (16). In sealed ampoules or rotors, the water can not escape the catalyst bed in the pulse-quench reactor, the water is swept out in the gas stream. Most of the experiments that motivated the calculations in this contribution were performed using "conventional" sealed-rotor methods, but the pentamethylbenzenium study (vide infra) would not have been possible without the pulse-quench reactor. 

Air samples for airborne portions may be taken with high volume air samples 0.67-0.85 m3 (20-30 ft /min.) through a glass fiber filter backed up with a resin type (Rohm and Haas XAD) absorbing column or liquid bubbler for gas phase trapping where desired. Any type application may be monitored downwind with these type of collectors at ground level, or vertical tower collection can be provided for Impacted drops or for those drawn into air samplers. 

Bjorklund, E., L. Mathiasson, P. Persson, et al. 2001. Collection capacity of a solid phase trap in supercritical fluid extraction for the extraction of lipids from a model fat sample. J. liquid Chromatogr. Rel. Technol. 24 2133-2143. 

Lopez, R, Batlle, R., Neiin, C., Cacho, J., and Ferreira, V. (2007). Use of new generation poly(styrene-divinylbenzene) resins for gas-phase trapping-thermal desorption - Application to the retention of seven volatile organic compounds. J. Chromatogr. A., 1139, 36 4. 

Materials for selective vapor-phase trapping have been considered by various workers (6,7). Traps used in the present study included molecular sieve 5A for subtraction of straight chain hydrocarbons, sodium bisulfate and phosphoric acid for subtraction of shale oil bases, and alumina for subtraction of acidic components. 

Fig. 9.18. Effects due to the interaction between a RAM phase and a mixture an antibody (Y shaped), antigen (triangle) and labelled antigen (triangle + star) (a) the phase traps small analyte and tracer molecules by mixed effects, i.e., size exclusion and hydrophobic interactions (b) if the hydrophobic forces are strong enough, the phase removes the tracer/anal5rte from their antibody complexes (c) the free antibody binds to free anal Tte/tracer previously trapped by the RAM phase.

Eritz, J.S. Macka, M. Sohd-phase trapping of solutes for further chromatographic or electrophoretic analysis. J. 40. Chromatogr., A 2000, 902, 137-166. 

Examples of miscible displacement are the intrusion of saltwater into fresh groundwater or a step change in feed composition in a chemical reactor. In one sense, miscible displacements are simpler processes than immiscible displacements because issues such as interfacial behavior and phase trapping are not relevant. However, they are complicated by hydrodynamic dispersion (which tends to smear the displacement front), and they are subject to similar viscous instabilities as those described earlier. 

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