Abstract:Abstract. When temperature programming is performed in supercritical fluid chromatography (SFC) under conditions where the solute-mobile phase interactions are approximately fixed, that is, with the use of constant mobile phase density, then positive temperature programs can be used. The results resemble those in temperature-programmed gas chromatography (GC) except that chromatograms are compressed and solutes elute at lower temperatures. For a given temperature-program rate, the extent of this compression ef… Show more
“…The selectivity behavior we see here is consistent with the stationary phase swelling model presented earlier [4]. If a nonpolar stationary phase like DB-1 (which interacts with solutes predominantly through dispersion forces) is swollen by CO, (which also interacts with solutes predominantly through dispersion forces), there is relatively little selectivity shift even for very polar solutes.…”
Section: Selectivity As a Function Of' Temperaturesupporting
confidence: 87%
“…forces to the solute retention mechanism. Thus, Since higher temperatures result in lower elution swelling polar stationary phases with CO, may pressures and densities for the relatively lowinduce large selectivity shifts [4]. molecular-weight solutes in this test mix, we conclude that the stationary phases simply cannot be swelled above 160°C at the elution pressures of these solutes.…”
Section: Selectivity As a Function Of' Temperaturementioning
Abstract. We studied solute retention for n-hydrocarbons, (low-molecular-weight) polystyrenes, an ethoxylated surfactant, and a selectivity test mix on open-tubular columns with methyl-, biphenyl-, and cyanopropyl-substituted stationary phases for pressures up to 680 atm and temperatures up to 240°C. The solute elution range varied tremendously with column choice, with the least retentive stationary phases providing the highest elution range. However, the best resolution and largest analysis range were obtained with the most retentive stationary phase and the highest pressures. Increasing the temperatures above 160°C did not cause a large increase in elution pressure for the solutes used in this study. Exceptionally large shifts in selectivity with temperatures up to about 160°C occurred for the biphenyl and cyanopropyl stationary phases. The availability of pressures higher than 680 atm, coupled with adequately retentive stationary phases used at optimal temperatures, would further increase the analysis scope of opentubular supercritical fluid chromatography.
“…The selectivity behavior we see here is consistent with the stationary phase swelling model presented earlier [4]. If a nonpolar stationary phase like DB-1 (which interacts with solutes predominantly through dispersion forces) is swollen by CO, (which also interacts with solutes predominantly through dispersion forces), there is relatively little selectivity shift even for very polar solutes.…”
Section: Selectivity As a Function Of' Temperaturesupporting
confidence: 87%
“…forces to the solute retention mechanism. Thus, Since higher temperatures result in lower elution swelling polar stationary phases with CO, may pressures and densities for the relatively lowinduce large selectivity shifts [4]. molecular-weight solutes in this test mix, we conclude that the stationary phases simply cannot be swelled above 160°C at the elution pressures of these solutes.…”
Section: Selectivity As a Function Of' Temperaturementioning
Abstract. We studied solute retention for n-hydrocarbons, (low-molecular-weight) polystyrenes, an ethoxylated surfactant, and a selectivity test mix on open-tubular columns with methyl-, biphenyl-, and cyanopropyl-substituted stationary phases for pressures up to 680 atm and temperatures up to 240°C. The solute elution range varied tremendously with column choice, with the least retentive stationary phases providing the highest elution range. However, the best resolution and largest analysis range were obtained with the most retentive stationary phase and the highest pressures. Increasing the temperatures above 160°C did not cause a large increase in elution pressure for the solutes used in this study. Exceptionally large shifts in selectivity with temperatures up to about 160°C occurred for the biphenyl and cyanopropyl stationary phases. The availability of pressures higher than 680 atm, coupled with adequately retentive stationary phases used at optimal temperatures, would further increase the analysis scope of opentubular supercritical fluid chromatography.
“…Kueppers et al examined the influence of velocity and multigradient programming on chromatographic efficiency, concluding that a pressure or density program needs a simultaneous negative velocity program to maintain column efficiency (139). Chester and Innis performed open-tubular SFC with positive linear temperature programming at constant density (140). The results using nonpolar columns resembled GC except that chromatograms were uniformly compressed to lower elution temperatures as the experiment was repeated at higher density.…”
Section: Sfc Instrumentation Techniques and Performancementioning
“…However, it has been often reported in the SFC literature that the density of the fluid (mobile) phase is the determinant factor for the retention of chromatographic solutes. In particular, the solute partition coefficients should vary logarithmically with density. ,− Figure illustrates that this model is valid for the benzene retention data reported in Table S-2 (Supporting Information). 4 Partition coefficient of benzene versus the density of the gas phase.…”
Experimental results for the investigation of chromatographic columns containing two mobile phases are presented. The eluent was composed of mixtures of methanol and carbon dioxide. The column was an uncoated fused-silica-lined stainless steel capillary column. At certain experimental conditions, the eluent divided into two phases, both of which moved through the column. The predominant component of the liquid phase was methanol whereas the gas phase was composed of at least 93 mol % CO2. The columns were studied over a range of feed compositions (45-95 mol % CO2), pressures (61-101 bar), and temperatures (30-100 degrees C). The compositions and densities of each phase were calculated from the Peng-Robinson equation of state. The residence times of the two mobile phases were determined by tracer pulse chromatography. The partition coefficients of a probe solute, benzene, were measured along with the retention times of neon and the total volume of the chromatographic column as a function of temperature, pressure, and stoichiometric feed composition. The calculated column volumes, that is the volume of the liquid and gas, were constant over the full range of feed composition. The partition coefficient of benzene was constant at fixed pressure and temperature, varied logarithmically with density at fixed temperature and feed composition, and displayed a maximum at intermediate temperatures at fixed pressure and feed composition. The measured retention times of neon were consistently equivalent to the calculated residence times of the gas phase, indicating that neon did not dissolve in the liquid phase and could thus serve as an accurate dead time marker. The implementation of chromatography with two mobile phases produces a chromatographic "window". There is a lower limit for the retention volume of all solutes, viz., the residence time of the gas phase, exactly the same as normal chromatography. However, elimination of the stationary phase produces an upper limit to the retention volumes of solutes. This upper limit is the residence time of the liquid phase, so there is a retention window such that tG < or = ti < or = tL for all solutes.
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