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  • Journal of Chromatography A
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  • 1
    Language: English
    In: Journal of Chromatography A, 2011, Vol.1218(28), pp.4559-4568
    Description: The first systematic study of the performance of a porous shell, hydrophylic interaction liquid chromatography (HILIC) column in supercritical fluid chromatography (SFC) is presented. Observed efficiency on 2.6-μm porous shell particles exceeded all reports using UHPLC on 100-mm long columns packed with 〈2-μm totally porous particles. A Kinetex 4.6 × 150 mm, 2.6 μm HILIC column significantly outperformed a 3 μm Luna totally porous silica of the same length and diameter. A 17 component, low molecular weight test mix, consisting of a range of small drug-like molecules was separated isocratically on each column, with similar selectivity, but the porous shell column required ½ the time (≈2 min vs. 4 min), with almost 50% higher efficiency. Even little retained compounds ( 〈 0.5) exhibited more than 30,000 plates under some conditions. Reduced plate heights were higher than previously reported on porous shell particles in both HILIC and rHPLC, with the lowest value of 1.62. Significant fronting was sometimes observed. The cause of the fronting was not determined. The least symmetrical peaks showed the highest apparent efficiency. Pressure drop at optimum velocity (2.5 ml/min) and low modifier concentrations was 〈60 bar, and only exceeded 250 bar at near double optimum flow and 65% modifier. Peak widths were mostly just over 0.01 min (20 Hz) wide. There was a loss of efficiency when the injection volume was increased. The chromatograph was shown to have extremely low extra-column dispersion, on the order of 5–10 μL , which is also the lowest reported in an SFC, in spite of using standard components. This is likely due to turbulent flow in the tubing and fittings.
    Keywords: Sfc ; Supercritical Fluid Chromatography ; Porous Shell Particles ; Low Pressure Drop ; Turbulent Flow ; Low Dispersion ; Chemistry
    ISSN: 0021-9673
    E-ISSN: 18733778
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  • 2
    Language: English
    In: Journal of Chromatography A, 20 November 2015, Vol.1421, pp.171-183
    Description: Analytical scale supercritical fluid chromatography (SFC) is largely a sub-discipline of high performance liquid chromatography (HPLC), in that most of the hardware and software can be used for either technique. The aspects that separate the 2 techniques stem from the use of carbon dioxide (CO ) as the main component of the mobile phase in SFC. The high compressibility and low viscosity of CO mean that pumps, and autosamplers designed for HPLC either need to be modified or an alternate means of dealing with compressibility needs to be found. The inclusion of a back pressure regulator and a high pressure flow cell for any UV–Vis detector are also necessary. Details of the various approaches, problems and solutions are described. Characteristics, such as adiabatic vs. isothermal compressibility, thermal gradients, and refractive index issues are dealt with in detail.
    Keywords: Compressibility ; Pumps ; Chillers ; Modify Autosampler ; Stagnant Air Oven ; Extra-Column Band Broadening ; Chemistry
    ISSN: 0021-9673
    E-ISSN: 18733778
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  • 3
    Language: English
    In: Journal of Chromatography A, 2011, Vol.1218(16), pp.2320-2326
    Description: Pressure fluctuations and resulting refractive index changes, induced by the back pressure regulator (BPR) can be a significant source of UV detector noise in supercritical fluid chromatography (SFC). The refractive index (RI) of pure carbon dioxide (CO ) changes ≈0.2%/bar at the most commonly used conditions in supercritical fluid chromatography (SFC) (40 °C and 100 bar), compared to 0.0045%/bar for water (CO IS 44× worse). Changes in RI cause changes in the focal length of the detector cell which results in changes in UV intensity entering the detector. The change in RI (ΔRI/bar) of CO decreases 8-fold at 200 bar, compared to 100 bar. A new back pressure regulator (BPR) design representing an order of magnitude improvement in the state of the art is shown to produce peak to peak pressure noise (PN ) as low as 0.1 bar, at 200 bar, and 20 Hz, compared to older equipment that attempted to maintain PN 〈 1 bar, at 〈5 Hz. With this lower PN , changes in baseline UV offsets could be measured as a function of very small changes in pressure. A pressure change of ±1 bar at 100 bar, common with some older BPR's, produced a UV baseline offset 〉0.5 mAU. A pressure change of ±0.5 bar representing the previous state-of-the-art, resulted in a UV offset of 0.3 mAU. Baseline noise 〈0.05 is required to validate methods for trace analysis. The new BPR, with a PN of 0.1 bar, demonstrated UV peak to peak noise (N ) 〈 0.02 mAU with a 〉0.03 min (10 Hz) electronic filter under some conditions. This new low noise level makes it possible to validate SFC methods for the first time.
    Keywords: Supercritical Fluid Chromatography (Sfc) ; Back Pressure Regulator (Bpr) ; Refractive Index (Ri) ; Uv Detector Noise ; Pressure Noise ; Validation ; Chemistry
    ISSN: 0021-9673
    E-ISSN: 18733778
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  • 4
    Language: English
    In: Journal of Chromatography A, Oct 17, 2014, Vol.1364, p.249(12)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.chroma.2014.08.067 Byline: Terry A. Berger Abstract: acents Suppression of UV noise caused by mis-match between the mobile phase temperature and the flow cell temperature. acents Noise with most older flow cells and detectors used in SFC can be significantly improved. acents Conical flow path not as important as in HPLC since refractive index in SFC is much lower. acents Thermal isolation of the flow cell from the optical bench is important. acents Quantify a 0.1% trace component with S/N10 at 80Hz (200bar). Author Affiliation: SFC Solutions, Inc., 9435 Downing St, Englewood, FL 34224, United States Article History: Received 25 February 2014; Revised 18 August 2014; Accepted 20 August 2014
    Keywords: Detection Equipment ; Chromatography
    ISSN: 0021-9673
    Source: Cengage Learning, Inc.
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  • 5
    Language: English
    In: Journal of Chromatography A, 11 August 2017, Vol.1510, pp.82-88
    Description: The performance of a 3 × 50 mm, 1.6 μm d column with an immobilized polysaccharide stationary phase (ChiralPak IA-U) was evaluated for efficiency, and pressure drop, with respect to flow rate and modifier concentration using supercritical fluid chromatography (SFC). This appears to be the first such report using such a column in SFC. A unique low dispersion (ultra-high performance) SFC was used for the evaluation. The minimum reduced plate height of 2.78, indicates that the maximum efficiency was similar to or better than coated polysaccharide columns. Selectivity was different from ChiralPak AD, with the same chiral selector, as reported by many others. At high flows and high methanol concentrations, pump pressures sometimes approached 600 bar. With 5% methanol, pressure vs. flow rate was non-linear suggesting turbulent flow in the connector tubing. The optimum flow rate (F ) at 40% methanol was ≈0.8 mL/min, where the column efficiency was highest. At 5% methanol, F increased to ≈1.6 mL/min, but efficiency degraded noticeably. The differences in F suggests that the solute diffusion coefficients are a strong function of modifier concentration. Several sub–1 min separations, including a 7.5 s separation, are presented.
    Keywords: Sub–2 Μm Particles ; Immobilized Polysaccharide Chiral Column ; Chiralpak Ia-U ; Ultra-High Performance Supercritical Fluid Chromatography (Sfc) ; Kinetic Performance ; Sub-1 Min Chromatograms ; Chemistry
    ISSN: 0021-9673
    E-ISSN: 18733778
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  • 6
    Language: English
    In: Journal of Chromatography A, 02 December 2016, Vol.1475, pp.86-94
    Description: It has been widely suggested that the outlet pressure be changed to maintain constant density (“isopycnic” conditions) when comparing the kinetic performance of different columns in supercritical fluid chromatography (SFC). However, at high flow rates, flow in the tubing is turbulent, causing large extra-column pressure drops that limit options for changing outlet pressure. Some of these pressure drops occur before and some after the column, obscuring the actual column inlet and outlet pressures. In this work, a 4.6 × 100 mm, 1.8 μm R,R-Whelk-O1 column was used with low dispersion LD (120 μm) plumbing to generate sub-1 min chiral separations. However, the optimum, or near optimum, flow rate was 5 mL-min , producing a system pressure of 580 bar (with 40% methanol, outlet pressure 120 bar). Both the flow rate and pump pressure required were near the limits of the instrument, and significantly exceeded the capability of many other SFC’s. Extra-column pressure drops (ΔP ) were as high as 200 bar, caused mostly by turbulent flow in the tubing. The ΔP increased by more than the square of the flow rate. Reynolds Numbers (Re) were calculated for tubing as a function of flow rate between 100 and 400 bar and 5–20% methanol in CO , and 40°–60 °C. This represents the most extensive analysis of turbulence in tubing in the SFC literature. Flow in 120 μm ID tubing was calculated to be laminar below 1.0 mL-min , mostly transitional up to 2.5 mL-min and virtually always turbulent at 3 mL-min and higher. Flow in 170 μm tubing is turbulent at lower flows but generates half the ΔP due to the lower mobile phase linear velocity. The results suggest that, while sub-minute chromatograms are easily generated, 4.6 mm columns are not very user friendly for use with sub-2 μm packings. The high flow rates required just to reach optimum result in high ΔP generated by the tubing, causing uncertainty in the true column inlet, outlet, and average column pressure/density. When comparing kinetic performance of columns with different dimensions, the pressure drops in the tubing must be considered.
    Keywords: Supercritical Fluid Chromatography (Sfc) ; Turbulent Flow in Tubing ; Extra-Column Pressure Drop ; High Pressure ; Sub-Minute Chiral Separations ; Chemistry
    ISSN: 0021-9673
    E-ISSN: 18733778
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  • 7
    Language: English
    In: Journal of Chromatography A, 24 August 2018, Vol.1564, pp.188-198
    Description: In a relatively recent reevaluation of the van Deemter Equation, Guiochon concluded that the mass transfer resistance in the mobile phase is independent of the retention factor. In the process he showed reduced plate heights ≈ 2 for a nearly unretained peak (k = 0.4) in high performance liquid chromatography (HPLC). In the present work, using supercritical fluid chromatography (SFC), efficiency was measured at various pressures, densities, and modifier concentrations. The highest efficiency, with a reduce plate height of h  = 1.63, was recorded with the lowest retention factor (k 〈 0.8). This is an extremely low h for totally porous particles, at very low k, and appears to support Guiochon's analysis. The density of methanol/carbon dioxide mixtures were calculated using the REFPROP program from the National Institute of Standards and Technology (NIST) over a wide range of pressures and % methanol. The density of higher methanol concentrations (〉20%), commonly used in SFC, was found to be lower than the density of lower concentrations (〈20%). At low methanol concentrations, density varies widely with pressure. However, at high methanol concentrations there is very little change in density, and very little change in retention with pressure. With increasing modifier concentration, density decreases, while viscosity increases (ΔP increases). The pump and back pressure regulator (BPR) pressures are not necessarily good indicators of pressures or densities in the column. At high flow rates the extra-column pressure drop (ΔP) can be much larger than the column ΔP and can be unevenly distributed in front of and behind the column. In one extreme the ΔP after the column was 3 times higher (105 bar) than the actual column ΔP (32 bar).
    Keywords: Supercritical Fluid Chromatography (Sfc) ; Density of Co2/Meoh ; Density Vs. Pressure ; Retention ; Efficiency ; Isopycnic Operation ; Chemistry
    ISSN: 0021-9673
    E-ISSN: 18733778
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  • 8
    Language: English
    In: Journal of Chromatography A, 12 August 2016, Vol.1459, pp.136-144
    Description: Reduced plate heights (h ) of 〈2 were observed for the first time during the chiral separation of enantiomers, on sub-2 μm particles with supercritical fluid chromatography (SFC). The enantiomers of -stilbene oxide, were separated on a 4.6 × 50 mm, 1.8 μm R,R-Whelk-O1 column, with h as low as 1.93. The plumbing of a commercial SFC instrument was modified to create a low dispersion version. Without the modification performance was considerably worse. vanDeemter like plots of reduced plate height vs. flow rate, for -stilbene oxide, indicate that the optimum flow varied with% modifier. On a 4.6 × 250 mm, 5 μm R,R- Whelk-O1 column, the optimum flow was 〉4 mL/min for 5% methanol in CO , decreasing to 〈2 mL/min for 40% methanol (more than a factor of 2). For a 4.6 × 50 mm column packed with 1.8 μm particles the optimum appeared to be near, or 〉5 mL/min with 2.5%, 5%, and 10% methanol, decreasing to between 3 and 3.5 mL/min at 40% methanol. This is the first time such shifts have been characterized. Since the solutes were the same in all cases, the differences are likely due to changes in solute diffusion coefficients caused by changes in modifier concentration, and pressure. Pump pressure requirements sometimes exceeded 500 bar. It is shown that a 5 mL/min flow rate is inadequate for use with 1.8 μm particles in a 4.6 mm ID column format. Instead, it is suggested to decrease the ID of the column to 3 mm, where the optimum flow rates are on the order of 2 mL/min with decreased tubing variance. Nevertheless, a number of sub-1 min chromatograms are presented.
    Keywords: Chiral Column ; 1.8 Μm Particles ; Hr 〈2dp ; Variable Vandeemter Minima ; Sub-1 Min Chromatograms ; Extra-Column Dispersion ; Chemistry
    ISSN: 0021-9673
    E-ISSN: 18733778
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  • 9
    Language: English
    In: Journal of Chromatography A, 29 April 2016, Vol.1444, pp.129-144
    Description: The concept of peak fidelity was shown to be helpful in modeling tubing and detector cell dimensions. Connection tubing and flow cell variances were modeled to determine appropriate internal ID’s, lengths, and volumes. A low dispersion plumbing configuration, based on these calculations, was assembled to replace the standard plumbing and produced the reported results. The modifications made were straightforward using commercially available parts. The full theoretical efficiency of a 3 × 100 mm column packed with 1.8 μm totally porous particles was achieved for the first time in supercritical fluid chromatography (SFC). Peak fidelity of 〉0.95 was maintained to below k = 2. A reduced plate height as low as 1.87 was measured. Thus, true “ultra high performance” SFC was achieved, with the results a major improvement from all previous SFC reports. Since there were no efficiency losses, none could be attributed to thermal gradients caused by the expansion of the fluid over large pressure drops, under the conditions used. Similarly, changes in diffusion coefficients caused by significant decreases in density during expansion are apparently balanced by the increase in linear velocity, keeping the ratio between the diffusion coefficient and the linear velocity a constant. Changing modifier concentration to change retention was shown to not be a significant problem. All these issues have been a concern in the past. Diffusion coefficients, and viscosity data needs to be collected at high pressures before the actual limits of SFC can be discovered.
    Keywords: Supercritical Fluid Chromatography (Sfc) ; 1.8 Μm Particles ; Theoretical Efficiency ; Hr 〈 2 @ K = 2 ; Peak Fidelity ; Temperature Gradients ; Chemistry
    ISSN: 0021-9673
    E-ISSN: 18733778
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  • 10
    Language: English
    In: Journal of Chromatography A, 17 October 2014, Vol.1364, pp.249-260
    Description: A mis-match between the post-column mobile phase temperature and the UV detector flow cell temperature can cause significant UV noise in supercritical fluid chromatography (SFC). Deviations as little as 5 °C can increase noise as much as 5 times, making the detector unsuited for trace analysis. Two approaches were used to minimize this noise. When a flow cell was in direct thermal contact (metal on metal) with the detector optical bench, the mobile phase temperature was actively controlled to the measured flow cell temperature, by using one of the heat exchangers (HX) in the column compartment. However, with some older, but still widely used flow cell designs, this required repeated, hourly monitoring of the flow cell temperature and repeated manual adjustment of the heat exchanger temperature, due to thermal drift. Flow cell design had a strong influence on susceptibility to this thermally induced noise. Thermally insulating the flow cell from the optical bench made some cells much less susceptible to such thermally induced noise. Five different flow cells, some insulated, some un-insulated, were evaluated. Most had a truncated conical flow path, but one had a cylindrical flow path. Using either approach, the ASTM noise, with a 10 mm, 13 μL conical flow cell, could be optimized to ≈0.007 mAU at 2.5 Hz, in SFC, which is very near the 0.006 mAU manufacturer's specification for HPLC. The insulated version of this flow cell required far less optimization, compared to the un-insulated version. At 150 bar, an experimental 3 mm, 2 μL flow cell, with only one side insulated, yielded noise slightly too high (≈0.16–0.18 mAU) for trace analysis, at 80 Hz. However, at 200 bar, noise at 80 Hz was 〈0.06 mAU, which should allow quantification of a 1 mAU tall trace component with a signal to noise ratio (S/N) 〉10. Even partially un-insulated, this flow cell design was much less susceptible to thermally induced noise. Further insulating this flow cell design failed to improve performance.
    Keywords: Supercritical Fluid Chromatography (Sfc) ; Uv Noise ; Post-Column Temperature Optimization ; Conical Vs. Cylindrical Flow Cell ; Flow Cell Insulation ; Sub-2 Μm Particles ; Chemistry
    ISSN: 0021-9673
    E-ISSN: 18733778
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