Supercritical fluid chromatography retention

Pyo, D. Li, W. Lee, M.L. Weckwerth, J.D. Carr, P.W. Addition of methanol to the mobile phase in packed capillary column supercritical fluid chromatography retention mechanisms from linear solvation energy relationships. J. Chromatogr. A. 1996, 753, 291-298. 

A method which uses supercritical fluid/solid phase extraction/supercritical fluid chromatography (SE/SPE/SEC) has been developed for the analysis of trace constituents in complex matrices (67). By using this technique, extraction and clean-up are accomplished in one step using unmodified SC CO2. This step is monitored by a photodiode-array detector which allows fractionation. Eigure 10.14 shows a schematic representation of the SE/SPE/SEC set-up. This system allowed selective retention of the sample matrices while eluting and depositing the analytes of interest in the cryogenic trap. Application to the analysis of pesticides from lipid sample matrices have been reported. In this case, the lipids were completely separated from the pesticides. 

Yan, C. and Martire, D.E., Molecular theory of chromatography for blocklike solutes in isotropic stationary phases and its application to supercritical fluid chromatographic retention of PAHs, J. Phys. Chem., 96, 7510, 1992. 

The separation and estimation of diloxanide furoate and metronidazole in solid dosage forms was reported by Bhoir et al., using packed column supercritical fluid chromatography [38], A JASCO Cig colunm (10 pm particle size, 25 cm x 4 mm) was used at 40°C, with an injection volume of 20 pL. The mobile phase consisted of 26% methanol in CO2 (flow rate of 2 mL/min), and operated at a pressure of 17.6 MPa. When detected on the basis of its ultraviolet absorbance at 230 nm, the retention time for the drug was 1.6 minutes. The linear region of the calibration graph was reported to be 20-70 pg/mL. 

Trimethylsilyl derivatives of ten hydroxy- and methoxyhydroxyflavonoids have been studied by the GC-FTIR technique." " The correlation found between retention and gas-phase IR data was used in structural identification of compounds having very similar chromatographic behavior. The shift of the carbonyl frequency gave information on the presence of substitution. Some hydroxy- and methoxy-substituted flavones have been studied following carbon dioxide supercritical fluid chromatography on polymethylsiloxane capillary columns using flame ionization and FTIR detection." " " ... 

Cosolvent-modifled supercritical fluids are also used routinely in supercritical fluid chromatography (SFC) to modify solute retention times (11-20). In these reports, cosolvents are used to alter the mobile and stationary phase chemistries (16t17t20). However, distinguishing between such effects in a chromatography... 

Use of Solvatochromic Dyes To Correlate Mobile Phase Solvent Strength to Chromatographic Retention in Supercritical Fluid Chromatography... 

T.A. Berge and J. F. Deye, Correlation between column surface area and retention of polar solutes in packed-column supercritical fluid chromatography, J. Chromatogr., 594 291 (1992). 

Biermanns et al. reported the chiral resolution of /3-blockers, including propranolol, metoprolol, and atenolol using packed-column supercritical fluid chromatography [38]. A Chiracel OD column with a mobile phase of 30% methanol with 0.5% isopropylamine in carbon dioxide was used for the separation. A baseline separation of isomers was obtained in less than 5 min at a mobile-phase flow rate of 2 ml/min. While keeping the column outlet pressure constant, the flow rate was increased to 4 ml/min and it was noted that, although the retention was reduced, the resolution remained the same. Both R- and S-propranolol gave linear responses from 0.25-2500 ppm with a correlation coefficient of >0.9999. The detection limit was approximately 250 ppb for a S/N ratio of 3. The reproducibility for both R- and S-propranolol was less than 1.5%. It was also noted that 0.09% R-propranolol can be quantitated in the presence of 2500 ppm of S-Propranolol. 

The complicated dependence of retention in supercritical fluid chromatography as a function of temperature and pressure is examined. Simple thermodynamic relationships are derived and discussed which allow the calculation of the slope of solute retention as a function of both temperature and pressure. 

The effect of temperature, pressure and density on solute retention (k1) in supercritical fluid chromatography (SFC) has been well studied.(1-6) Retention in SFC depends upon both solute solubility in the fluid and solute interaction with the stationary phase. The functional relationship between retention and pressure at constant temperature has been described by Van Wasen and Schneider. ( 1 ) The trend in retention is seen to depend on the partial molar volume of... 

The checkers determined enantiomeric purity by supercritical fluid chromatography (SFC) using a Chiralpak AD (4.6 x 250 mm) column. Eluent carbon dioxide (300 Bar) modifier methanol (24%) flow rate 1.5 mL/min detection UV (210 nm). Retention times were as follows "diphenylcyclopentanone (3.9 min) (R)-diphenylcyclopentanol (5.9 min) (S)-diphenylcyclopentanol (10.4 min). 

Wright, B.W., Kalinoski, H.T., and Smith, R.D., 1985. Investigation of retention and selectivity effects using various mobile phases in capillary supercritical fluid chromatography. Analytical Chemistry, 57 2823-9. 

In addition to normal- and reversed-phase chromatography, silver ion HPLC, RP ion-pair HPLC, chiral separation, and supercritical fluid chromatography have been used for analysis of lipids [3]. In silver-ion HPLC, the counterion of an ion-exchange column such as sulfonate is exchanged with silver ions. Only the degree of unsaturation in the lipid molecule determines retention. In RP ion-pair HPLC, different ion-pairing agents, such... 

Retention and selectivity in supercritical fluid chromatography (SFC) are a complex function of many experimental variables and are not as easily rationalized as in the case of gas and liquid chromatography. Retention in SFC is dependent on temperature, density (and pressure drop), stationary-phase composition, and the mobile-phase composition. Many of these variables are interactive and do not change in a simple or easily predicted manner [1]. 

Figure 33-2 shows plots of plate heights // as a function of average linear velocity u in cm/s for high-performance liquid chromatography and supercritical-fluid chromatography. In both cases, the solute was pyrene, and the stationary phase was a reversed-phase octadecyl silane maintained at 40°C. The mobile phase for HPLC was acetonitrile and water, while for SFC the mobile phase was carbon dioxide. These conditions yielded about the same retention factor (k) for both mobile phases. Note that the minimum in plate height occurred at a flow rate of 0.13 cm/s... 

Supercritical fluid chromatography has at least one extra control variable compared to HPLC. Many people still associate the name SFC with syringe-pump-based systems in which pure carbon dioxide was used with pressure programming. Modern SFC sometimes still operates in this manner but only for nonpolar solutes. SFC is somewhat different from HPLC. It is useful to discuss the relative effects of different control variable on retention and selectivity. The relative effects of several control variables are represented schematically in Fig. 5. 

Supercritical fluid chromatography can be used as a qualitative or quantitative technique. Qualitative information allows a substance to be identified on the basis of retention time comparisons between a standard and the sample. When used quantitatively, the SFC can employ external or internal standards to generate a standard curve. Then, the peak area of the sample peak on the chromatogram can be used to obtain a concentration. An example of an SFC instrument is shown in Figure 3.35. 

Hanson, M. Aspects of retention behaviour of steroids in packed column supercritical fluid chromatography. Chromatographia, 1995, 40, 58-68... 

A number of experimental techniques have been described for the determination of isotherms based on frontal analysis, frontal analysis by characteristic point, elution by characteristic point, and perturbation methods [12,21,27,169,176-179]. Most authors report single-component isotherm results. Multiple-component isotherm data are more complicated because all components are simultaneously in competition for the sorption sites on the stationary phase. The retention time and peak shapes of any solute is dependent on the concentration and properties of all other solutes in the mixture [12,170,180]. For multicomponent mobile phases in liquid and supercritical fluid chromatography this includes each component of the mobile phase. 

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