Chromatography with Two Mobile Phases

Abstract

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.

Chromatography Using a Water Stationary Phase and a Carbon Dioxide Mobile Phase

“…However, since we did not observe citric acid under either of these conditions after several hours of operation, it stands to reason that the water flow rate is not causing movement of the stationary phase but rather providing humidification to prevent evaporation. This characteristic is in contrast to other biphasic CO 2 systems that are based upon a pseudostationary wall coating moving at a slower rate than the CO 2 mobile phase, which dictates that analytes can elute within a specified time frame. ,, …”

“…For example, CCC using a supercritical CO 2 mobile phase was employed to separate acetophenone and benzophenone in 3 h when incorporating a rotating coil operating at 435 rpm . More recently, however, methanol/supercritical CO 2 systems have been explored by Parcher et.al., which employ a slowly moving, pseudostationary methanol phase formed along the inner wall of an uncoated capillary. , In this instance, the separation of several light n-alkanes was clearly and successfully demonstrated. To our knowledge, a system as described above, which operates with a purely aqueous stationary phase and a CO 2 mobile phase in capillary separations has not been reported.…”

Mass Spectrometric Tracer Pulse Chromatographic Investigations of Eluent Sorption with Bonded RPLC Packings

“…Mass spectrometric detection of stable isotopes, such as H, 13 C, O, and N, has been used extensively to investigate retention mechanisms with gas and supercritical fluid systems. , However, the application of tracer pulse methods to RPLC systems was inhibited by the lack of suitable LC/MS interface and ionization instrumentation. This situation has improved with the development of commercial, atmospheric pressure electrospray, and chemical ionization sources.…”