Mobile-phase density

One of the primary benefits of pressure/denslty programming is peak compression that results in later eluting peaks having the same width, or an even narrower width, than the earliest peaks in the chromatogram. Qualitatively, this can be ascribed to either positional variations in mobile phase density or velocity along... 

Although cSFC shows relatively poor figures of merit (speed, sensitivity, detection dynamic range and sample capacity) as well as a limited application area, its applications tend to be unique. These include solutes that can be solvated with pure SCCO2 and quantified with FID. Linear density programs typical in cSFC are ideal for homologous series found in surfactants, many prepolymers, etc. Selectivity in cSFC, which can be achieved by mobile phase density and temperature programming, relies on selective interactions with the stationary phase. Quantitative analysis in cSFC may be rendered difficult by small injected volumes the use of internal standards is recommended. 

Density Optimization. The retention surfaces as a function of mobile phase density at a temperature of 80 °C are shown in Figure 4. The quadratic dependence of In k on density is apparent over a wide range in density. More importantly, however, there are numerous peak reversals which demonstrate the need for a systematic optimization approach. 

Veuthey and Haerdi reported the separation of amphetamines using packed-column SFC [26]. The amphetamines were derivatized with 9-fluorenylmethyl chloroformate and chromatographed with a methanol or 2-propanol-modified carbon dioxide as the mobil phase. The separations were compared on bare silica and aminopropyl-bonded silica columns. Both columns gave comparable results and the separation of all five amphetamines (methylamphetamine, amphetamine, phenethylamine, ephed-rine, and norephedrine) was achieved in less than 5 min. Both methanol and 2-propanol-modified carbon dioxide gave comparable results. It was observed that the modifier concentration had more effect on the solvating power than the mobile-phase density. 

Figure 3.33 Retention (In ft) as a function of (a) pressure, (b) mobile phase density and (c) the logarithm of the mobile phase density in SFC at three different temperatures. Mobile phase carbon dioxide. Stationary phase ODS. Solute naphthalene. Figure taken from ref. [390]. Reprinted with permission. Experimental data from ref. [391].

To a first approximation [393] the selectivity (a) on a given stationary phase may be expected to be independent of the mobile phase density. Consequently, the problem of stationary phase selection is similar to that encountered in GC. In GC each stationary phase will require a given temperature at which the capacity factors are in the optimum range. In SFC, each stationary phase will require a given mobile phase density. Different phases may be compared at their individual optimum conditions. 

The mobile phase density is determined by the combination of the pressure and the temperature. 

SFC Mobile phase density Density programming pressure programming... 

SFC Mobile phase density Mobile phase composition Nature of mobile phase stationary phase Nature of modifier(s)... 

The experimental curves in Figure la can be used to calculate Dj2 pyrene in carbon dioxide at the cited temperature and mobile phase density D12 = 0.00008 cmVs, a value that is quite... 

Sedimentation Field Flow Fractionator. The chromatography-related principle of this particle size and size distribution analyzer is based upon the interaction of the particle suspension under centrifugal field motion in a thin channel. The elution time of the particles is a function of particle size, particle density, flow rate of mobile phase, density of mobile phase, and the centrifugal force applied. After the size separation has occurred, the particles are detected in the mobile phase using a turbidity detection system. The dynamic range of the instrument is dependent on particle density and operating conditions and is typically within 0.03 /rm— 1 /rm range. 

Figure 29 8 illustrates the separation of polycyclic aromatic hydrocarbons extracted from a carbon black, Detection was by fluorescence excited at two differeni wavelengths. Note the selectivity provided by this technique, The chromatogram was obtained b> using a 40 m X. SO pm inside-diameter capillary coated with a 0,25-pm film of S0% phenylpolysiloxane. The mobile phase was pentane at 2l(rc. and the following pro-gram was used initial mobile-phase density held at... 

Even though the movement of the intercept has not been studied in depth in capillary SFC, the general trend of retention curves at various densities at temperatures away from the origin are well known Increasing the mobile phase density at any temperature reduces the retention. Several conclusions follow from this knowledge ... 

Figure 2. Retention behavior of an ideal analyte in supercritical fluid chromatography with changing mobile phase density, P.

---