Anal. Chem. 1995,67, 273-279
Determination of Solubilities of Organic Solutes in Supercritical COn by On=LineFlame Ionization Detection David J. Miller* and Steven B. Hawthome Energy & Environmental Research Center, Universijl of North Dakota, Box 9018, Grand Forks, North Dakota 58202-9018
A simple and rapid method has been developed to determine the solubility of organic compounds in supercritical C02 by coupling a saturation cell directly to a flame ionization detector. After temperature equilibration,the detector's response to cell pressure changes is rapid, which allows several solubility determinations to be performed per hour. Solubilities determined by this method are in good agreement with published data obtained from gravimetric or spectroscopic methods for anthracene and tripalmitin. The estimated detection limit for this method is 5 min (i.e., it took ca. 5 min for the FID signal to stabilize at a maximum value after reaching 200 "C). However, when the programming rate was lowered to 2 "C/min, no lag in the FID response was observed. 'Therefore, solubility changes with temperature were obtained with temperature ramps of 2 "C/min (which requires a total of 75 min for a ramp from 50 to 200 "C). Figure 7 shows solubility profiles with temperature programming at 2 "C/min for anthracene, chrysene, coronene, and rubrene. Since C02 flow changes by only (7% during temperature programming (in contrast to pressure programming, where large changes in flow occur), solubility data are directly proportional to the change in FID response. For anthracene (Figure 7), the temperature programming FID profile is directly related to the data given in Table 2. Since FID response per mass of hydrocarbon is essentially the same for anthracene and the other species shown in Figure 7, the anthracene solubilities can, therefore, be used to directly estimate the solubilities of the other hydrocarbons by simply adjusting for differences in molecular weight (Le., to obtain mol/mol solubilities). Figure 7 shows the temperature-programmedsolubility profiles (pressure constant at 400 bar) for chrysene (MW = 228), coronene (MW = 300), and rubrene (MW = 532), along with the estimated solubility at 200 "C (400 bar). Chrysene has some solubility in supercritical COz at 40 "C, and the solubility continues
to rise with temperature to an estimated solubility of 4.60 x mol/mol. The solubility of coronene appears to be below the detection limit (lo-* mol/mol) of the on-line FID method at temperatures
