Viscosity Measurement of CO2-Solvent Mixtures for the Study of the Morphology and Size of Crystalline Particles Obtained Using Supercritical Antisolvent Precipitation.

The viscosity values of CO2-dimethylphormamide, chloroform, methanol, isopropanol, ethyl acetate, acetone, and dimethyl sulfoxide mixtures were measured at a pressure of 150 bar and a temperature of 313 K. The correlation of the mean size of levofloxacin hydrochloride and malonic acid particles precipitated using the SAS method with the viscosity of the used CO2-solvent mixtures is shown. The high viscosity of the mixtures leads to slower mixing of the solution and the antisolvent. Therefore, crystallization occurs at large fractions of the solvent, and as a consequence at a lower supersaturation. This causes the formation of larger particles when using more viscous solvents in SAS.

Solute Diffusion into Polymer Swollen by Supercritical CO2 by High-Pressure Electron Paramagnetic Resonance Spectroscopy and Chromatography.

High-pressure electron paramagnetic resonance (EPR) was used to measure translational diffusion coefficients (Dtr) of a TEMPONE spin probe in poly(D,L-lactide) (PDLLA) and swollen in supercritical CO2. Dtr was measured on two scales: macroscopic scale (>1 µm), by measuring spin probe uptake by the sample; and microscopic scale (<10 nm), by using concentration-dependent spectrum broadening. Both methods yield similar translational diffusion coefficients (in the range 5-10 × 10-12 m2/s at 40-60 °C and 8-10 MPa). Swollen PDLLA was found to be homogeneous on the nanometer scale, although the TEMPONE spin probe in the polymer exhibited higher rotational mobility (τcorr = 6 × 10-11 s) than expected, based on its Dtr. To measure distribution coefficients of the solute between the swollen polymer and the supercritical medium, supercritical chromatography with sampling directly from the high-pressure vessel was used. A distinct difference between powder and bulk polymer samples was only observed at the start of the impregnation process.

Unusual effect of flow rate on retention in analytical supercritical fluid chromatography exemplified by polyethylene glycol separation.

We report a case of a peculiar effect of flow rate on retention in a separation of polyethylene glycol oligomers via supercritical fluid chromatography. During method development, we tested flow rate gradients and notices that for some PEG oligomers retention times at flow rate gradient were lower than at constant flow with the largest flow rate value used in a gradient. For instance, at BEH stationary phase and CO2-MeOH gradient from 10 to 35% at 20 min a PEG oligomer having mass of 1225 Da has a retention time 14 min at 1 mL/min flow rate, 10.3 at 2 mL/min and 9.5 min at 1-2 mL/min flow rate gradient. The effect is not unified for all PEG oligomers, it occurs only starting from a particular PEG molecular weight which depends on the stationary phase type and/or mobile phase conditions. We believe that such an unusual flow rate effects can happen in SFC on various occasions, not exclusively for flow rate gradients, and thus should be taken into account during method development or method transfer.

Unexpected separate elution of cation and anion of an ammonium salt in supercritical fluid chromatography.

Amines are frequently used as additives in supercritical fluid chromatography (SFC). They allow eluting basic analytes with reasonable retention times and less distorted peak shapes. Since amines are chemically active compounds, their introduction into SFC mobile phase always raises a question on whether they can react with analytes or mobile phase constituents and, if so, can it affect chromatography separation. Primary and secondary amines are known to react with carbon dioxide, also all amines, being bases, can interact with CO2-alcohol mixtures which are known to be slightly acidic. In this work, we report a case of separate elution of an ammonium salt, salbutamol sulfate, anion and cation in SFC. Retention time of a peak which molecular mass registered by mass-spectrometry in ES(-) mode corresponds to HSO4- differs substantially from the retention time of a peak corresponding to salbutamol [M+H] registered in ES(+) regime. Moreover, sulfate anion retention time depends both on amine additive type and concentration whereas salbutamol retention time doesn't. Similar effect is observed on other columns as well as with other ammonium salts. We suppose that such behavior is caused by the exchange chemical reaction happening between ammonium salt analyte and amine additive. An additive converts a salt into a free base form and turns into a salt form itself. If this hypothesis is true, it might be important to take the possibility of such interactions into account during preparative SFC work since the compound injected might not be equivalent to a compound eluted.