Methanol/CO2 phase behavior in supercritical fluid chromatography and extraction

Advantages of dimethyl carbonate as organic modifier for enantioseparation of novel psychoactive substances in sub/supercritical fluid chromatography

BACKGROUND

Sub/supercritical fluid chromatography is regarded as a greener separation technique due to the use of carbon dioxide as the main component of the mobile phase compared to conventional liquid chromatography techniques. Organic co-solvents are usually added to carbon dioxide to increase elution strength of the mobile phase. Therefore, it is of great importance to test applicability of green co-solvents in separation methods and to include them among commonly used mobile phase components.

RESULTS

A comprehensive study of the suitability of green solvent dimethyl carbonate as a co-solvent for enantioseparation in sub/supercritical fluid chromatography was conducted with a set of novel psychoactive substances from various groups. The experiments were performed on polysaccharide-based columns. For successful enantioseparation of these compounds, the presence of basic or mixed mobile phase additives was essential. The obtained results clearly show that dimethyl carbonate is a suitable co-solvent for enantioseparation on polysaccharide-based columns in sub/supercritical fluid chromatography and in some cases surpasses commonly used co-solvents as methanol and propan-2-ol.

SIGNIFICANCE

The use of more sustainable co-solvents, such as dimethyl carbonate, instead of conventional ones to carbon dioxide presents a greener approach to analytical applications and reduces the overall environmental impact of analytical processes.

Supercritical Fluid Nanospray Mass Spectrometry

Supercritical fluids are typically electrosprayed using an organic solvent makeup flow to facilitate continuous electrical connection and enhancement of electrospray stability. This results in sample dilution, loss in sensitivity, and potential phase separation. Premixing the supercritical fluid with organic solvent has shown substantial benefits to electrospray efficiency and increased analyte charge state. Presented here is a nanospray mass spectrometry system for supercritical fluids (nSF-MS). This split flow system used small i.d. capillaries, heated interface, inline frit, and submicron emitter tips to electrospray quaternary alkyl amines solvated in supercritical CO₂ with a 10% methanol modifier. Analyte signal response was evaluated as a function of total system flow rate (0.5-1.5 mL/min) that is split to nanospray a supercritical fluid with linear flow rates between 0.07 and 0.42 cm/sec and pressure ranges (15-25 MPa). The nSF system showed mass-sensitive detection based on increased signal intensity for increasing capillary i.d. and analyte injection volume. These effects indicate efficient solvent evaporation for the analysis of quaternary amines. Carrier additives generally decreased signal intensity. Comparison of the nSF-MS system to the conventional SF makeup flow ESI showed 10-fold signal intensity enhancement across all the capillary i.d.s. The nSF-MS system likely achieves rapid solvent evaporation of the SF at the emitter point. The developed system combined the benefits of the nanoemitters, sCO₂, and the low modifier percentage which gave rise to enhancement in MS detection sensitivity.

Thermodynamics of mixing methanol with supercritical CO2 as seen from computer simulations and thermodynamic integration

The changes in extensive thermodynamic quantities, such as volume, energy, Helmholtz free energy and entropy, occurring upon mixing liquid methanol with supercritical CO₂, are calculated using Monte Carlo simulations and thermodynamic integration for all eight combinations of four methanol and two CO₂ potential models in the entire composition range at 313 K. The obtained results are also compared with experimental data whenever possible. The transition of the system from liquid to a supercritical state is found to occur at this temperature around a CO₂ mole fraction value of 0.95 with all model combinations considered. This liquid to supercritical transition is always accompanied by positive Helmholtz free energy of mixing values and, consequently, by the non-miscibility of the two components. Furthermore, both this non-miscibility around the liquid to supercritical transition and also the miscibility of the two components below this transition, in the liquid regime, are found to be primarily of the energetic rather than entropic origin; the entropy of mixing turns out to be very close to zero, and around the liquid to supercritical transition even its qualitative behaviour is strongly model dependent. Finally, it is found that the methanol expansion coefficient is not sensitive to the details of the potential models, and it is always in excellent agreement with the experimental data. On the other hand, both the volume and the energy of mixing depend strongly on the molar volume of neat CO₂ in the model being used, and in this respect the TraPPE model of CO₂ [J. J. Potoff and J. I. Siepmann, AIChE J., 2001, 47, 1676] performs considerably better than that of Zhang and Duan [Z. Zhang and Z. Duan, J. Chem. Phys., 2005, 122, 214507].

Determination of thermodynamic properties by supercritical fluid chromatography

This survey attempts to summarise thermodynamic applications of supercritical fluid chromatography (SFC) with an emphasis on the results published during the last 10 years. In addition to a review of thermodynamic measurements by SFC, it contains brief sections on instrumental considerations and on the sources of auxiliary information needed when processing the retention data.

Supercritical fluids in separation science--the dreams, the reality and the future

The last 20 years have seen an intense interest in the use of supercritical fluids in separation science. This started with the introduction of commercial instruments first for packed and then for capillary chromatography and it looked as if this would be a technique to rival gas-liquid chromatography and HPLC. The activity developed quite rapidly into packed column supercritical fluid separations then into supercritical fluid extraction. However, in recent years there has been a decline in publications. These later techniques continue to be used but are now principally applied to a limited group of applications where they offer significant advantages over alternative techniques. This review looks back over this period and analyses how these methods were developed and the fluids, detectors and applications that were examined. It suggests why many of the initial applications have vanished and why the initial apparent promise was not fulfilled. The rise and fall of supercritical fluids represents a lesson in the way analysts approach new techniques and how we might view other new separation developments at the end of this millennium. The review looks forward to the future of supercritical fluids and their role at the end of the first century of separation science. Probably the most important idea that supercritical fluids have brought to separation science is a recognition that there is unity in the separation methods and that a continuum exists from gases to liquids.