High-Pressure Flow Probes

The main, and most crucial, part of the experimental set-up is the probe, which has to be pressure-and temperature-stable at the same time and must have a flow cell with an inlet and an outlet to supply and take away the samples. For the development of SFC-NMR probes, one has to keep in mind the aim of the application, i.e. the structure elucidation of unknown compounds. For the investigation of new, unknown substances, the resolution of the probe has to 

The non-rotation of the sapphire flow cell, together with the detection volume and the employed flow rate, determine the resulting NMR line width. Whereas with rotation of the NMR tube in a conventional NMR probe, the signal line width of chloroform at the height of the 13C satellites in degassed acetone-d6 is about 3-4 Hz, SFC continuous-flow probes show values of the order of 15-18 Hz in the liquid and in the supercritical state. This hump test also indicates that there is no change in the signal line width in the H NMR spectra in the liquid and in the supercritical state. 

---

This article treats the benefits, possibilities and drawbacks of supercritical fluid chromatography (SFC) and supercritical fluid extraction (SFE) coupled to nuclear magnetic resonance spectroscopy. After a general overview and consideration of the motivation for such techniques, the design of high-pressure flow probes, as well as the principle experimental set-ups, are described. By means of several applications and comparison to HPLC-NMR, the utility of these hyphenated techniques is demonstrated. 

The application of NMR to the study of chemical reactions has been expanded to a wide range of experimental conditions, including high pressure and temperatures. In 1993, Funahashi et al. [16] reported the construction of a high pressure 3H NMR probe for stopped-flow measurements at pressures <200 MPa. In the last decade, commercial flow NMR instrumentation and probes have been developed. Currently there are commercially available NMR probes for pressures of 0.1-35 MPa and temperatures of 270-350 K (Bruker) and 0.1-3.0 MPa and 270-400 K (Varian). As reported recently, such probes can be used to perform quantitative studies of complicated reacting multicomponent mixtures [17]. 

SFC-NMR is available from 200 to 800 MHz, and is suitable for all common NMR-detected nuclei. SFC/SFE-NMR requires dedicated probe-heads for high pressure (up to 350 bar) and elevated temperature (up to 100 °C). SFC-NMR is carried out with conventional packed columns, using modifier, pressure and temperature gradients. The resolution of 1H NMR spectra obtained in SFE-NMR and SFC-NMR coupling under continuous-flow conditions approaches that of conventionally recorded NMR spectra. However, due to the supercritical measuring conditions, the 111 spin-lattice relaxation times 7) are doubled. 

Special mention may still be made of the Ingold199 range of industrial immersion and continuous-flow probes for pH measurements at high pressures, as... 

The gas flow rates are adjusted to ensure a constant pressure inside the cell and to minimize degradation of the spectral resolution by the gas bubbles. Typical line-widths at half height are <4 Hz on NMR (200 MHz) and <2 Hz on (80.3 MHz). The probe fits into the room temperature shims of a wide-bore NMR magnet and the pressure and temperature range of the high pressure NMR flow are 0.1 to 20 MPa and -40 to 175 °C, respectively. 

One of the mayor drawbacks is that only volatile and temperature-resistant compounds can be investigated. Gases are magnetized faster than liquids, because they have shorter spin-lattice relaxation times (T ), due to an effective spin rotation mechanism. Therefore, pulse repetition times in flow experiments can be in the range of 1 s and some dozen transients can be accumulated per separated peak. Nevertheless, the sample amounts used nowadays in capillary GC are far from the detection limit of NMR spectroscopy, and therefore the sensitivity is low or insufficient, due to the small number of gas molecules per volume at atmospheric pressure in the NMR flow cell. In addition, high-boiling components (> 100 °C) are not easy to handle in NMR flow probes and can condense on colder parts of the apparatus, thus reducing their sensitivity in NMR spectroscopy. 

Phase equilibria and pressure-temperature coordinates of critical points in ternary systems were taken with a high-pressure apparatus based on a thermostated view cell equipped with two liquid flow loops which has been described in detail elsewhere [3]. The loops feed a sample valve which takes small amounts of probes for gas-chromatographic analysis. In addition to temperature, pressure and composition data, the densities of the coexisting liquid phases are measured with a vibrating tube densimeter. Critical points were determined by visual oberservation of the critical opalescence. 

Aerometrics Eclipse is a white light blockage system designed for both liquid and gas systems. The probe head includes a light source, flow cell and photodiode. The processor can accommodate two heads covering the size ranges 2-100 pm, 10-500 pm, 20-1000 pm and 50-2500 pm. The flow cells are compatible with most liquids and can be specially adapted for high-pressure environments. 

Supercritical fluid chromatography was coupled online to Proton High Field Nuclear Magnetic Resonance Spectroscopy by an specially designed pressure-proof continuous-flow probe head. Separation of phthalate esters was carried out under supercritical conditions using carbon dioxide as eluent. ... 

---