Continuous-flow NMR probes

Figure 1.8 Schematics of (a) conventional and (b) continuous-flow NMR probes suitable for cryo magnets...

Nuclear magnetic resonance spectroscopy (NMR) coupling to LC has seen significant progress in the past five years [11]. Continuous-flow NMR probes have been designed with a typical detection volume of 40-120 pi or smaller. The NMR spectmm is often recorded in stop-flow mode, although continuous-flow applications have been reported as well. 

For continuous-flow NMR measurements, two probe designs have become established. For LC-NMR with analytical HPLC columns (4.6 x 250 mm), probes with an active volume between 60 and 120 iL are used. In contrast to the conventional tube design, the double-saddle Helmholtz coil is arranged vertically, directly affixed to a U-shaped glass tube in a glass Dewar vessel. The inlet and outlet capillaries are coimected by shrinkable Teflon tubings (Fig. lb). 

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. 

Hunger, M. and Horvath, T.J. (1995) A new mas NMR probe for in-situ investigations of hydrocarbon conversion on solid catalysts under continuous-flow conditions, Chem. Soc. Client. Comm., 14, 1423. 

The process for introducing the HPLC-separated analyte peaks into the NMR probe can be configured using two different modes of operation on-flow (continuous flow) or stopped-flow. 

The continuous flow method is still necessary when one must use probe methods which respond only relatively slowly to concentration changes. These include pH, Oj-sensitive electrodes, metal-ion selective electrodes,thermistors and thermocouples, " epr and nmr detection. Resonance Raman and absorption spectra have been recorded in a flowing sample a few seconds after mixing horseradish peroxidase and oxidants. In this way spectra of transients (eompounds I and II) can be recorded, and the effext of any photoreduction by the laser minimized. ... 

While NMR spectra possess a high information content, this confers a level of complexity that can be obstructive if a quick and simple answer is required. The use of NMR flow probes has reduced the time taken to acquire a spectrum to 2-3 min and flow NMR has consequently found considerable application in robot synthesis, multiple parallel synthesis (MPS) or combinatorial chemistry. Since H possesses 5600 times the receptivity of it is inevitable that it will continue to be the nucleus of choice in this area, complicating the analysis. Unless automatic analysis is able to transcend the aid to interpretation role and into the confirm or refute the structure role, the higher information content of the NMR spectrum constitutes a threat rather than an opportunity. 

Figure 1.9 H NMR signal line shape of chloroform in acetone-c/f, (hump test), measured with a 120 pi continuous-flow probe (600 MHz)...

The main prerequisite for on-line LC-NMR, besides the NMR and HPLC instrumentation, are the continuous-flow probe and a valve installed before the probe for the registration of either continuous-flow or stopped-flow NMR spectra. 

If the retention times of the compounds to be separated are known, or if they can be detected by using UV (including diode arrays), radiochemical or fluorescence detectors, stop-flow LC-NMR becomes an option. Upon detection, the PC controlling the liquid chromatograph allows the pumps to continue running, moving the peak of interest into the NMR probe. Once the pumps have stopped, normal high-resolution NMR spectroscopy is possible. It could be... 

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. 

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