NMR Flow Cell Design

Figure 1 shows the design of NMR cells employed for various coupling techniques. For online HPLC-NMR and GPC-NMR coupling, a vertically oriented flow cell with a directly fixed double-saddle Helmholtz coil is used (Figure 1A). The whole arrangement is centred in the glass dewar of a conventional probe body in which a thermocouple is inserted, allowing temperature-dependent measurements to be made. The internal diameter of the glass tube is 2, 3 or 4 mm, resulting in detection volumes of 60, 120 and 180 |L. The glass walls of the flow cell are parallel within the length of the proton detection coil, and taper at both ends to fit polytetrafluoroethylene (PTFE) tubing (i.d. 0.25 mm). PTFE and glass tubing are connected by shrink-fit tubing. Inverse continuous-flow probes contain an additional coaxial coil (tuned to the 13C resonance frequency) surrounding the 1H detection coil for heteronuclear JH/13C shift-correlated experiments. This design leads to optimal NMR resolution values with a typical line width of chloroform at the height of the 13C satellites of 9-12 Hz, allowing the determination of coupling constants of 1 Hz in continuous-flow NMR spectra. The disadvantage of this design is the high detection volume, leading to a degraded chromatographic resolution. For analytical HPLC columns (250 x 4.6 mmi.d.) the plate height is increased for solutes with capacity factors less than 2.5 at detection volumes higher than 48 | L.

The probe design employed for SFC-NMR coupling is shown in Figure 1B. The inner glass tube of the original LC-NMR probe is substituted with a sapphire tube (o.d. 5 mm, i.d. 3 mm, detection volume 120 | L) and the polyetheretherketone (PEEK) capillaries used in the LC-NMR probe are replaced by Titan tubings. A double-tuned proton deuterium coil is directly fixed to the sapphire flow cell. The whole arrangement is centred in the glass dewar of

Figure 1 Design of NMR flow cell for: (A) HPLC-NMR and GPC-NMR experiments; (B) SFC-NMR experiments; (C) capillary NMR experiments.

a conventional probe body, in which a thermocouple is inserted, allowing temperature-controlled experiments.

Figure 1C shows the schematic diagram of the capillary NMR detection probe. Here a fused silica capillary is directly inserted within the NMR radio-frequency coil of a 2.0 mm microprobe. Within the area of the NMR detection coil the polyimide coating of the capillary is removed; either capillaries with an i.d. of 180 |im or bubble cell types with an increased i.d. of 220 |im are used.

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