¹H-NMR Spectroscopy on the Nanoliter Scale for Static and On-Line Measurements

NMR spectroscopy is one of the most powerful methods available for structural elucidation and for determination of the physical environment of an analyte. However, there has bed limited success in scaling NMR to work with smaller rotate samples. We report the development of nanoliter-scale NMR detection cells, formed by wrapping a radio frequency microcoil directly around a fused silica capillary. This creates 5-200-nL volume NMR detection cells from an ∼ 1 mm length of 75-530-μm-i.d. fused silica capillary. The design and construction of optimized coils are described in detail. Using such NMR detection cells in a static mode, <50-ng limits of detection for amino acids are achieved for 1-min data acquisition times. The NMR line widths are in the several hundred hertz range for thin-walled (30-μm walls) fused silica capillaries but can be reduced to under 10 Hz using thick-walled (140-μm walls) capillaries. Such small-volume cells allow on-line NMR detection for capillary electrophoresis separations. The complex dependence of the NMR band intensities on electrophoretic current, applied potential, and dynamic flow are also described.