High-resolution microcoil NMR for analysis of mass-limited, nanoliter samples

An improved nanoliter volume NMR probe design places the microcoil and capillary at the magic angle (54.7 degrees) with respect to the external magnetic field, Using an NMR probe which requires a total sample volume of just 200 nL, high-resolution 300-MHz H-1-NMR spectra (line width, 0.6 Hz) are presented of 10 mM alpha-bag cell peptide for an observe quantity of 45 ng (50 pmol in 5 nL). For the volume of sample inside the microcoil (the observe volume, V-obs), the 3 sigma limit of detection (LOD) is 9 ng (10 pmol, 2 mM) for data obtained in 15 h, To reduce the data acquisition time, a probe With a greater V-obs is developed. As an example of a rapid, mass-limited analysis, a concentration corresponding to 400 ng of menthol dissolved in V-obs = 31 nL (82.6 mM) yields a spectrum in 9 min (LOD = 6.9 ng, 44 pmol, 1.4 mM). To illustrate improvements in concentration sensitivity, a spectrum is acquired in 45 min for 400 ng of menthol dissolved in a total sample volume of 200 nL (12.8 mM). Compared to a commercial nanoprobe for the same mass of menthol, these two examples reduce data acquisition time by at least 95%. Both model compounds demonstrate substantially improved concentration LODs compared to those obtained in previous high-resolution, microcoil NMR work, These advances illustrate the utility of enhanced sensitivity provided by NMR microcoils applied to nanoliter volumes of mass-limited samples.