Design and construction of a microcoil NMR probe for the routine analysis of 20‐μL samples

Abstract

Recent advances in microcoil NMR have provided commercially available, robust methodologies for analyzing mass and volume limited samples in the low microliter regime, and the technology has been applied in a number of areas. Unfortunately, due to constraints on sample size and the limited solubility of some compounds of interest, the application of this approach to certain areas of development, such as the structural analysis of chromatography eluates, is restricted. A current challenge is to provide an option within a previously unexplored sample size regime (tens of microliters) while still taking advantage of the increase in mass sensitivity afforded by solenoidal microcoil NMR. In this article, we present the design and construction of a microcoil NMR probe with a custom detection cell for the routine analysis of 20-lL samples. The detection cell is comprised of a CO 2 -laser-heated HF-etched borosilicate active volume with fused silica transfer lines added to provide sample input and output. This setup produces an enlarged sample bubble within the detection coil and provides easy connection with 1/16 in. standard LC connections, lending itself to applications with HPLC-NMR, online SPE and similar separation techniques, as well as higher-throughput robotic automation. NMR performance characteristics determined using standard compounds showed the probe exhibited reasonable resolution (<0.01 ppm), although sensitivity was less than optimal due to tuning constraints. Future improvements and opportunities are also discussed.

Mass spectrometry for small molecule pharmaceutical product development: A review

“…MS can be used for confirmation of the isolated peaks before further characterization by NMR (Babjak et al, 2002). With the advances in NMR technologies, for example, introduction of flow probes to allow for LC-NMR analysis, higher sensitivity probes based on the use of microcoils (Olsen et al, 1995;Lacey et al, 2001;Henry et al, 2008) and cryocooling of the active probe (Keun et al, 2002;Rashid et al, 2002) the power of both MS and NMR can be coupled for more complete impurity profiling. Mistry et al (1997), have shown how this methodology can reduce time-consuming isolation of individual impurities and eliminate the potential for degradation during the work-up process.…”

Saturation transfer double‐difference NMR spectroscopy using a dual solenoid microcoil difference probe

“…Further optimization of the probe circuitry will lead to a more effective cancellation without the need to weight the protein concentrations, and better resolution for more complex systems. The use of an increased solenoidal coil sample volume [32] would improve the concentration sensitivity of the method. Additionally, while we used a series of shaped Gaussian pulses for the saturation in order to follow the approach of Meyers et al , the substitution of a long soft pulse could potentially achieve the same level of irradiation while eliminating the extra time involved with optimizing the parameters for the shaped pulses.…”

Advances in NMR-based biofluid analysis and metabolite profiling

“…The on-line LC-NMR mode requires the use of a continuous-flow probe with a suitably matched sample detection volume to provide sufficient sensitivity for low concentration analyte detection. 53 Pre-concentration systems prior to NMR detection including SPE and column trapping 88 offer clear advantages in signal enhancement. Further, incorporation of a cryo-flow probe for such applications not only increases the sensitivity but also allows for rapid characterization of metabolites.…”