Benchtop Permanent Magnet High Resolution NMR Systems

The latest buzz in the NMR news release world has been the emergence of the 45 MHz picoSpin (www.picospin.com) miniature NMR system that boasts shoe box size dimensions, a resolution of 80 ppb (3.5 Hz at peak half height), and a tiny 300 micron probe dimension. The sample can be injected with a syringe or pump. A number of application examples and spectra are posted on the application pages of the company website. It is noted that the spectra require the signal averaging of 24-200 pulses requiring an estimated experimental time of 3-10 minutes

The system claims to be the first miniature NMR system but I guess that depends on how you define “miniature”. A number of “relatively small” high resolution permanent magnet NMR systems operating at 60 MHz have been around since the early ’90s (Elbit-ATI, FoxboroNMR, Qualion, ASPeCT-MR, and ACT). The picoSpin NMR is definitely the first spectrometer to deliver high resolution NMR from such a small footprint permanent magnet combined with a capillary probe. The S/N of the picoSpin system is approximately 300:1 on a one pulse spectrum of water. On our 60 MHz systems we are routinely obtaining, non-spinning, one pulse spectra without signal apodization with a S/N of 1600:1 for a 5 mm sample size, and 3600:1 for a 10 mm sample size. Very little degradation of spectrum quality is observed moving from 5-10 mm probe systems. The 5mm lineshape spec is typically LW(50%)=2 Hz (34 ppb), LW(10%)=6 Hz, LW(0.5%)=20 Hz. For the 10 mm probe the spec is typically LW(50%)=2.5 Hz (43 ppb), LW(10%)=13 Hz, LW(0.5%)=80 Hz. We have many example spectra posted on this blog and on our website.

The S/N obtained with 5mm and 10mm sample dimensions represent a 28 and 144 fold improvement of sensitivity compared to the S/N obtained on the picoSpin system. This is a vitally important difference between these two technologies. One yields a useful, repeatable spectrum at a high S/N level after every pulse (4-5 seconds between spectra) while the other requires 100x longer time frame to obtain the same result. In the realm of reaction monitoring this time difference is a huge factor as a reaction can be at completion in a few minutes but an NMR analyzer providing a spectrum every 5 seconds allows dozens of observations to be performed in a short 2 minute reaction. The larger sample dimension in the 60 MHz systems also has the advantage of allowing a much wider range of industrial sector samples to be analyzed with respect to sample viscosity, contamination levels, sample temperature, and “particulate content”.

The picoSpin spectrometer does have a wonderful and truly portable package and will find many applications in university general chemistry labs and in QA/QC of liquid products, but I am not sure that it will have the stability and high sensitivity to allow it’s use in real time process control and reaction monitoring where the samples are complex mixtures, at high temperature, often with particulates present. Also the small probe dimension will mean that the analysis of flowing samples will be extremely difficult because of the small sample volume being analyzed.

Process NMR Associates is currently investigating the possibility of reducing the foot print of their 60 MHz NMR system to a platform that would support a 5 mm sample dimension operating at 60-80 MHz. The magnet would be closer to 50 lbs in weight in this scenario, and with a small FPGA based spectrometer would be a powerful mobile NMR system. However, I do not have a feel for whether the NMR and broader analytical community will be willing to accept the idea of low priced NMR systems selling in the $20-70K range. The question remains….if you build it, will they come? I would be interested to hear any comments on the utility of NMR in the field based on permanent magnet technologies at NMR frequencies of 45-80 MHz. Please address any comments to John Edwards.