Separate and Detailed Treatment of Absolute Signal and Noise Enables NMR Under Adverse Circumstances

Abstract

When deploying a spectrometer in an adverse environment, such as during a typical ODNP experiment or experiments that require low-volume low-field measurements, a clear and modern protocol for characterizing and quantifying the absolute signal and noise levels proves essential. This paper provides such a protocol. It also highlights the clarity and insight that comes from (1) discussing NMR signal intensities in (conserved) units of square root instantaneous power that are derived from a theory and notation developed initially for ESR spectroscopy; as well as (2) characterizing the spectral distribution of the noise. Crucially, the strategy introduced here applies not only to ODNP measurements, but to all low-field NMR. Low-field NMR offers immense flexibility: it enables integration with other instrumentation and deploys in practical applications not accessible to higher-field instrumentation. More generally, the protocol introduced here should apply to a wide range of instruments, and should prove especially useful in cases subject to design constraints that requires integration with multiple other modules that are not dedicated to NMR but that control other forms of spectroscopy or other crucial aspects of the measurement. However, in the specific case of ODNP, this protocol demonstrates that the absolute signal and noise level can be estimated from the clarified theory presented here, and uses that theory to identify the inefficient distribution of fields in the hairpin loop probe as the main remaining bottleneck for the improvement of low-field low-volume ODNP SNR.