Phase-Incremented, Steady-State Solution NMR: Maximizing Spectral Sensitivity Without Compromising Resolution

TL;DR

This paper introduces a phase-incremented steady-state free-precession (PI-SSFP) method in NMR that enhances spectral sensitivity and SNR per unit time without sacrificing resolution, overcoming previous limitations of SSFP techniques.

Contribution

The study develops a new PI-SSFP approach with a processing pipeline that achieves high spectral resolution at large flip angles, improving sensitivity over traditional Ernst-angle FT NMR.

Findings

PI-SSFP often outperforms FT-NMR in SNRt.

Large flip angles improve sensitivity but can distort line shapes.

The new method maintains high resolution with enhanced sensitivity.

Abstract

NMR acquisitions based on Ernst-angle excitations are widely used in analytical spectroscopy, as for over half a century they have been considered the optimal way for maximizing spectral sensitivity without compromising bandwidth or peak resolution. However, if as often happens in liquid state NMR relaxation times T1, T2 are long and similar, steady-state free-precession (SSFP) experiments can actually provide higher signal-to-noise ratios per square root of acquisition time (SNRt) than Ernst-angle-based counterparts. Although a strong offset dependence and a requirement for pulsing at repetition times TR << T2 leading to poor spectral resolution have impeded widespread analytical applications of SSFP, phase-incremented (PI) SSFP schemes could overcome these drawbacks. The present study explores if, when and how, can this approach to high resolution NMR improve SNRt over the performance afforded by Ernst-angle-based FT acquisitions. It is found that PI-SSFP can indeed often provide a superior SNRt than FT-NMR, but that achieving this requires implementing the acquisitions using relatively large flip angles. As also explained, however, this can restrict PI-SSFP's spectral resolution, and lead to distorted line shapes. To deal with this problem we introduce here a new outlook on SSFP experiments that can overcome this dichotomy, and lead to high spectral resolution even when utilizing relatively the large flip angles that provide optimal sensitivity. This new outlook also leads to a processing pipeline for PI-SSFP acquisitions, which is here introduced and exemplified. The enhanced SNRt that the ensuing method can provide over FT-based NMR counterparts collected under Ernst-angle excitation conditions, is examined with a series of 13C and 15N natural abundance investigations on organic compounds.