Microflow NMR: Concepts and capabilities

The principles and parameters to consider when choosing an NMR probe for analysis of a volume- or mass-limited sample are identified and discussed. In particular, a capillary-based microflow probe is described which has a mass sensitivity comparable to cryoprobes (observe volume ∼40 μL), but with several distinct advantages. The microflow probe has a flowcell volume of 5 μL and an observe volume of 1.5 μL and is equipped with proton and carbon observe channels, deuterium lock, and z-gradient capability. The entire flow path is fused silica; inlet and outlet capillary inner diameters are 50 μm to minimize sample dispersion, making it well-suited to volume-limited samples. An injected sample of 1 nmol of sucrose (0.34 μg in 3 μL, 0.33 mM; MW = 342 g/mol) yields a 1D proton spectrum in 10 min on a spectrometer of 500 MHz or higher. In another example, 15 μg of sucrose (in 3 μL; 15 mM, 45 nmol) is injected and parked in the probe to yield a heteronuclear multiple-quantum coherence (HMQC) spectrum in less than 15 h. The natural product muristerone A (75 μg in 3 μL, 50 mM, 150 nmol; MW = 497 g/mol) was delivered to the flow cell, and a gradient correlation spectroscopy spectrum was acquired in 7 min, a gradient HMQC in 4 h, and a gradient heteronuclear multiple-bond correlation in 11 h. Four basic modes of sample injection into the probe vary in degree of user intervention, speed, solvent consumption, and sample delivery efficiency. Manual, manual-assisted (employing a micropump), automated (using an autosampler), and capillary HPLC modes of operation are described.