Comparing the Potential of Helmholtz and Planar NMR Microcoils for Analysis of Intact Biological Samples

Abstract

Nuclear magnetic resonance (NMR) spectroscopy has played an integral role in medical and environmental metabolic research. However, smaller biological entities, such as eggs and small tissue samples, are becoming increasingly important to better understand toxicity, biological growth/development, and diseases. Unfortunately, their small sizes make them difficult to study using conventional 5 mm NMR probes due to limited sensitivity. The use of microcoil NMR holds great potential for the analysis of such samples, where the coil can be designed to match the sample size to significantly improve NMR mass sensitivity and the filling factor. Here, we compare the potential of planar and Helmholtz microcoil designs to execute complex experiments for the analysis of intact, mass-limited biological samples. The planar coil offers the advantage of an open access design, potentially allowing flow systems to be incorporated and varying sample sizes to be studied; however, its relatively inhomogeneous B 1 field leads to reduced NMR performance. The Helmholtz microcoil overcomes this drawback with its symmetrical design, improving B 1 homogeneity across the sample but with the caveat that the size and shape of the sample is limited to the spacing between the two parallel coils. The line shape, sensitivity, and RF performance are compared on both coils using standard samples and biological samples. This study found that the Helmholtz microcoil used here considerably outperforms the planar coil in multipulse experiments and has great potential to study complex biological samples in the 50−200 nL range.

Integrated Digital Microfluidics NMR Spectroscopy: A Key Step toward Automated In Vivo Metabolomics

“…However, if a planar coil is already oriented horizontally to create a field along X , Y , it must generate a field that runs across its surface rather than perpendicular to it. Practically, all NMR microcoils designed to date are aligned vertically and generate perpendicular B 1 fields. ,,,− As such, before a horizontal DMF system can be considered, it must be determined whether a horizontal planar coil can be designed with acceptable lineshape, sensitivity, and penetration depth. Ultimately, the questions become “how compromised is the NMR performance of horizontal coils when compared to vertical orientations?” as well as “are these losses acceptable for in vivo DMF-research?”…”

“…Microcoil NMR is one possible method to address these mass sensitivity issues. Surface microcoils are particularly interesting as the sample sits directly on top, increasing mass sensitivity due to the proximity to the coil surface, and they can be easily redesigned to match a wide range of specific sample sizes. , Previous research has shown that matching the size of the coil to the sample size can increase sensitivity by two to three magnitudes. − In turn, the additional mass sensitivity should require fewer organisms. This is especially important as in vivo NMR monitoring requires 2D 1 H- 13 C experiments to provide the additional chemical shift dispersion and in turn allow a range of metabolites to be assigned and monitored .…”

“…D. magna are quite small (∼2 mm), and one of the ways they reproduce is laying resting eggs during harsh conditions (such as winter), which then hatch when ideal conditions return. 17 These eggs can be as small as ∼300 μm, 18 and the response of D. magna to toxins over different life stages is known to vary. 19 As such, it is important to develop NMR approaches with potential to study all life stages from egg to adult.…”

Lenz Lenses in a Cryoprobe: Boosting NMR Sensitivity Toward Environmental Monitoring of Mass-Limited Samples

“…The shape of the nutation curve provides valuable insight into both the intensity and, in some cases, the homogeneity of the magnetic field. For example, poor B 1 homogeneity is a common challenge with planar surface microcoils <1000 μm, where samples often experience inhomogeneous excitation leading to nutation curves with poor or no inversion. ,− This becomes problematic when analyzing biological samples where the complex metabolic profile and high water content often necessitate the use of more complex pulse sequences that require inversion and/or phase-cycled cancellation. , This makes it difficult to use 500 μm planar microcoils to study small biological samples. In contrast, the Lenz lens designed here for samples <500 μm produces a sinusoidal nutation curve with excellent inversion over the critical 0–360° window, consistent with a uniform and intense field across the entire sample.…”

“…32,39−42 This becomes problematic when analyzing biological samples where the complex metabolic profile and high water content often necessitate the use of more complex pulse sequences that require inversion and/or phase-cycled cancellation. 40,43 This makes it difficult to use 500 μm planar microcoils to study small biological samples. In contrast, the Lenz lens designed here for samples <500 μm produces a sinusoidal nutation curve with excellent inversion over the critical 0−360°window, consistent with a uniform and intense field across the entire sample.…”