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

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 B₁ field leads to reduced NMR performance. The Helmholtz microcoil overcomes this drawback with its symmetrical design, improving B₁ 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.