Active shims to achieve high-resolution spectra are crucial parts of the NMR instrumentation. Typically, shims are designed to produce a magnetic correction field corresponding to individual spherical harmonics. Other methods have been proposed (e.g. matrix shims) and the approach described in this article simplify uses flat ribbon cables.
Meerten, S. G. J. van, P. J. M. van Bentum, and A. P. M. Kentgens. “Shim-on-Chip Design for Microfluidic NMR Detectors.” Analytical Chemistry 90, no. 17 (September 4, 2018): 10134–38.
https://doi.org/10.1021/acs.analchem.8b02284.
In this contribution we present a novel system for shimming capillary samples such as used in microuidic NMR probe heads. Due to the small sample size shimming microliter samples using regular shim coils is complicated. Here we demonstrate the use of a series of parallel wires placed perpendicular to B0 as a Shim-on-Chip shim system. This is achieved by placing a ribbon at cable horizontally over the NMR detector, in our case a stripline. The current through each wire of the ribbon cable can be controlled independently employing a 16 channel DAC. This makes for a simple, cheap and easy to construct alternative to regular shim systems. The Shim-on-Chip is, nevertheless, quite exible in creating a magnetic eld which matches the inhomogeneity of the magnet in 1 dimension. The capillary sample geometry is well suited for this type of shimming since its length (8mm) is much larger than its width (100 µm to 250 µm). With this Shim-on-Chip system we have reached linewidths of 2:2 Hz (at 50%) and 27 Hz (at 0:55%) on a 144MHz NMR spectrometer without any other room temperature shims. Unlike regular shims the Shim-on-Chip is located inside the NMR probe. It is always centered on the NMR sample, because of this the shims have an intuitive eect on the lineshape. Therefore the manual shimming is simpler when compared to a regular shim system, as it is dicult to position a microliter sample in the exact center of the shim coils. We furthermore demonstrate the use of a Shim-on-Chip method in a 400MHz Rapid-Melt DNP system. Decent linewidths were achieved even for a sample which is located o-center inside the NMR magnet.