Spectroscopic Solutions Series (S3) Workshops: What is Shimming?
Each year, the Thermo Fisher Scientific Molecular Spectroscopy sales team hosts our annual ‘S3’ workshops across North America for our current and prospective customers, a total of 16 workshops across the nation! During the workshops, the local sales engineer and applications scientists review the basics of Thermo Fisher Scientific’s core Molecular Spectroscopy instrumentation. Our workshops focus on FT-IR, Raman, Benchtop NMR, UV-Visible Spectroscopy, and Material Characterization. The S3 workshops provide our instrument users time to meet with applications scientists for training on hardware and/or software, and brainstorm applications. The S3 workshops also provide an opportunity for everyone to have valuable hands-on time with the range of spectrometers presented. In addition, the workshops also provide an opportunity to have any individual questions answered.
One of the workshops presented this spring talked about the importance of shimming. What is shimming? Shimming is the most essential factor for producing an optimally resolved NMR spectrum. In a NMR experiment, there must be a homogeneous field over the entire sample volume located within the detection coil to avoid distorted line shapes or poor resolution, both of which can also have a detrimental effect on sensitivity. The process of adjusting the homogeneity of the magnetic field is called “shimming.” All NMR spectrometers, whether high-field or low-field, rely on shim coils located strategically around the sample in the magnet. An electrical current passing through a coil produces its own relatively small magnetic field when compared to the main field. Each of the coils is designed to apply a different corrective magnetic field in a particular direction relative to the main field. The application of combinations of electrical currents to the set of shim coils corrects for any inhomogeneities to produce the purest magnetic field possible.
In the case of the Thermo Scientific™ picoSpin™ NMR spectrometers, our detection coil is a copper wire that is wrapped around a quartz capillary within the cartridge. The picoSpin capillary cartridges have 8 shim coils embedded in the cartridge board around the quartz capillary – 3 linear and 5 quadratic, and the cartridge sits between the poles of the magnet. The capillary cartridge is not exchanged between samples and requires a very small sample volume of 40µL to completely fill the loop. The fixed quartz capillary and small sample volume ensure that all picoSpin NMR spectrometers have a stable shim, or homogeneous magnetic field, from sample to sample. For maintenance purposes, one only needs to shim a picoSpin instrument once per day rather than shimming every sample.
To shim a picoSpin NMR spectrometer you simply inject tap water and use the supplied autoShim script. This script uses a simplex algorithm to make incremental changes to the shims until maximum water peak intensity is reached. One can observe the numerical and graphical results of each change made to the shims on the screen in real time. Upon completion of the autoShim script one should evaluate the results by delivering a single RF pulse. Using the parameters found on your factory test report, the FID should decay to approximately 50% of the initial intensity over the acquisition time and the spectrum peak height should be within 15% of the results on the report. Your picoSpin instrument is shimmed and you are ready to run your NMR experiments!
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