Precision NMR measurement of 7Li and limits on spatial anisotropy

⁷Li NMR is observed with ⁷LiCl in D2O using a Nicolet NT-360 NMR Spectrometer over a 3.5 month period. The linewidth is typically about 40 mHz. Fitting to a triple Lorentzian line shape gives the splitting Δf ≲ 2.2 mHz which is roughly 20 times better than previous experiments. This result puts a limit on mass anisotropy Δm /m and the nonuniversality |Hij − gij| of the gravitational coupling Hij to electromagnetism and the gravitational coupling gij, to particle mass as follows: δm/m < 2 × 10⁻²⁴, Hμν − gμν | / U ≤ 10⁻¹³, | H0μ − g0μ | / U ≤ 10⁻⁸ ∼ 10⁻⁹, and | H00 − H00 | / U ≤ 10⁻⁵.