Enhanced signal-to-noise ratio in a compact earth's field NMR spectrometer: design and construction

Earth's Field Nuclear Magnetic Resonance (EF-NMR) provides a portable and cost-effective alternative to conventional high-field NMR by exploiting the Earth's natural magnetic field. However, the extremely low field strength severely limits the signal-to-noise ratio (SNR), restricting practical spectroscopic applications. In this work, we present the design and implementation of a low-cost desktop EF-NMR spectrometer optimized for high SNR operation. The system achieves a normalized SNR improvement of 55% compared to previously reported EF-NMR systems. This performance gain is primarily enabled by a novel coil switching architecture that significantly increases the receiver quality factor (Q ≈ 23.3), resulting in an ultra-narrow detection bandwidth of 72.9 Hz. Additional performance enhancements are obtained through optimized pre-polarization timing, aluminum shielding, electromagnetic interference filtering, first-order Maxwell shimming, and precise excitation pulse calibration. Experimental results demonstrate an SNR of 10.7 for an 87 mL water sample, confirming the effectiveness of the proposed design. The presented architecture offers a practical pathway toward compact, affordable EF-NMR systems for spectroscopy, sensing, and educational applications.