A 2.0-GHz compact ESR spectrometer for monitoring automobile lubrication oil degradation

Abstract

This article reports a simple, compact, and cost-effective electron spin resonance (ESR) spectrometer for monitoring automobile lubrication oil degradation. Lubrication oil degradation strongly correlates with the concentration of stable free radicals caused by hydrocarbon chain decomposition due to heating. For the prototype spectrometer, the amplitude shift in a marginal oscillator output detects ESR absorption in a sample. The spectrometer’s spin sensitivity of 2.3 × 1014 spins for a used oil sample was achieved using the marginal oscillator with a loop-gap resonator. For the prototype spectrometer, the oscillation frequency was 2.09 GHz. The volume of the prototype spectrometer was 1.3 L, including a permanent magnet, microwave circuits, and digital communication circuitry on printed circuit boards. The weight of the spectrometer setup was 1.45 kg. This prototype spectrometer successfully detected the ESR signal from a 50 μL oil sample (spin concentration 8.3 x1019 spins/L) with a signal-to-noise ratio of 37 and an acquisition time of 30 s.