Described is an electron spin resonance (ESR) spectrometer comprising a miniaturized radio-frequency (RF) microwave cavity resonator. The miniaturized RF microwave cavity resonator receives a carrier signal from a circulator, modulates the signal path signal, and reflects the carrier signal back to the circulator to amplify the carrier signal prior to demodulation. A mixer receives and demodulates the carrier signal and outputs an audio signal to generate an ESR spectrum for analyzing a chemical composition of a fluid sample. The ESR spectrum represents a magnetic susceptibility of the fluid sample to a magnetic resonance cause variation in a resonant frequency of the miniaturized RF microwave cavity resonator. In a desired aspect, the carrier signal is split into two paths prior to demodulation. The two paths are demodulated by different mixers to produce two separate outputs, an absorption spectrum and a dispersion spectrum.