A method of using a resonant cavity for measuring a complex permittivity ε and identifying of a sample (solid or liquid) of microliter volume size includes using a network analyzer to measure over a defined millimeter wave frequency range, a first resonance frequency at a cavity resonance mode, and calculating an unloaded quality factor of an enclosed resonant waveguide cavity of a defined internal dimensions, placing a sample on a surface of a bottom wall of the resonant waveguide cavity and measure a second resonance frequency and calculating a loaded quality factor; determining, a resonance frequency shift Δf=(fs−fo), determining a complex permittivity ε of the sample according to the resonance frequency shift Δf, the loaded quality factor, the unloaded quality factor and the defined internal dimensions; and identifying the sample using a database through the complex permittivity ε.