  The accurate measurement of complex liquid permittivity in a frequency range
provides important information on liquid properties in comparison to single
frequency permittivity investigations. The multifrequency microwave
characterization technique based on a single quartz whispering-gallery-mode
(WGM) resonator with a microfluidic chip are proposed, developed, and
demonstrated. This technique allows the complex permittivity of small volumes
of liquid to be measured at six resonant frequencies in the 30-40 GHz frequency
range. The calibration is performed by simulating the measurement cell and by
plotting the calibration nomogram charts for all six investigated frequencies.
The novel approach is applied in studies of the complex permittivity of the
L-lysine in water solutions. The results open up possibilities to investigate
the complex permittivity of biological liquids at several frequencies and to
further develop the microwave dielectrometry of small liquid volumes in a
certain frequency range using the quasi-optical nature of a single high-Q WGM
resonator.
