  Studies of biological solutions require high measurement accuracy, the
ability to detect low changes in substance concentration, and small amounts of
the liquid under test. A microwave complex permittivity measurement technique
based on a high-quality-factor whispering-gallery-mode resonator with a
microfluidic chip allows small amounts of dielectric liquids to be investigated
with high accuracy. An existing technique based on a sapphire resonator does
not provide for a low-concentration detection limit for substances with low
molecular weight. Here, we present an advanced technique based on a quartz
dielectric resonator. The detection limit obtained for glucose in water
solution was found to be about one order of magnitude lower for the quartz
resonator cell than for the sapphire-resonator-based measurement cell. The
limit is about or lower than the concentration of glucose in human blood. This
fact means that the technique can be used as the sensor for investigations of
biological solutions in the microwave range. A study of glucose, lactalbumin,
and bovine serum albumin was successfully performed in the Ka -band using the
developed sensor.
