Concentration of a target chemical in the presence of other substances in a specimen is determined by subjecting the specimen to radio frequency electromagnetic components, sequentially or otherwise, ranging to about 5 GHz. The reflected and/or transmitted signal real and imaginary components at the specimen are spectrally examined as a function of frequency to identify the presence and/or concentration of the chemical of interest. Such examination includes analysis of the effective complex impedance presented by the specimen, and/or effective phase shift between the transmitted and reflected signal at the specimen. The effects upon glucose concentration measurements of varying electrolytes, primarily NaCl, can be nulled-out by examining impedance magnitude at a cross-over frequency, for example about 2.5 GHz. NaCl concentration exhibits a very linear relationship with phase shift change at frequencies in the 2 GHz-3 GHz range. In a specimen that is blood, such phase shift measurements provide data proportional to NaCl concentration. Impedance magnitude measurements using 1 MHz to 400 MHz frequencies provides a measure of combined concentration of glucose and NaCl. The phase shift data may then be used to substrate out the NaCl concentration from the combined concentration, to yield a good measure of glucose concentration. Such tests may be conducted in-vitro or in-vivo and lend themselves to blood level glucose analyses by diabetics.