Dielectric measurements of aqueous DNA solutions up to 110 GHz

A measurement system for broadband dielectric spectroscopy of biological samples for frequencies between 25 MHz and 110 GHz is presented. It is based on a vector network analyzer and a 1.19 mm-diameter open-ended coaxial probe. Complex reflection coefficients of aqueous Na-DNA solutions are measured in the frequency domain at a constant temperature of 25 °C. Complex permittivity spectra are analysed at various solute concentrations and two dispersions are observed. The first one is located at about 19 GHz and is due to the reorientation of water molecules. The second one is located at approximately 100 MHz and is interpreted as being caused by DNA counterion fluctuations. The relaxation frequency of free water in solutions appears to be practically unaffected by the presence of DNA. For the relaxation in the MHz region the dielectric loss maximum shifts to higher frequencies and the distribution of relaxation times becomes broader with increasing polymer concentration.