Apparatus for assessing a particular property in a medium

An apparatus for assessing a particular property of a medium comprising:

• a chamber having a thermally conductive portion;
• a vibratile element thermally coupled within said chamber to said thermally conductive portion;
• optical excitation means for delivering optical excitation energy incident to said vibratile element so as to cause the vibratile element to vibrate at its resonant frequency without inducing thermal changes in the chamber;
• optical means for monitoring and detecting the vibration frequency of the vibratile element;
• and
• thermally activated sensor means, thermally coupled to an exterior surface of said thermally conductive portion thereby being exposed to enable contact between said thermally activated sensor means and the medium, said thermally activated sensor means being a material which is thermally activated when contacted with said medium having said particular property for changing the temperature of the chamber and hence the resonant frequency of the vibratile element by heat transfer through the thermally conductive portion between the thermally activated sensor means and the chamber, said changes in said resonant frequency being monitored and detected by the optical means which provides an assessment of the presence of said particular property of said medium.