This invention relates to an an apparatus for monitoring a fluid medium comprising a piezoelectric element provided with a layer including an absorbent material or a catalyst material. The apparatus further includes means for generating a Surface Skimming Bulk Wave for propagation along said piezoelectric element, said layer being capable of confining said wave to said element and being capable of changing its acoustic properties in response to absorption thereby and retention therein by chemical linkage of one or more analytes from the fluid medium or in response to chemical and/or physical changes in the fluid medium and means for monitoring changes in the propagated acoustic Wave caused by changes in the acoustic properties of said layer.