The invention relates to an apparatus and method for the detection of the spectral absorption properties of a fluid in a microenvironment, the apparatus comprising a radiation source, a microfluidic device and detection means, wherein the apparatus additionally comprises means for increasing the path length of the radiation through the fluid. Preferably, the means for increasing the path length of the radiation through the fluid comprises two highly reflective mirrors which cause the base path length through the fluid to be increased by many orders of magnitude, and the method comprises Cavity Enhanced Absorption Spectroscopy. The method is especially useful for the handling of small volumes of fluids for chemical and biological processing.