Flow cell for optical detection having reduced sensitivity to refractive index variation

An apparatus for cavity enhanced optical detection comprising:

• a) a source of optical radiation
• b) a resonant optical cavity which provides a round trip path for said optical radiation said cavity comprising:
• i) a plurality of mirrors, a first mirror of said plurality being an input mirror which receives said optical radiation from said source and inputs same into said cavity;
• ii) a flow cell positioned within said cavity, said flow cell comprising at least a first analysis channel which accommodates a flow of analyte fluid there through, the exterior wall of said at least first flow channel closest to said source of optical radiation intersecting said round trip radiation path at an angle approximately equal to Brewster's angle;
• iii) a second mirror of said plurality of mirrors, which second mirror receives the radiation from said optical source after passage of said radiation through both said input mirror and said at least first analysis channel and reflects at least a portion of said received radiation;
• whereby said resonant optical cavity provides a round trip path for analyte fluid having at least two different refractive index values and the location of the point at which said reflected radiation impinges on said input mirror is substantially the same as the point from which said first mirror inputs said radiation into said cavity notwithstanding changes in the refractive index of said analyte fluid or the wavelength of said radiation.