An apparatus includes a housing and a chamber. The chamber is configured to hold analyte. The chamber includes a reflective element, either a reflective coated layer or a reflective mirror, to return optical signals entering the analyte. A window is coated with an anti-reflective material or has negligible optical interference. A cap seals the chamber and the housing. A flow channel is provided within the housing and the chamber assembly to allow the analyte to flow through with reduced bubble entrapment as the cap is tightened to seal the chamber assembly.