I am trying to measure the plasmon resonance angle of gold film and pure water using the Kretschmann configuration and a 633nm laser. Without flowing water over the gold, I can detect a clear absorption peak at 44 degrees. However, when I inject water into the flow cell I see no reflectance peak whatsoever, instead I see a (mostly) flat line with little to no change in absorption from 40 to 60 degrees. I am using ultra pure water so there are no impurities. The gold film has no impurities either and there are no bubbles in the flow cell. I have tried multiple thicknesses of the water layer ranging from less than half a millimeter to over 2 millimeters in depth. My beam spot size is 6mm in diameter and my fluidic well is 9mm. What could be causing this issue and how can I improve my setup?