I made an SPR setup with equilateral prism made of SF11. The light source is 650nm LED. The sensor chip Nanospr with glass of 1.61 refractive index. I see a lot of dark bands where the SPR dark band expected. But none of them are SPR band. I know it because I ariented the polarizer 0 degree and 90 degre. If it was SPR band it should disappear at s polarization. But it doesn't. This is done in water. When I work at air the SPR band looks as expected at enother angle. I suspect about the immersion oil. My oil is n=1.515. I don't have immersion oil with higher refractive index. There is at cargille.com a lot of options but it will take too long to arrive to me. I don't have too much time.

*Do you think the problem is the immersion oil?

*If this is the problem, what can I use as a high refractive index matching liquid?

*If this is not the problem what can it be?

Thanks for any help.