Method and apparatus for monitoring surface phenomena

A method, comprising:

• measuring a first surface plasmon resonance angle value of a sample region,
• measuring a first critical angle value of the sample region,
• causing a change of surface concentration of an analyte at the sample region,
• changing the bulk composition at the sample region,
• measuring a second surface plasmon resonance angle value of the sample region,
• measuring a second critical angle value of the sample region,
• determining an indicator value indicative of the change of the surface concentration, wherein the indicator value is determined from the second surface plasmon resonance angle value by compensating an effect of the bulk composition, and wherein the magnitude of said effect is determined by using the second critical angle value;
• wherein the sample region comprises a substrate and a conductive layer,
• wherein the method comprises using a coupling material to couple an input light beam from a coupling element into the substrate, and using the coupling material to couple a reflected beam from the substrate into the coupling element,
• wherein the magnitude of the effect of the bulk composition is determined from the second critical angle value according to the following equation:
• ϕ
• COMP
• ⁡
• (
• t
• )
• =
• arcsin
• ⁡
• [
• 1
• n
• G
• ⁢
• ɛ
• 2
• ·
• ɛ
• x
• ⁡
• (
• t
• )
• ɛ
• 2
• +
• ɛ
• x
• ⁡
• (
• t
• )
• ]
• where ϕCOMP denotes a compensating angle value, nG denotes the refractive index of the substrate, ε2 denotes the real part of the dielectric permittivity of the conductive layer, and εx(t) denotes a dielectric permittivity value calculated according to the following equation:
• εx(t)=nG2 sin2 ϕTIR(t)
• where ϕTIR(t) denotes the second critical angle value.