bulk effect

How to interpret the bulk effect?
Help

Not sure what you actually are asking.
Every solution has a refractive index (dn/dc) and when injecting a solution (analyte) with a different refractive index than the running buffer you will see a jump in the curves a.k.a. a bulk effect. In general this will not alter the kinetics but makes the analysis more difficult especially when the differences in refractive index are high. The bulk effect can be compensated with a reference channel but this is not always sufficient. MP-SPR can solve the bulk effect by using the TIR-angle of the original SPR-curve ( www.sprpages.nl/spr-overview/spr-theory ). Double referencing can compensate maybe better.
Best way to avoid the bulk effect is to match analyte and running buffer by dialysing the analyte or adding salts to either analyte or running buffer. When using analyte dilutions from glycerol / DMSO stocks you can add these compounds to the running buffer. Be aware that kinetics are dependent on the solution that is used.
Since you do not state what you did it is difficult to comment on the sensorgrams. The sad-face sensorgram has a high negative bulk effect which should be compensated by a proper reference channel (e,g. the red one). But if the happy-face sensorgram is the result of this I would not believe it since the ‘binding-curves’ are the result of improper referencing. When you want to compensate a bulk effect with a reference channel, the curve should be almost flat (no binding).
Kind regards
Arnoud