Provided herein are systems and methods of optical particle counters which account and adjust for the refractive index of the carrier fluid being analyzed. The provided systems are robust and may be implemented in a variety of optical particle counters including obscured light, reflected light, emitted light and scattered light particle counters. The described systems may be useful with any fluid, including gases or liquids. In some cases, the system can account for the differences in refractive index between two liquids, for example, ultrapure water and an acid, such as sulfuric, hydrochloric, hydrofluoric, acetic, phosphoric, chromic phosphoric, and the like. By accounting for the refractive index of the carrier fluid, the described systems and methods are also more sensitive and able to more accurately detect and characterize smaller particles, including nanoscale sized particles.