A refractive index (RI) detector was introduced by Tiselius and Claesson [21]. This detection technique requires isocratic elution. Further developments were made by Zaukelies and Frost [175], and later by Vandenheuvel and Sipas [176]. The RI detector is a bulk property detector. There are several designs for RI detectors that are based on different techniques: the angle of deviation method, the critical angle method, the Fresnel method utilized by Conlon [177], and the Christiansen effect, based on Christiansen’s own work with crystal filters [178,179].