Laser-based refractive index detection using the sheath flow cuvette

Abstract

The sheath flow cuvette is used for refractive index determinations of neat solutions within picoliter probe volumes. In this detector, a sample stream is injected as a narrow stream into the center of a flowing sheath stream under laminar flow conditions. The sample stream retains its identity as a thin cylinder through the center of a 250-μm square flow chamber. The propagation properties of a focused helium-neon laser are perturbed by interaction with the sample stream. Detection limits ofΔ RI=3×10⁻⁶ were obtained within a 20 micrometer radius sample stream, corresponding to about a 400 picoliter probe volume. For analyte with refractive index significantly different than the solvent, detection limits are possible which correspond to a few picograms of analyte within the probe volume.