Microvolume index of refraction determinations by interferometric backscatter

A new method has been applied to the determination of fluid bulk properties in small detection volumes. Through the use of an unfocused He-Ne laser beam and a cylindrical tube of capillary dimensions, relative refractive-index measurements are possible. The backscattered light from the illumination of a tube of capillary dimensions produces an interference pattern that is spatially defined and that contains information related to the bulk properties of the fluid contained in the tube. Positional changes in the intensity-modulated beam profile (interference fringes) are directly related to the refractive index of the fluid in the tube. The determination of dn/n at the 10(-7) level is possible in probe volumes of 350 pL. The technique has been applied to tubes as small as 75 µm inner diameter and as large as 1.0 mm inner diameter. No modification of the simple optical bench is required for facilitating the determination of refractive index for the complete range of tube diameters.