Ultrasmall volume refractive index detection using microinterferometry

A microinterferometric backscatter detector (MIBD) has been developed to pe rform subnanoliter volume refractive index measurements using a simple, fol ded optical train based on the interaction of a laser beam and a fused sili ca capillary tube. Positional changes of the interference pattern extrema ( fringes) allow for the determination of Delta n at the 10(-7) level, corres ponding to 5.3 pmole or 0.48 ng of solute, when thermal noise is controlled at 8 x 10(-3) degrees C. MIBD is relatively path-length insensitive for ca pillaries ranging in inner diameter from 75 to 775 mu m, allowing a large r ange of detection volumes, from 350 pL to 40 nL, to be produced. A theoreti cal model of the microinterferometric backscatter detector has also been de veloped and evaluated and has been found to be in agreement with experiment al data. This model indicates increased sensitivity of the instrument as th e wavelength of the probe beam and the wall thickness of the capillary tube are reduced. (C) 2000 American Institute of Physics. [S0034-6748(00)03907- 1].