Micro-interferometric backscatter detection using a diode laser

Micro-interferometric backscatter detection (MIBD) is performed with a simp le, folded optical train based on the interaction of a diode laser beam and a fused silica capillary tube allowing for refractive index (RT) determina tions and detection of optically active molecules in small volumes. Side il lumination of the capillary by a laser produces a 360 degrees fan of scatte red light that contains two sets of high contrast interference fringes. The se light and dark spots are viewed on a flat plane in the direct backscatte r configuration. Signal interrogation for polarimetry is based on quantifyi ng the relative intensities (depth of modulation (DOM)) of adjacent high fr equency (HF) interference fringes for polarimetry and relative fringe posit ion for RT detection. Positional changes of the interference pattern extrem a (fringes) allow for the determination of Delta n at the 10(-7) level or 5 .3 pmol or 0.48 ng of solute. The MIBD-RI detection volume is just 5.0 nl. DOM changes allow for optical activity detection limits of 5.7 x 10(-50) (m andelic acid, [alpha](23) = -153 degrees, and D-glucose, [alpha](25) = +52. 5 degrees), and a 2 sigma detection limit of 7.5 x 10(-4) M (D-glucose) and 1.14 x 10(-3) M (R-mandelic acid). The probe volume of MIBD-polarimetry wa s 38 nl, and within the probed volume at the limit of detection, about 28.7 pmol of mandelic acid or about 43.7 pmol of D-glucose is present. Furtherm ore, DOM (polarimetry signal) is unchanged when a non-optically active solu te is interrogated by the MIBD-polarimeter. Finally, an optical model was d erived and used to evaluate the advantages and pitfalls of using diode lase r for MIBD. (C) 1999 Elsevier Science B.V. All rights reserved.