Micro-interferometric backscatter detection using a diode laser Original Research Article

Micro-interferometric backscatter detection (MIBD) is performed with a simple, folded optical train based on the interaction of a diode laser beam and a fused silica capillary tube allowing for refractive index (RI) determinations and detection of optically active molecules in small volumes. Side illumination of the capillary by a laser produces a 360° fan of scattered light that contains two sets of high contrast interference fringes. These light and dark spots are viewed on a flat plane in the direct backscatter configuration. Signal interrogation for polarimetry is based on quantifying the relative intensities (depth of modulation (DOM)) of adjacent high frequency (HF) interference fringes for polarimetry and relative fringe position for RI detection. Positional changes of the interference pattern extrema (fringes) allow for the determination of Δn 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 × 10−5° (mandelic acid, [α]23 = −153°, and D-glucose, [α]25 = +52.5°), and a 2σ detection limit of 7.5 × 10−4 M (D-glucose) and 1.14 × 10−3 M (R-mandelic acid). The probe volume of MIBD-polarimetry was 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. Furthermore, DOM (polarimetry signal) is unchanged when a non-optically active solute is interrogated by the MIBD-polarimeter. Finally, an optical model was derived and used to evaluate the advantages and pitfalls of using diode laser for MIBD.