Akin to optical beam chopping, we demonstrate that formation and routing of aqueous droplets in oil can chop a fluidic sample to permit phase sensitive detection. This hand-operated microfluidic sample chopper (Î¼Chopper) greatly reduces the detection limit of molecular absorbance in a 27 Î¼m optical path. With direct dependence on path length, absorbance is fundamentally incompatible with microfluidics. While other microfluidic absorbance approaches use complex additions to fabrication, such as fiber coupling and increased optical paths, this self-regulated Î¼Chopper uses opposing droplet generators to passively alternate sample and reference droplets at â¼10 Hz each. Each dropletâs identity is automatically locked-in to its generator, allowing downstream lock-in analysis to nearly eliminate large signal drift or 1/f noise. With a lock-in time constant of 1.9 s and total interrogated volume of 59 nL (122 droplets), a detection limit of 3.0 Ã 10â4 absorbance units or 500 nM bromophenol blue (BPB) (29 fmol) was achieved using only an optical microscope and a standard, single-depth (27 Î¼m) microfluidic device. The system was further applied to nanoliter pH sensing and validated with a spectrophotometer. The Î¼Chopper represents a fluidic analog to an optical beam chopper, and the self-regulated sample/reference droplet alternation promotes ease of use.