New technique for high resolution DNA sizing in epi-illumination

We present a high-resolution DNA-sizing technique based on the principles of flow cytometry, using a high numerical aperture objective and epi-illumination. The new technique, designed for small fluorescing samples/particles (sub-micron diameter) suspended in a weakly fluorescent medium, makes use of an additional focus for high-precision particle localisation. This way, only those particles are considered that flow exactly through a well-defined volume. Results are presented for fluorescent beads, as well as for YOYO-stained plasmids containing 5,500 basepairs. The latter were measured with 6.2% resolution, setting a new limit to flow-based sizing of DNA.