An absorption spectroscopy process uses a single radiation beam with two or more pulsed beams (including at least a signal beam and a reference beam) that are passed into a liquid sample to a variable effective depth and then reflected out of the liquid sample where it is detected and processed to obtain a value over a preselected time. As values are determined for multiple effective depths, a sampling dataset is obtained which is used to calculate a concentration level of a targeted particle in the liquid sample by use of calibration dataset obtained from use of known samples.