Abstract: An ultra high sensitive on-line water monitor is disclosed. The on-line water monitor can determine the concentration of a wide range of inorganic contaminants in water at parts per trillion level in real time. The list of inorganic elements detectable by the instrument is as follows: Be, Mg, Ca, Th, V, Ti, Cr, W, Mn, Fe, Co, Ni, Cu, Au, Zn, Cd, Al, Pb, Sb, Bi, Hg, and other elements which are suitable for detection and assaying with spectrophotometric (also known as colorimetric) technique. The on-line water monitor described herein surpasses the currently available analytical instruments (such as inductively coupled mass spectrometers (ICP-MS), atomic absorption spectrometers, and atomic emission spectrometers) in a number of factors: cost, on-line operation, speed, ease of automation, low power requirements, sensitivity, and selectivity.