Subnanomolar detection of acid-labile sulfides by the classical methylene blue method coupled to HPLC

Methylene blue reactive sulfides (MBRS) are ubiquitous in surface waters of local lakes and streams. However, in most cases they exist in concentrations too low to measure by traditional methods. Both the classical methylene blue and the similar Cline method have been used to determine concentrations of a particular pool of acid-labile sulfides in aquatic systems. The estimated lowest limit of MBRS detection is ca. 10 nM under ideal conditions. By coupling the classical methylene blue method to HPLC, it is possible to make direct, reliable measurements of MBRS in fully oxygenated freshwaters. Using this modified approach, a detection limit for MBRS of ca. 0.3 nM is achieved. Measurements in a variety of local freshwaters reveal levels in the range of 0-74 nM, with systematic variations explainable in terms of standard biogeochemical characteristics.