Methylene blue derivatization then LC-MS analysis for measurement of trace levels of sulfide in aquatic samples

The methylene blue method has been widely used for analysis of sulfide for more than 100 years. Direct measurement of methylene blue at nanomolar concentrations is impossible without a preconcentration step, however. In this study the response of LC-MS with electrospray ionization (ESI) to methylene blue was evaluated. HPLC with simple isocratic elution was followed by ESI-MS quantification, which was compared with traditional UV-visible detection. The limit of detection for sulfide was approximately 50 ng L-1, or 1.5 nmol L-1. Analysis time was substantially reduced by use of isocratic elution. Interfering compounds produced by side reactions can be eliminated by use of the mass filter. A polysulfide sample was also analyzed to determine which products are formed and whether or not polysulfides react stoichiometrically with methylene blue reagent. It seems that polysulfides do not react quantitatively with methylene blue and so cannot be quantified reliably by use of this method.