Determination of 3-mercaptopropionic acid by HPLC: A sensitive method for environmental applications
Abstract
The organic sulfur compound 3-mercaptopropionic acid (3-MPA) is an important thiol intermediate in organic sulfur metabolism in natural environments. It is generated during degradation of sulfur-containing amino acids (e.g. methionine) and from demethylation of dimethylsulfoniopropionate (DMSP). This pathway is an alternative enzymatic process in the DMSP catabolism that routes sulfur away from the climatically-active dimethyl sulfide (DMS). 3-MPA detection and subsequent quantification in different matrices is difficult due to its extreme reactivity. We therefore developed a sensitive method for determination of 3-MPA based on pre-column derivatization with monobromobimane and analysis by high-performance liquid chromatography (HPLC) with fluorescence detection. This methodology was first tested with 3-MPA standards under low (0.005–0.2?μmol?L?1) and high (1–25?μmol?L?1) concentrations. For the optimization of the reaction, CHES and, alternatively, Tris–HCl buffers were evaluated in the derivatization step, with Tris–HCl showing more effective separation of thiol derivatives and a better 3-MPA peak shape. The detection limit was 4.3?nmol?L?1 with a 10?μL sample injection, and mean recoveries of 3-MPA ranged from 97 to 105% in estuarine waters with different salinities (0.17 and 35.9?ppt). The linearity (r?>?0.99) and repeatability of detector response, with intra- and inter-day precision (% CV) of 2.68–7.01% and 4.86–12.5%, respectively, confirmed the reliability of the method. Previous 3-MPA analytical methods required immediate analysis due to unstable derivatives, but in this method we achieved high stability of the derivatized samples when stored at 4?°C, with only a 3–5% loss after more than one year of storage. This method was successfully applied to measure 3-MPA concentrations and rates of 3-MPA production in a variety of intertidal estuarine sediment slurries. Dissolved 3-MPA concentrations in these sediment slurries varied between 2 and 237?μmol?L?1 and, 3-MPA net fluxes ranged in wet sediments between ?3.6?±?1.7 and 30?±?5?μmol?L?1?g?1?h?1. Thus, the application of this optimized methodology showed an efficient performance for measuring 3-MPA in environmental samples, with a straightforward sample derivatization and a simple analysis of stable 3-MPA derivatives.