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An automatic mesofluidic cold-vapor atomic fluorescence spectrometric method is herein developed for on-line sample processing and trace level determination of inorganic mercury capitalized on a lab-on-a-valve (LOV) manifold, which integrates a readily exchangeable commercially available sorptive micro-cartridge, a microscale reaction chamber for in-line vapour generation and a membraneless gas-liquid separator. The uptake of mercury(II) is accomplished onto the surfaces of reversed-phase co-polymeric Oasis (TM) HLB beads, containing a balanced ratio of hydrophilic and lipophilic monomers [namely, poly(divinylbenzene-co-N-vinylpyrrolidone)], without need of any extra chelate reagent. The preconcentrated analyte is eluted by a mixture of hydrochloric and nitric acid, merged downstream with a metered volume of tin(II) chloride and transported into the integrated reaction chamber/gas-liquid separator (RC-GLS). The generated cold vapor is monitored using a peripheral atomic fluorescence spectrometer. The detection limit and repeatability (RSD) are found to be 0.04 mu g L-1 and 3.8% (at 10.0 mu g L(-1)concentration level), respectively, for 9 mL sample. The proposed LOV mesofluidic system can also be effectively used without sorptive preconcentration by loading a metered volume of sample (<= 3 mL) into the RC-GLS. The detection limit and precision for direct analysis of samples are estimated to be 0.1 mu g L-1 and 2.9%, respectively. The applicability and reliability of the automated miniaturized method are ascertained through the analysis of spiked environmental waters.