A refined approach for the experimental determination of fluid composition at elevated pressure and temperature conditions is developed. It effectively determines partition coefficients of trace elements between aqueous fluid and minerals. This approach is developed on top of many previously known technical achievements. However, it differs from the previous ones by the use of calibrated base and acid leaching of quenched samples to strip off the precipitates formed from the fluid during the quench. This permits the reconstruction of a fluid composition from an experimental charge that consists of a complete mixture of solid and fluid phases. This is a significant difference from the previous analogous studies, in which the fluid was separated from the solids all through the experiment. Mass balance calculations and error propagation assess the merits of the present method applied to a fluid-antigorite system.