A method of operating a tissue processor for processing tissue samples. The tissue processor includes at least one retort for receiving tissue samples, at least one container for storing a reagent, and at least one sensor arranged for fluid communication with one or both of the at least one container and the at least one retort for measuring a measured purity level of a reagent. The method includes conducting reagent from the at least one container or the at least one retort to the at least one sensor, automatically measuring a measured purity level of the reagent, checking whether the measured purity level meets a predetermined purity level of the reagent associated with the at least one container, and thereby automatically determining whether the reagent is suitable for processing tissue samples in the tissue processor. Also, a tissue processor for processing tissue samples and a container for storing tissue samples.