A process for optimising the removal of calcium from a hydrocarbon feedstock in a refinery desalting process, the refinery desalting process comprising the following steps: (a) mixing one or more wash water streams with one or more hydrocarbon feedstock streams; (b) at least partially separating the wash water from the hydrocarbons in a refinery desalter; and (c) removing the separated water and hydrocarbons from the refinery desalter as one or more desalted hydrocarbon streams and one or more effluent water streams; the process optimisation comprising: (i) providing at least one x-ray fluorescence analyser at at least one point in the refinery desalting process; (ii) measuring the concentration of calcium at the at least one point in the process using the at least one x-ray fluorescence analyser; and (iii) optionally adjusting at least one process condition of the refinery desalting process in response to the calcium concentration measurement in step (ii). An apparatus comprises a desalter; a line through which one or more hydrocarbon feedstock streams are passed to the desalter; optionally a line through which one or more wash water streams are passed to the desalter; and one or more x-ray fluorescence analysers configured so as to measure the concentration of calcium in water or hydrocarbons at one or more positions within the apparatus.