A sensor for determining the overall content of a pre-determined chemical element in a liquid body uses X-ray fluorescence technology and includes an X-ray source, an X-ray detector, and a cell intended to contain a sample of lubricant to be analyzed, and is provided with a wall forming a window for passage of X-rays, the wall of the sensor cell being produced from polyethylene terephthalate, the cell also including a casing defining an internal volume for receiving the sample. A procedure for calibrating the sensor, and an automated method for online monitoring are also described and claimed.