In X-ray fluorescence analysis, the specific secondary radiation intensities of the elements are generated and measured by a spectrometer. Usually, the measured intensities are corrected for instrument drift first, then converted into element concentrations. In routine analysis, the intensity is always affected by the instability of the instrument. Instrumental instability consists of two components: short and long term drift Short-term instability is caused by fluctuations in the ambient laboratory conditions and the instrument's components. Long-term drift is caused by aging of the instrument's components (mainly X-ray tube, crystals, detectors, electronic circuitry) and results in a gradual intensity decrease. For example, the X-ray tube output decreases due to pitting of the target and sublimation of metal on the inside of the window. The quantum efficiency of detectors gradually decreases due to drift in potential supply and changes in ambient pressure and temperature.