Once the initial screening has identified the presence of a banned substance, further verification takes place. Using lead as an example, the precise concentration present is determined using inductively-coupled plasma atomic emission spectrometry (ICP-AES) or atomic absorption spectrometry (AAS). Both give similar results, the difference being in the principle of operation, the former using energy radiation, and the latter energy absorption to produce results. The usual approach to lead screening is to start the process with energy dispersive x-ray fluorescence to start the process with energy dispersive x-ray fluorescence spectrometry (ED-XRF), a qualitative and quantitative process in which the interaction of x-rays with the test pieces secondary (fluorescent) x-rays to be generated. Each element present in the test piece produces x-rays with different energies, which are detected and displayed as a spectrum of intensity against energy; the position and height of each peak identifies the element present and its concentration in the sample. ED-XRF is accurate and fast, but only sensitive enough to measure concentrations down to about 0.1%. One limitation of the technique is that the depth to which the surface is penetrated will vary according to the nature of the material being analyzed. On plated or painted metals, the surface layer may need to be mechanically or chemically removed before analysis to endure that the eradiation reaches the substrate to be tested, while other materials may allow the radiation to reach beyond the desired depth, giving a misleading result. The skill of the equipment operator and the intelligent interpretation of results are therefore critical to the accuracy of the finding. Wavelength-dispersive x-ray fluorescence (WD-XRF) may be used in a similar fashion to ED=XRF, although this method offers varying sensitivity, it is less accurate and therefore not as commonly used in routine analysis.