ESCAPE PEAK
In both the EDS detector and the WDS gas-filled proportional counter, there is the statistically probability that besides photoelectrons (and holes in EDS, and Ar cations in P10-filled WDS detector), there may be some secondary fluorescence. That is, the incident x-ray (let's say Fe ka for example) could knock out an inner shell electron and produce another x-ray (Si ka in typical Si-Li detector; Ar ka in P10-filled detector). Most of these x-rays will be "consumed" within the detector, yielding photoelectrons, but some will find a means to escape the detector. Their characteristic energy will be lost to the detector for this one x-ray, resulting in one pulse that is at a lower energy level than is expected, which will then show up as a small peak at a lower voltage.
Taking the Fe Ka X-ray as an example, in the Si-Li EDS spectrum, there will be a small peak at 4.66 keV, 1.74 keV below the Fe Ka peak of 6.40 keV. In the WDS detector using P10 gas (90% Ar, 10% CH4), Ar-escape peaks will occur on the PHA energy display at a proportionally lower value, i.e., if the Fe Ka pulse is centered at 2.5 volts, then there will be a small Ar-escape peak at around 1.3 volts, because Ar Ka = 2.96 keV, so the escape peak will be 6.40 - 2.96 = 3.44 keV; 3.44/6.40 =0.54; 0.54 * 2.5 volts = 1.34 volts.
In WDS, sometimes it is impossible to acquire the complete Ar-escape peak, because the lower part is too low in energy and falls below the baseline cutoff. In that case, it is recommended that the baseline be raised to cut off all of the escape peak; either acquire all of it, or none of it. See section on PHA for more details.