Species which diffuse to the surface may or may not evapo-rate. If they enrich the surface the EDX measurement, which has atypical sampling depth of 1–2 m, will detect an enhanced signalunrepresentative of the bulk material and if they evaporate the EDXmeasurement will underestimate the concentration of that speciesbecause it is both absent from the surface and has been depletedfrom the near surface layer