Homogeneity and stability of materials distributed within the wageningen evaluating programmes for analytical laboratories

Participation in laboratory evaluating programmes (Proficiency Testing sche mes) is becoming increasingly important, especially for accredited laborato ries. The Wageningen Agricultural University meets this need by organizing several world-wide international laboratory evaluating programmes on chemic al analysis of soils (ISE), plants (IPE), manure, refuses (MARSEP) and sedi ments (SETOC). Since 1995, the existing programmes have been placed under a n umbrella organization called WEPAL: Wageningen Evaluating Programmes for Analytical Laboratories. Because of the increasing importance of a good per formance within the Proficiency Testing scheme, it is crucial that the orga nizer of the programmes can continue to guarantee a quality product. To ach ieve this, WEPAL reorganized and documented the complete system in order to get a formal accreditation (based on ISO/IEC Guide 43, part 1) as a qualif ied organizer of proficiency testing schemes. Much attention was paid to th e topic of homogeneity and stability of the distributed materials. Based on a comparison of results obtained by i) participants of the schemes, ii) th e Central Laboratory of the Sub-department Soil Science and Plant Nutrition and iii) a long-history of results, it may be concluded that the homogenei ty and stability of the materials distributed through the ISE- and SETOC-sc hemes are sufficient for the goal of Proficiency Testing.