Proficiency testing schemes: solutions for homogeneity control

The organisation of the homogeneity control of samples prepared for proficiency testing schemes (PTS) is a challenge for the PTS organisers and engenders logistical and financial problems. Samples used in PTS need to be homogeneous in order to be sure that if a laboratory has a result different from the other laboratories, its error can be attributed to its analysis method and not to its sample. The present paper describes the solutions found by the Bureau Interprofessionnel d'Etudes Analytiques (Bipea) in order to answer the homogeneity control requirement. The methodologies of homogeneity control are adapted to specific characteristics of the products proposed in Bipea's PTS. The first methodology of control, the "preparation approach", concerns specific PTS where the samples are formulated and where the assigned values are known and chosen. The second methodology, the "target approach", concerns mainly the field of the environment and implies the choice of homogeneity targets. The third methodology, the "global approach", concerns the agricultural and agro-food fields and involves comparison of near-IR spectra samples. Each methodology of control has its advantages and disadvantage and is suited to specific products prepared with specific preparation procedures. It has to be stressed that the solutions developed answer quality requirements but are not a substitute for the use of adapted and detailed preparation procedures with validated and regularly checked devices and for the a posteriori control of the results of the laboratories.