Coordination of organochlorine pesticides in water proficiency testing schemes in South Africa

The National Metrology Institute of South Africa (NMISA) developed a proficiency testing (PT) scheme for the analysis of organochlorine pesticides (OCPs) in drinking water, for a limited number of participants. Two identical 1 L water samples fortified with OCPs at the ng/L concentration level were distributed to each participant. The concentration range of pesticides was selected to mimic regulated trace levels in drinking water. The assigned value was based on the traceable, gravimetrically prepared value of each of the analytes spiked into the water samples. The samples were adequately homogeneous and stable over the 3-week reporting period. The standard deviation for proficiency assessment was determined using the alternate Horwitz model, and participant performance was reported using z-scores. En numbers were also included in the PT reports to motivate laboratories to evaluate the suitability of their measurement uncertainty estimations. One of the biggest challenges associated with coordinating this PT scheme remains courier costs and delays in sample distribution within Africa, whilst the accurate determination of pesticides at trace concentration levels remains the biggest challenge for PT scheme participants.