Monitoring of purity and stability of CRMs of multiclass pesticides during prolonged storage before and after expiration

Certified reference materials (CRMs) are the backbone for qualitative and quantitative determination of pesticides in technical materials, formulations, and food matrices. However, their stringent expiry date, limited shelf life, and diminishing purity with time, limits their use beyond expiry. Stability study of 89 CRMs of multiclass pesticides, stored at - 25 degrees C, has been conducted to assess the purity of CRMs beyond their expiry date. It was evident that > 96 % of the CRMs remained optimally stable with respect to their observed purity even after their expiry date as per certificate of analysis. Percentage deviations in purity of expired and valid CRMs posited well below 7 % and are within +/- 10 % acceptable range as recommended by the SANTE. During 2013-2017, performance of the valid and expired CRMs was evaluated through 44 z-scores obtained in 14 international and national proficiency testing programmes. All the z-scores received for 15 expired and 29 valid CRMs were found satisfactory. Inter-laboratory comparison of 6 randomly selected expired and valid CRMs was tested at three different ISO/IEC 17025 accredited laboratories and their % difference between purities ranged from - 2.35 % to + 0.95 %. It was inferred that by maintaining proper storage conditions and continuous monitoring of purity, the expired CRMs can give comparable results as valid CRMs.