The lack of fit (or linearity) shall be carried out for all determinands at least once at the point
of calibration (certification / installation). Thereafter the lack of fit shall be determined as part
of the annual performance testing for each analyser for those determinands with applicable
SRMs. Traceable, certified standards shall be used either individually, or as a mixture, with
concentrations at 100% of the respective calibration / certification ranges. Each standard
shall be measured across at least five concentration points that shall include 0 and 100%.
The concentration points will be separated by approximately even increments and achieved
either by diluting the 100% of calibration / certification range standard by a suitably certified
procedure or by acquiring a set of traceable, certified materials at the required lower
concentrations.