Use of Air — SO₂ Mixtures for Intercomparison of SO₂ Monitors in the EEC

Abstract

For an intercomparison programme among 29 continuous SO₂ analysers in eight EC Member States, cylinders of air-SO₂ mixtures were used as comparison standards. Two or three samples of two batches for each participating Member State were measured successively by the Central Laboratory (CL) of the JRC Ispra, calibrating with the TCM reference method, as referred in the EC Directive No. 80/779 EEC, and by one or more National Laboratories (NL) using their own calibration methods. Regular checks of the mixtures carried out by the CL showed an exponential decay of SO₂ concentration in the cylinders. In the two batches (190 and 40 ppb SO₂, respectively), mean losses of 0.48 and 3.7% per month were found with coefficients of variation of 28 and 48%. The CL values were corrected for the observed SO₂ decay and a statistical analysis performed between $\overline {{\text{NL}}}$ and $\overline {{\text{CLm}}} ,$ $\overline {{\text{CLm}}}$ being the corrected CL concentrations at the measuring month of NL. In the average, 68% of the 186 $\overline {{\text{NL}}} - \overline {{\text{CLm}}}$ cross schecks showed comparability (± 6.3%) versus the CL reference standard. This standard, dynamically generated by permeation, showed a quite good agreement with the TCM method. Considering on the one hand reference laboratory instruments (RI) and, on the other, network instruments (NI), the range of comparability included 73% RI and only 40% LI cross checks. The calibration procedures are discussed in so far as the main quality parameter is concerned.