Within the DETR's UK NO₂ Network, there are currently 33 analytical laboratories responsible for preparation and analysis of the NO₂ diffusion tubes used by Local and Unitary Authorities. It is important that the data obtained by this Network is of the highest possible quality, therefore QA/QC of laboratory analyses is very important to the Network's operation.

Since 1996, the UK NO₂ Network has employed a laboratory performance testing programme, which uses artificially doped diffusion tubes to test the accuracy of laboratory analyses on a monthly basis (Bush et al)¹. This scheme provides valuable information on the analytical performance of laboratories. However, owing to the use of artificially doped diffusion tubes in this programme, uncertainties arising from the sampling phase of diffusion tube monitoring cannot be assessed. Field intercomparison exercises were therefore reintroduced into the Network's laboratory QA/QC programme, in order to better define the sampling and analytical performance of diffusion tubes under normal operating conditions.

The 1999 field intercomparison exercise follows on from previous intercomparisons held in 1993, 1994, 1995 (Stevenson et al)^2,3,4 and most recently 1998(Bush) ⁵. It was designed to complement the existing laboratory quality assurance programme for the UK NO₂ Network, which currently utilises information supplied by the WASP scheme under the management of the Health and Safety Laboratory (HSL).

Full details of the performance of individual laboratories in the WASP scheme and the 1999 field intercomparison exercise are available direct from the laboratory.

In 1998, a three-month field intercomparison exercise was carried out, involving the thirty-eight laboratories participating at that time. The main conclusions that may be drawn from the 1998 field intercomparison exercise are as follows:

1. 79% of the 38 laboratories performed to the data quality objectives set out in the EU Daughter Directive for NO₂: that is, within +25%.

2. 68% of the 38 laboratories displayed an average measurement precision of £ 2.54 ppb.

3. However, the range in bias of measurement data was large: -39% to +58%.

4. The average precision associated with the measurement data reported by the UK NO₂ Network was estimated at 2.54 ppb.

5. The recommended limits of acceptable bias were established at ± 25%. A recommended target for precision of 2.5 ppb was suggested.

6. Poor extraction efficiency during analysis was identified as the most likely contributory factor resulting in under estimation by diffusion tubes relative to the chemiluminescent technique and variability in laboratory performance.

7. Laboratories showing a negative bias (under estimating) were recommended to investigate their procedures for tube preparation and nitrite extraction from the tube. They were advised to ensure that excess triethanolamine (TEA) was loaded on the absorbent grid of the diffusion tubes and also that all absorbed nitrite was made available for analysis by the extraction phase.

8. It was predicted that improved extraction and sampling efficiency may result in average bias in diffusion tube measurements relative to the EU prescribed chemiluminescence method tending to the positive. Also, the reduction in the overall variability in analyses may also be expected, thus improving the comparability of measurements in the UK NO₂ Network.

The 1999 intercomparison was intended to build upon the experience of the 1998 study, and further improve performance of laboratories participating in the UK NO₂ Network. The objectives of the 1999 field intercomparison exercise were as follows:

1. To estimate bias and precision, under normal field operating conditions, for all laboratories performing analysis in the UK NO₂ Network during 1999.

2. To compare the estimated bias and precision in 1999 with the results from the field intercomparisons in 1998 and earlier years.

3. To investigate whether the method of tube preparation introduces systematic differences in performance of diffusion tubes, for the two main methods.

4. To investigate how techniques for preparing, exposing and analysing diffusion tubes could be improved to reduce uncertainty.

Contents Page         Chapter 2