+ de critères

Quality control and improving measurements

Nils König (NW-FVA: Northwest German Forest Research Institute) takes the floor

ICP Forests: long experience in quality control and improvement, for monitoring forest in a comparable way

Summary

Detecting changes in forest ecosystems at the European scale supposes measurements to be done in some homogeneous way over decades and accross countries. Samples must be analysed in labs e.g for surveying atmospheric deposition, tree nutrition or soil chemistry. So the evaluation and the improvement of the quality in labs are a key issue.

Over 100 different labs have been analysing water, soil and foliage samples within the monitoring program of ICP Forests. To guarantee the comparability of the analytical results between different laboratories, a quality assurance (QA) program is necessary with participation of all laboratories. The ICP Forests QA program is based on three pillars:

  • the use of harmonized, well-defined and documented analytical methods
  • an internal quality control (QC) program within each lab
  • an external QC program coordinated by the monitoring program organizers.

Analytical data can be comparable only if laboratories are using the same or comparable methods for extracts, digestion and element determinations. Therefore each new digestion or extraction method which is not the mandatory one has to be tested if the results are comparable. This is very important when new instruments or analytical technics come to the market; the comparability has to be tested before these new technics can be allowed for analysing ICP Forest samples.

Proposals for the internal quality control program for each laboratory have been developed by the Working Group QA/QC in Labs and the meetings of the heads of the labs of ICP Forests and are documented in the ICP Forests manual. It consists of different tools to check the plausibility of each measurement in regard to range limits, and to test the consistency between values obtained for different parameters.

For instance for water samples the positive and negative electric charges of the measured ions should be balanced. If not, that means that at least one ion concentration must be wrong. For plausible range checks the monitoring data of different countries have been evaluated to decide which range of results is realistic for the different parameters. If one value exceeds the plausible range, it must be corrected or confirmed by another measurement.

The main part of the external QC program is made of ring tests between all labs. They are planned regularly and generally based on 5 natural samples representing contrasted conditions. The percentage of non-tolerable results in ring tests can be seen as a degree of quality and comparability of results from participating labs.

The Figure 1 shows the results for several parameters of water ring tests within the last 15 years. The development of the quality of the labs, but also the limitation due to different analytical methods can be seen from these results.

Figure : Percentage of results outsit to the tolerance level during the ring tests organized to the water analyze by the ICP Forests programme. Parameter : electrical conductivity (Cond), pH, alcalinity (Alk),dissolved total nitrogen (TDN), and dissolved organic carbon (DOC)
Figure : Percentage of results outsit to the tolerance level during the ring tests organized to the water analyze by the ICP Forests programme. Parameter : electrical conductivity (Cond), pH, alcalinity (Alk),dissolved total nitrogen (TDN), and dissolved organic carbon (DOC) © Nils König / NW-FVA

The results of the ring tests are discussed during the meetings of the Working Group QA/QC and the meetings of the heads of the labs. These meetings offer the possibility to find the mistakes and problems for those parameters with unacceptable ring test results. New methods are discussed and method comparisons are presented. Indeed sharing the knowledge and experience of so many labs is a great opportunity to make quality improve in all of them.

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