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General Information on VDLUFA Interlaboratory Tests

The VDLUFA sections Soil Analysis, Fertiliser Analysis, Seeds, Milk, Feedstuff Analysis and Analysis of Environmental Parameters and Trace Compounds regularly carry out interlaboratory tests with the aim of   

  • validating and interpreting analytical methods (collaborative tests)
  • assessing the analytical quality of laboratories (proficiency tests) and
  • certifying reference materials.

Only members of VDLUFA may participate in these interlaboratory tests.
These tests mainly focus on feeding stuff and fertilisers, soils, horticultural soils and substracts and seeds. This not only raises the quality standards of participating laboratories but also allows a continuous review of the methods involved based on the diversity of samples and matrices tested and on the different analyte concentrations in the samples.
VDLUFA also offers two public interlaboratory tests which are also open to non-members:

  • VDLUFA EU Fertiliser Ring Test
  • Feeding Stuff Ring Test

Collaborative Tests

Validation of analytical methods provides significant quality parameters such as the variation coefficient of repeatability, variation coefficient of reproducibility, recovery and measurement uncertainty. Each interlaboratory test should be performed by a minimum of eight laboratories with the same level of expertise and qualification in the analytical discipline relevant to the method to be tested. The results from these interlaboratory tests allow conclusions to be drawn on the robustness of the method, the testing performance of the laboratories and on the samples involved. Depending on the content, statistical parameters such as variation coefficient of reproducibility, measurement uncertainty and Horwitz ratio obtained from interlaboratory tests are used to define analysis tolerances.
The objectives, protocols, implementation and evaluation of method performance tests have been explained in detail in a position paper from 2007 (in German).

Proficiency Tests

Successful participation in proficiency testing programmes is required for being granted official approval (accreditation) for performing expert testing and official controls of materials. Accreditation is required for performing official testing under the following German regulations: Düngemittelverordnung (Ordinance on Fertilisers), Bioabfallverordnung (Ordinance on Biowastes), Klärschlammverordnung (Ordinance on Sewage Sludge) and Bundes-Bodenschutz- und Altlastenverordnung (Federal Ordinance on Contaminated Sites and Soil Protection).


VDLUFA carries out interlaboratory tests in accordance with state-of-the-art, generally applicable rules and current standards (e.g AQS data sheet A3 (9/1998), DIN 38402 A45, International Harmonized Protocol for the proficiency testing of analytical chemistry laboratories, IUPAC Technical Report (2006), Tattersfield method for seeds). The test results are interpreted using software authorised by VDLUFA (at present the ProLab software package from quodata (Dresden)). VDLUFA has a set of minimum requirements for the documentation and archiving of interlaboratory tests.

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