Proficiency tests - Drinking water

Recommendation of the Federal Environment Agency - Cooperation with other PT providers - Evaluation and assessment

The Federal Environment Agency has developed recommendations for the provision of proficiency tests (PTs) in the field of drinking water analysis and published them in the "Bundesgesundheitsblatt". These were published in November 2003 for the chemical sector. Among other things, the following key points are laid down (reproduced here in abbreviated form):

  • Regular and successful participation in PTs is to be verified in particular by the responsible bodies according to § 15 paragraph 5 TrinkwV 2001 in the context of the verification of compliance with the requirements of paragraph 4 sentence 1 at the drinking waterlaboratories  established in the respective Land.
  • The PT schemes must be offered by qualified providers who meet the requirements for PT schemes according to DIN 38402-45, edition: 2003-09 "Interlaboratory comparisons for proficiency testing of laboratories (A 45)".
  • The interlaboratory comparisons for quality and proficiency control of drinking water laboratories must meet the requirements of DIN 38402-45 with regard to their planning, execution and evaluation.
  • The PT samples should be, as far as possible, real drinking water samples or drinking water samples spiked with the respective analytes.
  • A method for the determination of a parameter should not be prescribed by the organiser, as the Drinking Water Ordinance 2001 does not specify any analytical methods for the parameters of this recommendation either.
  • All chemical parameters and indicator parameters contained in the Drinking Water Ordinance 2001 should, as far as they can be tested within the framework of PTs, be offered by one or, if necessary, several PTproviders in a period of 2 to 3 years ("PT cycle").
  • It is to be required that the drinking water laboratories must prove successful participation within a PT cycle (2 to 3 years) for all parameters for which they are or wish to be accredited within the scope of drinking water testing according to TrinkwV 2001. If a PT for a parameter is not passed, the laboratory must repeat the determination of this parameter as soon as possible and, if necessary, with another provider.
  • The evaluation of raw data from drinking water ring tests must be carried out according to the requirements of section 10 of DIN 38402-45. If possible, the evaluation procedure should record and present all concentrations assigned in the PT in a cross-concentration, closed procedure with regard to recovery and dispersion.
  • The so-called zU score is to be used as the primary success criterion for assessing the performance of a drinking water laboratory.
  • Success criteria:
    • Acceptable measured value: |zU| ≤ 2
    • Successfully determined parameter: Based on the evaluation and the exclusion criterion |zU| ≤ 2, more than half of the samples (concentration levels) for each parameter should be correctly determined.
    • There is no evaluation of the entire PT, but only of individual parameters.
  • A dated and meaningful certificate of achievement should be issued to the participant for the successfully passed parameters in PTs. It should also indicate which parameters were not passed.

In order to enable laboratories to meet the requirements of the above-mentioned UBA recommendations, the proficiency test providers active in this field have decided to adopt a harmonized approach.
This resulted in two proficiency testing systems that perform these proficiency tests in a coordinated manner. One of these systems is located in North Rhine-Westphalia ( Landesamt für Natur, Umwelt und Verbraucherschutz Nordrhein-Westfalen in cooperation with IWW Rheinisch-Westfälisches Institut für Wasser), the other is operated by AQS Baden-Württemberg in cooperation with the Institut für Hygiene und Umwelt der Freien und Hansestadt Hamburg.

The parameters to be analyzed according to the Drinking Water Ordinance were distributed among 10 PT rounds:

  • A 1. anions:bromate, chloride, fluoride, nitrate, nitrite, sulfate, phosphorus.
  • A 2. trace elements: antimony, arsenic, lead, cadmium, chromium, copper, nickel, mercury, selenium, uranium.
  • A 3. cations: aluminum, boron, calcium, iron, magnesium, manganese, sodium, potassium, silicate.
  • A 4. general parameters: color (SAC436), conductivity, pH, turbidity.
  • A 5. other inorganic parameters: ammonium, cyanide, oxidizability, TOC.

  • O 1. pesticides PSM GC/MS or HPLC/UV: atrazine, desethylatrazine, metribuzin,
    tebuconazole, terbutylazine, bromacil, diuron, ethofumesate, flufenacet,
    hexazinone, isoproturon, metalaxyl, metamitron, methabenzthiazuron,
    dichlorobenzamide, desethylterbutylazine, metolachlor
  • O 2. VOCs/benzene: 1,2-dichloroethane, tetrachloroethene, trichloroethene, trichlorme-
    than, bromodichloromethane, dibromochloromethane, tribromomethane, benzene,
    Vinyl chloride
  • O 3. PAHs (according to drinking water ordinance* and others): acenaphthene, anthracene, benzo(a)-
    anthracene, benzo(a)pyrene*, benzo(b)fluoranthene*, benzo(k)fluoranthene*,
    benzo(ghi)perylene*, chrysene, dibenz(ah)anthracene, fluoranthene, fluorene,
    indeno(1,2,3-cd) pyrene*, naphthalene, phenanthrene, pyrene.
  • O 4. acidic pesticides: bentazone, bromoxynil, dichlorprop, MCPA, MCPB, mecoprop,
    2,4-D, quinmerac, 2,4,5-T
  • O 5. pesticides HPLC/MS: N,N-dimethysulfamide, methyl desphenylchloridazone,
    desphenyl chloridazone, metazachlorous acid, metazachlorosulfonic acid,
    meta-achloroic acid, meta-achlorosulfonic acid, glyphosate, AMPA.

Both proficiency testing systems offer the complete program of these proficiency tests staggered by one year. Four proficiency tests are performed per year, so that the entire program is completed within a period of 2.5 years. For the joint PT system of AQS Baden-Württemberg and IHU, it was agreed that AQS Baden-Württemberg would offer three PTs per year and IHU one. With this approach, it is possible for the laboratories to complete the entire program in each of the two systems and thus fulfill the requirements of the UBA recommendation. If a parameter is not successfully analyzed in one proficiency test, a repetition with the other proficiency test system is possible after 1.5 years at the latest. On the other hand, the entire program can also be completed within 1.5 years, which is particularly important for laboratories that are new to the market.

An overview of the harmonization of drinking water proficiency testing in Germany is compiled in a flyer.

In addition to these harmonized drinking water PTs, AQS Baden-Württemberg also regularly offers so-called special PTs for drinking water in cooperation with the IWW Zentrum Wasser in Mülheim an der Ruhr. These are usually parameters that are only measured by comparatively few laboratories or newly emerging substances with current relevance. In many of these interlaboratory comparisons, the samples are prepared by the IWW Zentrum Wasser on behalf of AQS Baden-Württemberg.

The evaluation of thesePTs is based on the specifications of DIN 38402 - A45, taking into account the recommendations of the Federal Environment Agency.
The assigned value is usually determined on the basis of the robust mean value of the measurement results of all participants. In individual cases, however, the proficiency test manager may also use values from formulation to calculate the assigned values. The content of analyte in the matrix used must then be determined from the results of the participants or from measurements of reference laboratories and taken into account in the calculation of the assigned value. The standard deviation necessary for the calculation of the zU scores is usually also obtained from the measurement results of the participating laboratories. Whenever possible, a closed evaluation over all concentration levels with the variance function described in DIN 38402 - A45 is used. The proficiency testing manager sets upper and lower limits for the standard deviation before the proficiency testing is carried out, which are intended to prevent the tolerance limits and thus the quality requirements for the laboratories from becoming too sharp or too lax.

All participating laboratories receive a data evaluation sheet after completion of the evaluation, which shows the evaluation of this individual value for all measured values by marking with "s" for successful, "q" for questionable and "n" for non satisfactory. In addition, it is documented for each parameter whether it was determined "successfully" or "unsuccessfully". Parameters are classified as "successfully" determined if at least 50% of the measured values are within the tolerance limits. Parameters for which no measured values were submitted are marked "not participated".

The participants also receive a certificate for the PT on which the parameter evaluation (successful, unsuccessful or not participated) is noted for each parameter.

A detailed evaluation report is also published on the AQS Baden-Württemberg website, which contains the results of all participating laboratories in anonymized form and is presented in tabular and graphical form.

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