Second Interlaboratory Study

Approximately 4 kg of flounder was purchased at Sletten Havn located in Niva Bay, 30 km north of Copenhagen, where high levels of total Hg in fish tissues had been reported previously. The fish sample was mixed with redistilled water, homogenised and stored at — 20°C. Six sub-samples of homogenate each of 0.2 g were analysed for total Hg by RNAA. The total content was found to be (191 20) pg kg (as Hg) on wet mass basis. Extracts were then prepared by the Danish Isotope Centre and the stability of 

MeHg was verified by the National Food Agency (Denmark). Another batch of aqueous solutions as described under the first intercomparison was prepared by the KFA and sent to the participants. The preparation of the samples was the following. 

Raw extract. Subsamples of 30 g of fish homogenate were mixed with 80 mL HCl and 20 mL CUSO4. This mixture was shaken, left to react for 15 min and extracted 3 times with toluene to obtain ca. 37 L of extract which was dried by addition of anhydrous Na2S04 and stored at 5-10 °C. The samples were bottled in 500 mL light-protected borosilicate bottles with PTFE gaskets in the screw cap. 

Raw extract spiked with MeHg. Approximately 2500 mL of the raw extract were spiked with MeHgCl dissolved in DMSO to obtain a concentration of about 11 pg L . Samples were bottled in 250 mL light-protected borosilicate bottles. 

Cleaned extract. Sub-samples of the raw extract were extracted twice with 150 mL of cysteine acetate. After acidification with HCl, the mixtures were back-extracted twice with toluene. This cleaned extract was dried by addition of anhydrous Na2S04 and samples were distributed in 100 mL bottles. 

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The results of the second interlaboratory study of PCN analytical methods using environmental matrices, undertaken by the US National Institute of Standards and Technology (NIST), should provide an indication of the comparability of published environmental PCN data and show where additional method enhancements are needed. Further method development efforts in the analysis of PCNs, and other complex mixtures are likely to focus on improving efficiencies by optimizing run times and separation. Examples may include time-of-flight mass spectrometry and multidimensional GC. New methods, such as isotope ratio mass spectrometry, may contribute to further source apportionment of complex mixtures [88]. 

For the first intercomparison exercise, participants were asked to dilute the solution 1000 fold, i.e. to determine levels of TriML of ca. 40 pg L. Some laboratories also analysed the solutions after a 10,000-fold dilution. The participating laboratories in the second interlaboratory study received two sets of solutions containing ca. 50 and 5 pg L of TriML respectively. They were requested to perform five replicate analyses of, respectively ... 

The second interlaboratory study generated detailed discussions one ETAAS technique employed did not include a separation step but the participant stated that EDTA extraction would only extract organic lead compounds this technique was considered to be suitable for the analysis of a simple solution containing only one lead compound but would not be suitable for mixtures of lead species, e.g. the technique would not allow the separation of TriML and TriEL in a natural rainwater sample. In cases where different organolead compounds are to be determined in natural samples or solutions containing different lead compounds, ETAAS should be coupled to a separation technique, e.g. GC or HPLC. 

Table 8.19 gives a summary of the results obtained in the second interlaboratory study. Specific remarks were made for some elements ... 

Previous exercises have shown the difficulties of determining Hg in seawater. As an example, coefficients of variation of 26.7 and 11.6% between eleven laboratories (CV of the mean of laboratory means) were found for Hg levels of respectively 6.0 and 24.2 ng L [31]. Therefore, it was chosen to consider seawater samples containing much higher Hg concentrations (coastal seawater samples spiked with mercury) in a first method performance study and to use natural coastal seawater, along with a spiked sample, in the second interlaboratory study. 

The programme to improve the quality control of organotin determinations in environmental matrices was started in 1988 [98] by a consultation of European experts. It was decided to follow a stepwise approach for the evaluation of the performance of methods used in butyltin analyses. The first exercise (simple solutions) was conducted in 1988-89 and a second interlaboratory study (spiked sediment) was organized in 1989-90. The first certification campaign (harbour sediment) was carried out in 1990-92, whereas the certification of a coastal sediment was conducted in 1992-93. The most recent certification focused on organotin determinations in mussel samples (1994-97). 

Validation of a new analytical method is typically done at two levels. The first is the level of prevalidation, aiming at fixing the scope of the validation. The second level is an extensive, full validation performed through a collaborative trial or interlaboratory study. The objective of full validation, involving a minimum number of laboratories, is to demonstrate that the method performs as was stated after the prevalidation. 

A comparison with the results obtained in the first interlaboratory study (Table 8.18) showed a clear improvement in the quality of the determination of most of the elements, particularly cadmium (considering the much lower concentrations determined in the second exercise), copper, lead and zinc. For iron and nickel, the CVs between laboratories were approximately the same. 

Two interlaboratory studies were organised prior to the certification campaign. The first one dealt with the analysis of artificial seawater and the second exercise concerned the analyses of natural and spiked seawater. The results obtained for Cd, Cu, Pb and Zn are compared in Table 8.23. The CV between all the laboratories appeared to be quite high in the second round-robin exercise. However, the participation in such intercomparison combined with critical discussions of methods and results was found to be a most useful tool in obtaining a high level of accuracy (which is reflected in the... 

In the first interlaboratory study, the examination of the raw data (14 sets of results of which 12 involved CVAAS, one RNAA and one MIP-AES) revealed a high spread of results due to two outliers. The mean obtained was 12.6 pg L of Hg with a coefficient of variation (CV) between laboratories of 33%. The two high results were attributed to a laboratory contamination. The accepted values showed a picture which was found more acceptable, i.e. the mean obtained was 10.8 pg L with a CV between laboratories of 6.6"/n [8]. At this stage, the agreement between the laboratories was found to be satisfactory however, the Hg content in this (spiked) sample was considered much too high for being representative of natural samples which justified the organization of a second interlaboratory exercise for which results are described below. 

The project to improve the quality control of lead speciation analysis was started in 1990 by a feasibility study on the stability of alkyllead species in solution [118], and was concluded in 1991. The first interlaboratory study was conducted in 1992 [119] and was followed by a second exercise carried out in 1993 [120]. The certification campaign of trimethyllead in artificial rainwater and urban dust was conducted in 1995-96. 

One laboratory (DPASV) repeated the analysis and observed a systematic difference due to two different sets of calibrants. The second set of data (5.94 0.33 pgkg- ) was obtained with a calibrant solution made with a newer calibrant from the same producer. This highlighted the need to verify the calibrant thoroughly, i.e. not to rely on calibrants from one producer of which the quality could vary from one set to another. Most of the laboratories actually used their own calibrants which were not verified for purity and stoichiometry. Only one laboratory used the calibrant previously verified and distributed in the first interlaboratory study. It was stressed that calibration was an important issue and that more effort should be put into the verification of calibrants in future exercises. It was agreed that the coordinator of the project would purchase calibrant from a chemical company and verify its purity sets of verified primary calibrants would then be made available to participants in a further exercise to verify their own calibrants. 

The sediment sample used in the second interlaboratory trial was collected in the River Besos, Spain. The material was sampled with a grab, air-dried, then sieved at 63 pm, homogenized and bottled. Homogeneity and stability studies of extractable trace metals were carried out and the material was found to be homogeneous and stable enough to be used in the intercomparison exercise [196]. 

The first thermal transition temperature precision data are based on duplication determinations on five different petroleum waxes in an interlaboratory study among six laboratories. The second thermal transition temperature precision data are based on duplicate determinations on two different petroleum waxes in an interlaboratory study among six laboratories. 

Several overall conclusions can be drawn based on the statistical evaluation of the data submitted by the participants of the DR CALUX intra-and interlaboratory validation study. First, differences in expertise between the laboratories are apparent based on the results for the calibration curves (both for the curves as provided by the coordinator and for the curves that were prepared by the participants) and on the differences in individual measurement variability. Second, the average results, over all participants, are very close to the true concentration, expressed in DR CALUX 2,3,7,8-TCDD TEQs for the analytical samples. Furthermore, the interlaboratory variation for the different sample types can be regarded as estimates for the method variability. The analytical method variability is estimated to be 10.5% for analytical samples and 22.0% for sediment extracts. Finally, responses appear dependent on the dilution of the final solution to be measured. This is hypothesized to be due to differences in dose-effect curves for different dioxin responsive element-active substances. For 2,3,7,8-TCDD, this effect is not observed. Overall, based on bioassay characteristics presented here and harmonized quality criteria published elsewhere (Behnisch et al., 2001a), the DR CALUX bioassay is regarded as an accurate and reliable tool for intensive monitoring of coastal sediments. 

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