## 3 Results  
### AOX Analytical Method  
Table 4 shows the results obtained by the four laboratories for the AOX content assessment of samples S1 and S2. Although all the laboratories applied the same analytical method, i.e., UNI EN ISO 9562:2004, they obtained very different values from each other. The marked differences--at least one order of magnitude--for samples with a high concentration, i.e., sample S2, led us to more deeply explore the topic by asking all the laboratories to provide the sensitive data necessary to calculate the AOX concentration, namely  
* The adopted adsorption method;* The amount of activated carbon;
* The sample volume;
* The volume of ultra-pure water used for washing before combustion;
* Any other data used for the quantification of AOXs in the sample. Lab-1 and Lab-2 adopted the shaking method, while Lab-3 and Lab-4 employed the column method. Lab-1 and Lab-2 used 50 mg of activated carbon. Lab-3 used two columns--provided by Analytik Jena--per sample, each one containing approximately 50 mg of activated car-bon. Lab-4 also used the columns provided by Analytik Jena, but claimed that the activated carbon mass was not indicated, and it would be about 50-200 mg. The third question allowed us to verify that all laboratories operated in the optimal range for the AOX assessment (0.01-0.3 mg L\({}^{-1}\) of Cl) and that they tested 100 mL of sample--as is or after diluting it--in the adsorption process on activated carbon. The results showed that all the laboratories followed the standard method and therefore, they used 100 mL of the diluted samples. However, the samples were diluted with different DFs. Lab-1 used a DF of 1:2500, obtaining a concentration of 12 ug L\({}^{-1}\) and certifying the concentration as 30.8 mg L\({}^{-1}\). Lab-2 used a DF of 1:1000, obtaining a concentration of 86.5 ug L\({}^{-1}\) (average value of the two tests, both using a DF of 1:1000) and certifying the concentration as 86.5 mg L\({}^{-1}\). Lab-3 used a DF of 1:200, obtaining a concentration of 119 ug L\({}^{-1}\) and certifying the concentration as 23.8 mg L\({}^{-1}\). Lastly, Lab-4 used a DF of 1:100, obtaining a concentration of 80 ug L\({}^{-1}\) and certifying the concentration as 8 mg L\({}^{-1}\). It can be noticed that the certified value increased as the dilution ratio increased, except for Lab-1. In order to verify whether this cause/effect relationship actually exists, Lab-1 was commissioned to carry out the AOX quantification using four different DFs. The results were as follows: a DF of 1:2000 gave a value of 30.0 mg L\({}^{-1}\), a DF of 1:1000 gave 25.4 mg L\({}^{-1}\), a DF of 1:500 gave 27.3 mg L\({}^{-1}\) and a DF of 1:200 gave 27.1 mg L\({}^{-1}\). Thus, it can be stated that the result was not influenced by the different dilution ratios. Moreover, the measurement uncertainty was higher for Lab-4 with respect to the other laboratories. The volume of ultra-pure water used for washing before combustion was different for all laboratories. Lab-1 employed 25 mL of a NaNO\({}_{3}\) wash solution. Lab-2 used 5-10 mL of a washing solution, which was obtained by diluting a nitrate solution (Na-NO\({}_{3}\) + HNO\({}_{3}\)) 1:20 with ultra-pure water to clean any remaining carbon from the flask and another 5 mL was used to rinse the walls of the flask. Lab-3 reported that the instrument washes the columns with a solution of sodium nitrate and nitric acid (according to the provider/method specifications) after adsorption. Lab-4 added 5 mL of a stock solution--a diluted solution of NaNO\({}_{3}\) and HNO\({}_{3}\)--to the acidified sample and after adsorption, it was washed with 25 mL of a washing solution, a solution that was prepared by diluting the stock solution. Lab-4 also reported that, in addition to the initial sample volume of 100 mL, in some cases, it was necessary to dilute the sample and therefore the DF may need to be taken into