Third Interlaboratory Study

6 kg of cod homogenate was prepared as described before from samples fished in the Koege Bay, south from Copengagen. The total Hg concentration determined by CVAAS in triplicate in the homogenate was ca. 190 pg kg.  

Spiked extract. 4.4 g of the stock solution was added to the toluene extract (raw extract) to obtain a concentration in the spiked extract of (0.9972 + 123.77) pgkg of MeHgCl where X is the concentration in the toluene extract. The four toluene extracts were bottled in 50 mL light-protected borosilicate bottles with PTFE gaskets in the screw cap. 

In addition to these samples, mussel and tuna samples were prepared at the Joint Research Centre of Ispra (Italy). Mussel flesh (wet weight) was collected and minced whereas the tuna fish was filleted the samples were frozen, freeze-dried, ground and homogenized. The two samples were bottled in brown borosilicate bottles each containing 15 to 20 g. 

The stability of the calibrant solution and extracts was verified over a period of 5 months at the National Food Agency (Denmark). Analyses were performed by packed column gas chromatography followed by electron capture detection in five replicates on each of three bottles. 

An additional source of error was calibration, which should be performed systematically using the compound to be determined (e.g. MeHgCl) and not with, for example,HgCl2- 

---

Given the instrumental transfer problems for CE methods, a robusmess test should include besides the instrument on which the method was developed at least one alternative instrument, when the method is to be applied on different instraments or in different laboratories. However, obtaining an analytical CE method that is considered robust from a test that included different instruments does not guarantee a successful transfer to a third instrument or the absence of problems during an interlaboratory study. 

---