Reference compounds interlaboratory studies

A complementary project has started in 1997 by an interlaboratory study on butyl-and phenyl-tin compounds in a freshwater sediment reference material. A certification campaign has been conducted in 1998 and the certification of the six compounds (mono-, di- and tri-butyl and phenyltins) is promising [15]. All precautions have been taken to ensure the material stability (storage at -20°C). 

Besides the classical interlaboratory studies, improvement schemes enable laboratories to develop and validate all steps of new or existing analytical procedure(s) in adequately organised successive exercises. Improvement schemes may be considered as preliminary studies for laboratory or method performance studies or certification of reference materials [5]. Such programmes are very valuable when the analytical procedures include several complex and critical steps, e.g. for the determination of trace organic compounds or chemical species. They require a long term involvement of the organiser and participants, as well as investment of resources. 

The main difficulty in preparing a sediment reference material for interlaboratory studies on chemical species is to achieve the stability of the relevant compounds [12]. With respect to methylmercury, the main source of instability is due to bacteria, either by demethylation [88,89] or formation of volatile dimethylmercury [90]. The conversion is indirectly provoked by the biological activity of various types of bacteria, such as (i) aerobic mesophilic heterotrophic microorganisms, (ii) anaerobic sulfate-reducing bacteria and (iii) anaerobic spore-forming bacteria. In order to control the remaining bacteria present after different irradiation treatments, a bacterial enumeration was performed on samples which were dehydrated after homogenization and irradiated at various 7-ray doses (0, 4, 8, 12, 25 and 50 kGy). The determination of the sulfate-... 

Validation of a SPME method for target analytes should be performed using standard reference materials with similar matrix, when available. Another possible and frequently used way is validation of a SPME method against well-accepted extraction techniques, such as purge-and trap [13,25,46] or static head-space [46]. Several interlaboratory studies demonstrated that SPME is a reliable technique for the quantitative analysis of volatile organic compounds [46] and pesticides in water samples [47 48]. We have validated our SPME-GC-MS method for the determination of nitrous oxide in urine by means of the comparison with static headspace [33]. 

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