Accred Qual Assur (2006) 10: 562–564 DOI 10.1007/s00769-005-0058-8 PRACTITIONER’S REPORT in situ interlaboratory comparisons for dissolved oxygen concentration and pH Lauri Jalukse Viktor Vabson Ivo Leito Received: 18 August 2005 Accepted: 19 November 2005 Published online: 14 January 2006 C  Springer-Verlag 2005 Electronic Supplementary Material Supplementary material is available for this article if you access the article at http://dx.doi.org/10.1007/s00769- 005-0058-8. The Final Report of the DO Concentration and pH Intercomparison is available in the electronic supplementary material. L. Jalukse · I. Leito () University of Tartu, Faculty of Physics and Chemistry, Jakobi 2, 51014 Tartu, Estonia e-mail: ivo.leito@ut.ee Tel.: +372-7-375-259 Fax: +372-7-375-264 V. Vabson AS Metrosert, Division of Standards Services, Riia 185b, 51014 Tartu, Estonia Abstract Organization, benefits, and possible drawbacks of in situ interlaboratory comparison are discussed using the example of dissolved oxygen concentration and pH measurements organized at the University of Tartu. In situ interlaboratory comparisons are intercomparison measurements, where all the participants (with their technical equipment and using their own competence) are measuring the same sample continuously at the same time, at the same site. In the field of proficiency testing–especially in chemical measurements–the in situ schemes are not yet widespread. Their main advantage emerges in the fields where, due to non-stability of samples, traditional comparison schemes can fail. Direct exchange of experience between the participants and presentations during measurements may add the value to in situ interlaboratory comparison. Dissolved oxygen concentration and pH measurements are among the most widespread chemical measurements. Both measurements are complex and less robust than often considered. In this paper, we describe in situ interlaboratory comparisons of dissolved oxygen concentration and pH organized at University of Tartu Testing Centre. Keywords In situ interlaboratory comparison . Proficiency testing . Dissolved oxygen . pH Introduction Dissolved oxygen (DO) concentration and pH measure- ments are central in monitoring environment, aquaculture, industrial sector, wastewater treatment, etc. Neither of these measurements is methodologically easy [1, 2]: both param- eters are highly unstable. Thus, preparation of reference so- lutions sufficiently stable for an extended period is difficult (with pH) or impossible (with DO). This complicates both the standardization of the measurements and organization of interlaboratory comparisons (ILC) in which the orga- nizer sends a set of samples to the participating laboratories (the usual arrangement in chemistry). Instead, the ILC has to be carried out in situ, at the same time on the same site. 1 1 We thank the referee for pointing out that if the term “laboratory” is interpreted in a narrower sense as operating only in specific premises, in situ proficiency testing (PT) is not yet widespread in chemistry and its organization has certain specific aspects. In this paper, we describe how this type of PT can be applied to DO concentration and pH measurement. in situ ILC of DO concentration and pH The in situ ILC of DO concentration and pH measurement took place on February 3, 2005, at the University of Tartu then the term “in situ interlaboratory comparison” is misleading, be- cause measurements are not done on the laboratory premises. We interpret the term “laboratory” as a legal entity. The personnel of such a laboratory can make measurements also on site, if necessary. This is very common for environmental and working environment laboratories. With this interpretation, in situ interlaboratory compar- ison is a perfectly adequate term.