Abstract A comparison of manual procedures for mea- surements of dissolved gaseous mercury (DGM) in sea- water was accomplished. The experiments were performed on board the Italian research vessel Urania during July 2000 as a subtask in the CNR “Med-Oceanor Project 2000”. Water samples for DGM were collected by Go-Flo bottles and subsequently analysed for DGM on board the ship. Determinations of DGM were made in parallel by two groups using different analytical routines. The two sets of data obtained compare favourably. Based on the field- work and an additional laboratory study, analytical proce- dures are discussed and an optimised method to determine DGM is presented. In addition, a method for automated in situ measurements of DGM positioned in the water body was tested. This method has the potential to simplify studies of DGM dynamics, that is variation in concentra- tion as a function of water temperature and solar radiation etc. Keywords In situ · Evasion · Phase equilibria · Med-Oceanor Introduction Mercury from cinnabar (HgS) ores has been prospected since Roman times resulting in the spread of semi-volatile mercury compounds in the environment. Mercury is still continuously emitted into the environment primary from coal combustion, incinerators and water discharges. The metal is subsequently transported and transformed into various forms in ambient air and water. The most danger- ous forms of mercury are organomercury compounds, that is species in which a carbon atom is directly bonded to the metal. Among the organomercurials, the methyl mercury ion CH 3 Hg + is probably the most ubiquitous. The com- pound is concentrated in plankton and so enters the fish food chain. Inorganic and organic Hg(II) complexes constitute the important part of mercury in seawater. As a consequence of diverse biotic and abiotic reactions volatile mercury spe- cies (i.e. dissolved gaseous mercury (DGM)) are formed [1, 2, 3, 4, 5]. One reason for measuring DGM (i.e. ele- mental mercury (Hg 0 ) and dimethyl mercury (DMHg)) in seawater confers to their ability to partition to the air and therefore readily cycle between compartments in the envi- ronment. The quantities of DGM in natural waters that can volatilise to the atmosphere are of interest for calcula- tions of mercury evasions from oceans and fresh water systems. In fact, this process may significantly contribute to the atmospheric cycling and environmental turnover of mercury [6]. In general, DGM is determined by purging water sam- ples by an inert gas and the volatile Hg species released are pre-concentrated on gold or another appropriate adsor- bent. In the literature various procedures for mercury measurements in natural waters has been reported and in- tercompared (e.g. [7, 8, 9, 10, 11] and [12], respectively). In these works different approaches in terms of sample volumes, purging times, flow rates of purging gas and storage of water samples etc. are used. Lindberg et al. [12] showed that fresh water samples collected over short peri- ods are not necessarily stable due to volatilisation and/or oxidation. In contrast, Rolfhus and Fitzgerald [13] ob- served no consistent evidence for DGM losses from sea- water samples during two days storage. They concluded that measurements made within 4–6 h are not critical. It is well known that solar radiation may induce photochemi- cal transformations of mercury, both oxidation and reduc- Katarina Gårdfeldt · Milena Horvat · Jonas Sommar · Joze Kotnik · Vesna Fajon · Ingvar Wängberg · Oliver Lindqvist Comparison of procedures for measurements of dissolved gaseous mercury in seawater performed on a Mediterranean cruise Anal Bioanal Chem (2002) 374 : 1002–1008 DOI 10.1007/s00216-002-1592-4 Received: 3 April 2002 / Revised: 9 September 2002 / Accepted: 12 September 2002 / Published online: 29 October 2002 SPECIAL ISSUE PAPER K. Gårdfeldt · J. Sommar · O. Lindqvist Inorganic Chemistry, Department of Chemistry, Göteborg University, 41296 Göteborg, Sweden e-mail: katarina@inoc.chalmers.se M. Horvat · J. Kotnik · V. Fajon Department of Environmental Sciences, Institute Jozef Stefan, Jamova 39, Ljubljana, Slovenia I. Wängberg IVL Swedish Environmental Research Institute, P.O. Box 47086, 40258 Göteborg, Sweden © Springer-Verlag 2002