Non-disturbing characterization of natural organic matter (NOM) contained in clay rock pore water by mass spectrometry using electrospray and atmospheric pressure chemical ionization modes Sandrine Huclier-Markai 1 * , Catherine Landesman 1 , He ´le ` ne Rogniaux 2 , Fabrice Monteau 3 , Agnes Vinsot 4 and Bernd Grambow 1 1 SUBATECH, Ecole des Mines de Nantes, Universite ´ de Nantes, CNRS-IN2P3, 4 rue A. Kastler, BP 20722, F-44307Nantes Cedex 03, France 2 UR1268 Biopolyme `res Interactions Assemblages, Plate-Forme BIBS, INRA, Rue de la Geraudiere BP 71627, F-44316 Nantes Cedex 3, France 3 Laboratoire LABERCA, Ecole Nationale Ve ´te ´rinaire Nantes, Atlanpo ˆle La Chantrerie, BP 50707, F-44307 Nantes Cedex 3, France 4 ANDRA, Laboratoire de recherche souterrain de Meuse/Haute Marne, RD 960, F-55290 Bure, France Received 21 July 2009; Revised 2 November 2009; Accepted 9 November 2009 We have investigated the composition of the mobile natural organic matter (NOM) present in Callovo-Oxfodian pore water using electrospray ionization mass spectrometry (ESI-MS), atmos- pheric pressure chemical ionization mass spectrometry (APCI-MS) and emission-excitation matrix (EEM) spectroscopy. The generation of knowledge of the composition, structure and size of mobile NOM is necessary if one wants to understand the interactions of these compounds with heavy metals/ radionuclides, in the context of environmental studies, and particularly how the mobility of these trace elements is affected by mobile NOM. The proposed methodology is very sensitive in unambiguously identifying the in situ composition of dissolved NOM in water even at very low NOM concentration, due to innovative non-disturbing water sampling and ionization (ESI/APCI- MS) techniques. It was possible to analyze a quite exhaustive inventory of the small organic compounds of clay pore water without proceeding to any chemical treatment at naturally occurring concentration levels. The structural features observed were mainly acidic compounds and fatty acids as well as aldehydes and amino acids. Copyright # 2009 John Wiley & Sons, Ltd. In the eastern part of the Paris Basin (France), the Callovo- Oxfordian formation (COx) is a rock composed of carbonate (23–42%, w/w), clay minerals (25–55%, w/w), quartz and feldspars (20–31%, w/w), with minor amount of pyrite (<3%, w/w). 1 This rock may also contain up to 1% w/w of organic carbon. Most of the organic matter (OM) is attached to the mineral particles although a small portion is present as dissolved organic matter (DOM) in the pore water. 2,3 In environmental studies, natural organic matter (NOM) plays a key role in the bioavailability and the toxicity of metallic compounds. It is necessary to know the structure of any organic substance present in order to assess which chemical and biological reactions occur under environmentally relevant conditions. 4 The 150 Myears solid-bound organic matter of the COx (kerogen) has already been investigated in several studies 5,6 and it has been found to originate from a mixture of marine and terrestrial sources. In addition, the CCl 4 -soluble organic fraction (bitumen) has been characterized by liquid (LC/MS) and gas chromatography (GC/MS) coupled to mass spectrometry. This has allowed the proportion and distri- bution of biological markers to be determined as polar compounds, with aromatic and saturated hydrocarbons. 7,8 DOM was first studied by Elie et al. 9 under oxic conditions and then by Courdouan et al. 10 under anoxic conditions. The DOM was extracted from a crushed clay rock of the COx formation with a high rock/water ratio of about 1500 g/L. Part of the OM from the COx is known to be sensitive to air oxidation which can significantly modify the nature of the bitumen by an overall shift towards lower molecular weight compounds. 7 Therefore, the characteristics of the DOM must be determined in in-situ-like conditions if one wants to assess the mobility of DOM in the clay pore space and to evaluate the mobility of heavy metals/radionuclides. The migration of heavy metals/ radionuclides in natura is in most cases dependent on their interactions with NOM. This has largely been studied for soils and in particular with humic substances. 11–14 Due to their high binding capacity with metal ions and their colloidal sizes in natural waters, 15 these macromolecules, through complexation reactions, might either enhance the mobility of trace elements, for instance of radionuclides, or reduce their migration rates by sorption processes in relation to their size and that of the porous medium. 16–19 Consequently, the characterization of DOM in anoxic (about 170 mV vs. standard hydrogen electrode (SHE)) pore water samples from the COx formation is of great importance. In this context, establishing accurate sequencing of structural units for the DOM will be attempted. RAPID COMMUNICATIONS IN MASS SPECTROMETRY Rapid Commun. Mass Spectrom. 2010; 24: 191–202 Published online in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/rcm.4374 *Correspondence to: S. Huclier-Markai, SUBATECH, Ecole des Mines de Nantes, Universite ´ de Nantes, CNRS-IN2P3, 4 rue A. Kastler, BP 20722, F-44307 Nantes Cedex 03, France. E-mail: huclier@subatech.in2p3.fr Copyright # 2009 John Wiley & Sons, Ltd.