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In order to study a pit from the thirteenth century in the archeological site of Louvre (Paris) some biogeochemical markers such as n-alkanes, fatty acids and sterols have been used. A number of criteria allows us to estimate the impact of bacterial biodégradation of the ofganic matter (Carbon Preference Index). Informations about utilization, functioning and evolution of this pit have been obtainde by organic markers studies.

Ce travail a consisté à analyser certains marqueurs biogéochimiques pour l'étude de paléoenvironnements d'intérêt archéologique. Le choix des molécules étudiées par chromatographie en phase gazeuse (CPG) et en couplage chromatographie- spectrométrie de masse (CPG/SM) s'est porté sur des constituants lipidiques tels que les alcanes, les acides gras (saturés, insaturés, ramifiés) et les stérols en raison de leur stabilité chimique et de leur ubiquité dans le règne végétal et animal. Certains critères nous ont permis de préciser l'impact de la dégradation bactérienne sur la matière organique étudiée (Carbon Preference Index, acides ramifiés iso et ante iso). En caractérisant l'origine des apports en matière organique, il devient alors possible d'opérer des distinctions dans les fonctions des structures archéologiques mises à jour. Il s'avère que l'utilisation de ces traceurs organiques permet d'apporter une contribution efficace à la reconstitution des de vie en milieu urbain depuis l'époque gallo-romaine.

Letter: Determination of double bond position in di-unsaturated alkadienes by means of mass spectrometry of dimethyl disulfide derivativesDear Sir The determination of the position of a double bond in unsaturated compounds using mass spectrometric methods after derivatization by dimethyl disulfide (DMDS) has been widely studied in olefins, fatty acids and wax esters.1-6 The use of DMDS addition to locate the double bonds for di-unsaturated compounds is uncommon. This method has been used with acetates, natural long chain alkadienes, 7,8 but has never been tried in a systematic way with short chain alkadienes. In this paper, compounds of the type CH 3 -CH=CH-(CH 2 ) n -CH=CH-CH 3 (n varies from zero to four and greater than four) have been studied. The structure of the DMDS adducts has been discussed with respect to the localization of the double bonds, but no consideration of stereochemistry has been given.Electron impact (EI) mass spectra were obtained on an Ion Trap Varian Saturn mass spectrometer coupled to a Varian 3400 gas chromatograph. Chemical ionization (CI) mass spectra were obtained by using ammonia as reagent gas. The samples were injected in the SPI mode. The column employed was a fused silica capillary (30 m long, 0.25 mm i.d.) coated with DB5 phase (Chrompack). The temperature increased at 2°C min -1 from 40°C to 250°C. The di-unsaturated compounds (2-6 octadiene and 2-4 octadiene) were synthesised in our laboratory by classical methods. Hexane and dimethyl disulfide were distilled. Iodine and Na 2 S 2 O 3 were used as received. Alkadiene (50 µg) was treated in 100 µl of hexane by addition of 100 µl of DMDS and 20 µl of iodine solution (60 mg of iodine in 1 ml of diethyl ether). The reaction was carried out in a closed tube for 48 h at 50°C. The excess of iodine was reduced with Na 2 S 2 O 3 solution (5% in water). The organic phase was removed and the excess of DMDS was evaporated. The dry extract was diluted with 50 µl of hexane and then analysed by CG/MS.Derivatives were formed by the addition of DMDS (Scheme 1).Depending on the values of n, a mono or a di-addition is observed and the pathway of fragmentation is different depending on the mode of addition. n = 0 (2-4 octadiene) A molecular ion (M +• = 204) is observed, confirmed by the spectrum in CI mode (Figure 1). Only one molecule of DMDS is added in 1-4 position (classical addition for conjugated diene). The allyl position of the sulfur atom only allows us to obtain the usual loss of CH 3 S or CH 3 SH from the molecular ion (m/z 157, 156, 109). It is impossible to locate the double bonds, but the spectrum is completely different from a monoalkene spectrum.The mass spectra of the DMDS derivative (Figure 2) shows a mixture, unseparated by GC, of the two mono-derivatized compounds M +• = 176 proved by CI spectra. On the contrary DMDS adducts of di-unsaturated acetates and alcohols give a di-addition. 7In each spectrum the characteristic cleavage of the bonds between the two carbon atoms linked to the sulfur atom is observed, ions at m/z 61 an...

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The determination of the positions of double bonds after dimethyl disulfide (DMDS) addition is presented for linear or branched di-unsaturated esters and a di-substituted y lactone. Different DMDS derivatives are studied according to the number, n, of methylene groups separating the double bonds. For n = 0, a mono derivative is obtained following treatment with DMDS; for n = 1 , 2 or 3, a cyclic thioether is synthesized. For R = 0, only one double bond is located after a classical cleavage. For n = 1, 2 and 3, a systematic fragmentation pathway is observed allowing the determination of the positions of the double bonds and the molecular structures of the compounds.Alkenes and unsaturated fatty acids are very important molecules in biogeochemistry. The positions of the double bonds are often dependent on their origin. The electron impact mass spectra of these underivatized compounds give no indication about these double-bond positions.Dimethyl disulfide (DMDS) adducts of monounsaturated compounds obtained by addition of DMDS in the presence of iodine (Scheme l), have previously been investigated. This method allows the determination of the double bond position in olefins, fatty acids and esters.'-6 The use of dimethyl disulfide to locate the double bonds in di-unsaturated compounds is effective with acetate^,^ natural long-chain alkadienes' and short-chain alkadienes.'Author for correspondenceThe reaction of DMDS with di-unsaturated compounds differs according to the number of methylene groups ( n ) which separates the double bonds. Thus, (i) for n = 1, two mono-derivatized compounds are formed by addition of DMDS; (ii) for n=2, a five-membered cyclic thioether is synthesized; and (iii) for n = 3, two compounds are obtained: a six-membered cyclic thioether and a monoderivatized compound.' From n = 4, four methylthio groups are fixed and classical cleavages are ~bserved.~.'

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The rate of deuterium exchange of 4 and 6 methyl protons in 4,5,6-trimethyl-2-pyrimidone has been followed in D2O over a range of acidity from pD 0.4 to 7.7 by means of proton magnetic resonance spectroscopy. The maximum of exchange rate represents equimolar proportions of the pyrimidone and its conjugate acid. The maximum is due to the neutral molecule acting as a base and removing a proton from one of the methyl groups of its conjugate acid. The exchange study of methyl protons in 1,4,5,6-tetramethyl-2-pyrimidone corroborates this mechanism.