Identification par couplage CG/SM des sous-produits de chloration de deux acides aminés libres, la proline et la méthionine Identification of chlorination by-products of two free amino acids, proline and methionine, using GC/MS

Résumé

L'objectif de ces travaux a consiste en l'identification des sous-produits de chloration de deux acides aminés libres, la proline et la méthionine, structures reconnues pour leur grande réactivité avec le chlore.

Les expériences ont été conduites a pH 8 pour un taux de chlore fixe à 8 moles de chlore par mole d'acide aminé et un temps de contact de 72 heures. Les sous-produits de chloration ont été extraits successivement par le pentane et le diethyl éther (pH acide et pH basique, extraction suivie d'une dérivation au diazométhane) et identifiés par couplage CG/SM.

L'essentiel des sous-produits de chloration identifiés a été observé dans l'extrait éthéré obtenu à pH acide, que ce soit pour la proline ou la méthionine. Les acides dichloroacétique et trichloroacétique, composés retrouvés dans les eaux de surface désinfectées au chlore, ont été détectés pour les deux molécules étudiées.

Les travaux effectués avec la proline ont permis d'identifier également la N- chlorodichloroacétamide et la N-chlorotrichloroacétamide. On peut noter également la formation de quelques chloroacides présentant un groupement terminal aldehyde ou nitrile caractéristique, ainsi que des composeé à structure pyrrole.

En ce qui concerne la méthionine, les analyses par couplage CG/SM ont permis d'identifier quelques composés organiques chlorosoufrés comme le chlorure de méthyle sulfonyle, le chlorure de chlorométhylesulfonyle et le dichloro-1,1 diméthyle sulfonyle, ainsi que du soufre moléculaire S8.

Mots-clés:

• Proline,
• méthionine,
• chloration,
• couplage CG/SM,
• sous-produits chlorés et chlorosoufrés

Abstract

The objective of our work was to identify the chlorination byproducts of two amino acids, proline and methionine, structures that were found to be very reactive with chlorine, showing high chlorine demand and high total organohalogenated compounds (TOX) and chloroform formation potentials.

Concentrated solutions of each amino acid were prepared in MilliQ water buffered with phosphate at pH 8 and chlorinated at a dose equal to 8 moles of chlorine per mole of amino acid. Chlorinated solutions were stored at 20°C (in the dark) for 72 hours before applying our extraction procedure. Two different solvents were used, pentane first, followed by diethyl ether under acidic and basic pH.

After concentration (rotary evaporation, nitrogen flow) and derivatization (a few drops of diazomethane were added to the diethyl ether extracts prior to analysis), the different extracts were analyzed by gas chromatography / mass spectrometry.

Most of the byproducts that were identified were analyzed from the diethyl ether extracts obtained at acidic pH. A large number of structures were detected in the extracts of the chlorinated proline solution, and only a few in the extracts from the methionine solution. Most of the identified compounds are chlorinated structures including a carboxyl function. Trichloroacetic acid and dichloroacetic acid were identified as common chlorination byproducts of the two amino acids studied. The two intermediates in the formation of these two acids, CCl3CONH2 and CHCl2CONH2, were also found.

N-chlorodichloroaldimine and N-chlorotrichloroaldimine have been identified as chlorinated proline by-products. The chlorination of this amino acid also leads to the formation of chloroacids compounds which present an aldehyde or a nitrile function as a terminal group; a pyrrole structure is also proposed. The chlorination of methionine has been shown to produce several chlorosulfur compounds. The structures identified in the chlorinated methionine solution were H2CClSO2Cl, HCCl2SO2Cl and HCCl2CONH2. Molecular sulfur (S8) was also obtained.

Keywords:

• Proline,
• methionine,
• chlorination,
• GC/MS analysis,
• chlorinated and chlorosulfur byproducts