CHARACTERIZATION OF ASPHALTENES IN SEC/MS AND SEC/FTIR COUPLING

Characterization of asphaltenes in SEC/MS and SEC/FTIR coupling

Bernard Desmazières¹, Isabelle Merdrignac², Peran Terrier¹, Arnaud Delobel³ and Olivier Laprévote³

1. Université d'Évry-Val d'Essonne, Laboratoire Analyse et Environnement, CNRS UMR8587, Bd François Mitterrand, F91025 ÉVRY CEDEX, France

2. Institut Français du Pétrole, Physique et Analyse, BP3, F69310 SOLAIZE, France

3. CNRS, Institut de Chimie des Substances Naturelles, F91198 GIF SUR YVETTE CEDEX, France

Estimations of the molecular weights of asphaltenes in Steric Exclusion Chromatography (SEC) are difficult due to aggregation and secondary effects. A systematic study showed that SEC profiles are a function of the mobile phase, the SEC gel, the sample amount and the temperature.

We performed off-line coupling of SEC with Laser Desorption Ionisation – Time Of Flight mass spectrometry (LDI-TOF) and on-line coupling with Atmospheric Pressure Chemical Ionisation (APCI) or Atmospheric Pressure Photonic Ionisation (APPI) using in both cases a Quadripole-Time Of Flight analyser (Q-TOF).

The three coupling techniques gave similar results, both in positive and negative ions modes:

- average molecular weights of raw asphaltenes vary from 600 to 850 Da and molecular weights are distributed between 300 and 2000 Da;

- hydrotreated samples have average molecular weights of 450 to 600 Da, decreasing with the severity of treatment.

Off-line coupling of SEC and Fourier Transform Infra Red spectroscopy (FT-IR) was performed using deposition on KBr. Complementary information was obtained:

- the intensity of aromatic signals varies significantly as a function of elution volume;

- supplementary bands appear for the compounds eluting after the total permeation volume and are likely characteristic of compounds containing hetero elements and interacting with the SEC gel;

- a significant increase of aromaticity is observed in hydrotreated fractions.

This study will be completed by identification attempts using SEC-MS coupling with a high resolution mass Spectrometer (FT-ICR).