Laser desorption/ionization techniques in the characterization of high molecular weight oil fractions. Part 1: asphaltenes

Abstract

The molecular weight distribution of the asphaltene fractions of two types of crude oils from two different Italian fields (samples 1 and 2) was investigated. The analytical tools used to perform these analyses were matrix assisted laser desorption ionization (MALDI) and laser desorption ionization (LDI) mass spectrometry. After observing that the use of the matrix (as well as the addition of Ag+) did not improve the quality of the data compared to that obtained in LDI conditions, all further measurements were performed with the latter technique. Operating under usual conditions of laser power and delay time, a very low resolution was observed, showing only macroscopic differences between the two samples in the molecular weight distribution of the different components. An accurate study on the possible reasons of this undesirable behavior indicates that it can originate from space charge phenomena occurring either in the ion source region or during the flight. A valid parameterization of the delay time and the laser power allowed higher quality spectra to be obtained. Surface-enhanced laser desorption ionization (SELDI) measurements were also performed using normal phase (silica) as the sample holder surface. Under these conditions, better results are obtained, proving that the sample-surface interaction is important to achieve, by means of laser irradiation, a homogeneous set of product ions. Both asphaltene samples were fractionated in five subfractions by gel-permeation chromatography (GPC) to obtain a better separation of the molecular weight distributions; the related spectra confirmed these findings. By using different approaches, relevant and reproducible differences between the asphaltene fractions of the two oil samples were observed.

Mass Spectrometric Advances in the Analysis of Large Aromatic Fractions of Heavy Fuel Oils and Carbon Particulates

“…Using APPI ionization, the fragmentation is largely reduced and, therefore, the masses detected should be more representative of the true MW of the sample. The mass range found, between m=z 250-1100 with a maximum at m=z 500-600, is in very good agreement with previous works (Hortal et al, 2007;Merdrignac, 2007;Rizzi et al, 2006), whereas the eventual presence of higher masses (>100,000 u), as found by other authors (Islas et al, 2003;Karaka et al, 2004), cannot be investigated because the mass limit of the ion trap mass spectrometer is about 3000-4000 u for single-ionized analytes.…”

“…It can be assumed that the low resolution is due to the simultaneous desorption and ionization of many species with very small mass differences that cannot be resolved by the time of flight (TOF) analyzer. The complexity of the spectrum can be also due to the occurrence of in-source fragmentations originating either from photon irradiation or thermal effects (Rizzi et al, 2006). Some authors suggested the necessity of using a matrix to generate ions from the larger mass materials of asphaltenes (Herod et al, 2008).…”

“…The asphaltene APPI-MS spectrum, reported in Figure 4b, presents a broad structure extending in the range m=z 250-1100 (the peaks around m=z 200 are due to toluene) that shows peaks at every nominal mass with periodicities of 2 and 14 units, indicating the presence of homologues with different unsaturation degrees (Rizzi et al, 2006). It can be noted that the APPI-MS spectrum of asphaltenes is shifted towards higher MWs with respect both to the PB-MS (Figure 2b) and the LDI-MS spectra (Figure 3b).…”

Contribution of mass spectrometry in assessing quality of petroleum fractions

“…The role of the matrix is to absorb UV laser radiation and to energize the analyte (often not in the absorbing UV region), in order to ionize it more gradually (Pash et al, 2003;Watson and Sparkman, 2007). However, many polycondensed systems, especially those with aromatic moieties and, therefore, strong UV absorption, have the so-called "self-matrix" property (Apicella et al, 2007;Rizzi et al, 2006), which indicates the capability of the sample (or a part of it) to act as a matrix by itself. In this case, direct photoionization takes place by laser irradiation without the necessity of an external matrix addition (Apicella et al, 2010).…”

Laser desorption ionization FT-ICR mass spectrometry and CARSPLS for predicting basic nitrogen and aromatics contents in crude oils

“…MALDI became an established mass spectrometry method for macromolecular compounds. LDI/MALDI coupled to time of flight mass spectrometry (TOF) has been applied in petroleomics for the characterization of petroporphyrins [12], crude oils [13], and their fractions [14,15], in addition to the determination of the molecular-weight distributions (MWDs) of asphaltenes [16][17][18][19][20][21].…”