Article pubs.acs.org/EF
Direct Insertion Probe−Mass Spectrometry (DIP−MS) Maps and Multivariate Analysis in the Characterization of Crude Oils C. Flego and C. Zannoni* Refining and Marketing Division, ENI, Via Maritano 26, I-20097 San Donato Milanese, Milan, Italy ABSTRACT: Crude oils are complex organic mixtures, with their composition changing not only with the geographical region but also with the oil field. Chemical fingerprinting of crude oils and the knowledge of dependence of their macrophysical properties from the major chemical characteristics are fundamental to both upstream and downstream operations. Direct insertion probe−mass spectrometry (DIP−MS), on the basis of the introduction of samples without previous manipulation directly into the ionization chamber, their vaporization, and eventual ionization by electronic impact, appears as a proper tool to map crude oil characteristics. The components of the oils are separated according to their boiling points up to masses of m/z 950. A set of crude oils of different origin and bulk physical properties were characterized by DIP−MS in a fast and easy way. The data represented in contour plots, with molecular weight and volatility (i.e., vaporization temperature) on the axes, gave immediately an idea of the nature of the oil in terms of its physical properties and allowed for its fingerprinting. Principal component analysis (PCA) and hierarchical cluster analysis (HCA) were applied to analyze contour plots, showing similarities and differences among the crude oils, confirmed by a comparison to bulk physical properties, such as American Petroleum Institute (API) gravity and viscosity.
1. INTRODUCTION Chemical fingerprinting of crude oils and the knowledge of dependence of their macrophysical properties from the major chemical characteristics are fundamental to both upstream (e.g., reservoir evaluation, migration and maturity, and degradation processes) and downstream (processing, tightening of refinery specifications, environmental impact, etc.) operations.1,2 Crude oils are complex organic mixtures, with their composition widely changing, even considering oils extracted in the same geographical region. Concerning upstream operations, because of the considerable difficulties in analyzing the live oils in reservoirs, valuable information can be commonly obtained by analyzing stabilized (or dead) crude oils. The physical properties of these stabilized crude oils [American Petroleum Institute (API) gravity, viscosity, etc.] represent the fingerprint of the mixture of thousands of different compounds, which characterizes each single crude oil and determines the reservoir potentiality. As far as downstream operations are concerned, the need for a more efficient exploitation of heavier or poorly known feedstocks led to an increased interest in the comprehension of the molecular nature of oil components, to forecast their behavior in thermal and catalytic refining processes. Strong improvements have been made in the last few years by application of Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR−MS) to molecular characterization of complex oil mixtures,3 leading to identification of more than 110 000 components, but its routine use for crude oil characterization on the field is far from being applicable. Multivariate analysis [principal component analysis (PCA) and hierarchical cluster analysis (HCA)] of conventional physicochemical properties has been proven to be a powerful technique to identify the origin of hydrocarbon mixtures from refinery streams, although it requires a large number of experimental data and analytical techniques.4−10 Statistical evaluation (chemometric and cluster analyses) of mass spectra [laser desorption ionization− (LDI−) or electrospray © 2012 American Chemical Society
ionization−mass spectrometry (ESI−MS)] has also been taken into account for extrapolating useful criteria of crude oil classification according to their main chemical characteristics.11−16 Among the analytical techniques used to study the complexity of crude oils, the application of a direct insertion probe−mass spectrometry (DIP−MS) technique has never been considered. It is based on the direct introduction of the sample into the ionization chamber of the mass spectrometer, followed by vaporization under controlled heating and eventual ionization by electronic impact. This technique, when compared to other analytical methods, shows the advantage of the lack of very complex and time-consuming pretreatment or separation procedures. In a previous work,17 DIP−MS was employed as a fast alternative method of analysis of asphaltenes and high-boiling and insoluble petroleum fractions not analyzable by conventional gas chromatographic methods. The high-boiling mixture evolves in the ionization chamber on the basis of the volatility of the components and the probe temperature, allowing for a rough separation of the chemical species. Families of chemical components at an increasing number of aromatic rings were identified from both intensity and distribution of the masses of selected mass peaks and allowed for differentiation of asphaltene samples. The purpose of DIP−MS analysis of crude oils is the achievement of a fast and direct evaluation of the organic matter through similarities and differences among samples, without drawing a comprehensive map of all of their chemical components. The DIP− MS results depicted as contour plots have been evaluated by multivariate analyses to reinforce the main findings evinced by this technique and compared to API gravity and viscosity data. Received: July 5, 2012 Revised: November 13, 2012 Published: November 13, 2012 46
dx.doi.org/10.1021/ef301124s | Energy Fuels 2013, 27, 46−55
Energy & Fuels
Article
Table 1. List of Crude Oils under Study and Some of Their Physical Properties: SARA Distribution (% wt), API Gravity, and Kinematic Viscositya sample
high boiling fraction (%)
saturates (%)
aromatics (%)
resins (%)
asphaltenes (%)
API gravity (deg)
kinematic viscosity (mm2/s)
type
G1 G2 G3 G4 G5 G6 G7 G8 G9 G10 G11 G12 G13 G14 G15 G16 G18 G19 G20 G21 G22 G23 G26 G28 G29 G30 G31 G32
36.2 30.8 32.3 11.4 21.9 40.2 13.1 41.5 52.8 70.8 41.4 61.3 43.9 59.4 56.4 39.7 37.3 38.1 39.5 45.8 36.4 52.4 43.3 58.8 54.4 75.2 57.1 80.9
65.2 55.1 57.8 12.3 61.2 32.3 46.3 53.5 63.8 11.6 17.8 25.1 27.1 9.6 7.1 15.6 47.9 43.4 50.4 31.3 62.2 57.9 46.4 28.1 49.1
32.6 38.8 38.0 78.2 34.7 41.8 49.4 35.8 7.8 48.3 68.9 41.0 50.2 30.5 22.3 45.7 29.2 35.8 31.6 44.6 27.3 35.1 30.5 50.3 31.7
2.7 3.6 2.9 8.1 2.8 20.5 4.8 9.3 4.4 23.2 9.8 16.3 15.8 40.1 38.0 19.3 18.0 14.5 15.8 18.9 7.3 4.9 19.4 16.8 17.6
1.9 2.8 1.8 2.9 1.7 5.8 0.0 1.4 24.1 17.0 3.6 17.8 7.0 20 32.0 19.4 6.0 6.8 3.2 5.5 3.7 2.4 4.4 5.0 2.3
2.418 3.824 2.978 0.704 3.203 9.154 1.635 5.757 74.54 425.5 4.809 117.2 8.812 7753 5859 230.8 3.810 7.100 2.940 9.310 2.990 3.540 7.960 22.49 50.46
15.2
33.4
35.1
16.3
44.6 44.45 43.11 50.21 41.53 33.94 45.38 38.30 35.88 19.34 33.6 24.7 31.35 12.70 13.3 21.1 40.7 35.7 42.9 33.7 44.1 43.8 32.2 29.4 26.5 5.0 10.9 8.5
L L L L L M L M M H M H M H H H L M L M L L M H H B B B
a
Crude oils are grouped as light (L), medium (M), heavy (H), and bitumens (B). detection, with a procedure similar to the fractional distillation, favored by the combined effect of vacuum and heat. The heating rate was selected to avoid a too rapid vaporization of the sample and the saturation of the signal. Eventually these species are ionized by electronic impact and monitored. A turbomolecular pump system coupled to the mass spectrometer creates a dynamic vacuum (7 × 10−5 mbar) in the ionization chamber and near the source zone. The very short distance between the ionization source and the vaporized molecules and the small amount of molecules undergoing the collision with the electron flux prevent molecule recombination and thermolitic degradation. A detailed description of the apparatus is reported in ref 17. It should be stressed that the vacuum dramatically decreases the boiling temperature of the compounds, allowing for vaporization of high-boiling-point species even when moderate temperatures are applied; e.g., the boiling point of benzocoronene is 566 °C at atmospheric pressure and decreases to 303 °C at 10 mbar and 48 °C at 7 × 10−5 mbar. For this reason, no direct relation between the vaporization temperature in the DIP apparatus and in the conventional simulated distillation measure at atmospheric pressure is immediately available. 2.3. Multivariate Analysis. DIP−MS contour plots were normalized to the total amount of vaporized sample and transformed into binary data suitable for further multivariate analysis. Each contour plot was parted into nine regions, i.e., three ranges of m/z values (m/z 33−200, 200−500, and 500−950) and three ranges of vaporization temperature (30−120, 120−340, and isotherm at 340 °C). The intensity of the signals was binned with the colors in the map into six levels of increasing value from 0 to 240, which are the numerical translation of the native MS counts (e.g., 0−40 means an intensity lower than
