2.1. Precipitation of Asphaltenes from Crude Oil

The precipitation and filtration of asphaltenes is generally carried out by using small molecules, n-alkanes. The procedure is performed with a variety of standard methods, as shown in Table 1. Each provides different results ^[19][20][19,20].

2.2. Analytical Techniques for Asphaltene Properties

Asphaltenes, as the most mysterious fraction in petroleum, have had their properties studied for decades. The bulk properties of asphaltenes, such as element content, density, and thermogravimetric analysis, are nowadays well-known and can be readily determined via correlative analytical instruments ^[21][22][21,22]. A more comprehensive understanding of asphaltenes requires the utilization of specialized analysis methods. For example, Nuclear Magnetic Resonance (NMR) ^[23][24] [23,24] and X-ray diffraction (XRD) [25] can be used to determine the average molecular parameters of asphaltenes. Vapor pressure osmometry (VPO), size exclusion chromatography (SEC), and mass spectrometry (MS) techniques can be used to measure the molecular weight distribution of asphaltenes ^{[26][27][28][29]}[26,27,28,29].

In recent years, with its continuous development, MS technology and especially ultra-high-resolution (UHR) mass spectrometers are an indispensable tool for analyzing the chemical properties and molecular composition of asphaltenes ^[30][31][32][30,31,32]. Electrospray ionization (ESI) equipped Fourier transform ion cyclotron resonance–mass spectrometry (FT-ICR–MS) has long been used to characterize the molecular composition of asphaltenes, as early as a decade ago. ESI preferentially ionizes heteroatom-containing compounds such as polar N-, S-, O-, and metal-containing species. ESI FT-ICR–MS has been widely used to analyze the molecular composition of heteroatomic compounds in asphaltenes and their corresponding saturates, aromatics, and resins ^[33][34][33,34]. In addition, it is widely used in the research of petroleum porphyrin and geochemical porphyrin due to the good response of metalloporphyrins in ESI ^[35][36][35,36]. Compared to ESI, atmospheric-pressure photoionization (APPI) ionizes a wider range of compounds, including nonpolar compounds [37]. The APPI ionization source may obtain asphaltene-composition information not seen by ESI, such as polycyclic aromatic hydrocarbons (PAHs) [38]. Thus, APPI is widely used to characterize the molecular composition of petroleum asphaltenes and other components ^[39][40][39,40]. Although the resolution of Orbitrap is not as high as that of FT-ICR, its resolution ability cannot be ignored, and its stronger isolation and collision capabilities have unique advantages in studying the molecular composition and structural information of asphaltenes [41]. In addition, other ionization techniques (e.g., APCI, APLI, LDI, MALDI, and ASAP) equipped with ultra-high-resolution mass spectrometers are also applied to characterize asphaltenes ^{[42][43][44][45][46]}[42,43,44,45,46].

Ultra-high-resolution mass spectrometers equipped with multiple ionization sources have an absolute advantage in the qualitative research of asphaltenes’ molecular composition, but their quantitative results are affected by differences in ionization response and other substrates. Due to the limitations of molecular MS for the direct and quantitative identification of asphaltene compounds, inductively coupled plasma–high-resolution mass spectrometry (ICP–HR-MS) is an efficient tool for studying the size distribution of vanadium-, nickel-, and sulfur-containing compounds in asphaltenes. When ICP–MS is used to study asphaltenes, liquid chromatography (LC) is often used for online separation, and gel-permeation chromatography (GPC) is the most commonly used separation principle. The asphaltene solution (usually dissolved in tetrahydrofuran) is separated into high- (HMW), medium- (MMW), and low-molecular-weight (LMW) fractions after passing through GPC; then, it is analyzed online by mass spectrometry and simultaneously detected by UV or other detectors [47]. Despite criticisms of GPC, coupling GPC with ICP–MS allows for the identification and quantification of relative sizes associated with various V, Ni, and S compounds in asphaltenes [48]. Reinjected experiments revealed that the dissociation of asphaltene aggregates and reaggregation of LMW fractions occur after the isolation, as shown in Figure 2 [47].

Figure 2. Chromatograms obtained by gel-permeation chromatography (GPC) inductively coupled plasma (ICP) MS of collected and reinjected (a) high-molecular-weight (HMW), (b) medium-molecular-weight (MMW), and (c) low-molecular-weight (LMW) fractions [47].

Recently, high-resolution scanning probe microscopy has emerged as effective method for elucidating molecular structures, offering the unique capability of imaging a single adsorbate on the atomic scale. Attempts were made to use scanning tunneling microscopy (STM) to characterize asphaltenes, but no atomic resolution could be achieved on asphaltene molecules [49]. The latest advances in atomic force microscopy (AFM) enabled the direct observation of individual asphaltene molecules. Schuler et al. ^[50] [50] studied more than 100 asphaltene molecules using AFM and STM, and provided the direct measurement of the tremendous range of molecular structures in asphaltenes. Images of asphaltene molecules derived from coal and petroleum are shown in Figure 3. This work is groundbreaking in the history of asphaltenes, allowing for us to have a direct visual understanding of the structure of some molecules in asphaltenes. This technique is also used to identify colloidal particles associated with asphaltene aggregates present in crude oils and the model system [51]. AFM even allows for direct observations on the specific structure of some biomarkers, such as the characterization of substitution patterns on petroporphyrins [52]. This technology constitutes a paradigm shift in the analysis of complex molecular mixtures, and may be applied to molecular electronics, organic light-emitting diodes, and photovoltaic devices [50].

Figure 3. Atomic force microscopy (AFM) images of coal-derived asphaltenes and petroleum asphaltenes [50].

2.3. Analytical Techniques of Metal Content in Asphaltene

Petroleum is a complex mixture containing not only carbon, hydrogen, sulfur, oxygen, and nitrogen, but also many trace elements, including 45 metals, such as V, Ni, and Fe [53]. It is well-known that a fair amount of metal in crude oil exists in the asphaltene fraction. These metals are detrimental to petroleum processing, especially for accelerating catalyst deactivation [54]. Metal content is one of the important parameters for evaluating crude oil. Modern instrumental approaches to determine trace metals and asphaltenes in crude oil is by means of physical methods. Analytical methods were developed for the determination of metals in crude oils and derivatives using various analytical techniques, including flame atomic absorption spectrometry (FAAS) [55], graphite furnace atomic absorption spectrometry (GFAAS) [56], inductively coupled plasma optical-emission spectroscopy (ICP–OES) [57], inductively coupled plasma–mass spectrometry (ICP–MS) [58], X-ray fluorescence spectroscopy [59], spectrophotometry [60], and high-performance liquid chromatography (HPLC) [61]. To date, concentrations of these metals in petroleum have mostly been determined by ICP–OES and ICP–MS because of their distinguished sensitivity, repeatability, and operability [62]. A large amount of data indicates that metal content in crude oil is between 1 and 10,000 μg/g, and concentrations of nickel and vanadium can reach up to several thousand ppm (w/w) [63]. The concentration of these metals in the asphaltene fraction is the most abundant. The concentration range of V, Ni, and Fe in asphaltenes from different studies is shown in Table 2.