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This review will discuss the latest developments and trends in integrated separation in the petroleum and chemical industries in the past 2-3 years. Special emphasis will be placed on the modulation of LC×LC and GC×GC, as well as their applications.

1. Introduction 1.1 Benefits of comprehensive analysis In the early 1980s, JC Giddings first wrote about the potential advantages of using multidimensional chromatography, where two-dimensional (2D) separation is controlled by a one-dimensional (1D) displacement process, especially selective and non-selective Replacement process [1]. In order to optimize the 2D separation for maximum peak capacity, the 1D replacement process should be selective and independent of each other. For liquid chromatography and gas chromatography, the simplest way to ensure a selective and independent one-dimensional displacement process is to use orthogonal stationary phases between the first and second dimensions. However, the stationary phase must be selected carefully, because Giddings also pointed out that the components separated in the first dimensional separation must be kept separated by the second dimensional separation [2]. Since the two separations must be independent of each other, the eluate leaving the first dimension column must be transferred to the second dimension column. This is usually done by capturing a small amount of eluent until the end of the first-dimensional separation, and then transferring the "cut" to the second-dimensional column (for example, heart-cutting), or by using a modulator to capture (almost) all eluting the first-dimensional chromatogram Column eluent, and immediately transfer it to the second-dimensional chromatography column (for example, integrated). In order to achieve full separation, the column fixation is orthogonal to each other, and liquid chromatography (LC) and gas chromatography (GC) can use various column sizes and stationary phases. LC and GC columns benefit from smaller inner diameters (ID), smaller particle sizes, thinner membranes, and stable stationary phase supports (for example, fused silica capillaries in GC and LC Fully porous/solid core and porous shell particles). All of these options can help the chromatographer when implementing integrated chromatography. GC and LC instruments have also become more powerful, reliable, and user-friendly, and are constantly evolving in achieving full separation. At the same time, the functionality and user-friendliness of the instrument control and data processing software platform are constantly improving. Commercially available instruments are provided by the main supplies of integrated LC (LC×LC) and integrated GC (GC×GC). Even with the progress made in the past two decades, full separation has not become commonplace in the petroleum and petrochemical industries; one-dimensional separation still dominates (conventional) applications. This is partly because full separation is still quite complicated. New users of LC or GC can quickly master the skills and become proficient in one-dimensional LC or GC separations. However, full separation is not the case. Newbies to this technology need to overcome a large learning curve. Not only need to optimize a large number of parameters during method development, but also data interpretation requires more complex software. Although there are few conventional applications, the number of applications described in the petroleum and petrochemical industries is increasing.  

1.2 Scope of this review This review will discuss the latest developments and trends in the integrated separation of the petroleum and chemical industries in the past 2-3 years. LC×LC and GC×GC will be discussed. The LC separation mode and modulator method will be summarized. In terms of chemical applications, LC×LC has been widely used in polymer/copolymer analysis. Other applications include surfactants, polymer additives, and small molecule analysis in chemical processes. The focus will be on the development and progress of GC×GC in petroleum and petrochemical analysis. In addition, key points to consider will be highlighted; for example, the selection of a column set based on the desired application, the type of modulator installed, and the detectors included in the instrument setup will be discussed. The modulator is the core of the GC×GC instrument, so special attention will be paid to the advancement of modulator technology in recent years, as well as the advantages and disadvantages of flow and thermal modulators. Finally, the author will discuss some emerging trends in the past few years, emphasizing that environmental pollution and petroleum omics are making progress in source identification.  

2. Integrated liquid chromatography 2.1 Background For two-dimensional liquid chromatography, there are indeed a variety of separation schemes, the main two include heart-cutting and comprehensive separation [3]. Both have related modes, such as multiple heart cutting and selective synthesis/high-resolution sampling. These two have received attention in recent years. Multi-heart cutting allows targeted analysis across multiple areas separated in one dimension. Peaks can be stored in a loop or small column for further analysis in 2D [4]. The selective synthesis mode is particularly suitable for detecting impurities under the main peak, or for obtaining additional peak capacity for only a part of the one-dimensional chromatogram [5].  

2.2 Comprehensive separation modes In liquid chromatography, you can choose from a variety of separation modes. The most prominent is reverse phase LC (RPLC). Other modes include hydrophilic interaction liquid chromatography (HILIC), size exclusion chromatography (SEC), ion exchange chromatography (IEX) and liquid chromatography under critical conditions (LCCC). By performing very different or independent separations in two dimensions, a high degree of orthogonality can be achieved. For example, the combination of HILIC×RPLC can achieve very high two-dimensional separation space coverage, as demonstrated by the separation of surfactants [6] or polyols [7]. For alkyl ethoxylates, HILIC is separated by polarity (ethylene oxide distribution), while RPLC is separated by non-polarity (alkyl chain length distribution). For polymer/copolymer analysis, LCCC×SEC also has a high degree of orthogonality, because it separates compounds based on the type of functional group (end group) in LCCC, while SEC separates based on molecular weight distribution [8].

2.3. Modulator A ten-way or eight-way valve with two circuits is very commonly used for modulation. When considering LC×LC, mobile phase compatibility is a key challenge because strong eluent from the first dimension (1D) can produce sample penetration in the second dimension (2D). There are several advanced solutions to alleviate these problems, as shown in Figure 1. One is active modulation (AM) by capturing components on solid phase extraction (SPE) cartridges [9]. The third pump is used to co-feed the weak eluent to facilitate trapping. This is an elegant way to achieve peak focus during modulation. One disadvantage is the uncertainty of recycling during the capture process and the unknown lifetime of the cartridge. Active solvent modulation (ASM) is based on a dedicated valve solution [10]. In ASM, the strong eluent from 1D is diluted by the weak eluent from 2D, which greatly reduces mobile phase mismatch. ASM does not need a third pump, but requires an additional valve switch during the modulation process, which may have higher sealing requirements. During column dilution (ACD), the problem of strong solvents can be alleviated by co-adding weak eluent from an additional pump [11]. Like AM, ACD requires three pumps for LCxLC operation. Other modes include thermal modulation or via vacuum modulation.

2.4 Applications and trends Industrial chemical processes can be complex, as shown by the analysis of process intermediate samples. Zhu et al. An ultra-peak capacity LC×LC method for the separation of aromatic amines has been developed [12]. The complexity of the sample is high because the compounds have heterogeneous and oligomeric structures. For the separation, six pentafluorophenyl (PFP) columns in series were used for the first dimension, and Zorbax PAH columns were used for the second dimension. Even if both dimensions are operated in the reverse mode, good separation orthogonality is obtained. This difference between the two columns used is due to the dipole interaction with the PFP column and the more shape-selective/hydrophobic type separation from the PAH column. Within 20 hours of analysis time, a peak capacity greater than 11,000 was achieved to separate approximately 900 individual peaks (Figure 2). Bio-oils (such as eucalyptus sawdust, waste coffee grounds) produced by biomass pyrolysis have high sample complexity. The main components in such samples include phenols and ketones. These have potential benefits as raw materials in the chemical industry. Lazari et al. Use the LC×LC separation scheme to quantitatively analyze up to 28 components in this type of matrix [13]. The water phase produced by pyrolysis was studied in detail. Reversed-phase separation is used in two dimensions; an X bridge amide column is used in the first dimension, and a Poroshell C18 column is used in the second dimension. Detect by UV absorbance and ESI-MS (ion trap). A calibration curve was prepared using various phenol, aldehyde or hydroxy ketone standards covering two orders of magnitude of concentration. An effective peak capacity of up to 1000 was achieved in approximately 1 hour of analysis time. The recovery rate ranges from 92% to 113%. An interesting study using LC×LC coupled inductively coupled mass spectrometry (ICP-QQQ) evaluated sulfur and vanadium species in petroleum residues [14]. The focus of the research is to meet the new requirements of demetalization and desulfurization processes. Another aspect is to bring additional information about the species present in these matrices. The results show that the offline SEC×RPLC method can provide a higher peak capacity (2600 vs. 1700) in the same analysis time of 150 minutes compared to the online RPLC×SEC. The dilution factors of the two methods are similar (close to 30), but much fewer fractions need to be analyzed (12 vs. 400). Asphaltenes, which constitute the heaviest fraction of crude oil, are analyzed by the developed offline SEC×RPLC method. The resulting two-dimensional contour map shows that co-elution can be removed, thereby obtaining new information about high molecular weight substances containing sulfur and vanadium for the first time. The LC×LC method with evaporative light scattering detection (ELSD) was established to study the composition of a complex mixture of oxidized polar and non-polar lipids [15]. The LC×LC-ELSD method uses size exclusion chromatography (SEC) in 1D to separate various lipids based on size. In two dimensions, normal phase liquid chromatography (NPLC) is used to separate fractions based on their degree of oxidation. The coupling of SEC and NPLC produces a good separation of oxidized triacylglycerol (TAG) from a large excess of non-oxidized TAG. In addition, it allows the separation of non-oxidizing substances that would normally interfere with the detection of various oxidation products (similar polarity). This method helps to clarify how the lipid composition affects the oxidation kinetics in emulsified foods and will help to develop more stable vegetable oil products. Copolymers made from polar and non-polar monomers can also pose considerable separation challenges. Yang et al. Recently, the use of active solvent modulation (ASM) in the SEC×RPLC separation of vinyl acetate/acrylic acid copolymer and vinyl acetate/itaconic acid/acrylic acid terpolymer has been reported [16]. Using ASM avoids sample penetration of more polar components in the second dimension. Therefore, the heterogeneous copolymer composition can be monitored in random copolymers and terpolymer samples produced under various process conditions. This information provides insights for industrial polymer process development and optimization. In temperature gradient interaction chromatography (TGIC), the column temperature is changed to control the retention of polymer components without using a mobile phase gradient. The advantage of TGIC over solvent gradient LC is its improved compatibility with RI and light scattering detectors. Murima et al. Emphasizes the use of the RP-TGIC×SEC separation scheme for comprehensive branching analysis of polydisperse star-shaped polystyrene [17]. Pillock et al. Explore the combination of hydrodynamic chromatography (HDC) and SEC for the separation of hydrophobic polystyrene (PS) and poly(-methyl methacrylate) (PMMA) nanoparticles [18]. The goal is to determine the particle size distribution (PSD) and molecular weight distribution in one analysis. The nanoparticles eluted from the one-dimensional HDC column are dissolved in tetrahydrofuran (THF) in an Agilent Jet Weaver mixer, and then injected into the two-dimensional SEC column. A fixed phase assisted modulation (SPAM) with a C18 silica trap filter element is used in the modulator loop. This is done to alleviate the effect of solvent incompatibility due to the very different mobile phases used in the first and second dimensions. A major trend of LC×LC is to pay more attention to data analysis tools. Current research involves retention modeling, peak tracking, and advanced data analysis using chemometrics [19, 20]. Mass spectrometry is increasingly used for more general peak detection and compound identification. For the LC×LC analysis of polymers, general detection is mainly done by evaporative light scattering detection (ELSD), which has limited linearity. Any improvement of the detection scheme will be conducive to quantitative analysis. It is expected that the software will be further improved-including data collection and data analysis. Although the two-dimensional liquid chromatography system is already very powerful, further simplified use through enhanced system intelligence will enhance more unattended operations. Finally, automated method development tools are an urgent need for LC×LC, because it may take weeks or even months to develop new methods [21].

3. Comprehensive gas chromatography 3.1 Background Multidimensional gas chromatography (MDGC) has many aspects, usually including heart-cutting gas chromatography (GC-GC) and comprehensive gas chromatography (GC×GC). Although this review focuses on synthesis techniques, the author would like to take a moment to talk about GC-GC. GC-GC is a multidimensional technology that only transfers a small portion of the eluent from a one-dimensional column to a two-dimensional column. In this method, the two separations are independent of each other, and the first-dimensional fractions are usually selected based on the co-elution of the target for further separation. Dean's Switch or valve can be used to collect the target fraction and transfer it to a two-dimensional separation. A perfect example is the first automated paraffin, naphthenic, and aromatic (PNA) analyzer developed in 1971, which quantitatively separates heavy naphtha into PNA group types [22]. This MDGC application was later extended to provide quantitative information about paraffins, isoparaffins, alkenes, cycloalkanes, and aromatics (also known as PIONA), which are applicable to light petroleum fractions [23]. The PIONA analyzer has been reviewed by the American Society for Testing and Materials (ASTM) and is an industry standard for evaluating petroleum fractions with a boiling point below 200°C (ASTM-D5443) [24]. Although GC-GC has made progress in conventional applications, there are still difficulties in implementing GC×GC in conventional environments. Although GC×GC has made great progress in the past 20 years, compared with one-dimensional separation, it is still a complex technology, in which column selection, modulator type, detector selection and software platform increase the user’s Confusion and complexity [25]. Several key points will be discussed, as well as recent examples related to the petrochemical industry.

3.2 Column Set: The terminology of reversed-phase combination and normal-phase combination GC×GC column set has caused some confusion over the years, especially for users of multi-dimensional LC and GC. Do not confuse normal phase and reversed phase GC×GC column sets with normal phase and reversed phase LC. The normal phase column group in GC×GC uses a non-polar stationary phase in one dimension, a polar stationary phase in two dimensions (non-polar × polarity), and a reversed phase column group will place polarity in one dimension Stationary phase, placing a non-polar stationary phase in two dimensions (polar × non-polar). In terms of hydrocarbon separation, the choice between a normal phase chromatography column set and a reverse phase chromatography column set will depend on the information output required. Compared with the reversed-phase column set, the normal-phase column set will improve the resolution and resolution of mono-, di-, and tri-aromatic hydrocarbons; however, for the same sample, the reversed-phase column set will provide higher resolution and paraffinic hydrocarbons (positive and Iso) and the separation of cycloalkanes [26, 27]. Figure 3 illustrates this concept using mixed petroleum fractions analyzed on a reversed-phase column set and a normal-phase column set. It can be seen that when a reversed-phase column set is used, alkanes elute above the aromatics in the 2D chromatogram, and when a normal-phase column set is used, aromatics elute above the alkanes in the 2D separation space. Although these separation conditions need to be further optimized, it has been observed that compared with the normal phase column group, the early-eluting paraffin using the reverse phase column group has better resolution. A large number of GC stationary phases and column sizes are commercially available. Once you choose to use a reversed-phase or normal-phase column set, you must choose the correct column size. The 1D column is much longer than the 2D column, but the actual size will depend on the type of modulator used. Table 1 provides some examples of column sets from petroleum/petrochemical applications where thermal modulation and flow modulation are compared.  

3.3 Modulator: The flow and heat modulator is a device that distinguishes GC-GC from GC×GC. In order for the separation to be completely comprehensive, the modulator must meet certain criteria: 1) it must capture all the eluent from the one-dimensional column as the separation continues, 2) the analyte must be refocused in time or space, and 3) modulation The processor must quickly inject data packets into the 2D column to further separate [31]. Choosing the correct modulation time is crucial. If the conditioning time is too long, the analytes separated in the one-dimensional column will be remixed during the refocusing step, resulting in suboptimal two-dimensional separation. If the conditioning time is too short, wrap around will be observed, which occurs when the conditioning time is shorter than the retention time of the most recently eluted compound in the two-dimensional separation. In addition to modulation time, sampling rate is another important parameter that must be considered. Each one-dimensional peak must be sampled three to four times to maintain the sensitivity of the resolution of the first-dimensional separation in the second dimension [32]. GC×GC can use two types of modulators: thermal modulator and flow modulator

3.3.1 Thermal modulator The first modulator of GC×GC is based on heat [33]. The thermal modulator uses cooling and heating to effectively capture the eluent flowing out of the one-dimensional chromatographic column and inject the contents into the two-dimensional chromatographic column. Different types of thermal modulators can be used, especially resistance heating traps, heating sweepers, and cryogenic focusing [34]. Compared with flow regulators, heat regulators may have difficulty capturing more volatile compounds. Cryogenic modulators using liquid nitrogen can capture analytes as low as C3. The liquid nitrogen pulse captures the analyte and then uses the thermal jet pulse to transfer the analyte to the two-dimensional column. In this setup, single-stage and two-stage modulation can be achieved without moving parts; however, not all laboratories are equipped with equipment for handling cryogenic liquids. Non-refrigerant heat regulators are also available on the market, and coolers can cool dry nitrogen to -90˚C. Such systems can successfully capture analytes as low as C8 and use thermal jet pulses to release the captured analytes onto the 2D column. This modulator configuration also has no moving parts; however, there is another option that uses a specific modulator column combined with a thermoelectric cooler and a microthermal heater to capture and release analytes as low as -50˚C . By moving the modulator column back and forth, entering and exiting the cold zone, two-level modulation is realized on the solid-state modulator. According to the boiling point range of the analyte, four types of modulated columns can be used: EV (C2-C12), HV (C5-C30), SV (C7-C40) and DV (C9-C40+) modulated columns. Compared with the flow regulator, the thermal regulator is easier to couple with the MS detector; this is because the thermal regulator does not require a high flow rate for two-dimensional separation.  

3.3.2 Flow regulators Differential and split flow regulators are commercially available flow regulation options, and their operation methods are quite different. The differential flow (or pulse flow) modulator uses a switching valve to collect the eluent flowing out of the 1D chromatographic column in a loop. The loop is then purged with carrier gas, and the contents are injected into the two-dimensional chromatographic column. The first flow regulator was operated using the forward fill/flush (FFF) method, where the circuit was filled and then flushed with flow in the same direction [35]. Now, the reverse fill-flush (RFF) modulator is preferred (Figure 4), which fills the loop in one direction and flushes the loop in the opposite direction [36]. In many respects, FFF and RFF modulators have similar performance when the detectability and sample solute concentration are low or similar; however, for samples with a wide dynamic concentration range, the RFF modulator is superior to the FFF modulator [37]. Differential flow modulation relies on very fast two-dimensional flow rates to provide peak capacity in the second dimension. The coupling with MS detection is more challenging and requires a split two-dimensional column, which increases the complexity of the instrument setup. LECO is currently developing new developments in differential flow regulation, where the back pressure regulator is used for pressure control instead of an auxiliary pneumatic control module. The prototype RFF modulator (Figure 5) is expected to simplify method development because there is no need to change the sample loop when changing the modulation conditions. Steering flow modulation uses auxiliary air flow to perform modulation. In the injection mode, the flow is transferred from the one-dimensional column to the two-dimensional column; however, in the turning mode, the auxiliary gas flow is opposite to the flow of the 1D column, and it is diverted to the waste liquid while supplementing the 2D column flow [38] . Split flow modulation allows the flow rate of the 2D column to be similar to that of the thermal modulator, making it easy to connect to the MS detector. However, one disadvantage of the split flow modulator is that the overall sensitivity is reduced compared to other modulator types. This is because the eluent from the one-dimensional column is transferred to the waste and therefore never reaches the second dimension. In general, compared to thermal modulators, flow modulators can handle a wider range of sample components, especially for highly volatile samples. The flow regulator can adjust the compound in the range of C1-C40+.  

3.4 Detector In principle, any GC detector can be connected to GC×GC separation; however, the detector must be carefully selected to ensure reliable data collection. The narrower the peak width in 2D separations, the higher the peak capacity. When considering two peaks with the same area, the narrower the peak, the higher the sensitivity; however, the narrower peak means that a faster data collection rate is required to ensure that enough data points are collected for reliable Quantify. The type of modulator and 2D separation conditions (column, flow rate, etc.) will affect the 2D peak width. Nevertheless, the typical modulation peak width is in the range of 50-600 ms at the base, and the detector should sample at least 6-10 data points for each peak. Therefore, a minimum data rate of 20 Hz is required, but 100 Hz is usually preferred in order to adequately sample the peaks to obtain adequate peak shapes. The detector of choice for GC×GC petrochemical applications (such as PIONA quantification) is the flame ionization detector (FID) [39]. FID has an excellent response to hydrocarbons, and the data collection rate is greater than 100 Hz. This is not to say that no other GC detector was used. Electron Capture Detector (ECD) [40, 41], Sulfur Chemiluminescence Detector (SCD) [42], Nitrogen Chemiluminescence Detector (NCD) [43, 44], Nitrogen Phosphorus Detector (NPD) [45] Play a role in GC×GC petrochemical applications. One of the most powerful detectors of GC×GC is the mass spectrometer. Time-of-flight mass spectrometer (TOF-MS) is one of the most widely used MS detectors in GC×GC applications; this is due to its fast acquisition rate, which can sample the narrowest (50 ms) modulation peaks. Other advantages include the compound library for identification and the possibility of deconvoluting co-eluting peaks in a two-dimensional chromatogram. Other MS detectors are also used in GC×GC applications, such as single quadrupole (qMS), high resolution (HR) TOF-MS, qTOF, and triple quadrupole (QqQ) MS, but are comparable to TOF-MS Lower [46]. When using differential flow modulation to couple the MS detector to the GC×GC, the flow leaving the 2D column must be considered. This flow can easily exceed 20 mL/min, which far exceeds the capability of the MS detector. Therefore, the splitter is usually only used to direct a small part of the two-dimensional chromatographic column flow to the MS detector and the larger part to the second detector, such as the FID. Although this does cause a decrease in sensitivity, there are additional benefits of obtaining both MS data for identification and FID data for quantification [29, 47].  

3.5 Applications and trends With the advancement of modulator design, robustness and data processing software, petrochemical applications have taken off in many different directions. Here will highlight some recent applications and trends. Modulation improvements are still under study. In order to provide longer two-dimensional chromatographic columns to increase the number of theoretical plates and peak capacity, stopped-flow GC×GC was introduced in the early 2000s [48]. As the name suggests, the flow of a one-dimensional column is temporarily stopped during the conditioning cycle while two-dimensional separation is performed. Although compared with traditional GC×GC, stop-flow GC×GC provides better resolution, but it is less attractive to some users, so it has not been widely implemented [49]. In 2020, an article was published introducing the quasi-stop flow modulation of GC×GC, which produced excellent repeatability and chromatographic performance for the evaluated gasoline and light cycle oil [50]. The novelty of this method is that neither an auxiliary pneumatic control module nor a complex microfluidic device is required, making it a simple, cost-effective and efficient solution for quality assurance laboratories. The trend of using GC×GC for chromatographic fingerprint analysis is increasing; in the petroleum/petrochemical industry. GC×GC was used for the first time in 1999 to determine the source of oil spills at sea [51]. Since then, GC×GC has proven to be very important in identifying the source of marine oil spills that use fingerprints to compare samples and match the oil to the source. Recently, GC×GC-FID and GC×GC-HR TOF-MS have been used to assess the complexity of source oil in two iconic oil spills in the Gulf of Mexico, especially the Ixtoc I blowout from 1979 to 1980 and 2010 Deepwater Horizon explosion; the difference between the dibenzothiophene and alkylated dibenzothiophene regions of the GC×GC chromatogram can distinguish Ixtoc I oil and Maconda oil in the Deepwater Horizon event [52]. GC×GC fingerprints are also used to evaluate bioremediation. Recent examples include the evaluation of the photodegradation of polycyclic aromatic hydrocarbons (PAH) in the surface oil of marine oil spills and the biodegradation of PAHs and heterocycles in refinery wastewater [53-55] . The complete characterization of petroleum and its derivatives is called petronomics. When it was first described, the focus was on the analysis of petroleum using (ultra) high resolution mass spectrometry, such as Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS) [56] . In recent years, the significant influence of GC×GC in the field of "omics" has been realized through several articles published in recent years [57-59]. Although complete characterization is often sought, the identification of biomarkers is also an important aspect of "omics" analysis; the same is true in the field of petroleum omics. Table 2 highlights two recent examples of biomarker identification [60, 61].

4. Conclusion The author expects that full separation will increase the number of published applications for integrated liquid chromatography (LC×LC) and integrated gas chromatography (GC×GC). It is expected that software (including data acquisition, data analysis, and method development) and hardware (such as modulator and detector technology) will be further improved.

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