Flow regime at ambient outlet pressure and its influence in comprehensive two-dimensional gas chromatography

Theoretical modeling and machine learning-based data processing workflows in comprehensive two-dimensional gas chromatography-A review

In recent years, comprehensive two-dimensional gas chromatography (GC × GC) has been gradually gaining prominence as a preferred method for the analysis of complex samples due to its higher peak capacity and resolution power compared to conventional gas chromatography (GC). Nonetheless, to fully benefit from the capabilities of GC × GC, a holistic approach to method development and data processing is essential for a successful and informative analysis. Method development enables the fine-tuning of the chromatographic separation, resulting in high-quality data. While generating such data is pivotal, it does not necessarily guarantee that meaningful information will be extracted from it. To this end, the first part of this manuscript reviews the importance of theoretical modeling in achieving good optimization of the separation conditions, ultimately improving the quality of the chromatographic separation. Multiple theoretical modeling approaches are discussed, with a special focus on thermodynamic-based modeling. The second part of this review highlights the importance of establishing robust data processing workflows, with a special emphasis on the use of advanced data processing tools such as, Machine Learning (ML) algorithms. Three widely used ML algorithms are discussed: Random Forest (RF), Support Vector Machine (SVM), and Partial Least Square-Discriminate Analysis (PLS-DA), highlighting their role in discovery-based analysis.

Comprehensive two-dimensional gas chromatography- A discussion on recent innovations

Although comprehensive 2-D GC is an established and often applied analytical method, the field is still highly dynamic thanks to a remarkable number of innovations. In this review, we discuss a number of recent developments in comprehensive 2-D GC technology. A variety of modulation methods are still being actively investigated and many exciting improvements are discussed in this review. We also review interesting developments in detection methods, retention modeling, and data analysis.

Flow-modulated comprehensive two-dimensional gas chromatography combined with time-of-flight mass spectrometry: use of hydrogen as a more sustainable alternative to helium

The present research is focused on the use and evaluation of hydrogen, as a more sustainable alternative to helium, within the context of fast flow modulation comprehensive two-dimensional gas chromatography-time-of-flight mass spectrometry. In such a respect, a comparison was made between the two mobile phases in terms of speed and overall chromatography performance. All experiments were carried out by using the following column set: low polarity with dimensions 10 m × 0.25 mm ID × 0.25 µm d_f and medium polarity with dimensions 2 m × 0.10 mm ID × 0.10 µm d_f. Fundamental gas chromatography parameters (efficiency, resolution) were measured under different experimental conditions, using the two carrier gases. Efficiency was measured in both the first and second dimensions, using a probe compound under isothermal conditions; after defining the optimum carrier gas conditions, a mixture containing 20 pesticides was analyzed to measure resolution, again in the first and second dimensions, using a temperature program. It was found (as expected) that a similar chromatography performance could be attained when using hydrogen, albeit with a circa 25% reduction in analysis time. Signal-to-noise ratios of the pesticides were calculated, using both carrier gases, with such values generally reduced (on average by 14%) when using hydrogen. Finally, a comparison was made between mass spectral profiles obtained analyzing the pesticides and fatty acid methyl esters using the two mobile phases. Even though mass spectral differences were observed, the ion profiles could be considered generally similar.

Pyrolysis of end-of-life polystyrene in a pilot-scale reactor: Maximizing styrene production

Chemical recycling of polystyrene (PS) via pyrolysis is of great industrial, and academic interest, with styrene being the primary product of interest. To identify the optimal process conditions, the pyrolysis of end-of-life PS was studied in a pilot-scale unit consisting of an extruder, and a continuous stirred tank reactor (CSTR). The PS was pyrolyzed with continuous feeding at a pressure range from 0.02 to 1.0bara, and a temperature range from 450 to 600 °C, giving primarily styrene, other mono-aromatics, and oligomers. The comprehensive two-dimensional gas chromatography (GC × GC) coupled with flame ionization detector (FID), and time-of-flight mass spectrometer (ToF-MS) as well as GC with thermal conductivity detector (TCD) were used to characterize the liquid, and gaseous products exhaustively. The styrene yield increased from 36 wt% at 1.0bara, and 450 °C to 56 wt% at 0.02bara, and 550 °C. Working under a vacuum enhanced the styrene recovery at all corresponding temperature levels. The yield of benzene, toluene, ethylbenzene, and xylene (BTEX) increased from 4 wt% at 450 °C, and 0.02 bara to 17 wt% at 450 °C, and 1.0 bara. The experimental results have been used in a mathematical model that can explain the combined effect of temperature, and pressure on the yield of the primary products. The present work illustrates the potential of a continuous pyrolysis process for end-of-life PS, and paves the way for this technology to be rapidly transferred from mere laboratory use to industrial processes in the circular (petro-) chemical industry.

Naphthenic Acids: Formation, Role in Emulsion Stability, and Recent Advances in Mass Spectrometry-Based Analytical Methods

Naphthenic acids (NAs) are compounds naturally present in most petroleum sources comprised of complex mixtures with a highly variable composition depending on their origin. Their occurrence in crude oil can cause severe corrosion problems and catalysts deactivation, decreasing oil quality and consequently impacting its productivity and economic value. NAs structures also allow them to behave as surfactants, causing the formation and stabilization of emulsions. In face of the ongoing challenge of treatment of water-in-oil (W/O) or oil-in-water (O/W) emulsions in the oil and gas industry, it is important to understand how NAs act in emulsified systems and which acids are present in the interface. Considering that, this review describes the properties of NAs, their role in the formation and stability of oil emulsions, and the modern analytical methods used for the qualitative analysis of such acids.

Detailed Group-Type Characterization of Plastic-Waste Pyrolysis Oils: By Comprehensive Two-Dimensional Gas Chromatography Including Linear, Branched, and Di-Olefins

Plastic-waste pyrolysis oils contain large amounts of linear, branched, and di-olefinic compounds. This makes it not obvious to determine the detailed group-type composition in particular to the presence of substantial amounts of N-, S-, and O-containing heteroatomic compounds. The thorough evaluation of different column combinations for two-dimensional gas chromatography (GC × GC), i.e., non-polar × polar and polar × non-polar, revealed that the second combination had the best performance, as indicated by the bi-dimensional resolution of the selected key compounds. By coupling the GC × GC to multiple detectors, such as the flame ionization detector (FID), a sulfur chemiluminescence detector (SCD), a nitrogen chemiluminescence detector (NCD), and a mass spectrometer (MS), the identification and quantification were possible of hydrocarbon, oxygen-, sulfur-, and nitrogen-containing compounds in both naphtha (C5–C11) and diesel fractions (C7–C23) originating from plastic-waste pyrolysis oils. Group-type quantification showed that large amounts of α-olefins (36.39 wt%, 35.08 wt%), iso-olefins (8.77 wt%, 9.06 wt%), and diolefins (4.21 wt%, 4.20 wt%) were present. Furthermore, oxygen-containing compounds (alcohols, ketones, and ethers) could be distinguished from abundant hydrocarbon matrix, by employing Stabilwax as the first column and Rxi-5ms as the second column. Ppm levels of sulfides, thiophenes, and pyridines could also be quantified by the use of selective SCD and NCD detectors.

A Fast GC-MS-Based Method for Efficacy Assessment of Natural Anti-Oxidants for Inhibiting Lipid Oxidation

BACKGROUND

For health reasons it is preferred to prepare food products with edible fats and oils that are high in unsaturated fatty acids. Unfortunately, these unsaturated acids are susceptible to lipid oxidation and the addition of natural antioxidants, e.g., rosemary extracts, etc. is needed.

OBJECTIVE

To assess the efficacy of natural oxidation inhibition strategies, fast, yet realistic, and objective methods are needed to study oxidation inhibition.

METHODS

A model system consisting of salt and sunflower oil is proposed as a model for dry soups and sauces. Hexanal formation is studied using fast GC-MS as a quantitative indicator for lipid oxidation.

RESULTS

A fast GC-MS method using a short, 6-m 150 µm inner-diameter column was developed that allowed elution of hexanal within approximately 20 s, with a total run time of 2 min. The GC method has quantification limits below 1 ppm and is hence much more sensitive than the human nose.

CONCLUSIONS

The new accelerated method with hexanal read-out was successfully applied in a study to identify spices and herbs mixtures that can act as natural inhibitors of lipid oxidation. The fast GC-MS method is extremely stable and allowed the analysis of thousands of samples with very little maintenance.

HIGHLIGHTS

With the right mixture of spices and herbs, lipid oxidation can be delayed more than 100 times as compared to non-stabilized systems.

On-line Analysis of Nitrogen Containing Compounds in Complex Hydrocarbon Matrixes

The shift to heavy crude oils and the use of alternative fossil resources such as shale oil are a challenge for the petrochemical industry. The composition of heavy crude oils and shale oils varies substantially depending on the origin of the mixture. In particular they contain an increased amount of nitrogen containing compounds compared to the conventionally used sweet crude oils. As nitrogen compounds have an influence on the operation of thermal processes occurring in coker units and steam crackers, and as some species are considered as environmentally hazardous, a detailed analysis of the reactions involving nitrogen containing compounds under pyrolysis conditions provides valuable information. Therefore a novel method has been developed and validated with a feedstock containing a high nitrogen content, i.e., a shale oil. First, the feed was characterized offline by comprehensive two-dimensional gas chromatography (GC × GC) coupled with a nitrogen chemiluminescence detector (NCD). In a second step the on-line analysis method was developed and tested on a steam cracking pilot plant by feeding pyridine dissolved in heptane. The former being a representative compound for one of the most abundant classes of compounds present in shale oil. The composition of the reactor effluent was determined via an in-house developed automated sampling system followed by immediate injection of the sample on a GC × GC coupled with a time-of-flight mass spectrometer (TOF-MS), flame ionization detector (FID) and NCD. A novel method for quantitative analysis of nitrogen containing compounds using NCD and 2-chloropyridine as an internal standard has been developed and demonstrated.

Regional features of northern Italian sparkling wines, identified using solid-phase micro extraction and comprehensive two-dimensional gas chromatography coupled with time-of-flight mass spectrometry

We carried out comprehensive mapping of volatile compounds in 70 wines, from 48 wineries and 6 vintages, representative of the two main production areas for Italian sparkling wines, by HS-SPME-GCxGC-TOF-MS and multivariate analysis. The final scope was to describe the metabolomics space of these wines, and to verify whether the grape cultivar signature, the pedoclimatic influence of the production area, and the complex technology were measurable in the final product. The wine chromatograms provided a wealth of information, with 1695 compounds being found. A large number of putative markers influenced by the cultivation area was observed. A subset of 196 biomarkers fully discriminated between the two types of sparkling wines investigated. Among the new compounds, safranal and α-isophorone were observed. We showed how correlation-based network analysis could be used as a tool to detect the differences in compound behaviour based on external/environmental influences.

Comprehensive two-dimensional gas chromatography under high outlet pressure conditions: a new approach to correct the flow-mismatch issue in the two dimensions

The typical column sets employed in comprehensive two-dimensional gas chromatography (GC × GC) suffer from the impossibility to fully exploit the efficiency of both dimensions simultaneously. Adding a restrictor at the end of the second dimension is a possible approach to adjust the linear velocity profile. Under these high outlet pressure conditions the second dimension becomes much slower while the effect on the primary column is limited. The gap in terms of optimum inlet pressures is thus reduced. A program written in Microsoft Excel was used to calculate the efficiencies of the two dimensions in GC × GC at different outlet pressures. A GC × GC set-up with a restrictor at the end of the second dimension column was successfully installed. Experiments proved that this is a possible way to have a better exploitation of the columns. The chromatograms obtained for a number of applications confirm that the separations achieved at elevated outlet pressure are more efficient than those obtained with the same column set under atmospheric outlet conditions. The price to pay is that the separations become considerably slower.

Measurement of fundamental chromatography parameters in conventional and split-flow comprehensive two-dimensional gas chromatography-mass spectrometry: A focus on the importance of second-dimension injection efficiency

The present manuscript reports a comparative study between conventional and split-flow comprehensive GC×GC-MS. Conventional GC×GC-MS was performed by using a single GC oven, and with a widely employed apolar (30 m × 0.25 mm id)-polar (1 m × 0.10 mm id) column combination. Split-flow GC×GC-MS experiments were carried out by using the same first and second analytical columns, and by diverting part of the primary-capillary flow to waste, via a waste line. The latter was located at the conjunction point between the two dimensions. The measurement of fundamental gas chromatography parameters (efficiency, resolution), in both dimensions and under different experimental conditions, enabled the attainment of detailed information related to GC×GC-MS method optimization. Finally, the results derived from the present investigation highlight an important GC×GC issue, namely the efficiency of band reinjection onto the second-dimension capillary.

Separation of disaccharides by comprehensive two-dimensional gas chromatography-time-of-flight mass spectrometry. Application to honey analysis

A new method based on comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry (GC×GC-ToF MS) has been developed for the first time for the analysis of complex mixtures of disaccharides previously converted to their trimethylsilyl oximes (TMSO). Among the different experimental parameters considered for optimization, both the column set combination and the dimensions of the second-dimension column were found to be the most significant with regard to the complete resolution of structurally similar disaccharides. Application of the optimized method to honey analysis allowed the separation of most of the honey disaccharides previously described in the literature. Furthermore, 12 other unknown disaccharides have been separated by this method and characterized from their mass spectral data.

Multiresidue method for pesticides and persistent organic pollutants (POPs) in milk and cream using comprehensive two-dimensional capillary gas chromatography-time-of-flight mass spectrometry

A method for the analysis of pesticides and their metabolites including most of the persistent organic pollutants (POPs) in milk and cream is described. The method was single-laboratory validated through milk fortification in quadruplicate with 34 pesticides, isomers, and metabolites including 12 of the insecticide POPs and their metabolites. Whole cow's milk was fortified at 0.2, 0.4, 1, 2, 10, or 50 microg/kg wet weight and extracted with acetone/cyclohexane/ethyl acetate (2:1:1) with the addition of Mg(2)SO(4) and NaCl. Fat recovered in the extract accurately reflected the fat content of the milk or cream. All test portions were purified on a gel permeation chromatograph (GPC) followed by solid phase extraction (SPE) cleanup on a mixed bed graphitized carbon black (GCB) and primary/secondary amine silica gel (PSA) column before determination using a comprehensive two-dimensional gas chromatograph interfaced to a time-of-flight mass spectrometer. Average recoveries were 77, 72, 73, 66, 77, and 84% for 0.2, 0.4, 1, 2, 10, and 50 microg/kg wet weight whole milk, respectively. The average relative standard deviations for 0.2, 0.4, 1, 2, 10, and 50 microg/kg were 10, 8, 7, 7, 3, and 3%, respectively. The limits of quantification (LOQs) for all pesticides were 0.2 or 0.4 microg/kg wet weight. An archived cream sample collected in 1982 on Oahu, Hawaii, was found to contain only hepatachlor epoxide (HE) and DDE-p,p' at 380 +/- 24 and 69 +/- 17 microg/kg fat, significantly elevated over the current action level of 50 microg/kg fat for HE.

Enhanced resolution comprehensive two-dimensional gas chromatography applied to the analysis of roasted coffee volatiles

The present research is based on the full exploitation of the separation power of a 0.05 mm internal diameter (ID) capillary, as a comprehensive two-dimensional (2D) GC (GC x GC) secondary column, with the objective of attaining very high-resolution second dimension separations. The aim was achieved by using a split-flow system developed in previous research [P.Q. Tranchida, A. Casilli, P. Dugo, G. Dugo, L. Mondello, Anal. Chem. 79 (2007) 2266], and a dual-oven GC x GC instrument. The column combination employed consisted of a polar 30 m x 0.25 mm ID column connected, by means of a T union, to a detector-linked high-resolution 1.1 m x 0.05 mm ID apolar analytical column and to a 0.33 m x 0.05 mm ID retention gap; the latter was connected to a manually operated split valve. As previously demonstrated, the use of a split valve enables the regulation of gas flows through both analytical columns, generating the most appropriate gas linear velocities. Comprehensive 2D GC experiments were carried out on Arabica roasted coffee volatiles (previously extracted by means of solid-phase microextraction) with the split-valve closed (equal to what can be defined as conventional GC x GC) and with the split-valve opened at various degrees. The reasons why it is absolutely not effective to use a 0.05 mm ID column as second dimension in a conventional GC x GC instrument will be discussed and demonstrated. On the contrary, the use of a 0.05 mm ID column as second dimension, under ideal conditions in a split-flow, twin-oven system, will also be illustrated and discussed.