Off-line two-dimensional separation involving supercritical fluid chromatography for the characterization of the wastewater from algae hydrothermal liquefaction

Optimizing conditions in online RPLC × SFC for the analysis of complex samples containing neutral compounds: Solving injection issues

The online combination of reversed-phase liquid chromatography and supercritical fluid chromatography (online RPLC × SFC) is an attractive technique for the characterization of complex samples containing neutral compounds as the two techniques are highly complementary, especially with a polar stationary phase in supercritical fluid chromatography (SFC). However, the setup is challenging due to the presence of hydro-organic solvents in RPLC, which become injection solvent in SFC. In this study, numerous key experimental parameters were identified and found to have a major effect on peak shape under RPLC × SFC conditions. These parameters included the organic modifier in reversed-phase liquid chromatography (RPLC), the co-solvent in SFC, the gradient conditions and the column ID in SFC, the configuration of the valve and finally, the injection volume in SFC that should be maximized. Acetonitrile (ACN) in RPLC, a mixture of ACN and methanol (MeOH) (50/50, v/v) in SFC, a minimum initial composition of 5% B in SFC, column IDs of 1.0 mm and 2.1 mm in RPLC and SFC respectively and flushing the interface loops with pure CO2 while adding the co-solvent after the valve, are all conditions that have been identified as perfectly suitable for online RPLC × SFC. They were successfully applied to the online RPLC × SFC separations of microalgae bio-oil samples. Despite unusual injection conditions, the peaks were symmetrical over the entire chromatogram, leading to a high separation power.

Comprehensive profiling and identification of C21 steroids in the root of Marsdenia tenacissima (Dai-Bai-Jie) using offline two-dimensional chromatography (LC × SFC) with Q-TOF/MS

Dai-Bai-Jie, the root of the plant Marsdenia tenacissima from the Asclepiadaceae family, is well-known for its therapeutic effects in clearing heat, detoxifying, reducing swelling, and relieving pain as one of the most commonly used Dai medicine. Due to numerous structurally similar C21 steroidal compounds in Dai-Bai-Jie, chemical composition profiling has been substantially challenged. In this study, an offline two-dimensional chromatographic method (LC × SFC separation system) was developed to address these issues. Using the Hypersil Gold (1stD LC column) and 2-PIC (2ndD SFC column) based on 40 reference standards, the orthogonality was as high as 83.83 %. Most profiled ion peaks were tentatively identified through quadrupole time-of-flight mass spectrometry and a self-built compound virtual library. Consequently, the integrated method effectively addressed and resolved the issues associated with co-elution, thus significantly expanding the peak capacity. This advancement identified 362 C21 steroidal components, 319 of which were speculated to be potentially novel compounds. Furthermore, 86 groups of isomeric compounds were distinguished. This method provides a comprehensive understanding of chemical composition of Dai-Bai-Jie and an integrated qualitative analysis method for the C21 steroids.

LCxSFC Valve Technologies: Guidelines toward a Successful Online Modulation

Online two-dimensional liquid chromatography can be used under various separation modes but is sometimes limited in terms of orthogonality, especially for the analysis of neutral compounds. The use of SFC in the second dimension offers a wide choice of mobile and stationary phases, suggesting retention properties complementary to those of the first dimension. Initial works on online LCxSFC coupling published in the literature highlighted the first difficulties of solvent compatibility or potential problems with bubbles created by CO2 in loops. The present work highlights the impact of the interface between the LC and SFC dimensions on the performance of the 2D separation. Six different configurations have been evaluated, differing by the addition of a makeup flow in the first dimension, a division, or a separation of flow paths in the second dimension. Their performances with empty loops eliminated the need for complex trapping columns, regardless of the LC mobile phase nature. Injection in partial fill mode was possible without any problems of CO2 bubble formation, solvent miscibility, pressure, or modulation repeatability in each of the studied configurations. Selected configurations even allowed for the use of a mobile phase split upstream of the valve, so that the online LCxSFC configuration could be as flexible as LCxLC instrumentation in terms of the first dimension flow rate or second dimension injected volume. The obtained results allowed for building guidelines presenting the best interfaces to set up depending on the nature of the mobile phases in both dimensions. Two applications on monomers of lignin and industrial polymers confirmed preliminary results on configuration testing and illustrated that online LCxSFC could now be easily implemented. Precision tests on retention time were conclusive and are promising for the future use of this technique in industrial routines.

Online Supercritical Fluid Chromatography Hyphenated to Fourier Transform Ion Cyclotron Resonance Mass Spectrometry with Quadrupole Detection for Microalgae Bio-Oil Characterization

Microalgae are an attractive feedstock for biofuel production thanks to their renewable nature, high growth rate, and ability to use anthropogenic CO2. The conversion of microalgae by hydrothermal liquefaction (HTL) leads to a solid residue, a gaseous phase, and a biocrude. However, the bio-oil is rich in heteroatoms and requires upgrading processes to be used as biofuels. For these treatments to be effective, detailed knowledge of the sample is crucial. The bio-oil was characterized by direct introduction into a Fourier transform ion cyclotron resonance mass spectrometer (DI-FTICR MS) with an electrospray ionization source (ESI). Thousands of molecular formulas were assigned with a high level of confidence, mainly compounds with nitrogen and oxygen atoms. Additionally, the bio-oil was analyzed by coupling supercritical fluid chromatography (SFC) and FTICR to combine the separation power of SFC to the high performances of a 12 T FTICR. Quadrupole detection (2ω) was used in FTICR to have a high resolving power with a lower transient time. The coupling allowed the separation of many isomers along the chromatogram, showing the isomeric complexity of microalgae bio-oils. Moreover, classes of compounds were separated according to their heteroatom class thanks to the SFC separation. In this work, the advantages of DI-FTICR MS and SFC-FTICR MS proved complementary, and DI was useful to study the bio-oil at the molecular scale thanks to the high performances, while SFC proved useful for the characterization at the isomeric scale. This demonstrated the significant potential of this new online hyphenated technique for the characterization of complex matrices.

Structural elucidation of complex polyesters polyols from bio-lubricant using off-line liquid chromatography x supercritical fluid chromatography coupled with Orbitrap mass spectrometry

Synthetic complex esters and polyol esters are incorporated as partially bio-based and biodegradable alternatives to petroleum base oils in lubricant formulations, to provide specific properties or performance and to help reducing their carbon footprint in certain cases. A sample can contain over 400 molecules of high chemical similarity including numerous isomers. To resolve such complexity, a separation technique with large peak capacity coupled to high-resolution mass spectrometry (HRMS) is essential. In this study, comprehensive off-line LCxSFC hyphenated with an Orbitrap analyzer was used for the structural elucidation of a synthetic bio-lubricant composed of a polyol reacted with fatty acids of varying length or with repetitive units of polyesters of ricinoleic acid. Retention in the LC first dimension was mostly due to the degree of oligomerization of ricinoleic acid within the polyester and to the chain length of the fatty acid. The SFC second dimension highlighted the esterification degree of the polyalcohol and the number and positions of fatty acids double bonds. The combination of both dimensions permitted the separation of isomers. The coupling of SFC with Orbitrap analyzer allowed an accurate assignment of molecular formulas. Finally, the fragmentation in the ionization source confirmed the attributed structures. By introducing a clear distribution of the chemical structures in the retention space, LCxSFC-HRMS provided a powerful analytical method for the comprehensive molecular characterization of the complex polyester polyols sample.

Discrimination of plastic waste pyrolysis oil feedstocks using supercritical fluid chromatography

Advanced chemical recycling techniques provide new avenues for handling and recycling mixed plastic waste; pyrolysis is a prominent approach involving heating plastic waste in an oxygen-free environment to create pyrolysis oils. Pyrolysis oils must be thoroughly characterized before being refined into fuels and chemical feedstocks. Here, a method based on supercritical fluid chromatography with ultraviolet detection was developed to analyze plastic waste pyrolysis oils. Multiple stationary phases were examined, and 2-ethyl pyridine was chosen as the best stationary phase for resolving pyrolysis oil components. Different standards and different plastic waste pyrolysis oils were compared across the different stationary phases. Up to three columns were serially coupled to increase efficiency and column capacity. It was found that a general method using ethanol as a modifier and two 2-ethyl pyridine columns could effectively resolve plastic waste pyrolysis oils. The potential for differentiating polyethylene and polypropylene feedstocks was demonstrated using principal component analysis.

A powerful two-dimensional chromatography method for the non-target analysis of depolymerised lignin

BACKGROUND

Lignin is an abundant natural polymer obtained as a by-product from the fractionation of lignocellulosic biomass. In the name of a circular economy, lignin should be valorised into valuable chemicals or biomaterials and utilised instead of petrochemicals. For the development of efficient valorisation processes, the structural characterisation of lignin can be highly beneficial. However, this is an arduous task, as the isolated (and sometimes processed) lignin mainly consists of various neutral monomers but also oligomers. In addition, the material contains isomers, which can be especially problematic to separate and identify.

RESULTS

We present a powerful off-line comprehensive two-dimensional (2D) chromatography method combining liquid chromatography (LC), supercritical fluid chromatography (SFC), and high-resolution mass spectrometry for the non-target analysis of depolymerised lignin. The implementation of a 1-aminoanthracene column in the second dimension enabled a class separation of potential lignin monomers, dimers, trimers, and tetramers with an additional separation based on the number of hydroxyl groups and steric effects. The pentafluorophenyl column in the first dimension additionally improved the separation based on hydrophobicity. The comparison of off-line 2D LC × SFC to 1D SFC showed that besides the overall improved performance, the first method is also superior for the separation of isomers. Advanced data analysis methods (MS-DIAL, SIRIUS, and Feature-Based Molecular Network) were integrated into the non-target workflow to rapidly visualise and study the detected compounds, which proved to be especially beneficial for the characterisation of the separated isomers.

SIGNIFICANCE

The method yielded the first 2D LC plot demonstrating a classification of lignin compounds, which can be applied to compare various lignin sources and processing methods. In addition, the technique demonstrated improved separation of compounds, including isomers, which was especially beneficial as 77 % of the detected compounds had at least one isomer in the same lignin sample.

Supercritical fluid chromatography is not (only) normal-phase chromatography

Supercritical fluid chromatography (SFC), now using carbon dioxide as a major component of the mobile phase, has been known for over 60 years but still some misunderstandings remain about its capabilities. Amongst them, SFC is often described as a normal-phase chromatographic technique, based on different considerations: polarity of the stationary phase, elution order of the analytes, relative non-polarity of the mobile phase, non-linear retention behaviour, or adsorption retention mechanisms. All of these assumptions are true to a certain extent, and in certain circumstances. But also, all of these assumptions are wrong in different circumstances. In this paper, the criteria to categorize SFC as a normal-phase chromatographic method will be examined individually, considering all knowledge acquired from the early years of its development. Finally, it will appear that the "normal-phase" glass lens is greatly reducing the true extent of SFC's possibilities.

Two-dimensional chromatography for the analysis of valorisable biowaste: A review

Various everyday areas such as agriculture, wood industry, and wastewater treatment yield residual biowastes in large amounts that can be utilised for the purpose of sustainability and circular economy. Depending on the type of biowaste, they can be used to extract valuable chemicals or converted into alternative fuels. However, for efficient valorisation, these processes need to be monitored, for which thorough chemical characterisation can be highly beneficial. For this aim, two-dimensional (2D) chromatography can be favourable, as it has a higher peak capacity and sensitivity than one-dimensional (1D) chromatography. Therefore, here we review the studies published since 2010 involving gas chromatography (GC) or liquid chromatography (LC) as one of the dimensions. For the first time, we present the 2D chromatographic characterisation of various biowastes valorised for different purposes (chemical, fuels), together with future prospects and challenges. The aspects related to the 2D chromatographic analysis of polar, poorly volatile, and thermally unstable compounds are highlighted. In addition, it is demonstrated how different 2D setups can be applied for monitoring the biowaste conversion processes.