Hyphenation of packed column supercritical fluid chromatography with mass spectrometry: where are we and what are the remaining challenges?

Resolving phytosterols in microalgae using offline two-dimensional reversed phase liquid chromatography-supercritical fluid chromatography coupled with quadrupole time-of-flight mass spectrometry

Sterols are unsaponifiable lipids resulting from plant metabolism that exhibit interesting bioactive properties. Microalgae are a major source of specific phytosterols, most of which are still not fully characterized. The similarity in sterol structures and the existence of positional isomers make the separation of phytosterols challenging. A method was developed based on an offline two-dimensional (2D) system, reversed-phase liquid chromatography (RPLC)-supercritical fluid chromatography (SFC)/quadrupole time-of-flight (Q-ToF) mass spectrometry, for the identification of sterols in microalgae. Subsequent positive-mode MS/MS was used to confirm the identified phytosterols. The 2D chromatogram exhibited a pattern related to the positions of the double bonds, which were confirmed by standard injection, enabling structural elucidation. The analysis of the unsaponifiable fraction of two algae, namely Scenedesmus obliquus, a freshwater microalgae, and Padina pavonica, a marine macroalgae, highlighted the ability of the method to distinguish a large number of sterol isomers.

A numerical study on microfluidic devices to maintain the concentration and purity of dielectrophoresis-induced separated fractions of analyte

Determining the physical and chemical properties of biologically important particles such as cells, organelles, viruses, exosomes, complexes, nucleotides, and proteins is needed to understand their function. These properties are determined with common analytical tools (mass spectrometry, cryo-EM, NMR, various spectroscopies, nucleotide sequencing, etc.) whose function can be improved when samples are pure and concentrated. Separations science plays a central role in conditioning samples, ranging from low-resolution benchtop operations like precipitations or extractions to higher-resolution chromatography and electrophoresis. In the last two decades, gradient insulator-based dielectrophoresis (g-iDEP) has emerged as a high-resolution separation technique capable of highly selective enrichment of cells, viruses, exosomes, and proteins. Specific evidence has been shown that pure homogeneous and concentrated fractions of cells and exosomes can be generated from complex mixtures. However, recovering those fractions for analysis has not been developed, limiting the technique to an analytical rather than a preparative one. Here, a finite element analysis was undertaken to identify geometries and operational parameters to efficiently remove the enriched fraction while retaining maximum concentration and providing total mass transfer. Geometric factors (e.g., side channel width and distance from the gradient-inducing gap) were studied, along with the addition of a second inlet side channel. Two flow-generating mechanisms-electroosmosis and hydrostatic pressure-were evaluated for semi-optimized device designs, including a comparison of the one- and two-inlet designs. Simulations indicate effectively one hundred percent mass transfer and a concentration increase by an order of magnitude for several device configurations and operational parameters.

First proof-of-concept of UC/HILIC for extending the versatility of the current art of supercritical fluid separation

Supercritical Fluid Chromatography (SFC), a high-throughput separation technique, has been widely applied as a promising routine method in pharmaceutical, pesticides, and metabolome analysis in the same way as conventional liquid chromatography and gas chromatography. Unified chromatography (UC), an advanced version of SFC, which applied gradient elution with mobile phase changing continuously from supercritical to subcritical and to liquid states, can further extend the SFC applications. UC mostly applying the popular mobile phase of 95%:5%/Methanol:Water with additives allows to analyze many hydrophilic compounds. However, many of phosphorylated metabolites or multi carboxylic acids show very poor peak shapes or even can't be eluted under UC conditions, thus hampering the UC's metabolome coverage. In this study, we proposed the first proof-of-concept of UC/HILIC, a novel strategy to extend the current UC metabolome coverage by employing an aqueous gradient right after the UC gradient on a single packed column in a single measurement. The proposed method showed significant improvement regarding the chromatographic performance and metabolome coverage, while still maintaining the precision and high throughput in comparison with conventional UC methods.

Supercritical Fluid Nanospray Mass Spectrometry

Supercritical fluids are typically electrosprayed using an organic solvent makeup flow to facilitate continuous electrical connection and enhancement of electrospray stability. This results in sample dilution, loss in sensitivity, and potential phase separation. Premixing the supercritical fluid with organic solvent has shown substantial benefits to electrospray efficiency and increased analyte charge state. Presented here is a nanospray mass spectrometry system for supercritical fluids (nSF-MS). This split flow system used small i.d. capillaries, heated interface, inline frit, and submicron emitter tips to electrospray quaternary alkyl amines solvated in supercritical CO₂ with a 10% methanol modifier. Analyte signal response was evaluated as a function of total system flow rate (0.5-1.5 mL/min) that is split to nanospray a supercritical fluid with linear flow rates between 0.07 and 0.42 cm/sec and pressure ranges (15-25 MPa). The nSF system showed mass-sensitive detection based on increased signal intensity for increasing capillary i.d. and analyte injection volume. These effects indicate efficient solvent evaporation for the analysis of quaternary amines. Carrier additives generally decreased signal intensity. Comparison of the nSF-MS system to the conventional SF makeup flow ESI showed 10-fold signal intensity enhancement across all the capillary i.d.s. The nSF-MS system likely achieves rapid solvent evaporation of the SF at the emitter point. The developed system combined the benefits of the nanoemitters, sCO₂, and the low modifier percentage which gave rise to enhancement in MS detection sensitivity.

Rapid Simultaneous Quantification of 1-Formyl-2,2-Dimethylhydrazine and Dimethylurea Isomers in Environmental Samples by Supercritical Fluid Chromatography-Tandem Mass Spectrometry

When released to the environment, the rocket fuel unsymmetrical dimethylhydrazine (UDMH) undergoes oxidative transformations, resulting in the formation of an extremely large number of nitrogen-containing transformation products, including isomeric compounds which are difficult to discriminate by common chromatography techniques. In the present work, supercritical fluid chromatography–tandem mass spectrometry (SFC-MS/MS) was proposed for resolving the problem of fast separation and simultaneous quantification of 1-formyl-2,2-dimethylhydrazine (FADMH) as one of the major UDMH transformation products, and its isomers—1,1-dimethylurea (UDMU) and 1,2-dimethylurea (SDMU). 2-Ethylpyridine stationary phase provided baseline separation of analytes in 1.5 min without the distortion of the chromatographic peaks. Optimization of SFC separation and MS/MS detection conditions allowed for the development of rapid, sensitive, and “green” method for the simultaneous determination of FADMH, UDMU, and SDMU in environmental samples with LOQs of 1–10 µg L⁻¹ and linear range covering three orders of magnitude. The method was validated and successfully tested on the real extracts of peaty and sandy soils polluted with rocket fuel and UDMH oxidation products. It was shown that both UDMU and SDMU are formed in noticeable amounts during UDMH oxidation. Despite relatively low toxicity, UDMU can be considered one of the major UDMH transformation products and a potential marker of soil pollution with toxic rocket fuel.

Fast Optimization of Supercritical Fluid Chromatography-Mass Spectrometry Interfacing Using Prediction Equations

The effect of makeup solvent composition in ultrahigh-performance supercritical fluid chromatography-triple quadrupole mass spectrometry using electrospray ionization was studied using a set of 91 compounds, 3 stationary phases, and 2 organic modifiers of the mobile phase. The 24 tested makeup solvents included pure alcohols and methanol in combination with commonly used additives such as water, formic and acetic acid, ammonia, and ammonia salts with varying molarity. The behavioral trends for different makeup solvent additives were established in the first step. Subsequently, the correlations between physicochemical properties and the MS responses were calculated using the Pearson correlation test and matrix plots. The regression analysis was performed using five descriptors: molecular weight, pK_a, log P, number of hydrogen donors/acceptors, and the MS responses obtained with methanol as the makeup solvent. The resulting regression equations had a high prediction rate calculated as R²-predicted coefficient, especially when 10 mmol/L ammonium in methanol was used as an organic modifier of the mobile phase in positive mode. The trueness of these equations was tested via the comparison between experimental and predicted responses expressed as R². Values of R² > 0.8 were found for 88% of the proposed equations. Thus, the MS response could be measured using only one makeup solvent and the responses of other makeup solvents could be easily estimated. The suitability and applicability of determined regression equations was confirmed by the analysis of 13 blind probes, i.e., compounds not included in the original set of analytes. Moreover, the predicted and experimental responses followed the same increasing/decreasing trend enabling one to predict makeup solvent compositions leading to the highest sensitivity.

Simultaneous Determination of Vitamin D and Its Hydroxylated and Esterified Metabolites by Ultrahigh-Performance Supercritical Fluid Chromatography-Tandem Mass Spectrometry

In this study, an analytical method has been developed that, for the first time, allows simultaneous determination of vitamin D₂ and vitamin D₃ along with their hydroxylated and esterified forms. A group of 12 vitamin D analogues including vitamin D₂ and vitamin D₃, seven hydroxylated metabolites, and three ester forms were separated in a single 8.0 min run using ultrahigh-performance supercritical fluid chromatography coupled with triple quadrupole tandem mass spectrometry. Electrospray ionization and atmospheric pressure chemical ionization were investigated as ion sources, of which the latter showed a higher ionization efficiency. Chromatographic conditions were thoroughly evaluated by a step-by-step method, whereas an experimental design was applied for the optimization of the ionization parameters. Calibration and repeatability studies were carried out to validate the instrumental methodology showing determination coefficients higher than 0.9992 and good intra- and interday precision with relative standard deviations for areas and retention times lower than 10 and 2.1%, respectively, for all target analytes. Limits of quantification were below 3.03 μg/L for all compounds. The methodology was then validated and applied for the evaluation of human plasma samples in order to demonstrate its applicability to the analysis of vitamin D analogues in biological samples. Samples of five individuals were analyzed. Results show that linoleate-D₃, vitamin D₂, vitamin D₃, 25-hydroxyvitamin D₂, 24,25-dihydroxyvitamin D₃, and 1,25-dihydroxyvitamin D₃ could be detected in most samples, while the two latter also were quantified in all analyzed samples.

A compound post-column re-focusing approach in supercritical fluid chromatography

Supercritical-fluid chromatography (SFC) is regaining popularity in various fields of analytical chemistry owning to significant advances in instrumentation made in the past decade. However, due to the CO₂ based mobile phase and the high flow rates often employed, detection of trace amounts of analytes and coupling with certain detectors or other chromatography techniques are still difficult under many circumstances. In this study we propose a post-column re-focusing approach for SFC analysis to achieve not only signal enhancement in UV-Vis detection, but also actual concentration enhancement of the analyte. By heart-cutting and transporting a selected fraction from the SFC flow into a trapping column with a flushing solvent, re-focusing of the collected analytes can be achieved by re-mobilization with another solvent once the depressurized CO₂ is eliminated. By carefully selecting the trapping stationary phase and the two solvents, signal-enhancement ratios between 2.2 and 6.4 were realized for four representative compounds eluting with very different percentages of SFC modifier (methanol). The actual concentration enhancement was lower (ratios between 1.7 to 2.9), because the UV response of the analytes was found to differ significantly under SFC and LC conditions.

A rapid and sensitive method for characterization and quantification of polyglycerol esters by supercritical fluid chromatography coupled to high-resolution mass spectrometry (SFC-HRMS)

Polyglycerol esters of fatty acids (PGEs), a very complex mixture of various isomers, are widely used as green surfactants in different industrial fields such as in cosmetic, pharmaceutic and food industries. However, no study related to the purification and the absolute quantification of these compounds has been described yet. In this study, we developed a rapid and efficient method for characterization and quantification of PGEs using Supercritical Fluid CO₂ coupled to High-Resolution Mass Spectrometry (SFC-HRMS). The SFC conditions were first considered including the stationary phase, the nature of mobile phase, the column temperature, the back-pressure regulator. The MS parameters (drying-gas temperature, capillary voltage, nozzle voltage, fragmentor voltage) were then investigated to get the best sensitivity for the PGE analysis. The MS/MS-based structural characterization of targeted PGE, triglycerol mono-oleate (PG3+1C18:1), was established and is helpful to study complex mixtures of PGEs with numerous isobaric PGEs. PG3+1C18:1 was then purified at lab scale and used as standard for quantification. This enabled to develop a rapid quantification method for PG3+1C18:1 within 12 min with good linearity (R² = 0.9997) as well as sensitivity (picogram level). The validated method was then successfully applied to quantify PG3+1C18:1 in commercial products in order to evaluate their composition.

Analysis of boswellic acids in dietary supplements containing Indian frankincense (Boswellia serrata) by Supercritical Fluid Chromatography

Boswellic acids, a class of triterpenes, are the bioactive constituents in Indian frankincense, an herbal drug with pronounced anti-inflammatory activity. In this study their separation and quantification in B. serrata extracts is reported for the first time by using Supercritical Fluid Chromatography. Under optimized conditions, i.e. a Viridis HSS C18 SB column and carbon dioxide, methanol, acetonitrile and ammonium hydroxide as mobile phase, six boswellic acids could be separated in under 6 min. The assay fulfilled all validation criteria with coefficients of determination higher than 0.999, a wide linear range (30-1000 μg/mL), recovery rates from 97.1-103.0 %, excellent precision, and detection limits typical for SFC with UV-detection (≤ 5.5 μg/mL). The method could easily be hyphenated to mass spectrometry, which was helpful to tentatively assign further compounds (mainly derivatives of tirucallic acid) and to increase the assay's sensitivity. Its practical applicability was confirmed by analyzing several commercial products, which mainly contained β-boswellic acid as dominant triterpene, yet in extremely variable amounts ranging from 0.9 to 16.9 %.

A review of retention mechanism studies for packed column supercritical fluid chromatography

The packed column supercritical fluid chromatography has risen as a promising alternative separation technique to the conventional liquid chromatography and gas chromatography. Although the packed column supercritical fluid chromatography has many advantages compared to other chromatographic techniques, its separation mechanism is not fully understood due to the complex combination effects of many chromatographic parameters on separation quality and the lacking of global strategies for studying separation mechanisms. This review aims to provide recent information regarding the chromatographic behaviors and the effects of the parameters on the separation, discuss the results, and point out the remaining bottlenecks in the packed column supercritical fluid chromatography retention mechanism studies.

Comparison of internal energy distributions generated by supercritical fluid chromatography versus liquid chromatography hyphenated with electrospray high resolution mass spectrometry

The internal energy distributions of thermometer ions dissolved in supercritical and liquid solvents and produced in the gas phase by electrospray (ESI) were measured and compared by the survival yield method. The influence of different chromatographic conditions such as the nature of solvents, the composition of the mobile phase, the pressure of the back pressure regulator was studied for supercritical fluid chromatography (SFC) whereas the influence of the composition and of the flow rate of the mobile phase was investigated for liquid chromatography (LC). The MS instrumental parameters were studied in parallel for SFC and LC showing that the drying gas temperature and the fragmentor voltage affected the internal energy distribution, whereas the capillary voltage did not modify the internal energy distribution. A comparison of the internal energy distributions generated by SFC and LC was carried out leading to conclude that SFC led to higher internal energy, i.e. more in-source fragment ions, than LC.

Supercritical fluid chromatography - Mass spectrometry in metabolomics: Past, present, and future perspectives

Metabolomics, which consists of the comprehensive analysis of metabolites within a biological system, has been playing a growing role in the implementation of personalized medicine in modern healthcare. A wide range of analytical approaches are used in metabolomics, notably mass spectrometry (MS) combined to liquid chromatography (LC), gas chromatography (GC), or capillary electrophoresis (CE). However, none of these methods enable a comprehensive analysis of the metabolome, due to its extreme complexity and the large differences in physico-chemical properties between metabolite classes. In this context, supercritical fluid chromatography (SFC) represents a promising alternative approach to improve the metabolome coverage, while further increasing the analysis throughput. SFC, which uses supercritical CO₂ as mobile phase, leads to numerous advantages such as improved kinetic performance and lower environmental impact. This chromatographic technique has gained a significant interest since the introduction of advanced instrumentation, together with the introduction of dedicated interfaces for hyphenating SFC to MS. Moreover, new developments in SFC column chemistry (including sub-2 µm particles), as well as the use of large amounts of organic modifiers and additives in the CO₂-based mobile phase, significantly extended the application range of SFC, enabling the simultaneous analysis of a large diversity of metabolites. Over the last years, several applications have been reported in metabolomics using SFC-MS - from lipophilic compounds, such as steroids and other lipids, to highly polar compounds, such as carbohydrates, amino acids, or nucleosides. With all these advantages, SFC-MS is promised to a bright future in the field of metabolomics.