The many faces of packed column supercritical fluid chromatography – A critical review

An improved subtraction model applied in supercritical fluid chromatography to characterise polar stationary phases

Herein, an improved subtraction model was proposed to characterise the polar stationary phases in supercritical fluid chromatography (SFC). Fifteen stationary phases were selected, including two types of aromatic columns, Waters Torus and Viridis series columns, as well as silica and amino columns. Ethylbenzene and Torus 1-AA were defined as the reference solute and column, respectively. Identifying the interaction with the maximum contribution to retention in SFC separation and using it as the initial term is a key step in modelling. The dipole, or induced dipole interaction (θ'P), replaced the hydrophobic interaction (η'H) as the starting term. The improved model was expressed as logα=η'H+β'A+α'B+κ'C+θ'P+ε'E+σ'S, where the term ε'E indicated that anion exchange interaction was intentionally supplemented. A 7-step modelling process, including bidirectional fitting and residual analysis, was proposed. The obtained column parameters had reasonable physical significance, with the adjusted determination coefficient (R2adj) greater than 0.999 and the standard error (SE) less than 0.029. Methodological validation was further performed using the other four columns and 12 solutes that were not involved in the modelling. The result revealed good predictions of solutes' retention, as demonstrated by R2adj from 0.9923 to 0.9979 and SE from 0.0636 to 0.1088. This study indicated the feasibility of using the improved subtraction model to characterise polar stationary phases in SFC, with the most crucial being the determination of an initial term, followed by the addition of a new descriptor and the selection of an appropriate reference column. The study expanded the application scope of the subtraction model in SFC, which will help gain an in-depth understanding of the SFC separation mechanism.

Large-scale purification of a deprotected macrocyclic peptide by supercritical fluid chromatography (SFC) integrated with liquid chromatography in discovery chemistry

A macrocyclic peptide A was successfully purified in large quantities (∼30 g) in >95 % purity by an integrated two-step orthogonal purification process combining supercritical fluid chromatography (SFC) with medium-pressure reverse-phase liquid chromatography (MP-RPLC). MP-RPLC was used to fractionate the crude peptide A, remove unwanted trifluoroacetic acid (TFA) originating from the peptide A cleavage off the resin, and convert the peptide A into ammonium acetate salt form, prior to the final purification by SFC. A co-solvent of methanol/acetonitrile containing ammonium acetate and water in CO2 was developed on a Waters BEH 2-Ethylpyridine column. The developed SFC method was readily scaled up onto a 5 cm diameter column to process multi-gram quantities of the MP-RPLC fraction to reach > 95 % purity with a throughput/productivity of 0.96 g/h. The incorporation of SFC with MP-RPLC has been demonstrated to have a broader application in other large-scale polypeptide purifications.

The Impact of Water as an Additive on the Elution of Some Basic Organic Compounds in Supercritical Fluid Chromatography

In this study, water was used as an additive in the methanol-modified carbon dioxide-based eluent for the elution of some basic organic compounds from a hybrid silica column via supercritical fluid chromatography (SFC). The experiments were applied to sulfonamides, propranolol, and other organic nitrogen compounds involving aromatic rings from different classes of amine, pyrimidine, and purine with different pKa values (the pKa values for the studied analytes range from 4.6 to 10.4). The results revealed different responses to the different percentages of water addition. Adding 1~2% of water to the modifier (methanol) led to a positive effect manifested by more symmetrical peak shapes and reduced retention times for most compounds. The key factor for this improvement in the properties of chromatographic peaks is due to the adsorption of water on the silanol groups of the stationary phase, consequently resembling the phenomena observed in hydrophilic interaction liquid chromatography (HILIC). Moreover, the availability of hydrogen bond acceptor and donor sites in the analyte structure is an important factor to be considered when adding water as an additive to the modifier for improving the chromatographic peaks. However, introducing water in an amount higher than 3% resulted in perturbed chromatographic signals. It was also found that water as an additive alone could not successfully elute propranolol from the hybrid silica column with an acceptable peak shape; thus, the addition of a strong base such as amine salts was also necessary. The proposed use of a particular amount of water in the mobile phase could have a positive effect compared to the same mobile phase without water, improving the chromatographic peak properties of the elution of some basic organic compounds from the hybrid silica column.

Synthesis and evaluation of aromatic stationary phases based on linear solvation energy relationship model for expanded application in supercritical fluid chromatography

In the last decade, the separation application based on aromatic stationary phases has been demonstrated in supercritical fluid chromatography (SFC). In this paper, four aromatic stationary phases involving aniline (S-aniline), 1-aminonaphthalene (S-1-ami-naph), 1-aminoanthracene (S-1-ami-anth) and 1-aminopyrene (S-1-ami-py) were synthesized based on full porous particles (FPP) silica, which were not end-capped for providing extra electrostatic interaction. Retention mechanism of these phases in SFC was investigated using a linear solvation energy relationship (LSER) model. The aromatic stationary phases with five positive parameters (a, b, s, e and d+) can provide hydrogen bonding, π-π, dipole-dipole and cation exchange interactions, which belong to the moderate polar phases. The LSER results obtained using routine test solutes demonstrated that the aforementioned interactions of four aromatic stationary phases were influenced by the type and bonding density of the ligand, but to a certain extent. Furthermore, the LSER data verified that the S-1-ami-anth column based on full porous particles silica had higher cation exchange capacity (d+ value), compared to the commercialized 1-AA column (based on the ethylene-bridged hybrid particles). The relationship between the d+ value and SFC additive was quantitatively proved so as to regulate electrostatic interaction reasonably. This value was greatly increased by phosphoric acid, slightly increased by trifluoroacetic acid and formic acid, but significantly reduced by ammonium formate and diethylamine. Taking the S-1-ami-naph column as an example, better peek shape of the flavonoids was obtained after the addition of 0.1 % phosphoric acid in MeOH while isoquinoline alkaloids were eluted successfully within 11 min after adding 0.1 % diethylamine in MeOH. Combined with the unique π-π interaction and controllable electrostatic interaction, the aromatic stationary phases in this study have been proven to have expandable application potential in SFC separation.

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.

Development of an ultra-high-performance supercritical fluid chromatography method for the analysis of phenols in the pyrolysis aqueous fraction

The pyrolysis process consists of the thermal decomposition of biomass in an inert atmosphere, which produces a liquid (bio-oil) composed of a complex mixture of organic compounds, including an oil and water phase. The aqueous fraction can reach up to 45% w/w, and understanding its composition is of utmost importance in determining its intended destination, whether for the reuse of compounds in industrial applications or for treating the effluent for disposal. In this study, a fast, direct, and efficient method using ultra-high-performance supercritical fluid chromatography (UHPSFC) was developed and optimized for monitoring phenols in aqueous samples obtained from the pyrolysis processing of six different biomass sources. The following parameters were evaluated for method optimization: stationary phase type, mobile phase flow, organic modifier, sample diluent, temperature, pressure, and modifier gradient time. With a total analysis time of 26 min, out of the fourteen (14) investigated phenolic compounds, eleven (11) were successfully separated after method optimization, and among them, five (5) were quantified in all six aqueous fractions. The aqueous fractions of residue from cowpea pod (1.89 mg.mL-1), sugar apple (3.09 mg.mL-1), and acerola (4.79 mg.mL-1) presented lower concentrations compared to grape (8.16 mg.mL-1), pine nuts (6.68 mg.mL-1), and guava (6.05 mg.mL-1) fractions. However, even at lower concentrations, all biomasses showed promising results regarding the phenolic compound content, analytes that have high added value for the chemical industry.

Columns in analytical-scale supercritical fluid chromatography: From traditional to unconventional chemistries

Within this review, we thoroughly explored supercritical fluid chromatography (SFC) columns used across > 3000 papers published from the first study carried out under SFC conditions in 1962 to the end of 2022. We focused on the open tubular capillary, packed capillary, and packed columns, their chemistries, dimensions, and trends in used stationary phases with correlation to their specific interactions, advantages, drawbacks, used instrumentation, and application field. Since the 1990s, packed columns with liquid chromatography and SFC-dedicated stationary phases for chiral and achiral separation are predominantly used. These stationary phases are based on silica support modified with a wide range of chemical moieties. Moreover, numerous unconventional stationary phases were evaluated, including porous graphitic carbon, titania, zirconia, alumina, liquid crystals, and ionic liquids. The applications of unconventional stationary phases are described in detail as they bring essential findings required for further development of the supercritical fluid chromatography technique.

Supercritical fluid chromatography based on reversed-phase/ ion chromatography mixed-mode stationary phase for separation of spirooxindole alkaloids

The present paper illustrates the versatility of the supercritical fluid chromatography (SFC) since, for the first time, four spirooxindole alkaloids (SOAs) including two pairs of isomers were separated by using two types of reversed-phase/ ion chromatography (RP/IC) mixed-mode stationary phases. Two mixed-mode stationary phases (C8SAX and C8SCX) was simultaneously provided dispersive and electrostatic interactions, which were suitable for the separation of such alkaloids. This study tried to provide an in-depth understanding of the SFC separation mechanism of the mixed-mode stationary phase through investigation of the impact of changes in mobile phase composition on alkaloids' retention behavior. On C8SAX, due to the strong electrostatic repulsion, there was a very narrow elution window of the alkaloids, of which behaviors were hardly affected by adding diethylamine in mobile phase. When adding formic acid or acidic ammonium formate, the prolonged retention time of alkaloids was presented because of the shielded effect of formate anions on the electrostatic repulsion. In particular, better peak shape and improved resolution were obtained by using acidic ammonium formate due to the deactivation of silanol groups by ammonium cations. On the other hand, both formic acid and acidic ammonium formate can strengthen the electrostatic attraction of C8SCX, causing difficult elution of the alkaloids. Ammonium cations from either the protonated diethylamine or the ionized ammonium formate, were considered as counter ions to effectively mask the electrostatic attraction of C8SCX, to significantly reduce the retention of alkaloids, but improve the resolution. Finally, utilizing two developed SFC methods, i.e., C8SAX with EtOH+ 10 mM acidic ammonium formate in CO2, or C8SCX with EtOH+0.1% diethylamine in CO2, the baseline separation of corynoxeine and isocorynoxeine, rhynchophylline and isorhynchophylline was achieved within 5 min.

Electrospray Ionization Efficiency Predictions and Analytical Standard Free Quantification for SFC/ESI/HRMS

Supercritical fluid chromatography (SFC) is a promising, sustainable, and complementary alternative to liquid chromatography (LC) and has often been coupled with high resolution mass spectrometry (HRMS) for nontarget screening (NTS). Recent developments in predicting the ionization efficiency for LC/ESI/HRMS have enabled quantification of chemicals detected in NTS even if the analytical standards of the detected and tentatively identified chemicals are unavailable. This poses the question of whether analytical standard free quantification can also be applied in SFC/ES/HRMS. We evaluate both the possibility to transfer an ionization efficiency predictions model, previously trained on LC/ESI/HRMS data, to SFC/ESI/HRMS as well as training a new predictive model on SFC/ESI/HRMS data for 127 chemicals. The response factors of these chemicals ranged over 4 orders of magnitude in spite of a postcolumn makeup flow, expectedly enhancing the ionization of the analytes. The ionization efficiency values were predicted based on a random forest regression model from PaDEL descriptors and predicted values showed statistically significant correlation with the measured response factors (p < 0.05) with Spearman's rho of 0.584 and 0.669 for SFC and LC data, respectively. Moreover, the most significant descriptors showed similarities independent of the chromatography used for collecting the training data. We also investigated the possibility to quantify the detected chemicals based on predicted ionization efficiency values. The model trained on SFC data showed very high prediction accuracy with median prediction error of 2.20×, while the model pretrained on LC/ESI/HRMS data yielded median prediction error of 5.11×. This is expected, as the training and test data for SFC/ESI/HRMS have been collected on the same instrument with the same chromatography. Still, the correlation observed between response factors measured with SFC/ESI/HRMS and predicted with a model trained on LC data hints that more abundant LC/ESI/HRMS data prove useful in understanding and predicting the ionization behavior in SFC/ESI/HRMS.

An attempt to apply a subtraction model for characterization of non-polar stationary phase in supercritical fluid chromatography

This study verified the feasibility of using a subtraction model to characterize the non-polar stationary phases (including C4, C8, and phenyl-type) in supercritical fluid chromatography (SFC). The model with 6 terms was expressed as log α = η'H + θ'P + β'A + α'B + κ'C + σ'S, where a term θ'P indicating dipole or induced dipole interaction was intentionally supplemented. Ethylbenzene and SunFire C8 were respectively defined as the reference solute and column. A 7-step modeling procedure was proposed: in the first 6 steps, except σ'S, by the use of a bidirectional fitting method, other parameters were calculated based on the equation: log α = log (k_i/k_ref) ≈ η'H + θ'P + β'A + α'B + κ'C; and in the 7th step, residual analysis was employed to describe the σ'S term according to the equation: σ'S = log α_exp. - log α_pre. Furthermore, six columns that were not involved in modeling process and 12 compounds with unknown retention were used for methodology validation. It showed good predictions of log k, as demonstrated by adjusted determination coefficient (R²_adj) from 0.9927 to 0.9998 (column) and from 0.9940 to 0.9999 (compound), respectively. The subtraction model emphasized the contribution of dipole or induced dipole interaction to the retention in SFC, and it obtained the σ'S term through residual analysis. Moreover, it made reasonable physical-chemical sense as the linear solvation energy relationship (LSER) model did, with the distinct advantages of better fitting and more accurate prediction. This study provided some new insights into the characterization of non-polar stationary phases in SFC.

Separation and determination of triadimefon and its metabolites triadimenol enantiomers in fruit puree by supercritical fluid chromatography

A method was established for the separation and determination of triadimefon and its metabolite triadimenol enantiomer residues in major complementary fruit puree for infants and young children (banana puree, pineapple puree, and grape puree) by supercritical fluid chromatography. After the samples were extracted with acetonitrile and purified with a solid phase extraction cartridge, Acquity Trefoil CEL2 chiral chromatographic column was adopted for separation, and gradient elution was conducted at the flow rate of 1.0 ml/min under the mobile phase of supercritical carbon dioxide - 0.5% ammonia methanol, the detection wavelength was 220 nm and quantification was conducted with the external standard method. The limits of quantitation of triadimefon and triadimenol enantiomers were both 0.05 mg/kg, the linear ranges were 0.5-50 mg/L, and the linear correlation coefficients were greater than 0.9993. The recoveries in the spiked samples at 0.05, 0.2, and 3.0 mg/kg were from 80.1 to 106%, and the relative standard deviation reached 3.3-7.6%. The method is efficient, rapid, reproducible, and environmentally friendly, enabling accurate analysis of pesticide enantiomers, which can detect the enantiomer residues of triadimefon and its metabolite triadimenol in major complementary fruit puree for infants and young children.

An accurate and reliable analytical strategy for simultaneous determination of target furanocoumarins and flavonoids in cosmetic and pharmaceutical samples by ultra-high performance supercritical fluid chromatography

Furanocoumarins and flavonoids have various important biological activities and wide application. In the present study, a rapid and reliable supercritical fluid chromatography method was proposed for the separation of 10 target components including 8 furanocoumarins and 2 flavonoids. After detailed condition optimization, the 10 target compounds can be baseline separated on a Trefoil CEL1 (3.0 mm × 150 mm, 2.5 µm) column using gradient elution. A 0.07% (v/v) trifluoroacetic acid in ethanol was determined to be the most proper mobile phase for the separation of target compounds. The column temperature, back pressure, flow rate were set at 36 ℃, 2000 psi, 1.0 mL min^-1 to 1.4 mL min^-1, respectively. The ten target compounds were analyzed within 24 min using the optimized conditions. Under the optimized conditions, all the target compounds showed good linearity with linear correlation coefficients higher than 0.995, and satisfactory recovery in the range of 83.52-112.92%. All these results showed that the developed ultra-high performance supercritical fluid chromatography method was reliable and effective. Finally, the application of the developed method to cosmetic, Psoraleae fructus and Angelicae dahuricae radix samples were presented. The results highlight the applicability of the ultra-high performance supercritical fluid chromatography method to the analysis of interested compounds in pharmaceutical and cosmetic samples.

An in-depth investigation of supercritical fluid chromatography retention mechanisms by evaluation of a series of specially designed alkylsiloxane-bonded stationary phases based on linear solvation energy relationship

Fundamental research on supercritical fluid chromatography (SFC) has gained considerable interest, with many studies focusing on its retention mechanism based on the linear solvation energy relationship (LSER) model. In this paper, a series of alkylsiloxane-bonded stationary phases were specifically designed and synthesized, then evaluated using the mobile phase composed of CO₂ with 10% (v/v) methanol. The study demonstrated the close relationship between the interactions (manner and magnitude) of stationary phases and the C-chain length, bonding density and the endcapping treatment. All C8 phases provide positive e, v and negative s, whose magnitude was regularly affected by bonding density. It was worth mentioning the non-endcapped C8 phases could provide H-bonding (positive a and b) by reducing the bonding density of the alkyl chain. Once it was endcapped, the interaction manner did not vary with bonding density adjustment. The non-endcapped C4 phases with higher bonding density could establish additional dispersion interaction (positive v). It can be seen that two synthesis strategies, 1) non-endcapped, long C-chain (C8) combined with low bonding density, and 2) non-endcapped, short C-chain (C4) combined with high bonding density, can obtain the alkylsiloxane-bonded stationary phases (C8-1 and C4-3) to provide both polar and dispersion interactions, showing different separation selectivity. Furthermore, the LSER model with ionic terms was applied to evaluate partial C8 columns, and its rationality was verified. The non-endcapped C8 showed great d⁺ values, which originated from the silanol groups. C8SCX also possessed a great d⁺ value due to the benzenesulfonic acid groups. A remarkable result showed that C8SAX exhibited prominent d^- and d⁺ values simultaneously due to the combined effect of silanol and quaternary ammonium groups, which indicates the unique selectivity when separating ionic compounds. This study provides in-depth insights into the retention mechanism of alkylsiloxane-bonded stationary phases in SFC, as well as a reference for the design of SFC stationary phases.

Chiral separation and quantitation of methylphenidate, ethylphenidate, and ritalinic acid in blood using supercritical fluid chromatography

Supercritical fluid chromatography (SFC) is a technique that analyzes compounds that are temperature-labile, have moderately low weight, or are chiral compounds. Methylphenidate (MPH) is a chiral compound with two chiral centers. MPH has two chiral metabolites, ethylphenidate (EPH) and ritalinic acid (RA). MPH is sold as a racemic mixture. The d-enantiomer of threo-MPH is responsible for medicinal effects. Due to the differing effects of the enantiomers, it is important to analyze the enantiomers individually to better understand their effects. This method utilizes SFCand solid-phase extraction (SPE) to separate and analyze the enantiomers of MPH, EPH, and RA in postmortem blood. The objective of this method was to assess a unique approach with SFC for enantiomeric separation of MPH, EPH, and RA. A SPE method was developed and optimized to isolate the analytes in blood and validated as fit-for-purpose following international guidelines. The linear range for MPH and EPH was 0.25-25 and 10-1000 ng/mL for RA in blood. Bias was -8.6% to 0.8%, and precision was within 15.4% for all analytes. Following method validation, this technique was applied to the analysis of 49 authentic samples previously analyzed with an achiral method. Quantitative results for RA were comparable to achiral technique, whereas there was loss of MPH and EPH over time. The l:d enantiomer ratio was calculated, and MPH demonstrated greater abundance of the d-enantiomer. This is the first known method to separate and quantify the enantiomers of all three analytes utilizing SFC and SPE.

Fast Triacylglycerol Fingerprinting in Edible Oils by Subcritical Solvent Chromatography

Supercritical fluid chromatography (SFC) has witnessed a resurge of interest in the last decade, motivated by substantial progress in hardware capabilities and the development of specifically tailored packing materials. Meanwhile, the technique has been transitioning to the use of mobile phases in which an organic co-solvent is mixed to carbon dioxide under subcritical conditions (subFC). The use of a mobile phase modifier will also affect the selectivity, and extend the range of SFC-amenable compounds, including lipids. In this research, a subFC method was developed for the separation of triacylglycerols (TAGs) in edible oils of vegetable origin, namely borage, corn, hazelnut, olive, palm, peanut, and soybean oil. For all the samples investigated, elution of the TAG species was achieved within 8 min, the only exception being borage oil (14 min run), characterized by TAGs spanning in a wide range of partition number (PN 36–56), as constituted by fatty acids differing for alkyl chain lengths (CN 14–24), and degree of unsaturation (DB 0–9). The coupling to mass spectrometry (MS) detection allowed for the average parameters of fatty acid composition to be derived in the oil TAG constituents. A total of 121 TAGs were identified by subFC-MS, with reduced analysis time and solvent consumption (1.5 mL per analysis) compared to common chromatographic approaches. Key features of the method developed hereby are low toxicity, costs, and environmental impact. Notably, the achieved separations were conducted at room temperature, which is beneficial in terms of column life.

Defining a generic column set for achiral supercritical fluid chromatography applied to pharmaceuticals or natural products

When starting a method development in supercritical fluid chromatography (SFC), the first step is usually to screen several stationary phases based on previous experience or simply based on what is available in the laboratory. However, as there are now a large number of stationary phases available for SFC, the choice of an adequate set of columns to rapidly achieve a satisfying result can be difficult. In this project, 16 columns comprising a wide diversity of stationary phases and polarities ranging from the most polar (like bare silica gel) to the least polar (like octadecylbonded-silica) were compared, based on the gradient analysis of 129 probe compounds. The set mostly comprised active pharmaceutical ingredients, natural products and a few metabolites. The columns were ranked with the help of Derringer desirability functions taking account of (i) the number of compounds eluted from the column, (ii) the elution time in a suitable time frame, (iii) the average peak width, (iv) the average peak symmetry and (v) the spreading of retention along the gradient time. The five criteria selected showed no correlation. Overall, it appeared that those columns that had a high overall score were good for several reasons, like bare silica gel, propanediol-bonded silica or pentabromobenzyloxy-bonded silica. Initially, the columns had been screened with a gradient elution starting from 5% co-solvent and ending with 50% co-solvent in CO₂. However, for some most retentive columns like amide-bonded silica, too many compounds remained non-eluted from the column. To examine this column more fairly, a second elution gradient was applied that ended with 100% co-solvent. This proved effective in restoring good overall performance through the elution of the most polar compounds.

Ultra-high-performance supercritical fluid chromatography-mass spectrometry for the analysis of organic contaminants in sediments

A nontarget screening method was developed based on D-optimal designs for ultra-high performance supercritical fluid chromatography with positive and negative electrospray ionization mode mass spectrometry. A mixture of organic contaminants such as pesticides, steroids, surfactants, phenolic and fatty acids, and polycyclic aromatic hydrocarbon derivatives, was used for the optimization. An aprotic mixture of dichloromethane and acetone [3:1] performed overall best as the injection solvent. The highest peak capacities (n) were accomplished at the shallowest gradient (1%B/min), ammonium formate (n = 378 in negative ionization mode), or ammonium acetate (n = 327 in positive ionization mode) in methanol as the modifier. Capillary voltage, make-up solvent flow rate, water, and additive concentration were the most significant factors for improving peak intensity: higher peak intensities were obtained at lower additive concentrations (5mM ammonium formate), and with 5% water in positive ionization mode. Conversely, water had detrimental effects in negative ionization mode. The optimized method was used to quantify organic contaminants in 17 freshwater sediment samples from Copenhagen, Denmark. Out of 50 monitored contaminants, 35 were detected in at least one sample. Further, the method has a potential for target and nontarget screening analysis of organic contaminants in solid matrices.

Polymethoxyflavones from citrus peel: advances in extraction methods, biological properties, and potential applications

Citrus peel, as an effective component of citrus by-products, contains a large number of natural active components, including pectin, vitamins, dietary fiber, essential oil, phenolic compounds, flavonoids, and so on. With the development of the circular economy, citrus peel has attracted extensive concern in the food industry. The exploitation of citrus peel would assist in excavating potential properties and alleviating the environmental burden. Polymethoxyflavones (PMFs) exist almost in citrus peel, which have remarkable biological activities including antioxidant, anti-inflammatory, anti-cancer, and anti-obesity. Therefore, PMFs from citrus peel have the potential to develop as dietary supplements in the near future. Collectively, it is essential to take action to optimize the extraction conditions of PMFs and make the most of the extracts. This review mainly compiles several extraction methods and bioactivities of PMFs from citrus peel and introduces different applications including food processing, pharmaceutical industry, and plant rhizosphere to develop better utilization of citrus PMFs.

Rapid Determination of Oxygenated and Parent Polycyclic Aromatic Hydrocarbons in Milk Using Supercritical Fluid Chromatography-Mass Spectrometry

Liquid milks are consumed worldwide in large amounts, especially by adolescents and infants. Thus, their health quality linked with polycyclic aromatic hydrocarbon (PAH) contamination has attracted great concern. This study developed a rapid and sensitive supercritical fluid chromatography (SFC)-MS method to determine two typical oxygenated PAHs (OPAHs) and EU 15+1PAHs except for benzo[k]fluoranthene (BkF) in three types of liquid milks: 10 ultra heat treated (UHT) milks, 8 pasteurized milks, and 4 extended-shelf-life pasteurized milks. The instrumental analysis was 15 min with a recovery of 67.66-118.46%, a precision of 1.45-14.68%, detection limits of 0.04-0.24 μg/kg, and quantification limits of 0.13-0.78 μg/kg. We found 9-fluorenone, anthraquinone, 15 EU priority PAHs, and benzo[a]pyrene toxic equivalent quantity (BaPeq) in the 22 milk samples, which were 0.32-1.56 μg/kg, 0.40-1.74 μg/kg, 0.57-8.48 μg/kg, and 0.01-17.42 μg/kg, respectively. The UHT milks and whole fat milks showed higher PAH concentrations than other investigated samples, where the maximum levels of BaP and PAH4 were 0.77 and 3.61 μg/kg, respectively. PAH4 dominantly contributed to the PAH8 concentration and was detected in 73% and 32% of samples at more than 1.0 and 2.0 μg/kg, respectively. The results suggest that raw milks should be strictly monitored and extensively investigated for PAH4 and BaP concentrations for future risk assessment, limitations, and dietary guidance.

Quality evaluation of Panax notoginseng (Burk.) F.H. Chen using supercritical fluid chromatography-mass spectrometry and chemical pattern recognition

An efficient supercritical fluid chromatography-mass spectrometry (SFC-MS) method was developed for the quality evaluation of Panax Notoginseng (Burk) F.H. Chen (P. notoginseng) by combination with chemical pattern recognition (CPR). Design of experiments (DoE) was applied to obtain optimal SFC-MS conditions. Several CPR methods including hierarchical cluster analysis (HCA), principal component analysis (PCA) and partial least squares discriminant analysis (PLS-DA) were employed to establish a classification model based on the peak areas and contents of 12 components in P. notoginseng in order to evaluate the quality difference according to the collecting time (Chunqi and Dongqi) and medicinal parts (fibrous root, rhizome, branch root, and main root). PLS-DA has proved to be a satisfactory method with accurate discrimination of the selected samples. The characteristic variables based on the variable importance in projection (VIP) values were selected using PLS-DA. Three characteristic components (ginsenoside Rg₂, ginsenoside Rg₁, ginsenoside Rb₁) with higher VIP values (>1) were chosen to further build the CPR model. Subsequently, the model was verified by testing another set of samples and the results indicated that the established model was satisfactory. PLS-DA models based on the peak areas of the 12 selected analytes in 30 batches of P. notoginseng could give accurate classification. The obtained results demonstrate that the developed method using SFC-MS and PLS-DA has a great potential for the quality assessment of P. notoginseng.

Rapid determination of phthalates in paper products by supercritical fluid chromatography / tandem mass spectrometry

Phthalic acid esters (PAEs) are widely used as components of industrial and consumer products including paper and cardboard packaging materials contacting with food or human skin. Being endocrine-disrupting chemicals, PAEs have a negative effect on human health and must be controlled in pulp and paper products. In the present study, supercritical fluid chromatography - tandem mass spectrometry in combination with pressurized liquid extraction was proposed for PAEs determination in such objects. Octadecyl stationary phase with non-endcapped silanol groups ensured rapid (4 min) separation of the ten priority PAEs in isocratic elution mode and allowed for effective elimination of interferences from the trace impurities of PAEs in the mobile phase. The attained LOQs are in the range of 0.7-10 μg/L in extracts and 0.02-0.3 μg/g in paper and cardboard samples. The developed method is distinguished by analysis rapidity, easy sample preparation procedure, high selectivity, low susceptibility to mobile phase contamination with analytes, low cost and environmental friendliness due to the use of carbon dioxide as a main component of the mobile phase. The method was successfully tested on real samples of toilet paper and food packaging paper and cardboard in which eight analytes were found at the levels of 0.03-43.5 μg/g. This article is protected by copyright. All rights reserved.

Insights on the Extraction and Analysis of Phenolic Compounds from Citrus Fruits: Green Perspectives and Current Status

Citrus fruits (CF) are highly consumed worldwide, fresh, processed, or prepared as juices and pies. To illustrate the high economic importance of CF, the global production of these commodities in 2021 was around 98 million tons. CF's composition is considered an excellent source of phenolic compounds (PC) as they have a large amount and variety. Since ancient times, PC has been highlighted to promote several benefits related to oxidative stress disorders, such as chronic diseases and cancer. Recent studies suggest that consuming citrus fruits can prevent some of these diseases. However, due to the complexity of citrus matrices, extracting compounds of interest from these types of samples, and identifying and quantifying them effectively, is not a simple task. In this context, several extractive and analytical proposals have been used. This review discusses current research involving CF, focusing mainly on PC extraction and analysis methods, regarding advantages and disadvantages from the perspective of Green Chemistry.

Supercritical fluid chromatography coupled to high-resolution tandem mass spectrometry: an innovative one-run method for the comprehensive assessment of chocolate quality and authenticity

To assess chocolate quality and authenticity comprehensively, a combination of various analytical procedures is involved, thereby making the process time-consuming and costly. Thus, we investigated the potential of ultra-high performance supercritical fluid chromatography coupled to quadrupole-time of flight mass spectrometry (UHPSFC-QTOF-MS) as an alternative to "classic" methods. By combining hexane and aqueous extracts from sequential extraction, a single 8-min analytical run enabled us (i) to determine cocoa butter equivalents (CBEs) and milk fat content based on the detection of selected triacylglycerols, (ii) to calculate dry non-fat cocoa solids based on determined theobromine and caffeine content, and (iii) to profile contained sugars. To obtain the most comprehensive information about sample composition, the MS method comprised a full MS scan for non-target screening and several time-scheduled targeted MS/MS functions ("parallel reaction monitoring") optimized according to the possible concentration ranges of the analytes. For 40 different chocolate samples, our results and those obtained by using standard methods (LC-UV for non-fat cocoa solids, and GC-FID for CBEs) were in good agreement. Compared to the conventional approach for chocolate quality and authenticity control, the presented SFC-MS method is a fast, cost-effective, and efficient alternative, and only samples suspicious for the presence of CBE should be referred to the standard GC-FID method for exact CBE quantification. In the study, also some challenges offered by SFC-MS have been addressed.

Advanced Development of Supercritical Fluid Chromatography in Herbal Medicine Analysis

The greatest challenge in the analysis of herbal components lies in their variety and complexity. Therefore, efficient analytical tools for the separation and qualitative and quantitative analysis of multi-components are essential. In recent years, various emerging analytical techniques have offered significant support for complicated component analysis, with breakthroughs in selectivity, sensitivity, and rapid analysis. Among these techniques, supercritical fluid chromatography (SFC) has attracted much attention because of its high column efficiency and environmental protection. SFC can be used to analyze a wide range of compounds, including non-polar and polar compounds, making it a prominent analytical platform. The applicability of SFC for the separation and determination of natural products in herbal medicines is overviewed in this article. The range of applications was expanded through the selection and optimization of stationary phases and mobile phases. We also focus on the two-dimensional SFC analysis. This paper provides new insight into SFC method development for herbal medicine analysis.

A story of peptides, lipophilicity and chromatography - back and forth in time

Peptides, as part of the beyond the rule of 5 (bRo5) chemical space, represent a unique class of pharmaceutical compounds. Because of their exceptional position in the chemical space between traditional small molecules (molecular weight (MW) < 500 Da) and large therapeutic proteins (MW > 5000 Da), peptides became promising candidates for targeting challenging binding sites, including even targets traditionally considered as undruggable - e.g. intracellular protein-protein interactions. However, basic knowledge about physicochemical properties that are important for a drug to be membrane permeable is missing but would enhance the drug discovery process of bRo5 molecules. Consequently, there is a demand for quick and simple lipophilicity determination methods for peptides. In comparison to the traditional lipophilicity determination methods via shake flask and in silico prediction, chromatography-based methods could have multiple benefits such as the requirement of low analyte amount, insensitivity to impurities and high throughput. Herein we elucidate the role of peptide lipophilicity and different lipophilicity values. Further, we summarize peptide analysis via common chromatographic techniques, in specific reversed phase liquid chromatography, hydrophilic interaction liquid chromatography and supercritical fluid chromatography and their role in drug discovery and development process.

An Assessment of Stationary Phase Selectivity in SFC

Supercritical fluid chromatography (SFC) has seen a recent resurgence in interest following investment in the development of instrument technology by numerous instrument manufacturers. Increased focus on sustainability in chromatographic science, coupled with the orthogonality to reversed phase HPLC, is likely to further drive the uptake of SFC in many sectors. As with any form of chromatography, optimizing separation selectivity is a key variable in providing adequate resolution and accurate identification and quantification of target analytes. Stationary phase chemistry can be readily exploited to substantially alter the separation selectivity obtained. This article examines and characterizes the selectivity differences offered by three prototype SFC phases.

Supercritical fluid chromatography with post-column addition of supporting electrolyte solution for electrochemical determination of tocopherol and tocotrienol isomers

A supercritical fluid chromatography with electrochemical detection system was developed for the simultaneous determination of tocopherol and tocotrienol isomers. The supercritical fluid chromatography with electrochemical detection system was connected with an additional pump to create a flow path to add a supporting electrolyte solution. The supporting electrolyte solution was mixed with a mobile phase in a post-column fashion, enabling the independent control of the separation and detection. After optimization of the measurement conditions, vitamin E isomers and an internal standard substance (2,2,5,7,8-pentamethyl-6-hydroxychroman) were separated within 30 min using a mixture of supercritical carbon dioxide and methanol (99:1, v/v) as a mobile phase and a cyanopropyl column (4.6 mm inner diameter × 250 mm length, 5 μm). For the electrochemical detection, methanol containing 1.0 mol/L ammonium acetate was used as a supporting electrolyte solution and the applied potential was set at +0.8 V. This analytical method showed good linearity (5 to 100 μg/mL) and repeatability (less than 2.5% relative standard deviation (n = 6)) and was applicable to the determination of tocopherol and tocotrienol isomers in nutrition supplements. This article is protected by copyright. All rights reserved.

Hyphenated structural identification of additives in transmission fluids

Transmission fluids and other lubricants are used to reduce friction in engines and other surfaces. Additives in these transmission fluids are important for prolonging their lifetime and obtaining the desired physical properties for the lubricant. In this work, we show the successful structural identification of an important class of additives in transmission fluids by hyphenating several analytical techniques in-line. First, a separation of the additives in a transmission fluid was achieved within 10 minutes by using supercritical fluid chromatography. SFC-¹H NMR was then used as a novel analysis method to structurally identify different additives in transmission fluids, in combination with mass spectrometry data. By this unique hyphenation of SFC, NMR spectroscopy and MS, several alkylated diphenylamines were identified, which are one of the most important classes of antioxidants.

A novel hyphenated analytical method to unravel complex mixtures, SFC-¹H NMR/MS, is introduced. It is applied for the structural analysis of additives in transmission fluids which can be used to prolong the lifetime of lubricants.

Analysis of short-chain bioactive peptides by unified chromatography-electrospray ionization mass spectrometry. Part I. Method development

A method to analyse short-chain bioactive peptides (MW < 800 Da) and their impurities was developed with a unified chromatography (UC) analysis, including a wide mobile phase gradient ranging from supercritical fluid to near-liquid conditions, with UV and electrospray ionization mass spectrometry detection (ESI-MS). Four stationary phases and three mobile phase compositions were examined. Ten model peptides were first selected to identify the best operating conditions, including five linear tripeptides and five cyclic pentapeptides, with log P values ranging from -5.9 to 3.6, and including isomeric species. Derringer desirability functions were designed to identify optimal operating conditions based on 7 criteria, namely the number of peaks detected (including all impurities resolved), the proportion of the chromatogram occupied by target peaks, the least favourable resolution observed between the main peptide and impurities, peak shape features (asymmetry and peak width at half height), and finally the signal-to-noise ratio observed both with UV (210 nm) and ESI-MS in positive ionization mode. The optimum conditions were obtained on Ascentis Express OH5 stationary phase, with a mobile phase composed of carbon dioxide and methanol, comprising 2% water and 20 mM ammonium hydroxide. The final gradient program ranged from 5 to 80% co-solvent in CO₂, with a reversed flow rate gradient ranging from 3.0 to 1.5 mL/min. Back-pressure was set at 120 bar and the column oven temperature at 60°C. Optimal conditions were applied to a large set of 76 peptides (34 linear tripeptides and 42 cyclic pentapeptides) and provided adequate scattering of the peaks in the retention space, together with some separation of isomeric species, particularly for the cyclic peptides.

Supercritical fluid chromatography for the analysis of natural dyes: From carotenoids to flavonoids

Plant-derived natural dyes are used in a variety of formulated products, from food to cosmetics and pharmaceutics. In addition to their color, they also provide some bioactivity. While they are mostly analyzed with high-performance liquid chromatography, supercritical fluid chromatography was also employed for several dye families, mostly for carotenoids and chlorophylls, and more recently for anthraquinones and flavonoids. These supercritical fluid chromatography methods are described in this review. Because the dyes have different structures and structural variations (polarity, isomers, etc.), the best chromatographic system to achieve their separation is not always the same. Hydrophobic stationary phases are preferred for the most hydrophobic dyes (chlorophylls and carotenoids) while polar stationary phases are preferred for the polar dyes (anthraquinones and flavonoids). Regarding the mobile phase composition, chlorophylls and carotenoids are best eluted with moderate proportions of co-solvent in CO₂ (about 40%), while the most polar glycosylated flavonoids require higher proportions of co-solvent and acidic additives. Because dyes are colorful, ultraviolet-visible detection is often sufficient, while mass spectrometry offers additional structural information. Furthermore, fundamental information can also be gained through chromatographic analysis of dyes: either solubility in supercritical fluids, in view of their extraction, or retention behavior providing an understanding of stationary phase properties.

Determination of Saccharides in Honey Using Supercritical Fluid Chromatography Coupled with Single Quadrupole Mass Spectrometry

Saccharides are one of the most important biomoleculars in the world, which are responsible for various roles in living organisms. In this work, a combined use of mass spectrometry (MS) together with supercritical fluid chromatography (SFC) was applied for the separation of eight saccharides. A satisfactory separation was achieved within 15 min on a Unitary Diol column using 10 mM ammonium acetate in methanol as mobile phase additive at a temperature of 60°C. Compared to liquid chromatography-MS (LC-MS), the SFC-MS presented faster running time and better selectivity. The SFC-MS was validated and applied to the analysis of three saccharides (fructose, glucose and sucrose) present in different honey samples. The limits of detection (LODs) and limits of quantification (LOQs) of SFC-MS method for fructose, glucose and sucrose were 5.2, 4.5, 7.4 μg/mL and 17.2, 14.9, 24.1 μg/mL, respectively. The relative standard deviations for intrabatch and interbatch precision were <2.5% and the recoveries ranged from 97.6% to 103.1% for three concentration levels. The values of LODs and LOQs using SFC-MS method were lower (1.5-2.4-fold) than using LC-MS method. The results demonstrated the potential of SFC-MS for fast and sensitive determination of saccharides in honey.

Crystalline sponge X-ray analysis coupled with supercritical fluid chromatography: a novel analytical platform for the rapid separation, isolation, and characterization of analytes

Crystalline sponge (CS) based X-ray diffraction (XRD) analysis allows for the observation of the structure of an analyte, including its absolute configuration. Herein we report a powerful analytical platform for the separation, isolation, and structural elucidation of a target analyte in a seamless way by coupling supercritical fluid chromatography (SFC) with CS-based XRD analysis (SFC-CSXRD). The efficacy of this methodology is demonstrated by the rapid characterization of regio- and stereoisomers using three types of CSs with differing tolerances to the solvents used in SFC and guest-soaking.