An attempt to estimate ionic interactions with phenyl and pentafluorophenyl stationary phases in supercritical fluid chromatography

On the discrepancy between crossovers of solubility in supercritical carbon dioxide and retention in supercritical fluid chromatography

Various crossover phenomena are immanent to supercritical fluids due to multidirectional temperature effects in highly compressible supercritical fluid media. Solubility crossover, i.e. controversial effect of temperature on solubility at different pressures, is probably the most well-known among them. A curious discrepancy in upper crossover pressure values between solubility in supercritical carbon dioxide and retention in supercritical fluid chromatography with pure CO2 as an eluent was unexpectedly observed for several non-polar compounds on different stationary phases. In some cases, retention crossover was found to happen at pressures almost twice as high as pressures for solubility crossover for the same compound. Retention data for three solutes with known solubility crossovers: anthracene, benzoic acid and vanillin, were collected at different pressures and temperatures for several stationary phases. The existence of upper retention crossovers, i.e. such pressure values beyond which temperature increase starts decreasing retention, were registered for all solute-sorbent combinations. Using known thermodynamic models of temperature effect on retention in supercritical fluid chromatography and on solubility in supercritical carbon dioxide, possible reasoning for the observed discrepancies is discussed. Major contribution of the balance between adsorption and partition retention mechanisms in defining retention crossover values is hypothesized.

Entropy driven separation of xylene isomers on graphitic carbon adsorbents

Separation of xylene isomers remains one of the most important and challenging applications of adsorption-based separations in petrochemical industry. Despite the sustainable success of zeolite-based separations a search for efficient adsorbents selective for xylenes, especially para-xylene, is constantly ongoing. In this work, a potentially scalable chromatographic separation of all three xylenes was achieved on graphitic carbon sorbents, including a self-packed sorbent based on an oligo-graphene. A curious feature of this separation is stronger retention of para-xylene than meta- and, in some conditions, even than ortho-xylene. Noticeably, separation selectivity between para- and meta-isomers does not depend on temperature. Apparently, lower entropy of para-xylene in solution due to its higher molecular symmetry leads to a lesser adsorption entropy loss, which makes its adsorption statistically more likely. The concept of using carbon adsorbents for entropy driven chromatography separations may be useful for the isolation of xylenes from their mixture and, possibly, for other positional isomers separation.

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.

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.

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.

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 new RP-HPLC method as an auxiliary tool for optimization of sample preparation procedures for tracing of PPCPs of different hydrophilicities

Recently, pharmaceutical and personal care products (PPCPs) have received considerable attention because of their increasing use. Analysis of PPCPs presents a significant analytical challenge, with high-performance liquid chromatography (HPLC) in reversed-phase mode, as the most widely used analytical technique. To facilitate the optimization of the procedures that are applied in the early stages of sample preparation, a simple and fast HPLC method is proposed in this work for the separation of some PPCPs with a wide range of hydrophilicity. Two columns were evaluated (Atlantis dC18 and Discovery HS F5); as for mobile phases: a formate buffer (40 mmol L-1, pH 4) and methanol were tested in a gradient mode. The fluorinated column allowed better separation in a shorter time and better resolution for all analytes (Rs > 1). The proposed method delivered good performance for the tracing of PPCPs and is a suitable alternative to traditional C18-based HPLC methods.

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.

Characterization of stationary phases in supercritical fluid chromatography including exploration of shape selectivity

Achiral packed column supercritical fluid chromatography (SFC) has shown an important regain of interest in academic and industrial laboratories in the recent years. In relation to this increased concern, major instrument manufacturers have designed some stationary phases specifically for SFC use. SFC stationary phases have been widely examined over the last two decades, based on the use of linear solvation energy relationships (LSER), which relate analyte retention to its properties and to the interaction capabilities of the chromatographic system. The method provides some understanding on retention mechanisms (normal phase, reversed phase or mixed-mode) and the possibility to compare stationary phases on a rational basis, especially through a spider diagram providing a visual classification. The latter can be used as a primary tool to select complementary stationary phases to be screened for any separation at early stages of method development, before optimization steps. In this context, the characterization of the 14 columns from the Shim-pack UC series (Shimadzu Corporation, Kyoto, Japan), which are dedicated to SFC and more broadly to unified chromatography (UC), was performed, using the LSER methodology. As in previous works, seven descriptors, including five Abraham descriptors (E, S, A, B, V) and two descriptors describing positive and negative charges (D^- and D⁺) were first employed to describe interactions with neutral and charged analytes. Secondly, two more descriptors were introduced, which were previously employed solely for the characterization of enantioselective systems and expressing shape features of the analytes (flexibility F and globularity G). They brought additional insight into the retention mechanisms, showing how spatial insertion of the analytes in some stationary phases is contributing to shape separation capabilities and how folding possibilities in flexible molecules is unfavorable to retention in other stationary phases.

Analytical strategies for the determination of amino acids: Past, present and future trends

This review describes the analytical methods that have been developed over the years to tackle the high polarity and non-chromophoric nature of amino acids (AAs). First, the historical methods are briefly presented, with a strong focus on the use of derivatization reagents to make AAs detectable with spectroscopic techniques (ultraviolet and fluorescence) and/or sufficiently retained in reversed phase liquid chromatography. Then, an overview of the current analytical strategies for achiral separation of AAs is provided, in which mass spectrometry (MS) becomes the most widely used detection mode in combination with innovative liquid chromatography or capillary electrophoresis conditions to detect AAs at very low concentration in complex matrixes. Finally, some future trends of AA analysis are provided in the last section of the review, including the use of supercritical fluid chromatography (SFC), multidimensional liquid chromatography and electrophoretic separations, hyphenation of ion exchange chromatography to mass spectrometry, and use of ion mobility spectrometry mass spectrometry (IM-MS). Various application examples will also be presented throughout the review to highlight the benefits and limitations of these different analytical approaches for AAs determination.

Characterization of three macrocyclic glycopeptide stationary phases in supercritical fluid chromatography

Macrocyclic glycopeptides have been used as chromatographic stationary phases for over twenty years, particularly for their ability to separate enantiomers. While they are mostly used with buffered aqueous liquid mobile phases, they can also be used in supercritical fluid chromatography (SFC) with mobile phases comprising pressurized carbon dioxide and a co-solvent (like methanol), possibly comprising acidic or basic additives. In the present study, we compared three macrocyclic glycopeptide stationary phases (Chirobiotic V2, Chirobiotic T and Chirobiotic TAG) in SFC with carbon dioxide - methanol (90:10) containing no additives. First, the interactions contributing to retention are evaluated with a modified version of the solvation parameter model, comprising five Abraham descriptors (E, S, A, B, V) and two additional descriptors to take account of interactions with ionizable species (D^- and D⁺). Linear solvation energy relationships (LSER) are established based on the retention of 145 achiral analytes. Secondly, the contributions of interactions to enantioseparations are discussed, based on the analysis of 67 racemates. The individual success rate on each phase was observed to be moderate, especially as these phases are known to be more efficient when acidic or basic additives are employed. Chirobiotic TAG proved more successful than the other two phases. Discriminant analyses were computed to gain some insight on retention mechanisms, but only Chirobiotic TAG provided interpretable results. Finally, the effects of a small proportion of acidic or basic additive on enantioseparation with Chirobiotic T stationary phase are briefly discussed.

Supercritical Fluid Chromatography of Drugs: Parallel Factor Analysis for Column Testing in a Wide Range of Operational Conditions

Retention mechanisms involved in supercritical fluid chromatography (SFC) are influenced by interdependent parameters (temperature, pressure, chemistry of the mobile phase, and nature of the stationary phase), a complexity which makes the selection of a proper stationary phase for a given separation a challenging step. For the first time in SFC studies, Parallel Factor Analysis (PARAFAC) was employed to evaluate the chromatographic behavior of eight different stationary phases in a wide range of chromatographic conditions (temperature, pressure, and gradient elution composition). Design of Experiment was used to optimize experiments involving 14 pharmaceutical compounds present in biological and/or environmental samples and with dissimilar physicochemical properties. The results showed the superiority of PARAFAC for the analysis of the three-way (column × drug × condition) data array over unfolding the multiway array to matrices and performing several classical principal component analyses. Thanks to the PARAFAC components, similarity in columns' function, chromatographic trend of drugs, and correlation between separation conditions could be simply depicted: columns were grouped according to their H-bonding forces, while gradient composition was dominating for condition classification. Also, the number of drugs could be efficiently reduced for columns classification as some of them exhibited a similar behavior, as shown by hierarchical clustering based on PARAFAC components.