Shape selectivity in reversed-phase liquid chromatography for the separation of planar and non-planar solutes

Improved hydrophobic subtraction model of reversed-phase liquid chromatography selectivity based on a large dataset with a focus on isomer selectivity

Reversed-phase (RP) liquid chromatography is an important tool for the characterization of materials and products in the pharmaceutical industry. Method development is still challenging in this application space, particularly when dealing with closely-related compounds. Models of chromatographic selectivity are useful for predicting which columns out of the hundreds that are available are likely to have very similar, or different, selectivity for the application at hand. The hydrophobic subtraction model (HSM1) has been widely employed for this purpose; the column database for this model currently stands at 750 columns. In previous work we explored a refinement of the original HSM1 (HSM2) and found that increasing the size of the dataset used to train the model dramatically reduced the number of gross errors in predictions of selectivity made using the model. In this paper we describe further work in this direction (HSM3), this time based on a much larger solute set (1014 solute/stationary phase combinations) containing selectivities for compounds covering a broader range of physicochemical properties compared to HSM1. The molecular weight range was doubled, and the range of the logarithm of the octanol/water partition coefficients was increased slightly. The number of active pharmaceutical ingredients and related synthetic intermediates and impurities was increased from four to 28, and ten pairs of closely related structures (e.g., geometric and cis-/trans- isomers) were included. The HSM3 model is based on retention measurements for 75 compounds using 13 RP stationary phases and a mobile phase of 40/60 acetonitrile/25 mM ammonium formate buffer at pH 3.2. This data-driven model produced predictions of ln α (chromatographic selectivity using ethylbenzene as the reference compound) with average absolute errors of approximately 0.033, which corresponds to errors in α of about 3 %. In some cases, the prediction of the trans-/cis- selectivities for positional and geometric isomers was relatively accurate, and the driving forces for the observed selectivity could be inferred by examination of the relative magnitudes of the terms in the HSM3 model. For some geometric isomer pairs the interactions mainly responsible for the observed selectivities could not be rationalized due to large uncertainties for particular terms in the model. This suggests that more work is needed in the future to explore other HSM-type models and continue expanding the training dataset in order to continue improving the predictive accuracy of these models. Additionally, we release with this paper a much larger data set (43,329 total retention measurements) at multiple mobile phase compositions, to enable other researchers to pursue their own lines of inquiry related to RP selectivity.

Design and evaluation of semicarbazide-embeddedd stationary phases for liquid chromatography

Semicarbazide, as a derivative of urea, constitutes a great variety of functional molecules for different needs. Herein, novel stationary phases with an incorporated semicarbazide group were proposed. Using aliphatic (docosanoyl, C22) and aromatic (benzoyl, Bz) hydrazides, the semicarbazide-embedded ligands were synthesized before chemical modification of the silica gel, allowing for an accurate interpretation of the chromatographic properties of the corresponding packings. The new stationary phases were water-wettable, due to the presence of highly polar groups. In particular, Bz-semicarbazide (Bz-SCD) stationary phase was sufficiently hydrophilic to run in hydrophilic interaction (HILIC) mode, whilst the C22 (C22-SCD) equivalent, in spite of its reversed-phase nature, was markedly less hydrophobic than the referenced polar-embedded ones. The versatility of C22-SCD was demonstrated with a large selection of analytes, including geometric isomers and standard mixtures of polycyclic aromatic hydrocarbons, sulfonamides, sulfonylurea, substituted ureas, pyridines and carbamates, fat-soluble colorants, antifungal metabolites, angiotensin II receptor blockers and calcium channel blockers.

Facile preparation of embedded polar group-containing pentafluorophenyl stationary phases for highly selective separations of diverse analytes

Pentafluorophenyl (PFP) stationary phase is one of the most important phases after the C18 phase in terms of its applications. Three embedded polar groups (EPG)-containing stationary phases were newly synthesized to act the EPGs as additional interaction sites. The silica surface was initially modified with (3-aminopropyl)trimethoxysilane (APS). The APS-modified silicas were coupled with 2,3,4,5,6-pentafluorobenzoic acid, 2,3,4,5,6-pentafluorophenylacetic acid, and 2,3,4,5,6-pentafluoro-anilino(oxo)acetic acid to obtain Sil-PFP-BA, Sil-PFP-AA, and Sil-PFP-AN phases, respectively. The new phases were characterized by elemental analysis, ATR-FTIR, scanning electron microscopy (SEM), and thermogravimetric analysis (TGA). The phases were evaluated with the Tanaka and Neue tests in reversed-phase liquid chromatography (RPLC). In addition, they were characterized as hydrophilic phases by the Tanaka test protocol used in hydrophilic interaction chromatography (HILIC) separation mode. The Sil-PFP-AA phase showed the highest molecular shape selectivity in RPLC, while Sil-PFP-AN achieved the highest separability in HILIC compared to the commercial PFP reference column. The Sil-PFP-AA phase was successfully applied for the analysis of capsaicinoids from real samples of fresh chili peppers (Capsicum spp.) in RPLC and the Sil-PFP-AN phase for vitamin C (ascorbic acid) in HILIC.

A Sensitive Ultrahigh-Performance Liquid Chromatography/Tandem Mass Spectrometry Method for the Simultaneous Analysis of Phytocannabinoids and Endocannabinoids in Plasma and Brain

Introduction: Δ9-tetrahydrocannabinol (THC) and cannabidiol (CBD) are major chemical constituents of cannabis, which may interact either directly or indirectly with the endocannabinoid and endocannabinoid-like ("paracannabinoid") systems, two lipid-based signaling complexes that play important roles in physiology. Legislative changes emphasize the need to understand how THC and CBD might impact endocannabinoid and paracannabinoid signaling, and to develop analytical approaches to study such impact. In this study, we describe a sensitive and accurate method for the simultaneous quantification of THC, its main oxidative metabolites [11-hydroxy-Δ9-THC (11-OH-THC) and 11-nor-9-carboxy-Δ9-THC (11-COOH-THC)], CBD, and a representative set of endocannabinoid [anandamide and 2-arachidonoyl-sn-glycerol (2-AG)] and paracannabinoid [palmitoylethanolamide (PEA) and oleoylethanolamide (OEA)] compounds. Analyte separation relies on the temperature-dependent shape selectivity properties of polymerically bonded C18 stationary phases. Materials and Methods: Analytes are extracted from tissues using acetonitrile precipitation followed by phospholipid removal. The ultrahigh-performance liquid chromatography/tandem mass spectrometry protocol utilizes a commercially available C18 polymeric-bonded phase column and a simple gradient elution system. Results: Ten-point calibration curves show excellent linearity (R2>0.99) over a wide range of analyte concentrations (0.02-500 ng/mL). Lowest limits of quantification are 0.05 ng/mL for anandamide, 0.1 ng/mL for 11-OH-THC and OEA, 0.2 ng/mL for THC and CBD, 0.5 ng/mL for 11-COOH-THC, 1.0 ng/mL for 2-AG, and 2.0 ng/mL for PEA. The lowest limits of detection are 0.02 ng/mL for anandamide, 0.05 ng/mL for 11-OH-THC and OEA, 0.1 ng/mL for THC and CBD, 0.2 ng/mL for 11-COOH-THC, 0.5 ng/mL for 2-AG, and 1.0 ng/mL for PEA. Conclusions: An application of the method is presented, which showed that phytocannabinoid administration elevates endocannabinoid levels in plasma and brain of adolescent male and female mice.

Surface Modification of Silica with β-Alanine Derivatives for Unique Applications in Liquid Chromatography

Column purchasing cost is an important issue for an analyst to analyze complex sample matrices. Here, we report the development of an amino acid (β-alanine)-derived stationary phase (Sil-Ala-C12) with strategic and effective interaction sites (amide and urea as embedded polar groups with C12 alkyl chain) able to separate various kinds of analytes. Owing to the balanced hydrophobicity and hydrophilicity of the phase, it showed exceptional separation abilities in both reversed-phase high-performance liquid chromatography (RP-HPLC) as a hydrophobic phase and hydrophilic interaction chromatography (HILIC) as a hydrophilic phase. Remarkably, the baseline separation was achieved for the challenging β- and γ-isomers of tocopherol. Usually, three columns such as pentafluorophenyl or C30, C18, and sulfobetaine HILIC are required for the analysis of vitamin E, capsaicinoids, and vitamin C in chili peppers (Capsicum spp.), respectively. However, only Sil-Ala-C12 was able to separate these analytes. A single column can serve 3-4 purposes, which suggests that Sil-Ala-C12 had the potential to reduce column purchasing costs.

Selectivity optimization in liquid chromatography via stationary phase tuning

In striving for the best possible separation, the selectivity of stationary phases as an optimization parameter is often underestimated although there are many ways to influence this powerful tool. This review serves to provide an insight into the various ways of adapting the selectivity of a separation in liquid chromatography. Approaches via temperature and flow rate tuning are discussed as a basis followed by focusing on the stationary phase as the superior optimization parameter. Highly selective stationary phases hereby provide an advantage for groups of similar analytes. For more complex mixtures, separations can be improved using mixed-mode technologies where different retention mechanisms are combined. Serial coupling, mixed-bed columns, and stationary phase optimized selectivity liquid chromatography provide solutions to various degrees. Finally, the advantages of stationary phase tuning over adaption of mobile phase and/or temperature are presented in terms of optimum application range.

Effect of stereoconfiguration of aromatic ligands on retention and selectivity of terphenyl isomer-bonded stationary phases

The structure of ligands has a significant influence on the separation properties of alkyl and aromatic phases in reversed-phase liquid chromatography. Compared with alkyl phases, the effect of stereoconfiguration of aromatic ligands on the retention and selectivity of stationary phases has rarely been addressed. To illustrate the issue, three terphenyl isomer-bonded stationary phases were prepared via the coupling chemistry of isocyanate with terphenyl amine isomers, 3,4-diphenylaniline, 2,4-diphenylaniline and 4-amino-p-terphenyl, respectively. The retention behaviors of stationary phases were assessed in terms of retention strength, selectivity, hydrophobic and π-π interaction by five kinds of solutes. It is found that the selectivity towards the solutes is slightly larger on the branched m-terphenyl-bonded phase (m-π³) than o-terphenyl-bonded phase (o-π³) but is significantly improved on the chain p-terphenyl-bonded phase (p-π³). The results can be interpreted by the ease self-adjustment of the conformation of the chain p-terphenyl ligand and the smaller steric effect of p-π³ towards the insertion of solutes into the ligand brushes. In addition, the p-π³ yields excellent selective separation towards aromatic solutes. These findings are of significance in the design of aromatic stationary phases.

p-Terphenyl-based rigid stationary phases with embedded polar groups for liquid chromatography

Terphenyls are important building blocks for a wide range of functional molecules. Among the three isomers, p-terphenyl (C₁₈H₁₄) is particularly useful for the construction of optical devices on account of its unique structure. Herein, two rigid stationary phases bearing p-terphenyl as an external moiety and variable embedded carbamate groups (p-TerC with one embedded carbamate group and p-TerC² with two embedded carbamate group) were presented. The proposed stationary phases were characterized by various means and evaluated in reversed-phase (RP) mode, using different classes of analytes, including polycyclic aromatic hydrocarbons (PAHs), alkylbenzenes, 4-alkylbiphenyls, substituted ureas, sulfonylureas, substituted sulfanilamides and aromatic acids. The comparison with conventional C18, several other polar-embedded aromatic and C18 equivalents indicated p-terphenyl-based stationary phases were featured by multiple retention mechanisms, involving π-π interaction, charge-transfer interaction, hydrogen-bonding and hydrophobic interaction in RP mode. A unusually high specificity to the analytes with linear structures was observed, as exemplified by an irreversible adsorption of tetracene and a readily separation of tetraphene and chrysene. The aliphatic linker used in the proposed stationary phases was an influential factor for retentivity, selectivity and column efficiency. Interestingly, p-TerC² was operable in normal-phase mode for the separation of certain PAHs through polar-related interactions. The linear, rigid polyphenyl structure of p-terphenyl endowed the new stationary phase with distinctive chromatographic properties, in contrast to those of the preceding counterparts bonded with alkyl and/or polynuclear aromatic moieties.

Simultaneous purification of fucoxanthin isomers from brown seaweeds by open-column and high-performance liquid chromatography

Simultaneous purification of fucoxanthin isomers from brown seaweeds by two steps of open-column chromatography (OCC) and reversed-phase (RP)-high-performance liquid chromatography (HPLC) is described. Analysis and identification of fucoxanthin isomers were performed by chromatographic and spectrophotometric properties such as retention time (tR), spectral shape, maximal absorption wavelength (λmax), Q-ratio, and mass spectrometry (MS) data including the ratio of fragment ions. The optimal conditions for a simultaneous separation and purification were examined by changing several parameters of HPLC, i.e., mobile phase composition, equilibration time, and column oven temperature. The purification procedure consisted of the following two steps: first, highly purified fucoxanthin fraction was obtained by a silica-gel OCC. Then, four major fucoxanthin isomers, all-trans, 13'-cis, 13-cis, and 9'-cis, were simultaneously separated and purified by RP-HPLC with an analytical C30 column and gradient elution in a mixture of water, methanol, and methyl tert-butyl ether. The purity of fucoxanthin isomers purified was >95% for all-trans and 9'-cis, 85% for 13'-cis, and >80% for 13-cis. A large-scale purification by RP-HPLC using a preparative C18 column was effective for the purification of all-trans and 9'-cis with a yield of 95%. This developed technique was fully applicable to analyze the enhanced production of fucoxanthin isomers by iodine-catalyzed stereomutation which composed of 9 isomer species including 9-cis.

Separations by Shape: Molecular Shape Recognition in Liquid Chromatography

Molecular shape can provide a basis for chromatographic separations that is distinct from other interaction mechanisms often considered in liquid chromatography. Molecular shape recognition, or shape selectivity, is most evident for the separation of isomeric compounds that have constrained molecular structures, such as polycyclic aromatic hydrocarbons. A central feature of shape-selective columns is conformational order within the stationary phase; this aspect of stationary phase morphology is revealed through spectroscopic studies and molecular dynamics simulations, and is correlated with chromatographic performance.

A carbonylative coupling approach to alkyl stationary phases with variable embedded carbamate groups for high-performance liquid chromatography

A highly efficient carbonylative coupling method for the preparation of alkyl stationary phases with variable numbers of carbamate groups was established. The effectiveness of such method was verified through elemental analysis, X-ray photoelectron spectroscopy and solid-state ¹³C nuclear magnetic resonance spectroscopy of three as-synthesized stationary phases bearing different alkyl chains and different numbers of carbamate groups (octadecyl/one carbamate group, C18C; docosyl/three carbamate groups, C22C³; triacontyl/two carbamate groups, C30C²). The comparative evaluation of these stationary phases using a great variety of analytes, including three sets of isomers of alkylbenzenes, two sets of standard mixtures of polycyclic aromatic hydrocarbons (SRM 1647e and 869b), nine polychlorinated biphenyls, fiveteen N-substituted ureas, ten sulfonylureas, five xanthines and some other phytonutrients, revealed their remarkable applicability in reversed-phase liquid chromatography. Notably, the intercalated carbamate groups rendered the resultant stationary phases compatible with 100% aqueous mobile phase. The suppression of silanol activity was positively related to the number of polar groups embedded in the bonded selector, and the smallest peak tailing factor (1.14) for amitriptyline was obtained by C22C³. The molecular shape-related selectivity was found to be more closely related to the length of the selector's aliphatic chain, as supported by the lowest α_TBN/BaP value (0.31) by C30C². These carbamate-embedded alkyl stationary phases constituted another class of polar-embedded stationary phases possessing a single type of functional ligand.

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.

Improvements in the predictive accuracy of the hydrophobic subtraction model of reversed-phase selectivity

The hydrophobic subtraction model (HSM) for characterizing the selectivity of reversed-phase liquid chromatography (LC) columns has been used extensively by the LC community since it was first developed in 2002. Continuing interest in the model is due in part to the large, publicly available set of column descriptors that has been assembled over the past 18 years. In the work described in this report, we sought to refine the HSM with the goal of improving the predictive accuracy of the model without compromising its physico-chemical interpretability. The approach taken here has the following facets. A set of retention measurements for 635 columns and the 16 probe solutes used to characterize new columns using the HSM was assembled. Principal components analysis (PCA) was used as a guide for the development of a refined version of the HSM. Several outlying columns (84) were eliminated from the analysis because they were either inconsistent with the PCA model or were outliers from the original HSM model. With the retention dataset for the 16 probe solutes on the remaining 551 columns, we determined that a six-component model is the most sophisticated form of the model that can be used without overfitting the data. In our refined version of the HSM, the S*σ term has been removed. Two new terms have been added, which more accurately account for the molecular volume of the solute (Vv), and the solute dipolarity (Dd), and the remaining terms have been adjusted to accommodate these changes. The refined model described here provides improved prediction of retention factors, with the model standard error being reduced from 1.0 for the original HSM to 0.35 for the refined model (16 solutes, 551 columns). Furthermore, the number of retention factors with errors greater than 10% are reduced from 231 to 25. A revised metric for column similarity, F, is also proposed as a part of this work.

Comparison of the Retention and Separation Selectivity of Aromatic Hydrocarbons with Polar Groups in RP-HPLC Systems with Different Stationary Phases and Eluents

In this manuscript, the retention of aromatic hydrocarbons with polar groups has been compared for systems with various nonpolar columns of the types from C3 to C18 and different mobile phases composed of methanol, acetonitrile, or tetrahydrofuran as modifiers. The selectivity separation of the solutes in systems with different adsorbents, when one eluent modifier is swapped by another, has been explained, taking into account molecular interactions of the solutes with components of the stationary phase region (i.e., extracted modifier depending on the chain length of the stationary phase).

Identification of 7-9 ring polycyclic aromatic hydrocarbons in coals and petrol coke using High performance liquid chromatography - Diode array detection coupled to Atmospheric pressure laser ionization - Mass spectrometry (HPLC-DAD-APLI-MS)

Polycyclic aromatic hydrocarbons containing at least 24 carbon atoms (≥C₂₄-PAH) are often associated with pyrogenic processes such as combustion of fuel, wood or coal, and occur in the environment in diesel particulate matter, black carbon and coal tar. Some of the ≥C₂₄-PAH, particularly the group of dibenzopyrenes (five isomers, six aromatic rings) are known to show high mutagenic and carcinogenic activita. Gas chromatography - mass spectrometry is a well-established method for the analysis of lower molecular weight PAH but is not optimally suited for the analysis of ≥C₂₄-PAH due to their low vapor pressures. Also, hundreds of ≥C₂₄-PAH isomers are possible but only a few compounds are commercially available as reference standards. Therefore, in this study, a combination of multidimensional liquid chromatography, UV-Vis diode array detection, PAH selective and highly sensitive atmospheric pressure laser ionization - mass spectrometry is used to detect and unequivocally identify PAH. For identification of PAH in two bituminous coals and one petrol coke sample, unique and compound specific UV-Vis spectra were acquired. It was possible to identify ten compounds (naphtho[1,2,3,4-ghi]perylene, dibenzo[b,ghi]perylene, dibenzo[e,ghi]perylene, dibenzo[cd,lm]perylene, benzo[a]coronene, phenanthrol[5,4,3,2-abcde]perylene, benzo[ghi]naphtho[8,1,2-bcd]perylene, benzo[pqr]naphtho[8,1,2-bcd]perylene, naphtho[8,1,2-abc]coronene and tribenzo[e,ghi,k]perylene) by comparison of acquired spectra with spectra from literature. Additionally, it was possible to detect similar distribution patterns in different samples and signals related to alkylated naphthopyrenes, naphthofluoranthenes or dibenzopyrenes. Subsequent effect-directed analysis of a bituminous coal sample using the microEROD (ethoxyresorufin-O-deethylase) bioassay showed high suitability and revealed lower EROD induction for the ≥C₂₄-PAH (TEQ range 0.67-10.07 ng/g) than for the allover < C₂₄-PAH containing fraction (TEQ 84.00 ng/g). Nevertheless, the toxicity of ≥C₂₄-PAH has a significant impact compared with <C₂₄-PAH and must be considered for risk assessment. The LC-DAD-APLI-MS method, presented in this study, is a powerful tool for the unequivocal identification of these ≥ C₂₄-PAH.

Sublingual Delivery of Astaxanthin through a Novel Ascorbyl Palmitate-Based Nanoemulsion: Preliminary Data

Astaxanthin is a carotenoid extracted from several seaweeds with ascertained therapeutic activity. With specific reference, astaxanthin is widely used in clinical practice to improve ocular tissue health and skin protection from UV ray damages. Despite its well-documented pleiotropic actions and demonstrated clinical efficacy, its bioavailability in humans is low and limited because of its hydrophobicity and poor dissolution in enteric fluids. Furthermore, astaxanthin is very unstable molecule and very sensitive to light exposure and thermal stress. Taken together, these pharmacological and chemical–physical features strongly limit pharmaceutical and nutraceutical development of astaxanthin-based products and as a consequence its full clinical usage. This work describes the preliminary in vitro investigation of sublingual absorption of astaxanthin through a novel ascorbyl palmitate (ASP) based nanoemulsion.

Column Characterization and Selection Systems in Reversed-Phase High-Performance Liquid Chromatography

Reversed-phase high-performance liquid chromatography (RP-HPLC) is the most popular chromatographic mode, accounting for more than 90% of all separations. HPLC itself owes its immense popularity to it being relatively simple and inexpensive, with the equipment being reliable and easy to operate. Due to extensive automation, it can be run virtually unattended with multiple samples at various separation conditions, even by relatively low-skilled personnel. Currently, there are >600 RP-HPLC columns available to end users for purchase, some of which exhibit very large differences in selectivity and production quality. Often, two similar RP-HPLC columns are not equally suitable for the requisite separation, and to date, there is no universal RP-HPLC column covering a variety of analytes. This forces analytical laboratories to keep a multitude of diverse columns. Therefore, column selection is a crucial segment of RP-HPLC method development, especially since sample complexity is constantly increasing. Rationally choosing an appropriate column is complicated. In addition to the differences in the primary intermolecular interactions with analytes of the dispersive (London) type, individual columns can also exhibit a unique character owing to specific polar, hydrogen bond, and electron pair donor-acceptor interactions. They can also vary depending on the type of packing, amount and type of residual silanols, "end-capping", bonding density of ligands, and pore size, among others. Consequently, the chromatographic performance of RP-HPLC systems is often considerably altered depending on the selected column. Although a wide spectrum of knowledge is available on this important subject, there is still a lack of a comprehensive review for an objective comparison and/or selection of chromatographic columns. We aim for this review to be a comprehensive, authoritative, critical, and easily readable monograph of the most relevant publications regarding column selection and characterization in RP-HPLC covering the past four decades. Future perspectives, which involve the integration of state-of-the-art molecular simulations (molecular dynamics or Monte Carlo) with minimal experiments, aimed at nearly "experiment-free" column selection methodology, are proposed.

Confocal Raman Microscopy for in Situ Measurement of Phospholipid-Water Partitioning into Model Phospholipid Bilayers within Individual Chromatographic Particles

The phospholipid-water partition coefficient is a commonly measured parameter that correlates with drug efficacy, small-molecule toxicity, and accumulation of molecules in biological systems in the environment. Despite the utility of this parameter, methods for measuring phospholipid-water partition coefficients are limited. This is due to the difficulty of making quantitative measurements in vesicle membranes or supported phospholipid bilayers, both of which are small-volume phases that challenge the sensitivity of many analytical techniques. In this work, we employ in situ confocal Raman microscopy to probe the partitioning of a model membrane-active compound, 2-(4-isobutylphenyl) propionic acid or ibuprofen, into both hybrid- and supported-phospholipid bilayers deposited on the pore walls of individual chromatographic particles. The large surface-area-to-volume ratio of chromatographic silica allows interrogation of a significant lipid bilayer area within a very small volume. The local phospholipid concentration within a confocal probe volume inside the particle can be as high as 0.5 M, which overcomes the sensitivity limitations of making measurements in the limited membrane areas of single vesicles or planar supported bilayers. Quantitative determination of ibuprofen partitioning is achieved by using the phospholipid acyl-chains of the within-particle bilayer as an internal standard. This approach is tested for measurements of pH-dependent partitioning of ibuprofen into both hybrid-lipid and supported-lipid bilayers within silica particles, and the results are compared with octanol-water partitioning and with partitioning into individual optically trapped phospholipid vesicle membranes. Additionally, the impact of ibuprofen partitioning on bilayer structure is evaluated for both within-particle model membranes and compared with the structural impacts of partitioning into vesicle lipid bilayers.

Normal-phase liquid chromatography retention behavior of polycyclic aromatic hydrocarbon and their methyl-substituted derivatives on an aminopropyl stationary phase

Retention indices for 124 polycyclic aromatic hydrocarbons (PAHs) and 62 methyl-substituted (Me-) PAHs were determined using normal-phase liquid chromatography (NPLC) on a aminopropyl (NH2) stationary phase. PAH retention behavior on the NH2 phase is correlated to the total number of aromatic carbons in the PAH structure. Within an isomer group, non-planar isomers generally elute earlier than planar isomers. MePAHs generally elute slightly later but in the same region as the parent PAHs. Correlations between PAH retention behavior on the NH2 phase and PAH thickness (T) values were investigated to determine the influence of non-planarity for isomeric PAHs with four to seven aromatic rings. Correlation coefficients ranged from r = 0.19 (five-ring peri-condensed molecular mass (MM) 252 Da) to r = -0.99 (five-ring cata-condensed MM 278 Da). In the case of the smaller PAHs (MM ≤ 252 Da), most of the PAHs had a planar structure and provided a low correlation. In the case of larger PAHs (MM ≥ 278 Da), nonplanarity had a significant influence on the retention behavior and good correlation between retention and T was obtained for the MM 278 Da, MM 302 Da, MM 328 Da, and MM 378 Da isomer sets. Graphical abstract NPLC separation of the three-, four-, five-, and six-ring PAH isomers with different number of aromatic carbon atoms and degrees of non-planarity (Thickness, T). The inserted figure plots the number of aromatic carbon atoms vs. the log I value for the 124 parent PAHs.

Qualitative characterization of SRM 1597a coal tar for polycyclic aromatic hydrocarbons and methyl-substituted derivatives via normal-phase liquid chromatography and gas chromatography/mass spectrometry

A normal-phase liquid chromatography (NPLC) fractionation procedure was developed for the characterization of a complex mixture of polycyclic aromatic hydrocarbons (PAHs) from a coal tar sample (Standard Reference Material (SRM) 1597a). Using a semi-preparative aminopropyl (NH₂) LC column, the coal tar sample was separated using NPLC based on the number of aromatic carbons; a total of 14 NPLC fractions were collected. SRM 1597a was analyzed before and after NPLC fractionation by using gas chromatography/mass spectrometry (GC/MS) with a 50% phenyl stationary phase. The NPLC-GC/MS method presented in this study allowed for the identification of 72 PAHs and 56 MePAHs. These identifications were based on the NPLC retention times for authentic reference standards, GC retention times for authentic reference standards, and the predominant molecular ion peak in the mass spectrum. Most noteworthy was the determination of dibenzo[a,l]pyrene, which could not be measured directly by GC/MS because of low concentration and co-elution with dibenzo[j,l]fluoranthene. The NPLC-GC/MS procedure also allowed for the tentative identification of 74 PAHs and 117 MePAHs based on the molecular ion peak only. This study represents the most comprehensive qualitative characterization of SRM 1597a to date. Graphical abstract NPLC-GC/MS analysis for the six-ring MM 302 Da PAH isomers in SRM 1597a.

Retention behavior of alkyl-substituted polycyclic aromatic sulfur heterocycles in reversed-phase liquid chromatography

Retention indices for 79 alkyl-substituted polycyclic aromatic sulfur heterocycles (PASHs) were determined by using reversed-phase liquid chromatography (LC) on a monomeric and polymeric octadecylsilane (C18) stationary phase. Molecular shape parameters [length, breadth, thickness (T), and length-to-breadth ratio (L/B)] were calculated for all the compounds studied. Based on separations of isomeric methylated polycyclic aromatic hydrocarbons on polymeric C18 phases, alkyl-substituted PASHs are expected to elute based on increasing L/B ratios. However, the correlation coefficients had a wide range of values from r=0.43 to r=0.93. Several structural features besides L/B ratios were identified to play an important role in the separation mechanism of PASHs on polymeric C18 phases. First, the location of the sulfur atom in a bay-like-region results in alkylated-PASHs being more retentive than non-bay-like-region alkylated-PASHs, and they elute later than expected based on L/B value. Second, the placement of the alkyl group in the k region of the structure resulted in a later elution than predicted by L/B. Third, highly nonplanar methyl-PASHs (i.e., 1-Me and 11-MeBbN12T) elute prior to the parent PASH (BbN12T).

A new methodology to determine the isoeluotropic conditions on ultra-performance flash purification stationary phases from analytical reversed liquid chromatography stationary phase

Nowadays, the determination of the experimental chromatographic conditions to be used in Reversed Phase Liquid Ultra-Performance Flash Purification is still challenging. This is due to four different items. In most cases, flash purification stationary phases are not available with geometry of column used in analytical chromatography. The flash purification columns are single-use only. From the point of view of selectivity and retention, few RPLC phases exist with properties of separation identical for analytical and flash purification supports. Characterization methods and databases used for comparing analytical RPLC columns do not include stationary phases for RP flash purification columns. The goal of this work is to develop a new method development strategy which permits the determination of the experimental chromatographic conditions on RP ultra-performance flash purification columns. It relies on the knowledge of any isocratic conditions obtained on any given initial reversed stationary phase. The final conditions to implement on the RP ultra-performance flash purification phase enable either to keep the retention range of a selected solute constant, or to set it around a previously chosen value. The rules of transfer in linear gradient mode are also described. The methodology was valid, whatever the initial RP stationary and mobile phases, for different chemical classes, whatever the bonding, particle diameter, porous or core shell particle, towards different RP alkyl and analogues stationary and mobile phases.

Role of chromatography in the development of Standard Reference Materials for organic analysis

The certification of chemical constituents in natural-matrix Standard Reference Materials (SRMs) at the National Institute of Standards and Technology (NIST) can require the use of two or more independent analytical methods. The independence among the methods is generally achieved by taking advantage of differences in extraction, separation, and detection selectivity. This review describes the development of the independent analytical methods approach at NIST, and its implementation in the measurement of organic constituents such as contaminants in environmental materials, nutrients and marker compounds in food and dietary supplement matrices, and health diagnostic and nutritional assessment markers in human serum. The focus of this review is the important and critical role that separation science techniques play in achieving the necessary independence of the analytical steps in the measurement of trace-level organic constituents in natural matrix SRMs.

Molecular simulations of retention in chromatographic systems: use of biased Monte Carlo techniques to access multiple time and length scales

The use of configurational-bias Monte Carlo simulations in the Gibbs ensemble allows for the sampling of phenomena that occur on vastly different time and length scales. In this review, applications of this simulation approach to probe retention in gas and reversed-phase liquid chromatographic systems are discussed. These simulations provide an unprecedented view of the retention processes at the molecular-level and show excellent agreement with experimental retention data.

Dynamics within alkylsiloxane self-assembled monolayers studied by sensitive dielectric spectroscopy

Self-assembled monolayers are a ubiquitous laboratory tool and have been the subject of many experimental investigations which have primarily focused on static properties of full coverage monolayers, with the maximum density and ordering possible. In this work, dynamics within low density, planar siloxane self-assembled monolayers are studied utilizing highly sensitive dielectric spectroscopy. Dilute, disordered films were intentionally fabricated in order to study the widest range of possible motions. At low coverage, an interacting relaxation is observed, which has similar dynamics to polyethylene-like glass transitions observed in phase-segregated side-chain polymers, despite the rigidity of the substrate and the constraint of ethyl groups in relatively short chains. As density is increased, a second local relaxation, previously observed in three-dimensional SAMs and associated with rotation within a small segment of the alkyl chain, is also observed.