Chromatographic classification and comparison of commercially available reversed-phase liquid chromatographic columns containing phenyl moieties using principal component analysis

Characterization of reversed-phase liquid chromatographic columns containing positively charged functionality

To date, the most commonly used column characterization databases do not determine the relative positive charge associated with new generation RP columns, or they fail to successfully discriminate between RP columns of purportedly low level positive and neutral characters. This paper rectifies this in that it describes a convenient and robust chromatographic procedure for the assessment of the low levels of positive charge on a range of RP columns. The low degree of positive charge was determined by their electrostatic attraction towards the negatively charged 4-n-octylbenzene sulfonic acid (4-OBSA) relative to their retention of the hydrophobic marker toluene (Tol). The new parameter (α4-OBSA/Tol) was determined for 15 commercially available RP-LC columns. When this was combined with existing Tanaka parameters it was possible to guide the chromatographer towards similar columns as "backup / equivalent phases" or dissimilar columns for exploitation in method development strategies. It should be noted that under certain chromatographic conditions the retention mechanism(s) may be too complex to allow direct location of a "backup / equivalent" column(s). The α4-OBSA/Tol results indicate that even the new generation neutral alkyl phases may exhibit a small degree of positive charge at low buffer concentrations. Mobile phases containing low % MeCN were demonstrated to promote mixed mode (anionic exchange / hydrophobic) retention whereas at high % MeCN anionic exchange retention dominated. The measure of electrostatic repulsion between positively charged columns and positively charged bases was assessed by evaluating the relative retention of a range of bases and neutral analytes. The greatest electrostatic repulsion was observed with hydrophilic bases. While there was no correlation between the positive charge associated with the phases assessed by electrostatic attraction or repulsion, the columns could be broadly divided into three subsets (i.e., significant positive character, medium to low positive character and insignificant positive character). Finally, the results were used to highlight the usefulness of the column characterization database containing the new anionic exchange retention parameter (α4-OBSA/Tol) for the selection of an equivalent column possessing a low level of positive character in the analysis of a real-life biopharmaceutical application.

Preparation and chromatographic evaluation of 9-oxa-10-phosphophenanthrene 10-oxide bonded phenyl stationary phase and investigation of its retention mechanism through nuclear magnetic resonance spectroscopy

BACKGROUND

In reversed-phase liquid chromatography, the C18 alkyl bonded phase, as the primary stationary phase, is widely used in pharmaceutical and food analysis. The phenyl bonded phase often serves as a complementary choice to the C18 phase to enhance the separation performance of specific categories of compounds. However, both C18 and the currently available phenyl bonded phase chromatography columns show room for further optimization in improving the separation efficiency of specific compound classes, such as dihydroflavonoids. Additionally, the potential role and impact of introducing phosphorus groups into chromatographic stationary phases have not been fully explored, indicating a promising direction for research.

RESULTS

In the present work, we prepared a novel phenyl stationary phase by bonding 9-oxa-10-phosphaphenanthrene 10-oxide onto silica gel. The obtained material was characterized by scanning electron microscopy, fourier transforms infrared spectroscopy, and elemental analysis. The results show that 9-oxa-10-phosphaphenanthrene 10-oxide was successfully bonded on the silica surface with a load of 3.90 %. Further chromatographic characterization in high-performance liquid chromatography exhibited high column efficiency (40,792 plates m-1 for the determination of biphenyl) and good stability (RSD of 0.28 %∼5.38 %). Moreover, we made a detailed study of the column separation mechanism by nuclear magnetic resonance spectroscopy titration experiment. Comparing to commercial phenyl column, the proposed stationary phase showed shorter retention time and higher throughput. In addition, the stationary phase has a strong ability to separate multiple types of compounds, which provides a new strategy for the separation of complex samples, such as active ingredients in traditional Chinese medicine.

SIGNIFICANCE

We have developed a novel phenyl column and conducted a comprehensive examination of its chromatographic performance, demonstrating excellent separation capabilities and high efficiency for both nonpolar and moderately polar aromatic compounds. Additionally, we explored the impact of phosphorus-containing groups on the separation performance of chromatographic stationary phases.

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.

Updating the European Pharmacopoeia impurity profiling method for cetirizine and suggesting alternative column, using design space comparison

The goal of the present study was to develop a generic workflow to evaluate the chromatographic resolution in a design space and find replacement column for the new method. To attain this objective, a limited number of initial experiments have been performed, and a modeling tool was employed to study and compare design spaces obtained with different columns. By overlaying the different individual resolution maps (design spaces), it is possible to quickly identify a robust zone where the different columns meet a given resolution criterion. This new feature of the modeling tool is very useful for finding alternative columns for a given separation, rather than the usual column tests. It was also demonstrated that two different columns can be used as equivalents (replacement columns), providing sufficient resolution at the same working point and with a high degree of robustness.

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.

Surface-bubble-modulated liquid chromatography: an experimental strategy for identification of molecular processes of solute retention in reversed-phase separation systems

Molecular level understanding of the chemistry at the aqueous/hydrophobe interface is crucial to separation processes in aqueous media, such as reversed-phase liquid chromatography (RPLC) and solid-phase extraction (SPE). Despite significant advances in our knowledge of the solute retention mechanism in these reversed-phase systems, direct observation of the behavior of molecules and ions at the interface in reversed-phase systems still remains a major challenge and experimental probing techniques that provide the spatial information of the distribution of molecules and ions are required. This review addresses surface-bubble-modulated liquid chromatography (SBMLC), which has a stationary gas phase in a column packed with hydrophobic porous materials and enables one to observe the molecular distribution in the heterogeneous reversed-phase systems consisting of the bulk liquid phase, the interfacial liquid layer, and the hydrophobic materials. The distribution coefficients of organic compounds referring to their accumulations onto the interface of alkyl- and phenyl-hexyl-bonded silica particles exposed to water or acetonitrile-water and into the bonded layers from the bulk liquid phase are determined by SBMLC. The experimental data obtained by SBMLC show that the water/hydrophobe interface exhibits an accumulation selectivity for organic compounds, which is quite different from that of the interior of the bonded chain layer, and the overall separation selectivity of the reversed-phase systems is determined by the relative sizes of the aqueous/hydrophobe interface and the hydrophobe. The solvent composition and the thickness of the interfacial liquid layer formed on octadecyl-bonded (C₁₈) silica surfaces are also estimated from the bulk liquid phase volume determined by the ion partition method employing small inorganic ions as probes. It is clarified that various hydrophilic organic compounds as well as inorganic ions recognize the interfacial liquid layer formed on the C₁₈-bonded silica surfaces as being different from the bulk liquid phase. The behavior of some solute compounds exhibiting substantially weak retention in RPLC or the so-called negative adsorption, such as urea, sugars, and inorganic ions, can rationally be interpreted with a partition between the bulk liquid phase and the interfacial liquid layer. The spatial distribution of solute molecules and the structural properties of the solvent layer on the C₁₈-bonded layer determined by the liquid chromatographic methods are discussed in comparison to the results obtained by other research groups using molecular simulation methods.

A novel fully-automated method to measure steroids in serum by liquid chromatography-tandem mass spectrometry

Background

Steroids play a key role in numerous physiological processes. Steroid determination is a useful tool to explore various endocrine diseases. Because of its specificity, mass spectrometry is considered to be a reference method for the determination of steroids in serum compared to radioimmunoassay. This technology could progress towards more automation for the optimal organization of clinical laboratories and ultimately for the benefit of patients.

Methods

A fully automated ultra-high-performance liquid chromatography-tandem mass spectrometry method was developed and fully validated to determine five steroids in serum. Sample preparation was based on protein precipitation with filtration followed by online solid phase extraction. Chromatographic separation was performed using a biphenyl stationary phase.

Results

The method was successfully validated according to European Medicine Agency guidelines. Coefficients of variation did not exceed, respectively, 8.4% and 8.1% for intra- and inter-assay precision. Method comparison with radioimmunoassay showed a proportional bias for all compounds, except for testosterone in men. Comparison with another LC-MS/MS method demonstrated acceptable concordance for all steroids, although a small bias was observed for androstenedione.

Conclusion

The novelty of this method is that it has been fully automated. Automation provides benefits in traceability and allows significant savings in cost and time.

Grafting of Tetraphenylethylene on Silica Surface, Characterizations, and Their Chromatographic Performance as Reversed-Phase Stationary Phases

Surface modification is an effective way to functionalize the materials so as to get some special properties. Tetraphenylethylene (TPE) has been widely investigated as a well-known reagent which has the nature of aggregation-induced emission (AIE), but has never been reported in the liquid chromatography stationary phase. In this work, TPE-grafted silica (Sil-TPE) was obtained successfully using the derivative of 1-(4-hydroxyphenyl)-1,2,2-triphenylethylene as a ligand, and then characterized by elemental analysis, Fourier transform infrared spectra, thermogravimetric analysis, and so forth. Laser scanning confocal microscopy images reflected the AIE phenomenon of grafted TPE because the internal vibration and rotation of TPE molecules were restrained in the confined silica space. The contact angle test showed superhydrophobic properties of Sil-TPE. In order to understand thoroughly the mechanism of chromatographic performance and retention behavior for Sil-TPE, Tanaka test mixture, alkylbenzenes, polycyclic aromatic hydrocarbons (PAHs), and phenols were separated. This reveals that Sil-TPE has strong aromaticity and certain shape selectivity, especially, has excellent separation performance for PAHs and phenols. The thermodynamic properties and repeatability of Sil-TPE were further studied, which showed the stability of Sil-TPE. This work shows that TPE can be successfully grafted on silica surface and it has the potential to be a new kind of promising stationary phases in the future.

Development and evaluation of a hydrogenated rosin (β-acryloxyl ethyl) ester-bonded silica stationary phase for high-performance liquid chromatography separation of paclitaxel from yew bark

Paclitaxel (PTX) is a complex diterpenoid anticancer drug whose separation from yew biomass poses a significant challenge. In this study, a new stationary phase comprising hydrogenated rosin (β-acryloxyl ethyl) ester (HRE)-bonded silica (HRE@SiO₂) is developed to separate and purify PTX from crude yew-bark extract using high-performance liquid chromatography. In HRE@SiO₂, HRE molecules, which are functional ligands, are bonded to the surface of a silica gel matrix using a coupling agent, (3-mercaptopropyl)trimethoxysilane. The proposed HRE@SiO₂ stationary phase was characterized by Fourier-transform infrared spectroscopy, elemental analysis, thermogravimetric analysis, scanning electron microscopy, laser diffraction granulometry, and nitrogen gas adsorption. The HRE@SiO₂ column exhibited excellent chromatographic performance, satisfactory performance reproducibility, and typical reversed-phase chromatographic behavior. An HRE@SiO₂ column was used to separate PTX and its analogs, achieving resolutions exceeding 7.43 for consecutively eluted species. Stoichiometric displacement theory for retention (SDT-R), the van Deemter equation, and van 't Hoff plots were used to analyze the separation mechanism and properties of the HRE@SiO₂ column. The results showed that hydrophobic interactions determine the analyte retention and the separation of PTX and its analogs on an HRE@SiO₂ column is an exothermic process driven by enthalpy. Furthermore, an HRE@SiO₂ column was employed to separate and purify PTX from crude yew-bark extract, increasing PTX purity from 6% to 82%. The findings of this study provide insights for developing rosin-based stationary phases for the separation of natural products.

Rapid, sensitive, and reliable quantitation of nicotine and its main metabolites cotinine and trans-3'-hydroxycotinine by LC-MS/MS: Method development and validation for human plasma

New nicotine delivery products are gaining market share. For evaluation of their characteristics, toxicokinetic investigations are in current research focus. For reliable determination of blood plasma levels of nicotine and its main metabolites cotinine and trans-3'-hydroxycotinine, a quantitation method based on LC-ESI-MS/MS was developed and validated. Addition of isotope labeled internal standards prior to rapid sample preparation using protein precipitation with methanol was chosen for sample preparation. Different stationary phases were tested and phenyl-hexyl separation was found to be superior to HILIC, C18, and C8 stationary phases. Ion suppression effects caused by hydrophilic early eluting matrix were eliminated by the adjustment of an adequate retention utilizing a phenyl-hexyl separation stationary phase. Exchange of acetonitrile as organic mobile phase by methanol and elevation of pH value of aqueous mobile phase containing 5 mM NH₄Ac to 4.50 improved the chromatographic resolution. The limits of quantitation for nicotine, cotinine, and hydroxycotinine were 0.15, 0.30, and 0.40 ng/mL, respectively. Linearity was proven by matrix matched calibration for the whole working range from 0.50 ng/mL to 35.0 ng/mL for nicotine and from 6.00 to 420 ng/mL for cotinine and hydroxycotinine (Mandel's fitting test with R² > 0.995). Quality control samples at four different levels (0.50, 1.50, 17.5, 28.0 ng/mL for nicotine and 6.00, 18.0, 210, 336 ng/mL for cotinine and hydroxycotinine) in plasma were analyzed six times on three days. Mean accuracies ranged from 87.7% to 105.8% for nicotine, from 90.3% to 102.9% for cotinine, and from 99.9% to 109.9% for hydroxycotinine. Intra- and inter-day precisions (RSD %) were below 15% for all analytes (<20% for LLOQ). As proof of concept, the method was successfully applied to a real plasma sample from a cigarette smoking volunteer.

Intrinsic difference between phenyl hexyl- and octadecyl-bonded silicas in the solute retention selectivity in reversed-phase liquid chromatography with aqueous mobile phase

For choosing an optimal column for a particular separation by reversed-phase liquid chromatography (RPLC), it is essential to quantitatively understand the effects of the chemical structure of hydrophobic bonded layer derived onto silica particles on the distribution equilibrium of a solute compound at the interface between the aqueous mobile phase and the packing material. However, there is still a lack of understanding of the solute distribution equilibrium in RPLC separation due to the complexities of the chemistry at the interface between the mobile phase and the bonded layer. We successfully determined the distribution coefficients of various organic compounds concerning to their accumulation onto the water/bonded layer interface and into the bonded layer from bulk water using surface-bubble-modulated liquid chromatography with octadecyl- and phenyl hexyl-bonded silica columns. The water/phenyl hexyl-bonded layer interface accumulates organic compounds much less than the water/octadecyl-bonded layer interface due to its lower interfacial tension, and this result suggests that phenyl hexyl group orient their benzene ring facing toward water. On the other hand, aromatic moiety of phenyl hexyl group enhances partitioning of the organic compounds into the bonded layer. Experimental findings in the present work demonstrated that the water/bonded layer interface and the bonded layer itself have independent contributions to the solute distribution and the water/phenyl hexyl-bonded layer interface shows quite different solute retention selectivity from the water/octadecyl-bonded layer interface.

Investigation into reversed phase chromatography peptide separation systems part III: Establishing a column characterisation database

The Peptide RPC Column Characterisation Protocol was applied to 38 stationary phases, varying in ligand chemistry, base silica, end capping and pore size, which are suitable for the analysis of peptides. The protocol at low and intermediate pH is based on measuring retention time differences between peptides of different functionality to calculate selectivity delta values. The characterisation was designed to explore increases / decreases in positive or negative charge (deamidation), steric effect (i.e. racemisation / switch in amino acid order), oxidation and addition / removal of aromatic moieties. The necessity of developing a characterisation protocol specifically for peptide analysis was highlighted by the fact that the small molecule databases (Snyder's Hydrophobic Subtraction Model and the extended Tanaka protocol) failed to correlate with the Peptide RPC Column Characterisation Protocol. Principal Component Analysis was used to demonstrate that the protocol could be used to identify columns with similar or dissimilar chromatographic selectivity for the purpose of selectivity back-up or method development columns respectively. This was validated using peptide fragments derived from the tryptic digest of bovine insulin and carbonic anhydrase. It was also demonstrated that the presence of positively charged functional groups on the stationary phase was advantageous as it yielded very different chromatographic selectivity and improved peak shape.

Updating the European Pharmacopoeia impurity profiling method for terazosin and suggesting alternative columns

This work was motivated by the demand of European Directorate for the Quality of Medicines and HealthCare (EDQM). A new liquid chromatographic (LC) method was developed for terazosin impurity profiling to replace the old European Pharmacopoeia (Ph. Eur.) method. This new method is published as part of the new Ph. Eur. monograph proposal of terazosin in Pharmeuropa issue 32.2. The aim of the method renewal was to cut the analysis time from 90 min (2 × 45 min) down to below 20 min. The Ph. Eur. monograph method is based on two different chromatographic separations to analyze the specified impurities of terazosin. The reason for the two methods is that two of the impurities are not sufficiently retained in reversed phase (RP) conditions, not even with 100% water as eluent. Therefore, next to RP, an ion-pair (IP) chromatographic method has to be applied to analyze those two impurities. With our new proposed method it was possible to appropriately increase the retention of the two critical compounds using alternative stationary phases (instead of a C18 phase which is suggested by the Ph. Eur. method). Applying a pentafluoro-phenyl (PFP) stationary phase, it was feasible to separate and adequately retain all the impurities. The detection wavelength was also changed compared to the Ph. Eur. method and is now appropriate for the detection and quantification of all impurities using perchloric acid in the mobile phase at low pH. Another goal of the present study was to develop a generic workflow and to evaluate the chromatographic resolution in a wide range of method variables and suggest some replacement columns for terazosin impurity profiling. Retention modeling was applied to study the chromatographic behavior of the compounds of interest and visualize resolution for the different columns, where a given criterion is fulfilled. A zone (set of chromatographic conditions) of a robust space could be then quickly identified by the overlay of the individual response surfaces (resolution maps). It was also demonstrated that two columns from different providers (Kinetex F5 and SpeedCore PFP) can be used as replacement columns, providing sufficient resolution at the same working point and a high degree of robustness.

Recent applications of chemometrics in one- and two-dimensional chromatography

The proliferation of increasingly more sophisticated analytical separation systems, often incorporating increasingly more powerful detection techniques, such as high-resolution mass spectrometry, causes an urgent need for highly efficient data-analysis and optimization strategies. This is especially true for comprehensive two-dimensional chromatography applied to the separation of very complex samples. In this contribution, the requirement for chemometric tools is explained and the latest developments in approaches for (pre-)processing and analyzing data arising from one- and two-dimensional chromatography systems are reviewed. The final part of this review focuses on the application of chemometrics for method development and optimization.

Suitability of stationary phase for LC analysis of biomolecules

Biologically active compounds such as carotenoids/isoprenoids, vitamins, steroids, saponins, sugars, long chain fatty acids, and amino acids play a very important role in coordinating functions in living organisms. Determination of those substances is indispensable in advanced biological sciences. Engineered stationary phase in LC for the analysis of biomolecules has become easier with the development of chromatographic science. In general, C₁₈ column is being used for routine analysis but specific columns are being used for specific molecule. Monolithic columns are found to have higher efficiency than normal column. Among recent introduction, triacontyl stationary phases, designed for the separation of carotenoid isomers, are widely used for the estimation of carotenoids. In comparison to conventional C₁₈ phases, C₃₀ phases exhibited superior shape selectivity for the separation of isomers of carotenoids. It is also found useful for better elution and analysis of tocopherols, vitamin K, sterols, and fatty acids. Vitamin K, E, and their isomers are also successfully resoluted and analyzed by using C₃₀ column. Amino bonded phase column is specifically used for better elution of sugars, whereas phenyl columns are suitable for the separation and analysis of curcuminoids and taxol. Like triacontyl stationary phase, pentafluorophenyl columns are also used for the separation and analysis of carotenoids. Similarly, HILIC column are best suited for sugar analysis. All the stationary phases are made possible to resolute and analyze the target biomolecules better, which are the future of liquid chromatography. The present article focuses on the differential interaction between stationary phase and target biomolecules. The applicability of these stationary phases are reported in different matrices.

A pH-stable, crosslinked stationary phase based on the thiol-yne reaction

Stationary phases that can withstand extremes of pH and temperature are needed to allow a single column to accommodate a wider set of solutes and separation criteria. We used a simple multi-step process using the thiol-yne reaction following the modification of the silica surface with a thiol-containing silane. The monomers 1,4-diethynylbenzene (DEB) and 1,6-hexanedithiol were used to create a crosslinked thiol-yne (CTY) stationary phase along the surface of the thiol functionalized silica. In the Tanaka test characterization, the CTY phase showed a low phase ratio, methylene selectivity typical of a reversed phase, and extremely high shape selectivity compared to commercial reversed phases. The hydrophobic subtraction model characterization showed a high positive steric resistance, a low hydrogen bond acidity, and a high cation-exchange capacity compared to most reversed phases. At pH 0.5 with an 85% aqueous mobile phase the phase showed no significant change over 114 h. With a 50% aqueous mobile phase the phase took four more days than a sterically protected C18 phase for the k' to decline 25%. At pH 12.6, 50% aqueous mobile phase, a sterically protected C18 phase showed a 20% decrease in k' and more than a 60% decrease in theoretical plates per meter in three hours. The CTY phase actually showed modest increases in k' and theoretical plates per meter after three hours.

Rational design of mixtures for chromatographic peak tracking applications via multivariate selectivity

Chromatographic characterization and parameterization studies targeting many solutes require the judicious choice of operating conditions to minimize analysis time without compromising the accuracy of the results. To minimize analysis time, solutes are often grouped into a small number of mixtures; however, this increases the risk of peak overlap. While multivariate curve resolution methods are often able to resolve analyte signals based on their spectral qualities, these methods require that the chromatographically overlapped compounds have dissimilar spectra. In this work, a strategy for grouping compounds into sample mixtures containing solutes with distinct spectral and, optionally, with distinct chromatographic properties, in order to ensure successful solute resolution either chromatographically or with curve resolution methods is proposed. We name this strategy rational design of mixtures (RDM). RDM utilizes multivariate selectivity as a metric for making decisions regarding group membership (i.e., whether to add a particular solute to a particular sample). A group of 97 solutes was used to demonstrate this strategy. Utilizing both estimated chromatographic properties and measured spectra to group these 97 analytes, only 12 groups were required to avoid a situation where two or more solutes in the same group could not be resolved either chromatographically (i.e., they have significantly different retention times) or spectrally (i.e., spectra are different enough to enable resolution by curve resolution methods). When only spectral properties were utilized (i.e., the chromatographic properties are unknown ahead of time) the number of groups required to avoid unresolvable overlaps increased to 20. The grouping strategy developed here will improve the time and instrument efficiency of studies that aim to obtain retention data for solutes as a function of operating conditions, whether for method development or determination of the chromatographic parameters of solutes of interest (e.g., k_w).

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A strategy for rational mixture design for chromatographic experiments is described.

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Multivariate selectivity is used to optimize mixture compositions.

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UV spectral information is used to distinguish overlapped chromatographic peaks.

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.

Separation and Identification of Isomeric and Structurally Related Synthetic Cannabinoids Using 2D Liquid Chromatography and High Resolution Mass Spectrometry

Novel psychoactive substances (NPS) are emerging drugs of abuse that are variations of existing compounds intended to cause a CNS psychotropic effect. Some NPS are so comparable in structure and physicochemical properties that they co-elute using traditional single column chromatographic techniques and therefore will not be detected as individual compounds. 2D liquid chromatography (2D-LC) has demonstrated applicability in difficult separations of small molecules and compounds in complex mixtures. It was hypothesized that this technique could also be used to separate co-eluting isomeric and structurally related, non-isomeric NPS, including synthetic cannabinoids (SC). Initial studies assessed several parameters, including column type, mobile phase, analysis time, gradient and flow rate, to optimize a 2D-LC method for separation and analysis of SC. The final comprehensive on-line 2D-LC method employed a Bonus-RP column in the first dimension (1D) coupled with UV detection and a biphenyl column in the second dimension (2D) coupled with QTOF-MS detection in full scan positive mode. To test the utility of the method, three SC mixes were created, each containing five compounds that were unresolvable in a traditional, 1D-LC separation; one mix with isomeric compounds and two with structurally related but non-isomeric compounds. Contour plots of UV absorbance in 1D and MS ion intensity in 2D demonstrated that all components in each mixture were successfully resolved using the 2D-LC separation method. This research serves as proof-of-concept for the application of 2D-LC to the separation of isomeric and structurally related SC. With further optimization and validation, 2D-LC may be a generally useful tool for separation of complex mixtures of NPS.

Chromatographic retention behaviour, modelling and optimization of a UHPLC-UV separation of the regioisomers of the Novel Psychoactive Substance (NPS) methoxphenidine (MXP)

A detailed investigation into the chromatographic retention behaviour and separation of the three regioisomers of the Novel Psychoactive Substance (NPS) methoxphenidine (i.e. 2-, 3- and 4-MXP isomers) has revealed the ionization state of the analyte and stationary phase, to be the controlling factor in dictating which retention mechanism is in operation. At low pH, poor separation and retention was observed. In contrast, at intermediate pH, enhanced retention and separation of the three MXP isomers was obtained; it appeared that there was a synergistic effect between the electrostatic and hydrophobic mechanisms. At high pH, the MXP isomers were retained by hydrophobic retention. Accurate retention time predictions (<0.5%) were achievable using non-linear retention models (3 × 3). This allowed the optimization of the gradient separation of the MXP isomers using a two-dimensional gradient and temperature design space. Prediction errors for peak width and resolution were, in most cases, lower than 5%. The use of linear models (2 × 2) still afforded retention time and resolution accuracies of <2.3 and 11% respectively. A rapid and highly sensitive LC-MS friendly method (i.e. R_smin > 5 within 4 min) was predicted and verified. The developed methodology should be highly suitable for the rapid, specific and sensitive detection and control of MXP regioisomers.

Application of phenyl-based stationary phases for the study of retention and separation of oligonucleotides

The main goal of our work was to apply three different phenyl-bonded stationary phases in ion pair chromatography for the analysis of synthetic oligonucleotides of various sequences. The influence of the stationary phase structure and the impact of ion-pairing reagent concentration on the retention of oligonucleotides were tested. Moreover the influence of oligonucleotide sequence on their interactions with phenyl-based stationary phases was also investigated. Such complex studies for analysis of oligonucleotides with these adsorbents were done for the first time. Investigations were implemented in the Quantitative Structure Retention Relationships analysis in order to improve the discussion on the retention mechanism of analyzed compounds. The retention of oligonucleotides was the lowest for polar embedded phenyl stationary phase, however its selectivity was high and allowed for complete separation of studied compounds in the shortest time. It was shown that the low retention factor value was observed for oligonucleotides forming secondary structures, such as hairpin loops. Moreover obtained data showed that except for electrostatic and hydrophobic interactions, π-π also influences on the retention mechanism. These interactions cause higher retention factor values for phenyl-based stationary phases compared to octadecyl ones.

Effect-directed analysis supporting monitoring of aquatic environments--An in-depth overview

Aquatic environments are often contaminated with complex mixtures of chemicals that may pose a risk to ecosystems and human health. This contamination cannot be addressed with target analysis alone but tools are required to reduce this complexity and identify those chemicals that might cause adverse effects. Effect-directed analysis (EDA) is designed to meet this challenge and faces increasing interest in water and sediment quality monitoring. Thus, the present paper summarizes current experience with the EDA approach and the tools required, and provides practical advice on their application. The paper highlights the need for proper problem formulation and gives general advice for study design. As the EDA approach is directed by toxicity, basic principles for the selection of bioassays are given as well as a comprehensive compilation of appropriate assays, including their strengths and weaknesses. A specific focus is given to strategies for sampling, extraction and bioassay dosing since they strongly impact prioritization of toxicants in EDA. Reduction of sample complexity mainly relies on fractionation procedures, which are discussed in this paper, including quality assurance and quality control. Automated combinations of fractionation, biotesting and chemical analysis using so-called hyphenated tools can enhance the throughput and might reduce the risk of artifacts in laboratory work. The key to determining the chemical structures causing effects is analytical toxicant identification. The latest approaches, tools, software and databases for target-, suspect and non-target screening as well as unknown identification are discussed together with analytical and toxicological confirmation approaches. A better understanding of optimal use and combination of EDA tools will help to design efficient and successful toxicant identification studies in the context of quality monitoring in multiply stressed environments.

Assessment of column selection systems using Partial Least Squares

Column selection systems based on calculation of a scalar measure based on Euclidean distance between chromatographic columns, suffer from the same issue. For diverse values of their parameters, identical or near-identical values can be calculated. Proper use of chemometric methods can not only provide a remedy, but also reveal underlying correlation between them. In this work, parameters of a well-established column selection system (CSS) developed at Katholieke Universiteit Leuven (KUL CSS) have been directly correlated to parameters of selectivity (retention time, resolution, and peak/valley ratio) toward pharmaceuticals, by employing Partial Least Squares (PLS). Two case studies were evaluated, separation of alfuzosin, lamotrigine, and their impurities, respectively. Within them, comprehensive correlation structure was revealed, which was thoroughly interpreted, confirming a causal relationship between KUL parameters and parameters of column performance. Furthermore, it was shown that the developed methodology can be applied to any distance-based column selection system.