Mixed-mode chromatographic stationary phases: Recent advancements and its applications for high-performance liquid chromatography

Zeolite-based chiral ion-exchangers for chromatographic enantioseparations and potential applications in membrane separation processes

Chiral resolution of racemic compounds represents an important task in research and development and, most importantly, in the large-scale production of pharmaceuticals. Zeolites, which are already frequently utilized for their unique properties, represent materials that can be used for the development of new chiral stationary phases for liquid chromatography, simulated moving bed or enantioselective membranes. The aim of this study was to modify a series of MWW zeolites by a chiral anion-exchange type selector thereby creating a chiral stationary phase for enantiomeric resolution of acidic compounds. To evaluate the applicability of the prepared chiral stationary phase in liquid chromatography, we used N-protected amino acids as model analytes. First, we tested the new sorbents preferential sorption using N-(3,5-dinitrobenzoyl)leucine. We observed outstanding sorption properties of a zeolite-based sorbent (MCM-36), which were comparable to spherical chromatographic silica. This particular material was subsequently packed into a chromatographic column, which was tested under polar organic mode HPLC conditions facilitating baseline resolution of 5 out of 8 N-protected amino acids. Although the chromatographic performance shows several drawbacks (high backpressure, low column efficiency), it clearly documents the potential of the novel materials in chiral separation. To the best of our knowledge, this is the first example of the preparation of the chiral stationary phase based on MWW zeolites ever.

Preparation and evaluation of a pyridine sulfonate betaine-based zwitterionic stationary phase for hydrophilic interaction chromatography

A zwitterionic stationary phase comprising pyridinium cations and sulfonate anions was successfully developed through thiol-ene click chemistry. Using seven polar small molecules as probes, the zwitterionic stationary phase showed high separation selectivity and excellent column efficiency (35,200-54,800 plates/m) compared with two commercial columns. The influence of water proportion, salt concentration, and pH in the mobile phase, and column temperature, on the retention of six polar compounds was examined. The retention mechanism was explored by three hydrophilic retention models, Tanaka test and linear solvation energy relationship analysis. For the analysis of sample dairy products (milk powder, milk, and yogurt), the stationary phase was operated in hydrophilic interaction chromatography mode without the addition of buffer salts, facilitating rapid and efficient detection and quantification of melamine. The LOD and LOQ are 0.04 mg⋅g-1 and 0.13 mg⋅g-1, respectively, and the recovery rate is 90.3 - 102.8 %. The zwitterionic stationary phase has the advantages of simple preparation, good method reproducibility, good selectivity and high precision.

Molecular Dynamics Simulation for the Acidic Compounds Retention Mechanism Study on Octyl-Quaternary Ammonium Mixed-Mode Stationary Phase

With the widespread application of mixed-mode chromatography in separation analysis, it is becoming increasingly important to study its retention mechanism. The retention behavior of acidic compounds on mixed-mode octyl-quaternary ammonium (Sil-C8-QA) columns was investigated by computer simulation. Firstly, the benzoic acid homologues were used as the analytes, and the simulation model was constructed by the Materials Studio. Geometric optimization, annealing and molecular dynamics (MD) simulation of these complexes resulted in optimized conformations. The binding energy, mean square displacement (MSD) and torsion angle distribution generated by MD simulation were then analyzed. The results showed that the more negative binding energy, the greater the MSD and the narrower the torsion angle distribution, indicating that the stationary phase behaves with stronger interaction and retention. The retention behavior of five acidic drugs on the Sil-C8-QA column was then successfully explained by simulation. Acidic drugs are more retentive on the mixed-mode column due to the more substantial interaction brought by the reversed-phase/ion-exchange mixed-mode mechanism compared to other single-mode columns. This simulation method is expected to provide ideas for studying the separation mechanism and predicting the retention behavior of more complex samples.

Use of Commercial Mixed-Mode Stationary Phases and Sorbents in the High-Performance Liquid Chromatography Analysis and Solid-Phase Extraction of Ionized and Hydrophilic Bioactive Compounds

Mixed-mode high-performance liquid chromatography (HPLC) is increasingly used for the analysis of ionic and highly hydrophilic drugs, which are difficult to separate by conventional single-mode HPLC. In the former case, chromatographic separation is achieved using one of the several commercially available mixed-mode stationary phases, typically combinations of reversed and ion-exchange phases. Moreover, mixed-mode stationary phases can be used as solid-phase extraction (SPE) sorbents. This review focuses on the recent applications of mixed-mode stationary phases in the chromatographic analysis of bioactive compounds, such as drugs, herbicides, and pesticides. Specifically, we briefly summarize HPLC methods utilizing mixed-mode stationary phases and SPE pretreatment procedures utilizing mixed-mode sorbents developed in the last decade, thus providing a reference work for overcoming the difficulties in analyzing ionized or hydrophilic drugs by conventional reversed-phase chromatography.

Sample preparation for suspect screening of persistent, mobile and toxic substances and their phase II metabolites in human urine by mixed-mode liquid chromatography

Persistent, mobile and toxic substances have drawn attention nowadays due to their particular properties, but they are overlooked in human monitorization works, limiting the knowledge of the human exposome. In that sense, human urine is an interesting matrix since not only parent compounds are eliminated, but also their phase II metabolites that could act as biomarkers. In this work, 11 sample preparation procedures involving preconcentration were tested to ensure maximum analytical coverage in human urine using mixed-mode liquid chromatography coupled with high-resolution tandem mass spectrometry. The optimized procedure consisted of a combination of solid-phase extraction and salt-assisted liquid-liquid extraction and it was employed for suspect screening. Additionally, a non-discriminatory dilute-and-shoot approach was also evaluated. After evaluating the workflow in terms of limits of identification and type II errors (i.e., false negatives), a pooled urine sample was analysed. From a list of 1450 suspects and in-silico simulated 1568 phase II metabolites (i.e. sulphates, glucuronides, and glycines), 44 and 14 substances were annotated, respectively. Most of the screened suspects were diverse industrial chemicals, but biocides, natural products and pharmaceuticals were also detected. Lastly, the complementarity of the sample preparation procedures, columns, and analysis conditions was assessed. As a result, dilute-and-shoot and the Acclaim Trinity P1 column at pH = 3 (positive ionization) and pH = 7 (negative ionization) allowed the maximum coverage since almost 70 % of the total suspects could be screened using those conditions.

Ionic liquid/covalent organic framework/silica composite material: Green synthesis and chromatographic evaluation

BACKGROUND

Due to their large surface area and distinctive adsorption affinity, covalent organic frameworks (COFs) appear to be good candidates as liquid chromatographic separation materials with good application prospect. The development of COF materials in chromatographic science is currently in an exploratory stage. Especially, the practicability of COF@silica composite materials as liquid chromatographic stationary phases needs further exploration. Reasonably integrating a functional component such as ionic liquid (IL) into the COF@silica composite materials may provide customized functionality to achieve the purpose of synthesizing multi-functional COF based stationary phases.

RESULTS

In this study, an IL modified COF bonded silica composite material (IL-COF@SiO2) was successfully synthesized by using an environmentally friendly deep eutectic solvent as the reaction medium instead of the frequently-used organic solvent. The synthesized IL-COF@SiO2 composite material combines the excellent separation ability of COF and the excellent mass transfer function of spherical porous silica microsphere, and meanwhile, the introduction of IL endows COF@SiO2 with preferable separation performance. The slurry-packed IL-COF@SiO2 liquid chromatographic column could be applied to effectively separate hydrophobic and hydrophilic compounds with preferable separation selectivity and high column efficiency. By investigating the retention behavior and influencing factors, a mixed-mode retention mechanism was found. Multiple interaction forces endow the IL-COF@SiO2 with a hydrophilic-hydrophobic balance performance, demonstrating a good application prospect as a versatile liquid chromatographic separation material.

SIGNIFICANCE

In this study, a new strategy is proposed for greenly synthesizing a novel IL-COF@SiO2 composite material under mild conditions, which expands the potential application of COF materials in chromatographic science. One particular point to note is that the reaction medium in each step of the preparation process is low toxic and degradable deep eutectic solvent, which conforms to the concept of green chemistry.

A composite hydrogel modified silica stationary phase for mixed‑mode liquid chromatography

A novel composite hydrogel functionalized silica core-shell stationary phase was prepared by the surface modification of silica sphere. The successful synthesis of the new stationary phase (T-Sil@PAM/SA/UiO-66-NH2) was proven by scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), etc. Due to the coexistence of amide, hydroxyl, long carbon chain and UiO-66-NH2 in composite hydrogel shell, the obtained stationary phase can be used in hydrophilic/reversed-phase liquid chromatography with multiple retention mechanisms, such as hydrophilic, hydrophobic and π - π interactions. The chromatographic retention behavior of T-Sil@PAM/SA/UiO-66-NH2 demonstrated that the new stationary phase showed excellent separation performance for both polar analytes (such as alkaloids, saccharides, etc.) and nonpolar analytes (such as substituted benzene and polycyclic aromatic hydrocarbon (PAHs), etc.). Furthermore, compared with NH2 column and commercial C18 column, the T-Sil@PAM/SA/UiO-66-NH2 exhibited a certain superiority. Moreover, the relative standard deviation (RSD) of PAHs' retention time with eight replicates consecutive elution was found to range from 0.03% to 0.17%. Therefore, the successful use of T-Sil@PAM/SA/UiO-66-NH2 in mixed‑mode liquid chromatography expanded the potential applications of hydrogels in the field of separation.

Hydrophobic/hydrophilic separation performance evaluation of a mixed-mode ionic liquid embedded stearyl thioglycolate functionalized silica stationary phase

In this work, a novel imidazolium ionic liquid embedded multifunctional chromatographic stationary phase (Sil-AVI-ST) was synthesized by the radical-mediated thiol-ene click reaction. A wide range of samples including hydrophilic sulfonamides, vitamins and nucleosides/bases as well as hydrophobic phthalates, bisphenols, alkylphenols and steroid hormones were selected to evaluate the separation ability of the newly obtained Sil-AVI-ST. As expected, an efficient separation of the above tested analytes was successfully achieved in different chromatographic modes. It was proved that multiple stationary phase-analyte interaction forces promoted the selective separation. The Sil-AVI-ST column provided multiple retention mechanisms, enabling the efficient separation of diverse analytes with different polarity. More importantly, embedding a polar ligand (1-allyl-3-vinyl-imidazolium) could improve the separation efficiency of long-chain alkyl bonded stationary phases for hydrophilic analytes, and the developed Sil-AVI-ST column could also realize the detection of hydrophobic analytes under water-rich conditions, which is impossible for the conventional hydrophobic columns. Therefore, the newly prepared Sil-AVI-ST stationary phase has a good practical application potential.

Understanding the retention mechanisms of a reversed-phase/anion exchange/cation-exchange column for the separation of epinephrine and norepinephrine

In this paper, we investigated the retention mechanisms of a reversed-phase/anion-exchange/cation-exchange column (Acclaim trinity P1, Thermo Fisher Scientific) for the separation of epinephrine (EPI) from norepinephrine (NOE). The impact of the acetonitrile (ACN) content, pH, and salt concentration on the retention of these two catecholamines was studied under an isocratic mode with a mobile phase mixture of ACN and ammonium formate or acetate (pH 3 to pH 5). To better understand the retention mechanisms, several retention models were explored, including linear solvent strength, adsorption, quadratic, and mixed-mode models, using various chemical compounds in addition to EPI and NOE. The quadratic and mixed-mode models were the most appropriate to explain the column retention mechanisms according to the Akaike information criterion (AIC). The research showed the importance of the ACN content on the retention of compounds according to the quadratic model, and satisfactory resolution between EPI and NOE (>1.4) was achieved with 50% ACN content. The most important retention parameters were integrated in the mixed-mode model, namely ACN content, pH, and salt concentration. Using a three-factor Box-Behnken design (BBD), other optimal conditions were obtained to separate EPI and NOE with a resolution Rs > 1.5. The ACN content and salt concentrations of the aqueous part of the mobile phase were the parameters with the greatest impact on the separation performance of the stationary phase for both catecholamines. Finally, a rapid and simple separation of a mixture of EPI, NOE, and tetracaine was obtained using a mobile phase composed of ACN/ammonium formate (pH 4; 10 mM) (60:40, v/v), with a satisfactory resolution (>1.5) between the analyte peaks.

Preparation and application of sulfated lily polysaccharide bridged polyhedral oligomeric silsesquioxane hybrid organosilicas as stationary phase

Periodic mesoporous organosilicas (PMO) hydrophilic microspheres were synthesized by co-condensation of sulfated polysaccharide from Lilum lancifolium Thunb. bridged silane (SLLTPBS) and polyhedral oligomeric silsesquioxane (POSS) as stationary phase (PMO(SLLTP-POSS)) for per aqueous liquid chromatography (PALC), which would overcome the disadvantages of using a large amount of acetonitrile on the hydrophilic interaction liquid chromatography (HILIC) columns. Average particle size of PMO (SLLTP-POSS) microspheres was 4.9 μm, which was suitable for stationary phase. The retention mechanism of the stationary phase in PALC was mainly hydrophobic interactions and also included some ion-exchange interactions and electrostatic interactions. The acid-base resistance was greatly improved compared to the C18 column. The PMO(SLLTP-POSS) column under PALC mode had increased the resolution when separating some hydrophilic compounds such as eight organic acids and eleven sweeteners compared with the C18 column and HILIC column. The new column was more efficient than the HILIC columns. Additionally, a PALC-triple quadrupole mass spectrometry approach for the simultaneous identification of the eleven sweeteners was developed. The averagere coveries of the eleven compounds were 70.20%-91.33% with the relative standard deviation (RSD) range of 1.74% to 4.27%. The results showed good precision and accuracy of the method.

Preparation of three regioisomeric ionic liquid stationary phases and investigation of their retention behavior

The structural properties of ionic liquid stationary phases have a considerable effect on their separation selectivity. However, the difference of the chromatographic retention behavior of different regioisomeric ionic liquid stationary phases has rarely been investigated. In this study, three regioisomeric ionic liquid silane reagents were prepared by photoinitiated ene-click chemistry and bonded to silica by one-pot method to fabricate three new stationary phases (Sil-C2Im-C8, Sil-C6Im-C4, and Sil-C9Im-C1). All three stationary phases showed promising retention repeatability and efficiency. The retention behavior of the three stationary phases was investigated under various chromatographic conditions. The retention mechanism was further investigated by the linear energy solvation relationship and Van't Hoff plots. The stationary phases exhibited mixed-mode retention mechanisms. The π-π, hydrogen bonding, ion-exchange, and hydrophilic interactions with analytes were the weakest when the imidazole ions were embedded in the innermost part of the alkyl chains, while the interactions were the strongest when the imidazole ions were embedded in the middle of the alkyl chains. The three stationary phases provided great but different separation performances towards nucleosides, nucleobases, aromatic acids, alkyl benzenes, and polycyclic aromatic hydrocarbons due to the influence of imidazole ion position.

Mixed Metal-Organic Framework Stationary Phases for Liquid Chromatography

Strategic design of the stationary phase in liquid chromatography (LC) is crucial for modern separation science. Herein, a design approach using mixed metal-organic frameworks (MOFs) as tunable LC stationary phases is proposed. Three MOFs with an isostructural pillared-layer structure are employed, with pore sizes tuned by the systematic design of the constituent ligands, using 1,4-benzenedicarboxylate (bdc), 1,4-naphthalenedicarboxylate (ndc), and 9,10-anthracenedicarboxylate (adc). Packed columns filled with the MOFs and their mixed-particle/solid-solution stationary phases are prepared and examined for the retention capability of polyethylene glycol (PEG) in LC. While the MOF-packed columns filled with binary mixtures of different MOF particles provide good control of the retention with respect to the particle mixing ratio, the columns filled with mixed-linker solid-solution MOFs show a significant multicomponent effect on the retention behavior. Specifically, mixed-linker solid-solution MOFs consisting of bdc/ndc binary ligands are found to show a strong retention that surpasses even their parent MOFs, namely, pure bdc- and ndc-MOF stationary phases. The retention behavior on the MOF-packed columns is explained by the specific nanostructures of the solid-solution MOFs, which affects the balance between substrate affinity and adsorption kinetics into the MOF pores, dictating the total retention capability. The results provide an extra dimension for stationary phase design using MOFs as a promising recognition medium for LC.

Mixed-mode chromatography-mass spectrometry enables targeted and untargeted screening of carboxylic acids in biological samples

Carboxylic acids are crucial metabolites in the tricarboxylic acid (TCA) cycle and thus participate in central carbon metabolism (CCM). Research dependent on the analysis of metabolites involved in central carbon metabolism requires fast separation and sensitive detection of carboxylic acids using liquid chromatography-mass spectrometry (LC-MS). However, successful separation of all carboxylic acids from the TCA cycle by liquid chromatography remains a challenging task because of their high polarity and thus low retention on the conventional reversed-phase columns. In this study, we tested a reversed-phase/anion exchange mixed-mode stationary phase (Waters BEH C₁₈ AX) using liquid chromatography-tandem mass spectrometry (LC-MS/MS). We developed and optimized a method that enables a 10 minute separation of all carboxylic acids from the TCA cycle and lactic acid without prior derivatization or addition of ion-pair reagents in the mobile phase. The developed method was validated for quantification of 8 acids in murine brown preadipocytes, 5 acids in human plasma and 6 acids in Arabidopsis thaliana leaves with limits of quantification ranging from 0.1 μM for malic acid to 10 μM for isocitric acid. Moreover, the mixed-mode chromatography enabled untargeted screening of medium- to long-chain fatty acids in murine brown preadipocytes, Arabidopsis thaliana, and human plasma, where 23 fatty acids were identified by using liquid chromatography with high-resolution mass spectrometry (HRMS).

In situ photoinitiated fabrication of phosphorylcholine-functionalized polyhedral oligomeric silsesquioxane hybrid monolithic column for mixed-mode capillary electrochromatography

A phosphorylcholine-functionalized POSS hybrid monolithic column was synthesized via UV curing. It exhibits hydrophilic interaction and weak cation exchange chromatography retention mechanism for the separation of typical polar and charged compounds.

Mixed-acidic cation-exchange material for the separation of underivatized amino acids

Mixed-acidic cation-exchange (MCX) columns with both strongly (SCX) and weakly (WCX) acidic functional groups were developed for the separation of standard amino acids. The resins were prepared by carboxylation of highly crosslinked monodisperse poly(styrene-divinylbenzene) copolymer particles with performic acid and subsequent sulfonation with sulfuric acid. The degree of functionalization was varied independently for each processing step and controlled by measuring pH dependent retention of the obtained resins. A series of mixed-acidic resins with different SCX/WCX-ratios was chromatographically characterized by variation of formic acid and acetonitrile concentration in the aqueous eluent. The overall cation-exchange capacity was varied from 33 to 68 µmol/mL. The comparison with two commercial columns (Metrohm Metrosep C6, WCX and Hamilton PRP X-200, SCX) revealed the additive character of the different functional group properties within MCX columns and a unique selectivity which can be adjusted by both eluent composition and SCX/WCX-ratio of the resin. The retention window between neutral and basic amino acids was altered by varying the amount of sulfonic acid groups attached to the polymer. Orthogonality plots demonstrated constant selectivity for neutral amino acids. Correlating the retention data with log P data demonstrated the influence of non-ionic hydrophobic and π-π-interactions for the separation of amino acids on PS/DVB-based cation-exchangers. An isocratic IC-ESI-MS method was developed to separate and quantitate 20 underivatized standard amino acids within 30 min. Limits of detection were between 4 and 64 nmol L^-1 and a high linearity of calibration curves was obtained for all analytes. The method was validated by comparing a certified reference standard with external calibration data.

The separation characteristics and performance evaluation of the silica-based poly(pentabromostyrene) stationary phase in capillary electrochromatography

A mixed-mode capillary column packed with silica-based poly(pentabromostyrene) particles (denoted as SiO₂@pPBS) was prepared and applied to capillary electrochromatography (CEC) separation. With the presence of benzene rings and bromine atoms in polymer chains, the SiO₂@pPBS column provides a reversed-phase/hydrophilic mixed-mode retention mechanism owing to hydrophilic, hydrophobic and π-π interactions between the stationary phase and various analytes, including alkylbenzenes, polycyclic aromatic hydrocarbons, nucleosides, phenols and anilines. In CEC mode, the separation behavior of charged solutes is not only related to the interaction with the stationary phase, but also influenced by electrophoretic effects, which may lead to different selectivities compared to high performance liquid chromatography (HPLC). A column efficiency of up to 1.22 × 10⁵ N m^-1 was achieved for p-chloroaniline. Besides, the RSDs of retention time of anilines for run to run (n = 5), day to day (n = 5) and column to column (n = 3) were all less than 4.4%. Finally, the SiO₂@pPBS capillary column was applied to the separation of coking wastewater with satisfactory results. All the results demonstrated that the SiO₂@pPBS capillary packed column with RP/HILIC mixed-mode has great application potential.

Facile fabrication of silica@covalent organic polymers core-shell composites as the mixed-mode stationary phase for hydrophilic interaction/reversed-phase/ion-exchange chromatography

Covalent organic polymers (COPs) are a promising class of cross-linked polymeric networks that attracted extensive attention in separation and analysis fields. Exploring facile and convenient strategy to prepare COPs-based mixed-mode stationary phases for high performance liquid chromatography (HPLC) has seriously lagged and has never been reported. Herein, we describe a facile in-situ grow strategy for fabrication of silica@COPs core-shell composites (SiO₂@TpBD-(OH)₂) as a novel mixed-mode stationary phase for HPLC. Owing to the co-existing of abundant hydroxyl, carbonyl, imine, cyclohexyl groups, and benzene rings in the skeleton of COPs shell, the developed mixed-mode stationary phase exhibits hydrophilic interaction liquid chromatography (HILIC)/reversed-phase liquid chromatography (RPLC)/ion-exchange chromatography (IEX) retention mechanisms. The content of acetonitrile, pH value, and salt concentration in the mobile phase were investigated on SiO₂@TpBD-(OH)₂ packed column. In comparison to conventional single-mode columns, the SiO₂@TpBD-(OH)₂ column showed flexible selectivity, enhanced separation performance, and superior resolution for benzene homologues, polycyclic aromatic hydrocarbons, nucleosides and bases, and acidic organic compounds. The column efficiency of p-nitrobenzoic acid was up to 54440 plates per meter. The packed column also possessed outstanding chromatographic repeatability for six nucleosides and bases with the RSDs of 0.07-0.23%, 0.58-1.77%, and 0.31-1.23% for retention time, peak area, and peak height, respectively. Besides, the SiO₂@TpBD-(OH)₂ column offered baseline separation of multiple organic pollutants in lake water, which verified its great potential in real sample analysis. Overall, the silica@COPs core-shell composites not only provide a new candidate of mixed-mode stationary phases, but also extend the potential application of COPs in separation science.

Separation of vigabatrin enantiomers using mixed-mode chromatography and its application to determine the vigabatrin enantiomer levels in rat plasma

Mixed-mode chromatography-comprising a mixed phase with reversed and ionic phases, enabling hydrophobic and ion-exchange interactions simultaneously-was applied to identify vigabatrin enantiomers by HPLC with pre-column fluorescence derivatization with 2,5-dioxopyrrolidin-1-yl (4-(((2-nitrophenyl)sulfonyl)oxy)-6-(3-oxomorpholino)quinoline-2-carbonyl)prolinate (Ns-MOK-(S)-Pro-OSu). The MOK-(S)-Pro-vigabatrin enantiomers were efficiently separated within 12 min (total analysis time per sample: 28 min, including washing and equilibrium time for the column). The mobile phase was H₂O/CH₃OH/10 mM ammonium formate (pH 2.0) (20/20/60, v/v/v). Column temperature was maintained at 60℃. The proposed HPLC method could successfully monitor plasma vigabatrin enantiomer levels in rats administered (±)-vigabatrin (50 mg/kg, p.o.).

T. M, Boissel C. Electrospray ionization mass spectrometry ion suppression/enhancement caused by column bleed for three mixed-mode reversed-phase/anion-exchange high-performance liquid chromatography columns

RATIONALE

Mixed-mode reversed-phase/anion exchange liquid chromatography is useful for separations of mixtures containing anions (e.g. ionized acids). However, when using this form of liquid chromatography with mass spectrometry detection, the bleed of amine-containing hydrolysis products from the columns may cause ion suppression or enhancement.

METHODS

Using electrospray ionization tandem quadrupole mass spectrometry detection, we determined the ion suppression or enhancement caused by column bleed for three mixed-mode reversed-phase/weak anion-exchange columns containing stationary phases that differ in chemical structure. Two of the stationary phases are based on silica particles, while the third uses ethylene-bridged hybrid organic/inorganic particles, which have improved hydrolytic stability. Mixtures of acidic and basic analytes were combined with the chromatography flow postcolumn, both with and without a column, and their mass spectrometry ion signal responses (peak areas) were determined. The ratio of signal response with and without a column is the matrix factor. Positive ion electrospray measurements were carried out using 0.1% formic acid (pH ~ 2.7) as a mobile phase additive, and 10mM ammonium formate (pH ~ 6.4) was used for negative ion electrospray detection.

RESULTS

The matrix factors under both positive and negative ionization modes were closest to 1 (0.74-1.16) for the hybrid particle-based columns, showing minimal ion suppression or enhancement. In contrast, the silica-based columns gave matrix factors ranging from 0.04 to 1.86, indicating high levels of ion suppression or enhancement. These results may be explained by the differences in the structures of the stationary phases, which affect the relative amounts of hydrolysis products that elute from the columns.

CONCLUSIONS

The low levels of mass spectrometry ion suppression or enhancement caused by column bleed from the hybrid particle-based columns should allow for accurate quantitative mass spectrometric detection combined with mixed-mode reversed-phase/weak anion-exchange chromatography.

Hydrophobicity of polymer based anion-exchange columns for ion chromatography

The regularities of the retention of alkanoic and alkanesulfonic acids homologues were investigated for the set of 36 anion-exchange columns produced by various manufacturers. The role of hydrophobic and electrostatic interactions in the retention and separation of organic anions was studied. The methylene selectivity increments α(CH2) were measured for the studied columns with 10 mM sodium hydroxide eluent. The influence of matrix, surface area, polar group structure, ion-exchange capacity, the density of charged functional groups on the surface and other characteristics of anion-exchangers on resin hydrophobicity was considered. A unified approach for the measurements of hydrophobic properties of anion-exchange resins is proposed and the ratio of chloride retention factor (k Cl) to α(CH2) was introduced as mixed-mode factor. The synergetic effect of electrostatic and hydrophobic interactions was observed.

Preparation of two amide-bonded stationary phases and comparative evaluation under mixed-mode chromatography

In this work, the conventional reactions were used to functionalize the silica surface with amide and hydrocarbon chain groups affording two different mixed-mode stationary phases (Sil-amide-C11 and Sil-C12-amide). The prepared stationary phases were analyzed by elemental analysis and thermogravimetric analysis. The retention of benzene, phenol, pyridine, and aniline was investigated and compared with synthesized and commercial columns, and this led to prove the existence of different interactions on the synthesized stationary phases. The mixed-mode stationary phases showed multiple interactions, and different chromatography modes were found under distinct chromatographic conditions. According to the type of amide group (either free or within the hydrocarbon chain), different interactions can be made on the columns. The alkylbenzenes and polycyclic aromatic hydrocarbons, as nonpolar hydrocarbons, were chromatographed under reversed-phase liquid chromatography modes, in which amide groups on the silica could efficiently separate polar analytes under hydrophilic interaction liquid chromatography mode in both prepared stationary phases. The performance of the columns was compared by the separation of the carboxylic acid group and biological samples. The bonding method and the type of amide group showed different interactions leading to different separation and performance.

Analytical Methods for Determination of Antiviral Drugs in Different Matrices: Recent Advances and Trends

Viruses are the main pathogenic substances that cause severe diseases in humans and other living things. They are among the most common microorganisms, and consequently, antiviral drugs have emerged to prevent and treat viral infections. Antiviral drugs are an essential drug group considering their prescription and consumption rates for different diseases and indications. Therefore, it is crucial to develop accurate and precise analytical methods to detect antiviral drugs in various matrices. Chromatographic techniques are used frequently for the quantification purpose since they allow simultaneous determination of antivirals. Electrochemical methods have also gained importance since the analysis can be performed quickly without the need for pretreatment. Spectrophotometric and spectrofluorimetric methods are used because they are simple, inexpensive, and less time-consuming methods. The purpose of this review is to present an overview of the analysis of currently used antiviral drugs from 2010 to 2021. Since studies on antiviral drugs are numerous, selected publications were reviewed in this article. The analysis of antiviral drugs was divided into three main groups: chromatographic, spectrometric, and electrochemical methods which were applied to different matrices, including pharmaceutical, biological, and environmental samples.

Characterization and comparison of mixed-mode and reversed-phase columns; interaction abilities and applicability for peptide separation

In this work, two mixed-mode columns from a different manufacturers and one marketed as a reversed-phase column were characterized and compared in the terms of their interaction abilities, retentivity, peak symmetry, and applicability for peptide separation. All the tested columns contain octadecyl ligand and positively charged modifier, i.e. pyridyl group for the reversed-phase column XSelect CSH C18, quaternary alkylamine for mixed-mode column Atlantis PREMIER BEH C18 AX, and permanently charged moiety (details not available from the manufacturer) for mixed-mode column Luna Omega PS C18. For detailed characterization and comparison of their interaction potential, several approaches were used. First, a simple Walters test was performed to estimate hydrophobic and silanophilic interactions of the tested columns. The highest values of both parameters were observed for column Atlantis PREMIER BEH C18 AX. To investigate the effect of pH and buffer concentration on retention, mobile phases composed of acetonitrile and buffer (ammonium formate, pH 3.0; ammonium acetate pH 4.7 and pH 6.9) in various concentrations (5mM; 10mM; 15mM and 20mM) were used. The analysis of permanently charged compounds was used to describe the electrostatic interaction abilities of the stationary phases. The most significant contribution of electrostatic interactions to the retention was observed for Atlantis PREMIER BEH C18 AX column in the mobile phase with buffer of pH 3.0. A set of ten dipeptides, three pentapeptides and one octapeptide was used to investigate the effects of pH and buffer concentration on retention and peak symmetry. Each of the tested columns provides the optimal peak shape under different buffer pH and concentration. The gradient separation of the 14 tested peptides was used to verify the application potential of the tested columns for peptide separation. The best separation was achieved within 4 minutes on column Atlantis PREMIER BEH C18 AX.

A docosyl-terminated polyamine amphiphile-bonded stationary phase for multimodal separations in liquid chromatography

A convenient synthetic approach to a linear alkyl-polyamine amphiphilic chromatographic selector was proposed. Successive immobilization of the amphiphile onto silica gel afforded a multimodal stationary phase for high-performance liquid chromatography (HPLC). The as-prepared silica material was studied comparatively with a conventional octadecyl (C18) and an amide-embedded C18 stationary phase. The new uniform docosyl-triamine tandem was featured by an enhanced shape selectivity towards geometric isomers, and a low silanol activity towards alkaline solutes. The presence of multiple amino groups rendered the new adsorbent operable in different modes, such as hydrophilic interaction and ion-exchange modes. The satisfactory performance of the said stationary phase in separating different classes of analytes, including polycyclic aromatic hydrocarbons, flavonoids, tricyclic antidepressants, calcium channel blockers, aromatic acids, inorganic anions, nucleosides and estrogens, revealed its great potential and high adaptability for multipurpose LC utility.

Characterization of a highly stable mixed-mode reversed-phase/weak anion-exchange stationary phase based on hybrid organic/inorganic particles

We have characterized Atlantis ethylene-bridged hybrid C18 anion-exchange, a mixed-mode reversed-phase/weak anion-exchange stationary phase designed to give greater retention for anions (e.g., ionized acids) compared to conventional reversed-phase materials. The retention and selectivity of this stationary phase were compared to that of three benchmark materials, using a mixture of six polar compounds that includes an acid, two bases, and three neutrals. The compatibility of the ethylene-bridged hybrid C18 anion-exchange material with 100% aqueous mobile phases was also evaluated. We investigated the batch-to-batch reproducibility of the ethylene-bridged hybrid C18 anion-exchange stationary phase for 27 batches across three different particle sizes (1.7, 2.5, and 5 μm) and found it to be comparable to that of one of the most reproducible C18 stationary phases. We also characterized the acid and base stability of the ethylene-bridged hybrid C18 anion-exchange stationary phase and the results show it to be usable over a wide pH range, from 2 to 10. The extended upper pH limit relative to silica-based reversed-phase/weak anion-exchange materials is enabled by the use of ethylene-bridged hybrid organic/inorganic particles. The improved base stability allows Atlantis ethylene-bridged hybrid C18 anion-exchange to be used with a wider range of mobile phase pH values, opening up a greater range of selectivity options.

Prospects of using hyperbranched stationary phase based on poly(styrene-divinylbenzene) in mixed-mode chromatography

Evaluation of the chromatographic properties of covalently bonded hyperbranched stationary phase based on poly(styrene-divinylbenzene) (PS-DVB) and containing zwitterionic fragments in the structure of functional layer was conducted in suppressed ion chromatography (IC), reversed phase high performance liquid chromatography (RP HPLC), and hydrophilic interaction liquid chromatography (HILIC) modes. Besides the possibility of resolving 20 inorganic anions and organic acids using KOH eluent in suppressed IC, prepared resin provided the separation of alkylbenzenes in RP HPLC, water-soluble vitamins, amino acids, and sugars in HILIC mode. Trends in the retention of hydrophobic and polar analytes on the prepared stationary phase indicated the dominating effect of analyte nature on the retention mechanism and proved satisfactory hydrophilization of PS-DVB surface with hyperbranched functional layer for retaining polar compounds. The obtained results revealed good prospects of using hydrophobic PS-DVB substrate for preparing stationary phases for mixed-mode chromatography.

Preparation and evaluation a mixed-mode stationary phase with C18 and 2-methylindole for HPLC

A modified C₁₈ column (Silpr-2MI-C18) was prepared using 2-methylindole and C₁₈ reagent. The extent of C₁₈ hydrocarbon chain, conjugative rings and anion exchange site provided multiple retention mechanisms, including reversed-phase liquid chromatography (RPLC), π-π interaction, hydrophilic interaction liquid chromatography (HILIC) and anion exchange chromatography (AEC). The separation of protected amino acids was investigated on the commercial C₁₈ and Silpr-2MI-C18 columns, while the chromatographic conditions, including methanol content and pH of the mobile phase, were studied. The separation arrangement of the hydrophilic amino acids was different on the Silpr-2MI-C18 column compared to the commercial C₁₈ column under RPLC mode. Furthermore, these amino acids were separated on the Silpr-2MI-C18 column under HILIC mode. The modified C₁₈ column was employed to separate amino acids, alkylbenzenes and polycyclic aromatic hydrocarbons under RPLC mode and inorganic anion under AEC mode. The results confirm that this new stationary phase of RPLC/HILIC/AEC has multiple interactions with different analytes. Effective retention of biological samples was found on the Silpr-2MI-C18 column by comparing the results obtained from the commercial C₁₈ column.

Silica modification with 9-methylacridine and 9-undecylacridine as mixed-mode stationary phases in HPLC

In this research, 9-methylacridine and 9-undecylacridine were synthesized through Bernthsen's reaction and well characterized using gas chromatography-mass spectrometry (GC-MS) and nuclear magnetic resonance (NMR). Two mixed-mode stationary phases were developed by functionalizing silica with 9-methylacridine and 9-undecylacridine. Then, two modified silicas were characterized by elemental analysis, thermogravimetric analysis (TGA), and fourier transform-infrared spectroscopy (FT-IR). Due to the extent of conjugative rings, the hydrophobic hydrocarbon chain, and anion exchange sites of 9-methylacridinium and 9-undecylacridinium group on the silica gel of columns, mixed-mode stationary phases were designed with multiple interactions including π-π stacking interaction, reverse phase, hydrophilic interaction, and anion exchange. According to the type of acridine, different interactions may be formed in the target column. Polycyclic aromatic hydrocarbons (PAHs), alkylbenzenes, pyridines and parabens were chromatographed on π-π stacking modes and RPLC, where anion exchange sites can be applied for the separation of inorganic anions on AEC mode. Considering the structure of the stationary phases, these columns were used to separate organic compounds with higher polarity on the HILIC retention. The performance of the columns was investigated by the chromatographic parameters in terms of column efficiency (N/m), asymmetry factor (A_f), retention factor (k), and resolution (Rs). The mixed-mode stationary phases can be successfully employed to conduct chromatographic separation on a wide range of samples with a single column.

Strong cation- and zwitterion-exchange-type mixed-mode stationary phases for separation of pharmaceuticals and biogenic amines in different chromatographic modes

A set of new mixed-mode ion-exchange stationary phases is presented. The backbone of organic selectors is formed by a linear hydrocarbon chain, which is divided into two parts of various lengths by a heteroatom (oxygen or nitrogen). In all studied cases, there is a sulfonic acid moiety as the terminal group. Therefore, selectors bearing oxygen gave rise to strong cation ion-exchange stationary phases, while selectors with an embedded nitrogen atom (inducing a weak anion exchange capacity) were used to create zwitterion ion-exchange stationary phases. The new mixed-mode stationary phases were chromatographically evaluated in high performance liquid chromatography (HPLC) and supercritical fluid chromatography (SFC) using isocratic elution conditions to disclose their chromatographic potential. In HPLC mode, aqueous-rich reversed phase chromatography, acetonitrile-rich hydrophilic interaction liquid chromatography and methanolic ion-exchange chromatography mobile phases were employed. In these chromatographic modes, retention factors and selectivity values for a test set of basic and zwitterionic analytes were determined. The results were compared and principal component analysis for each chromatographic mode was performed. For all chromatographic modes, the component 1 in the principal component analysis reflected the elution order. The application of different mobile phases on a particular column resulted not only in variation in retention, but also in modified selectivity, and different elution order of the analytes. The orthogonality of the elution order depending on the employed mobile phase conditions was especially reflected for structurally closely related analytes, such as melatonin and N-acetyl-serotonin, tryptamine and serotonin or noradrenalin and octopamine. However, ion-exchange interactions remain the main driving force for retention. From all investigated stationary phases, the SCX 2 (C5-linker and C4-spacer) seems to be the best choice for the separation of basic analytes using different mobile phase conditions.

Trends in sample preparation and separation methods for the analysis of very polar and ionic compounds in environmental water and biota samples

Recent years showed a boost in knowledge about the presence and fate of micropollutants in the environment. Instrumental and methodological developments mainly in liquid chromatography coupled to mass spectrometry hold a large share in this success story. These techniques soon complemented gas chromatography and enabled the analysis of more polar compounds including pesticides but also household chemicals, food additives, and pharmaceuticals often present as traces in surface waters. In parallel, sample preparation techniques evolved to extract and enrich these compounds from biota and water samples. This review article looks at very polar and ionic compounds using the criterion log P ≤ 1. Considering about 240 compounds, we show that (simulated) log D values are often even lower than the corresponding log P values due to ionization of the compounds at our reference pH of 7.4. High polarity and charge are still challenging characteristics in the analysis of micropollutants and these compounds are hardly covered in current monitoring strategies of water samples. The situation is even more challenging in biota analysis given the large number of matrix constituents with similar properties. Currently, a large number of sample preparation and separation approaches are developed to meet the challenges of the analysis of very polar and ionic compounds. In addition to reviewing them, we discuss some trends: for sample preparation, preconcentration and purification efforts by SPE will continue, possibly using upcoming mixed-mode stationary phases and mixed beds in order to increase comprehensiveness in monitoring applications. For biota analysis, miniaturization and parallelization are aspects of future research. For ionic or ionizable compounds, we see electromembrane extraction as a method of choice with a high potential to increase throughput by automation. For separation, predominantly coupled to mass spectrometry, hydrophilic interaction liquid chromatography applications will increase as the polarity range ideally complements reversed phase liquid chromatography, and instrumentation and expertise are available in most laboratories. Two-dimensional applications have not yet reached maturity in liquid-phase separations to be applied in higher throughput. Possibly, the development and commercial availability of mixed-mode stationary phases make 2D applications obsolete in semi-targeted applications. An interesting alternative will enter routine analysis soon: supercritical fluid chromatography demonstrated an impressive analyte coverage but also the possibility to tailor selectivity for targeted approaches. For ionic and ionizable micropollutants, ion chromatography and capillary electrophoresis are amenable but may be used only for specialized applications such as the analysis of halogenated acids when aspects like desalting and preconcentration are solved and the key advantages are fully elaborated by further research. Graphical abstract.

Polysaccharide-based chromatographic adsorbents for virus purification and viral clearance

Viruses still pose a significant threat to human and animal health worldwide. In the fight against viral infections, high-purity viral stocks are needed for manufacture of safer vaccines. It is also a priority to ensure the viral safety of biopharmaceuticals such as blood products. Chromatography techniques are widely implemented at both academic and industrial levels in the purification of viral particles, whole viruses and virus-like particles to remove viral contaminants from biopharmaceutical products. This paper focuses on polysaccharide adsorbents, particulate resins and membrane adsorbers, used in virus purification/removal chromatography processes. Different chromatographic modes are surveyed, with particular attention to ion exchange and affinity/pseudo-affinity adsorbents among which commercially available agarose-based resins (Sepharose®) and cellulose-based membrane adsorbers (Sartobind®) occupy a dominant position. Mainly built on the development of new ligands coupled to conventional agarose/cellulose matrices, the development perspectives of polysaccharide-based chromatography media in this antiviral area are stressed in the conclusive part.