Mixed-mode stationary phases as a complementary selectivity concept in liquid chromatography–tandem mass spectrometry-based bioanalytical assays

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.

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.

Mixed-mode chromatography in pharmaceutical and biopharmaceutical applications

Mixed-mode chromatography (MMC) is a fast growing area in recent years, thanks to the new generation of mixed-mode stationary phases and better understanding of multimode interactions. MMC has superior applications in the separation of compounds that are not retained or not well resolved by typical reversed-phase LC methods, especially for polar and charged molecules. Due to the multiple retention modes that a single MMC column can offer, often MMC provides additional dimension to a separation method by adjusting the mobile phase conditions. Mixed-mode media is also an effective way to clean up complex sample matrices for purification purposes or for sensitive detection of trace amounts of analytes. In this article, we discuss mixed-mode stationary phases and separation mechanisms and review recent advances in pharmaceutical and biopharmaceutical applications including the analysis and/or purification of counterions, small molecule drugs, impurities, formulation excipients, peptides and proteins.

Retention prediction of highly polar ionizable solutes under gradient conditions on a mixed-mode reversed-phase and weak anion-exchange stationary phase

In the present work the retention of three highly polar and ionizable solutes - uric acid, nicotinic acid and ascorbic acid - was investigated on a mixed-mode reversed-phase and weak anion-exchange (RP/WAX) stationary phase in buffered aqueous acetonitrile (ACN) mobile phases. A U-shaped retention behavior was observed for all solutes with respect to the eluent organic modifier content studied in a range of 5-95% (v/v). This retention behavior clearly demonstrates the presence of a HILIC-type retention mechanism at ACN-rich hydro-organic eluents and an RP-like retention at aqueous-rich hydro-organic eluents. Hence, this column should be promising for application under both RP and HILIC gradient elution modes. For this reason, a series of programmed elution runs were carried out with increasing (RP) and decreasing (HILIC) organic solvent concentration in the mobile phase. This dual gradient process was successfully modeled by two retention models exhibiting a quadratic or a cubic dependence of the logarithm of the solute retention factor (lnk) upon the organic modifier volume fraction (φ). It was found that both models produced by gradient retention data allow the prediction of solute retention times for both types of programmed elution on the mixed-mode column. Four, in the case of the quadratic model, or five, in the case of the cubic model, initial HILIC- and RP-type gradient runs gave satisfactory retention predictions of any similar kind elution program, even with different flow rate, with an overall error of only 2.5 or 1.7%, respectively.

Investigation into the molecular and thermodynamic basis of protein interactions in multimodal chromatography using functionalized nanoparticles

Although multimodal chromatography offers significant potential for bioseparations, there is a lack of molecular level understanding of the nature of protein binding in these systems. In this study a nanoparticle system is employed that can simulate a chromatographic resin surface while also being amenable to isothermal titration calorimetry (ITC) and solution NMR. ITC and NMR titration experiments are carried out with (15)N-labeled ubiquitin to investigate the interactions of ubiquitin with nanoparticles functionalized with two industrially important multimodal ligands. The ITC results suggest that binding to both multimodal ligand surfaces is entropically driven over a range of temperatures and that this is due primarily to the release of surface bound waters. In order to reveal structural details of the interaction process, binding-induced chemical shift changes obtained from the NMR experiments are employed to obtain dissociation constants of individual amino acid residues on the protein surface. The residue level information obtained from NMR is then used to identify a preferred binding face on ubiquitin for interaction to both multimodal ligand surfaces. In addition, electrostatic potential and spatial aggregation propensity maps are used to determine important protein surface property data that are shown to correlate well with the molecular level information obtained from NMR. Importantly, the data demonstrate that the cluster of interacting residues on the protein surface act co-operatively to give rise to multimodal binding affinities several orders of magnitude greater than those obtained previously for interactions with free solution ligands. The use of NMR and ITC to study protein interactions with functionalized nanoparticles offers a new tool for obtaining important molecular and thermodynamic insights into protein affinity in multimodal chromatographic systems.

Preparation of a novel carboxyl stationary phase by "thiol-ene" click chemistry for hydrophilic interaction chromatography

A novel carboxyl-bonded silica stationary phase was prepared by "thiol-ene" click chemistry. The resultant Thiol-Click-COOH phase was evaluated under hydrophilic interaction liquid chromatography (HILIC) mobile phase conditions. A comparison of the chromatographic performance of Thiol-Click-COOH and pure silica columns was performed according to the retention behaviors of analytes and the charged state of the stationary phases. The results indicated that the newly developed Thiol-Click-COOH column has a higher surface charge and stronger hydrophilicity than the pure silica column. Furthermore, the chromatographic behaviors of five nucleosides on the Thiol-Click-COOH phase were investigated in detail. Finally, a good separation of 13 nucleosides and bases, and four water-soluble vitamins was achieved.

Multi-modal applicability of a reversed-phase/weak-anion exchange material in reversed-phase, anion-exchange, ion-exclusion, hydrophilic interaction and hydrophobic interaction chromatography modes

We recently introduced a mixed-mode reversed-phase/weak anion-exchange type separation material based on silica particles which consisted of a hydrophobic alkyl strand with polar embedded groups (thioether and amide functionalities) and a terminal weak anion-exchange-type quinuclidine moiety. This stationary phase was designed to separate molecules by lipophilicity and charge differences and was mainly devised for peptide separations with hydroorganic reversed-phase type elution conditions. Herein, we demonstrate the extraordinary flexibility of this RP/WAX phase, in particular for peptide separations, by illustrating its applicability in various chromatographic modes. The column packed with this material can, depending on the solute character and employed elution conditions, exploit attractive or repulsive electrostatic interactions, and/or hydrophobic or hydrophilic interactions as retention and selectivity increments. As a consequence, the column can be operated in a reversed-phase mode (neutral compounds), anion-exchange mode (acidic compounds), ion-exclusion chromatography mode (cationic solutes), hydrophilic interaction chromatography mode (polar compounds), and hydrophobic interaction chromatography mode (e.g., hydrophobic peptides). Mixed-modes of these chromatographic retention principles may be materialized as well. This allows an exceptionally flexible adjustment of retention and selectivity by tuning experimental conditions. The distinct separation mechanisms will be outlined by selected examples of peptide separations in the different modes.

Novel ultra stable silica-based stationary phases for reversed phase liquid chromatography--study of a hydrophobically assisted weak acid cation exchange phase

A mixed-mode reversed-phase/weak cation exchange (RP/WCX) phase has been developed by introducing a small amount of carboxylate functionality into a hydrophobic hyper-crosslinked (HC) platform. This silica-based HC platform was designed to form an extensive polystyrene network completely confined to the particle's surface. The fully connected polymer network prevents the loss of bonded phase, which leads to superior hydrolytic stability of the new phase when compared to conventional silica-based phases. Compared to previously introduced HC phases the added carboxylic groups impart a new weak cation exchange selectivity to the base hydrophobic HC platform. The phase thus prepared shows a mixed-mode retention mechanism, allowing for both neutral organic compounds and bases of a wide polarity range to be simultaneously separated on the same phase under the same conditions. In addition, the new phase offers the flexibility that gradients in organic modifier, pH or ionic competitors can be used to affect the separation of a wide range of solutes. Moreover, the inherent weak acid cation exchange groups allow formic and acetic acid buffers to be used as eluents thereby avoiding the mass spectrometric ionization suppression problems concomitant to the use of non-volatile additives such as strong amine modifiers (e.g. triethylamine) or salts (e.g. NaCl) to elute basic solutes from the strong cation exchange phase which was previously developed in this lab. The use of the new phase for achieving strong retention of rather hydrophilic neurotransmitters and drugs of abuse without the need for ion pairing agents is demonstrated.

Evaluation of protein adsorption and preferred binding regions in multimodal chromatography using NMR

NMR titration experiments with labeled human ubiquitin were employed in concert with chromatographic data obtained with a library of ubiquitin mutants to study the nature of protein adsorption in multimodal (MM) chromatography. The elution order of the mutants on the MM resin was significantly different from that obtained by ion-exchange chromatography. Further, the chromatographic results with the protein library indicated that mutations in a defined region induced greater changes in protein affinity to the solid support. Chemical shift mapping and determination of dissociation constants from NMR titration experiments with the MM ligand and isotopically enriched ubiquitin were used to determine and rank the relative binding affinities of interaction sites on the protein surface. The results with NMR confirmed that the protein possessed a distinct preferred binding region for the MM ligand in agreement with the chromatographic results. Finally, coarse-grained ligand docking simulations were employed to study the modes of interaction between the MM ligand and ubiquitin. The use of NMR titration experiments in concert with chromatographic data obtained with protein libraries represents a previously undescribed approach for elucidating the structural basis of protein binding affinity in MM chromatographic systems.

Critical topics in ensuring data quality in bioanalytical LC–MS method development

The use of LC–MS for bioanalysis of pharmaceuticals is entering its third decade and may be considered to be a mature technology. In many respects this is true, considering the advances made in such areas as instrument performance, electronics, software and automation of use. However, there remain instrumental and noninstrumental areas that require significant attention to ensure data quality. Increasing regulatory focus on analytical method performance and unaddressed method issues require the bioanalyst to understand those areas that most greatly impact data quality. This review will focus on instrumental and noninstrumental areas that can influence data quality, including reference standard and internal standard quality and physicochemical properties, matrix effects, stability in matrix, sample preparation, LC and MS.

Quantification of polar drugs in human plasma with liquid chromatography–tandem mass spectrometry

Liquid chromatography–tandem mass spectrometry (LC–MS/MS) has played an important role in quantitative bioanalytical assays. This review summarizes the recent progress on quantification of polar drugs in plasma with LC–MS/MS. Various types of polar analytes were extracted using protein precipitation or solid-phase extraction and precolumn derivatization was utilized in some cases. The analytes were then separated using different types of chromatographic method, which included reversed-phase chromatography, aqueous normal-phase chromatography, hydrophilic interaction liquid chromatography and ion-pairing chromatography. Stationary phases of mixed mode and porous graphitic carbon materials are gaining acceptance in bioanalytical applications. These technologies can be valuable supplements in the quantification of polar drugs in human plasma with LC–MS/MS. Matrix effects have also been discussed in this review.

Serial coupling of reversed-phase and hydrophilic interaction liquid chromatography to broaden the elution window for the analysis of pharmaceutical compounds

It is presently a common practice in drug discovery to analyse samples by reversed-phase liquid chromatography (RPLC) and hydrophilic interaction chromatography (HILIC). To increase throughput, HILIC was connected in series to RPLC by means of a T-piece with make-up flow. The first column is a 2mm I.D. column having an optimal flow between 0.1 and 0.2mL/min. Via the T-piece, the flow for the second dimension column with an I.D. of 4.6mm is adjusted to 1.5-2.0mL/min with a high acetonitrile content (i.e. >/=80%) mobile phase. Therefore, even in gradient RPLC analysis starting with a mobile phase with high water content, the HILIC column is always operated at high acetonitrile concentration which is required to obtain retention on the HILIC column. The performance of the hyphenated RPLC/HILIC set-up is illustrated with the analysis of two model samples of pharmaceutical interest. Optimization of the conditions in the HILIC dimension is discussed.