Study of overloading of basic drugs and peptides in reversed-phase high-performance liquid chromatography using pH adjustment of weak acid mobile phases suitable for mass spectrometry

The detection of ACTH and insulin in equine plasma by solid-phase extraction and micro-flow LC/MSMS

Previous liquid chromatography/mass spectrometry (LC/MS) methods for the detection of insulin and other similar peptide hormones in equine plasma relied on the use of antibody affinity extraction. As a result, these methods were not suitable for routine high-throughput analysis. A solid-phase extraction (SPE) method incorporating size exclusion as well as reversed-phase interactions allows the selective extraction of peptide hormones such as adrenocorticotropic hormone (ACTH), insulin and their synthetic analogues from equine plasma with approximately 80% extraction efficiencies. This extraction was combined with on-column derivatisation with acetic anhydride, followed by tryptic digestion and analysis by micro-LC/MSMS for high-sensitivity peptide hormone detection. The analysis of tryptic peptides provides greater sensitivity and more robust chromatography compared with the analysis of intact insulin and ACTH. For quantitative analysis, isotopically labelled internal standards of target peptides can be prepared in the laboratory through the use of deuterated acetic anhydride. The utility of the method was assessed for the analysis of ACTH and insulin in samples from horses suffering from pituitary pars intermedia dysfunction (PPID).

A stationary phase with a positively charged surface allows for minimizing formic acid concentration in the mobile phase, enhancing electrospray ionization in LC-MS proteomic experiments

The default choice of mobile phase acidifier for bottom-up LC-MS proteomic analyses is 0.10% formic acid because of its decent acidity, decent ion pairing ability, and low suppression of electrospray ionization. In recent years, state-of-the-art columns have been designed specifically to provide efficient separation even when using an MS-friendly mobile phase of low ionic strength. Despite this, no attempts have been made to improve the sensitivity of the MS-based analytical methods by reducing the amount of formic acid in the mobile phase. In this study, we evaluated the effect of reduced formic acid concentration in the mobile phase on the chromatographic behavior and MS response of peptides when separated using columns packed with a C18 stationary phase with a positively charged surface. Using 0.01% formic acid in the mobile phase maintained excellent chromatographic performance and increased MS signal response compared to the standard of 0.10%. The enhanced MS response translated to about 50% improved peptide identifications depending on the complexity and amount of sample injected. The increased retention of peptides at a reduced formic acid concentration was directly proportional to the number of acidic residues in the peptide sequence. The study was carried out by covering a spectrum of protein samples with varied complexity using analytical flow, micro-, and nanoflow regimes to expand the applicability in routine practice.

Reversed-Phase Liquid Chromatography of Peptides for Bottom-Up Proteomics: A Tutorial

The performance of the current bottom-up liquid chromatography hyphenated with mass spectrometry (LC-MS) analyses has undoubtedly been fueled by spectacular progress in mass spectrometry. It is thus not surprising that the MS instrument attracts the most attention during LC-MS method development, whereas optimizing conditions for peptide separation using reversed-phase liquid chromatography (RPLC) remains somewhat in its shadow. Consequently, the wisdom of the fundaments of chromatography is slowly vanishing from some laboratories. However, the full potential of advanced MS instruments cannot be achieved without highly efficient RPLC. This is impossible to attain without understanding fundamental processes in the chromatographic system and the properties of peptides important for their chromatographic behavior. We wrote this tutorial intending to give practitioners an overview of critical aspects of peptide separation using RPLC to facilitate setting the LC parameters so that they can leverage the full capabilities of their MS instruments. After briefly introducing the gradient separation of peptides, we discuss their properties that affect the quality of LC-MS chromatograms the most. Next, we address the in-column and extra-column broadening. The last section is devoted to key parameters of LC-MS methods. We also extracted trends in practice from recent bottom-up proteomics studies and correlated them with the current knowledge on peptide RPLC separation.

Investigation into reversed-phase chromatography peptide separation systems Part IV: Characterisation of mobile phase selectivity differences

The differentiation of mobile phase compositions between sub-classes which exhibit distinct chromatographic selectivity (i.e. termed characterisation) towards a range of peptide probes with diverse functionality and hence the possibility for multi-modal retention mechanisms has been undertaken. Due to the complexity of peptide retention mechanisms in given mobile phase conditions, no attempt has been made to explain these, instead mobile phases have simply been classified into distinct groups with an aim of identifying those yielding differing selectivities for use in strategic method development roadmaps for the analysis of peptide mixtures. The selectivity differences between nine synthetic peptides (fragments of [Ile27]-Bovine GLP-2) were used to assess how fifty-one RPC mobile phase compositions of differing pH (range 1.8 - 7.8), salt types, ionic strengths, ion-pair reagents and chaotropic / kosmotropic additives affected chromatographic selectivity on a new generation C18 stationary phase (Ascentis Express C18). The mobile phase compositions consisted of commonly used and novel UV or MS compatible additives. The chemometric tool of Principal Component Analysis (PCA) was used to visualise the differences in selectivity generated between the various mobile phases evaluated. The results highlight the importance of screening numerous mobile phases of differing pH, ion-pair reagents and ionic strength in order to maximise the probability of achieving separation of all the peptides of interest within a complex mixture. PCA permitted a ranking of the relative importance of the various mobile phase parameters evaluated. The concept of using this approach was proven in the analysis of a sample of Bovine GLP-2 (1-15) containing synthesis related impurities. Mobile phases with high ionic strength were demonstrated to be crucial for the generation of symmetrical peaks. The observations made on the C18 phase were compared on three additional stationary phases (i.e. alkyl amide, fluorophenyl and biphenyl), which had previously been shown to possess large selectivity differences towards these peptides, on a limited sub-set of mobile phases. With the exception of the ion-pair reagent, similar trends were obtained for the C18, fluorophenyl and biphenyl phases intimating the applicability of these findings to the vast majority of RPC columns (i.e. neutral or weakly polar in character) which are suitable for the analysis of peptides. The conclusions were not relevant for columns with a more disparate nature (i.e. containing a high degree of positive charge).

Experimental study on the reduction of loratadine particle size through confined liquid impinging jets

The confined liquid impinging jets (CLIJ) technique was applied as a simple and effective approach to reducing the particle size of loratadine to enhance its solubility. The effect of anti-solvent (AS) to solution (S) flow rate ratio, organic phase concentration, Reynolds number (Re), and stabilizer concentration was investigated in this reduction process. After the synthesis, the chemical and physical properties of loratadine nanoparticles were determined through different characterization and analytical techniques. The results indicated that the particle size of loratadine decreases from 320 nm to 80 nm by increasing the AS/S ratio from 1 to 25. It was found that the particle size of loratadine was unchanged at the higher AS/S ratios. The loratadine nanoparticle size was optimized by changing the solution concentration, Re, and Tween 80 as a stabilizer. The finest loratadine nanoparticle size of about 53 nm was obtained with a narrow size distribution, which corresponds to solution concentration of 35 mg/mL, Re of 5687, and 0.1% (w/v) stabilizer concentration. It was revealed that the optimized loratadine nanoparticles completely dissolved after 11 min, indicating the loratadine nanoparticle dissolution rate 50 times faster than raw loratadine.

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.

Investigation into reversed phase chromatography peptide separation systems part I: Development of a protocol for column characterisation

A protocol was defined which utilised peptides as probes for the characterisation of reversed phase chromatography peptide separation systems. These peptide probes successfully distinguished between differing stationary phases through the probe's hydrophobic, electrostatic, hydrogen bonding and aromatic interactions with the stationary phase, in addition, to more subtle interactions such as the phase's ability to separate racemic or isomeric probes. The dominating forces responsible for the chromatographic selectivity of peptides appear to be hydrophobic as well as electrostatic and polar in nature. This highlights the need for other types of stationary phase ligands with possibly mixed mode functionalities / electrostatic / polar interactions for peptide separations rather than the hydrophobic ligands which dominate small molecule separations. Selectivity differences are observed between phases, but it appears that it is the accessibility differences between these phases which play a crucial role in peptide separations i.e. accessibility to silanols, the hydrophobic acetonitrile / ligand layer or a thin adsorbed water layer on the silica surface.

Estimation of Nonlinear Adsorption Isotherms in Gradient Elution RP-LC of Peptides in the Presence of an Adsorbing Additive

ABSTRACT

In electrostatic repulsive interaction chromatography, using a charged surface hybrid sorbent carrying positive charges can improve the peak shape of peptides in reversed-phase liquid chromatography (RP-LC), especially in overloaded conditions, compared with standard C18 sorbents. However, the positive surface charges can interact with anionic additives commonly used in peptide separations, e.g., trifluoroacetic acid (TFA), complicating adsorption isotherm estimation. We investigated how the competition for available adsorption sites between TFA and two peptides influenced the adsorption isotherm in gradient elution. A model accounting for the competition with TFA was compared with a model neglecting TFA adsorption. We found that the two models predicted elution profiles with the same accuracy. We also found that the adsorption isotherms were extremely similar in shape, leading to the conclusion that neglecting the competition with TFA is a valid approximation enabling faster and more robust adsorption isotherm estimation for the studied type of sorbent.

GRAPHICAL ABSTRACT

Preparative separation of a challenging anthocyanin from Lycium ruthenicum Murr. by two-dimensional reversed-phase liquid chromatography/hydrophilic interaction chromatography

The preparative separation of anthocyanins by HPLC often suffers from insufficient separation selectivity.

Overloading study of basic compounds with a positively charged C18 column in liquid chromatography

While tailing and overloading of basic compounds remain problematic on most RP columns, a new kind of positively charged RP column named XCharge C18 was found to be superior good for the separation of alkaloids in our practical use. In this work, the surface charge property of the XCharge C18 column was evaluated by the retention of NO(3)(-) under different pH values and buffer concentrations. A considerable and pH-dependent positive charge was confirmed on the column. Then overloading behaviors of bases were systematically studied using amitriptyline as a basic probe. Good peak shapes (Tf<1.5) and extra high loadability with a C(0.5) of about 30,000 mg/L were observed on the column, with commonly used 0.1% formic acid as mobile phase additive. However, increasing the ionic strength of buffer with phosphates led to tailing peaks at high sample amount and sharp decline in loadability (C(0.5) of 2000-3000 mg/L), although it brought higher column efficiency at low sample amount. Higher pH also induced worse performance and lower loadability. The overall results demonstrated the importance of an appropriate level of ionic repulsion for the XCharge C18 column to achieve the good performance for bases, which could be explained by the multiple-site adsorption theory as ionic repulsion would shield the solute from occupying high-energy sites deeper in C18 layer.

Annealing of silica to reduce the concentration of isolated silanols and peak tailing in reverse phase liquid chromatography

Non-porous, colloidal silica particles were annealed at three different temperatures, 800, 900 and 1050 °C. The adsorption of lysozyme, a probe of surface roughness, was consistent with progressively reduced surface roughness as temperature increased. The heat treated silica particles were rehydroxylated and then used to pack UHPLC columns. The cationic protein lysozyme was used to probe silanol activity, which exhibited progressively less tailing as the annealing temperature increased. FTIR spectroscopy confirmed that the abundance of isolated silanols on the surface was reduced by annealing at 900 °C or 1050 °C. FTIR also revealed that there was markedly increased hydrogen bonding of the isolated silanols to neighbors after rehydroxylation. These results combine to support the hypothesis that (a) isolated silanols on silica cause tailing in RP-LC and (b) nonplanar topography gives rise to isolated silanols.

On-line coupling of anion exchange and ion-pair chromatography for measurement of intracellular triphosphate metabolites of reverse transcriptase inhibitors

We developed an automated on-line weak anion exchange (WAX) solid-phase extraction (SPE) method coupled with ion-pair (IP) chromatography-tandem mass spectrometry (MS/MS) detection for quantitatively measuring triphosphorylated metabolites of three reverse transcriptase inhibitors (RTI). The administered pro-drugs were Tenofovir disoproxil fumarate (TDF), Emtricitabine (FTC) and Lamivudine (3TC). Their intracellular metabolites Tenofovir-diphosphate (TFV-DP), Emtricitabine-triphosphate (FTC-TP), and Lamivudine-triphosphate (3TC-TP) were measured in peripheral blood mononuclear cells (PBMC). We coupled the WAX and IP chromatography systems using a combination of 6-port and 10-port switching valves, and we mixed the WAX elute with 1,5-dimethyl-hexyl-amine before IP chromatography separation. Multiple waste outlets allowed for eliminating potential matrix components interfering with MS/MS detection. Limits of detection were 9, 200 and 75 pg per sample for TFV-DP (448/176 m/z), FTC-TP (488/130 m/z) and 3TC-TP (468/119 m/z), respectively.

High-speed gradient elution reversed-phase liquid chromatography of bases in buffered eluents. Part I. Retention repeatability and column re-equilibration

We studied the run-to-run repeatability of the retention times of both non-ionizable and basic compounds chromatographed using buffered eluents. The effect of flow rate, organic modifier and other additives, buffer type/concentration, stationary phase type, batch-to-batch preparation of the initial eluent, gradient time, sample type and intra-day changes on retention repeatability were examined. We also assessed the effect of column storage solvent conditions on the inter-day repeatability. Although retention repeatability is strongly influenced by many parameters (flow rate, solvent compressibility compensation, precision of temperature control, and buffer/stationary phase type), our primary finding is that with a reasonable size column (15cmx4.6mm (i.d.)) two column volumes of re-equilibration with initial eluent suffices to provide acceptable repeatability (no worse than 0.004min) for both non-ionizable and basic analytes under a wide variety of conditions. Under ideal conditions (e.g. the right buffer, flow rate, etc.) it is possible to obtain truly extraordinary repeatability often as good as 0.0004min. These absolute fluctuations in retention translate to worst case changes in resolution of 0.2 units and average changes of only 0.02 units.

Thermodynamic characterization of separations on alkaline-stable silica-based C18 columns: Why basic solutes may have better capacity and peak performance at higher pH

A systematic study was made to explain the large improvements in separation performance and capacity of basic compounds at alkaline conditions. The adsorption of three probe components was investigated on four alkaline-stable silica-based C18 columns at three different pH-levels: 3, 7 and 11. The probes were 3-phenyl-1-propanol (neutral), 2-phenylbutyric acid (acidic) and metoprolol (basic). Adsorption isotherms were acquired over a broad concentration range, in order to detect both high and low energy sites. Before the choice of the proper adsorption isotherm model, the adsorption energy distribution (AED) was calculated yielding the number of different kinds of interaction sites between the solute and the stationary phase. The neutral probe was entirely unaffected by pH and its AED was unimodal (one site) indicating homogenous adsorption. For the acidic probe the interactions were unimodal at pH 3 where the probe is uncharged and at least bimodal (two sites) at pH 7 and 11 where the probe is charged. For the basic probe, the interactions were heterogeneous at both pH 3 and 11. The equilibrium constants of the high and low energy sites were different by a factor of 55-100 at pH 3 and only 6-7 at pH 11. The difference in saturation capacities between the two sites was much smaller at pH 11 where 20% of the total capacity is from the high energy site, as compared to pH 3 where the high energy site was only 2-5% of the total capacity. This explains why peaks of amines (basic solutes) tail at low pH while their peaks are symmetrical at alkaline pH. The Langmuir model fit the unimodal data and the bi-Langmuir model fit the bimodal AED data. The calculated band profiles based on these parameters agreed excellently with the experimental data. The electrostatic-modified Langmuir, on the other hand, did not describe this adsorption process well.

Single-Molecule Probing of Adsorption and Diffusion on Silica Surfaces

Single-molecule spectroscopy has emerged as a valuable tool in probing kinetics and dynamic equilibria in adsorption because advances in instrumentation and technology have enabled researchers to obtain high signal-to-noise ratios for common dyes at room temperature. Single-molecule spectroscopy was applied to the study of an important problem in chromatography: peak broadening and asymmetry in the chromatograms of pharmaceuticals, peptides, and proteins. Using DiI, a cationic dye that exhibits the same problematic chromatographic behavior, investigators showed that the adsorption sites that cause chromatographic problems are located at defects on the silica crystal surface.

Chromatography of amino acids and short peptides. New advances

The newest results in the application of various chromatographic methods (gas-liquid chromatography, liquid chromatographic techniques, electrically driven systems) for the separation and quantitative determination of amino acids and short peptides in pure state and in complicated matrices are compiled. The results are concisely described and critically evaluated. The future trends of the chromatographic analysis of amino acids and short peptides are briefly discussed.

A study of retention and overloading of basic compounds with mixed-mode reversed-phase/cation-exchange columns in high performance liquid chromatography

The retention and overload of bases were studied on two new mixed-mode, silica based phases possessing ionic carboxylate functionalities of different acidity embedded within a hydrophobic ligand (SiELC Primesep). At low pH, good peak shapes were obtained for small solute mass, suggesting that the mere presence of a mixed-mode hydrophobic/ionic retention mechanism is not responsible for the poor peak shape that can occur on conventional reversed-phases with ionised silanols. Somewhat inferior, but still acceptable peak shape for bases was obtained on a column containing a mixture of discrete ion exchange and reversed-phase particles (Hypersil Duet). In both types of column, the ionic sites favourably increased the capacity for ionised bases, reducing considerably the deterioration of peak shape with load observed with conventional RP columns. The combined ionic and reversed-phase interaction can give strong retention of bases under certain conditions, necessitating careful choice of stationary and mobile phase.

Enhanced Performance Test Mix for High-Throughput LC/MS Analysis of Pharmaceutical Compounds

LC/MS is being used for the routine analysis of small molecules in both the discovery and development stages within the pharmaceutical industry. In drug discovery, LC/MS is relied upon to confirm the identity and assess the purity of chemical entities. To ensure the quality of LC/MS analysis, it is important that the LC/MS system is operating within defined performance criteria. Performance monitoring of the system with a standard compound mix offers many advantages over other alternatives, since it monitors the LC/MS system as an integrated unit under the same working conditions as those used for the analysis of samples. It is also a convenient approach, because the test mix can be injected as part of the automated sequence. Use of a test mix for similar purposes has been described previously (Tang, L.; Fitch, W. L.; Alexander, M. S.; Dolan, J. W. Anal. Chem. 2000, 72, 5211-5218). To monitor the performance of ArQule's LC/MS operation (with UV and ELS detection) in greater detail, a set of eight compounds was selected from a collection of 137 commercially available "druglike" compounds. The compounds are generally stable and compliant with the rule-of-five criteria. This enhanced mix has a balanced selection of pKa values and covers the typical range of hydrophobicity and molecular masses of pharmaceutical compounds. Moreover, the selected compounds can generally be ionized using ESI and APCI modes with positive and negative polarity. The test mix can be used under formic acid or ammonium hydroxide conditions and with methanol or acetonitrile as an organic modifier. Performance monitoring with the enhanced mix is demonstrated with respect to ionization and mass measurement, as well as changes in gradient profile, flow rate, buffer pH, and ionic strength.

Study of overload for basic compounds in reversed-phase high performance liquid chromatography as a function of mobile phase pH

The retention and overloading properties for eight basic solutes and two quaternary ammonium compounds were studied over the pH range 2.7-10.0 using phosphate and carbonate buffers. At low pH, a hybrid inorganic-organic silica-ODS phase (XTerra RP-18, 15 cm x 0.46 cm) showed substantial loss in efficiency when sample masses exceeded about 0.5 microg; these results were similar to those obtained previously on pure silica ODS and wholly polymeric phases, suggesting a common overloading mechanism. At pH 7-8.5, substantial improvements in loading capacity were obtained on XTerra due apparently to the unexpectedly strong influence of small decreases in solute ionisation. Data from the quaternary compounds suggested that silanol ionisation on this phase was still small even at intermediate pH. For many bases, loading capacity continued to improve as the pH was raised to 10, in line with the decrease in the proportion of ionised solute. However, for the highest pK(a) solutes, peak shape worsened at high pH, possibly due to the negative influence of increasing column silanol ionisation.

Overload for Ionized Solutes in Reversed-Phase High-Performance Liquid Chromatography

Overloading occurs for submicrogram quantities of ionized solutes particularly when using low ionic strength mobile phases at low pH (e.g., formic acid), even with highly inert silica RP-HPLC columns of normal dimensions. Much higher loads can produce a sharp L-shaped peak with retention above the column void volume, in line with the hypothesis that a small number of high-energy sites fill first and are rapidly overloaded, followed by a much larger number of weaker sites. However, charged acids and bases show identical overloading behavior; overloading is reduced as the mobile-phase ionic strength is increased. These findings raise questions about the physical nature of the strong sites. The rapid overloading of silica and purely polymeric phases could be explained by mutual repulsion of ionic species or their inability to fully penetrate the hydrophobic structure of the phase. However, these alternative hypotheses cannot readily explain the high total saturation capacities obtained using frontal analysis. Ion pairing with trifluoroacetic acid may reduce overload, while the effect is less important for formate or phosphate buffers. A surface layer of acetonitrile is not a prerequisite for rapid overloading, as shown by studies using purely aqueous buffers.