Reversed-phase Separation Research Articles

Deep Proteome Profiling with Reduced Carryover Using Superficially Porous Microfabricated nanoLC Columns.

In the field of liquid chromatography-mass spectrometry (LC-MS)-based proteomics, increases in the sampling depth and proteome coverage have mainly been accomplished by rapid advances in mass spectrometer technology. The comprehensiveness and quality of the data that can be generated do, however, also depend on the performance provided by nano-liquid chromatography (nanoLC) separations. Proper selection of reversed-phase separation columns can be important to provide the MS instrument with peptides at the highest possible concentration and separated at the highest possible resolution. In the current contribution, we evaluate the use of the prototype generation 2 μPAC nanoLC columns, which use C18-functionalized superficially porous micropillars as a stationary phase. When compared to traditionally used fully porous silica stationary phases, more precursors could be characterized when performing single shot data-dependent LC-MS/MS analyses of a human cell line tryptic digest. Up to 30% more protein groups and 60% more unique peptides were identified for short gradients (10 min) and limited sample amounts (10-100 ng of cell lysate digest). With LC-MS gradient times of 10, 60, 120, and 180 min, respectively, we identified 2252, 6513, 7382, and 8174 protein groups with 25, 500, 1000, and 2000 ng of the sample loaded on the column. Reduction of sample carryover to the next run (up to 2 to 3%) and decreased levels of methionine oxidation (up to 3-fold) were identified as additional figures of merit. When analyzing a disuccinimidyl dibutyric urea-crosslinked synthetic library, 29 to 59 more unique crosslinked peptides could be identified at an experimentally validated false discovery rate of 1-2%.

Two-Dimensional Chiral HPLC Analysis of Lactate, Hydroxybutyrates and Malate in Human Plasma

For the simultaneous determination of lactate (LA), 2-hydroxybutyrate (2HB), 3-hydroxybutyrate (3HB) and malate (MA) enantiomers, a new separation technique in a two-dimensional high-performance liquid chromatography (2D-HPLC) system following the fluorescence derivatization with 4-nitro-7-piperazino-2,1,3-benzoxadiazole has been developed. The 2D-HPLC system was composed of the reversed-phase (1D) and enantioselective (2D) separations. In the first dimension, the target hydroxy acids were separated as their D plus L mixtures on a Singularity RP18 column (1.0 mm i.d. x 250 mm), and the enantiomers were separated in the second dimension using a polysaccharide-based chiral stationary phase, Chiralpak IG (2.0 mm i.d. x 250 mm). By using the Chiralpak IG column, the LA, 2HB, 3HB and MA enantiomers were separated with resolution values of 3.47, 3.63, 6.81 and 3.72, respectively. The developed 2D-HPLC system was applied to human plasma, and trace levels of D-LA and L-3HB in addition to their major antipodes were determined. The acquired %D of LA (the percentage of D-LA over total LA) and %L of 3HB (the percentage of L-3HB over total 3HB) were 1.0 and 2.7, respectively. For 2HB, only the L-form was detected, while the MA enantiomers were not found in the human plasma.

Segmented MS/MS acquisition of a1 ion-based strategy for in-depth proteome quantitation

In-depth proteome quantitation is of great significance for understanding protein functions, advancing biological, medical, environmental and metabolic engineering research. Herein, benefiting from the high formation efficiencies and intensities of dimethyl-labeled a1 ions for accurate quantitation, we developed an in-depth a1 ion-based proteome quantitation method, named deep-APQ, by a sequential MS/MS acquisition of the high mass range for identification and the low mass range for a1 ion intensity extraction to increase quantitative protein number and sequence coverage. By the analysis of HeLa protein digests, our developed method showed deeper quantitative coverage than our previously reported a1 ion-based quantitation method without mass range segmentation and lower missing values than widely-used label-free quantitation method. It also exhibited excellent accuracy and precision within a 20-fold dynamic range. We further integrated a workflow combining the deep-APQ method with highly efficient sample preparation, high-pH and low-pH reversed-phase separation and high-field asymmetric waveform ion mobility spectrometry (FAIMS) to study E. coli proteome responses under the nutritional conditions of glucose and acetate. A total of 3447 proteins were quantified, representing 82% of protein-coding genes, with the average sequence coverage up to 40%, demonstrating the high coverage of quantitation results. We found that most of the quantified proteins related to chemotaxis were differentially expressed, including the low-abundance proteins such as tap, trg, aer, cheA and cheB, indicating that chemotaxis would play an important role for E. coli cell to survive from acetate toxicity. The above results demonstrated that the deep-APQ method is of great promising to achieve the deep-coverage proteome quantitation with high confidence.

Nanohydrophobic Interaction Chromatography Coupled to Ultraviolet Photodissociation Mass Spectrometry for the Analysis of Intact Proteins in Low Charge States.

The direct correlation between proteoforms and biological phenotype necessitates the exploration of mass spectrometry (MS)-based methods more suitable for proteoform detection and characterization. Here, we couple nano-hydrophobic interaction chromatography (nano-HIC) to ultraviolet photodissociation MS (UVPD-MS) for separation and characterization of intact proteins and proteoforms. High linearity, sensitivity, and sequence coverage are obtained with this method for a variety of proteins. Investigation of collisional cross sections of intact proteins during nano-HIC indicates semifolded conformations in low charge states, enabling a different dimension of separation in comparison to traditional, fully denaturing reversed-phase separations. This method is demonstrated for a mixture of intact proteins from Escherichia coli ribosomes; high sequence coverage is obtained for a variety of modified and unmodified proteoforms.

Comparing the chromatographic performance of benzene and aniline end capped stationary phase synthesized by click chemistry

Two stationary phases named Sil-styrene and Sil-PAS were synthesized using styrene and p-aminostyrene as end capped functional monomers by the reaction of click chemistry, respectively. Intermediates and stationary phase materials were characterized by various methods. Four types of compounds including polycyclic aromatic hydrocarbons (PAHs), alkylbenzenes, positional isomers, nucleotides and nucleosides were chosen to assess reversed-phase and hydrophilic interaction separation modes of two prepared columns. Retention mechanism of these analytes was researched through investigating effects of mobile phase composition on retention. Prepared columns could achieve better separation effects including higher resolution, shorter retention time and less organic solvent compared with traditional commercial columns. Moreover, the PAHs in barbecued meat samples was achieved separation under the reverse-phase mode on the Sil-styrene column. The experimental results shows that amino groups on the benzene ring can affect the hydrophilic and hydrophobic interactions with analytes, and the prepared Sil-styrene column shows enormous potential for separation of PAHs in actual samples.

Rapid determination of key impurities in high purity bisphenol A with reversed phase separation and triple quadrupole mass spectrometry

Liquid chromatography with diode array detection (DAD) or ultraviolet spectroscopic (UV) detection as the most important analytical technique for the accurate quantification of impurities in bisphenol compounds normally requires long analysis time for baseline separation of all components as well as highly concentrated sample solutions for the detection of trace levels. To expand the application possibilities to all stages of polymerisation processes, an easy and robust reversed phase separation for 7 known impurities of bisphenol-A (BPA) including 4-isopropenylphenol and its dimeric isomers, o, p-bisphenol-A and trisphenol was established in this work. The method has been validated for the detection with triple quadrupole mass spectrometry (qqqMS) and DAD. In the investigated concentration range 0.5 – 100 mg/kg, the linearity is verified for both detection techniques. The limit of quantification (LOQ) for each impurity is with 0.5 – 1.5 mg/kg for qqqMS and 15 mg/kg for DAD sufficient for the evaluation of BPA as a raw material for polymerisation processes. The separation time for all impurities is 10 min whereas earlier reported methods need a minimum of 25 to 40 min. In addition the necessary sample concentration of BPA could be reduced to 5 mg/mL compared to existing methods where the sample concentration typically is > 50 mg/mL. For all those reasons the validated method can be efficiently applied for frequent process monitoring. Furthermore, 4 additional impurities were detected and identified. Mainly these are reaction products from the isopropenylphenol structure in combination with confirmed impurities as trisphenol or chroman. The quantification of these structures was established with trisphenol as reference and two structures were detected in all BPA qualities of this study in a concentration range from 20 – 400 mg/kg.

Off-line two-dimensional LC-MS/MS determination of tryptophan enantiomers in mammalian urine and alteration of their amounts in d-amino acid oxidase deficient mice

D-Tryptophan (D-Trp) is one of the minor D-enantiomers of amino acids discovered in microbes and mollusca. In the present study, a highly-selective 2D chiral LC-MS/MS method has been designed and developed focusing on the determination of Trp enantiomers to investigate the presence and regulation of free D-Trp in mammals. The developed system consisted of a reversed-phase separation for the first dimension, an enantioselective separation for the second dimension and also the detection using a triple quadrupole mass spectrometer for the third/fourth dimensions. Using the present method, urinary D-Trp in mammals, including healthy human volunteers and mice, were successfully determined. Although only l-Trp was observed in a mixed urine sample of healthy volunteers, small amounts of D-Trp were detected in the C57BL/6J mice (n = 5, %D=6.18 ± 0.47). In B6DAO- mice lacking the activity of d-amino acid oxidase (DAO), relatively high levels of D-Trp were observed (n = 6, %d=27.43 ± 3.26). The obtained %d values of Trp in the urine of the C57BL/6J mice and B6DAO- mice were confirmed using various enantioselective columns having different separation properties. These results indicate that the urinary D-Trp level is regulated by DAO in mammals, and further investigations, such as tissue distribution and physiological significance of the intrinsic D-Trp, are expected.

Pressure-Enhanced Liquid Chromatography, a Proof of Concept: Tuning Selectivity with Pressure Changes and Gradients.

Many chromatographers have observed that the operating pressure can dramatically change the chromatographic retention of solutes. Small molecules show observables changes, yet even more sizable effects are encountered with large biomolecules. With this work, we have explored the use of pressure as a method development parameter to alter the reversed-phase selectivity of peptide and protein separations. An apparatus for the facile manipulation of column pressure was assembled through a two-pump system and postcolumn flow restriction. The primary pump provided an eluent flow through the column, while the secondary pump provided a pressure-modulating flow at a tee junction after the column but ahead of a flow restrictor. Using this setup, we were able to quickly program various constant pressure changes and even pressure gradients. It was reconfirmed that pressure changes impact the retention of large molecules to a much greater degree than small molecules, making it especially interesting to consider the use of pressure to selectively separate solutes of different sizes. The addition of pressure to bring the column operating pressure beyond 500 bar was enough to change the elution order of insulin (a peptide hormone) and cytochrome C (a small serum protein). Moreover, with the proposed setup, it was possible to combine eluent and pressure gradients in the same analytical run. This advanced technique was applied to improve the separation of insulin from one of its forced degradation impurities. We have referred to this method as pressure-enhanced liquid chromatography and believe that it can offer unseen selectivity, starting with peptide and protein reversed-phase separations.

Front Cover: Comparison of the separation of proteins of wide‐ranging molecular weight via trilobal polypropylene capillary‐channeled polymer fiber, commercial superficiously porous, and commercial size exclusion columns

DOI: 10.1002/jssc.202100891 The reversed-phase (RP) separation of proteins, without size-exclusion contributions, is affected on capillary-channeled polymer (C-CP) fiber stationary phases. Polyester C-CP fibers are demonstrated to perform the rapid RP separation of peptides and proteins ranging in molecule weight of 1.44-to-660 kDa. Comparisons with commercial RP superficially porous and size exclusion columns designed for protein separations show that the across-the-board separation quality realized by the fibers is superior to the other column formats, with column material costs being far less than the standard columns.

Reversed-phase chromatography as an effective tool for the chiral separation of anionic and zwitterionic carboranes using polysaccharide-based chiral selectors

The aspect of the different spatial arrangement of anionic cobalt bis(dicarbollides) and dicarba-nido-undecaboranes remains grossly overlooked despite increased scientific interest. Regarding their growing potential, which does not limit only to medicinal chemistry, suitable enantioseparation methods are needed. The presented paper explores the possibilities of chiral separations of anionic cobalt bis(dicarbollide) and dicarba-7,8-nido-undecaborane derivatives on four polysaccharide-based columns under reversed-phase conditions. The chromatographic behavior of anionic derivatives was evaluated and compared with that of zwitterionic clusters. The isocratic procedure for HPLC method development was suggested. The main parameters for the optimization of separations were described. Successful chiral separations were critically compared to previously reported results in normal phase and polar-organic mode. Reversed-phase separations are superior in resolution, lower consumption of organic solvents, and flexibility during the method development. Moreover, the first chiral discrimination of some hydroxyalkyl derivatives of anionic cobalt bis(dicarbollides) is reported. The outcomes of this work may be used for method development in any area, where the chirality of these interesting molecules is of concern.

Analysis of chemical constituents in classical prescription Danggui Buxue Decoction based on RP-Q-TOF-MS and HILIC-Q-TOF-MS

The chemical constituents of classical prescription Danggui Buxue Decoction were analyzed by reversed-phase(RP) chromatography and hydrophilic interaction chromatography(HILIC) coupled with quadrupole time-of-flight mass spectrometry. RP separation of Danggui Buxue Decoction was performed on ACQUITY UPLC HSS T3(2.1 mm×100 mm, 1.8 μm), while HILIC separation was on Waters BEH Amide(2.1 mm×100 mm, 1.7 μm). Mass spectrometry(MS) data were acquired in both negative and positive ion modes. Chemical constituents of Astragali Radix and Angelicae Sinensis Radix were searched from Reaxys and thus the in-house library was established. MS data were further analyzed by MassLynx 4.1 combined with in-house library, HMDB, Reaxys, and comparison with reference substances. In conclusion, a total of 154 compounds were identified and characterized: 16 saponins, 44 flavonoids, 10 phthalides, 7 phenylpropanoids, 15 bases and the corresponding nucleosides, 30 oligosaccharides, and 32 other compounds. Among them, 65 compounds were detected by HILIC-MS/MS. This study provides experimental evidences for the material basis research, quality control, and preparation development of Danggui Buxue Decoction and a reference method for comprehensive characterization of Chinese medicine decoctions typified by classical prescriptions.

A Comprehensive 2D-LC/MS Online Platform for Screening of Acetylcholinesterase Inhibitors

The continuous interest in discovering new bioactive molecules derived from natural products (NP) has stimulated the development of improved screening assays to help overcome challenges in NP-based drug discovery. Here, we describe a unique platform for the online screening of acetylcholinesterase inhibitors without the need for pre-treating the sample. In the current study, we have demonstrated the ability to combine reversed-phase separation with a capillary immobilized enzyme reactor (cIMER) in two-dimensional liquid chromatography system coupled with mass spectrometry detection. We systematically investigated the effects of method parameters that are of practical significance and are known to affect the enzyme assay and interfere in the analysis such as: bioreactor dimensions, loop sizes, amount of immobilized enzyme, second dimension flow rates, reaction time, substrate concentration, presence of organic modifier, limit of detection and signal suppression. The performance of this new platform was evaluated using a mixture containing three known AChE inhibitors (tacrine, galanthamine and donepezil) and an ethanolic extract obtained from the dry bulbs of Hippeastrum calyptratum (Amaryllidaceae) was investigated to provide a proof of concept of the applicability of the platform for the analysis of complex mixtures such as those derived from NPs.

Rapid and sensitive simultaneous separation and electrochemical detection of tetracaine hydrochloride and oxymetazoline hydrochloride in pharmaceutical formulations via core-shell reversed-phase liquid chromatography

Tetracaine hydrochloride (TCH) is a nasal anesthetic and oxymetazoline hydrochloride (OZH) is a nasal decongestant. A moderate to acute overdosage of OZH and TCH can lead to mydriasis, nausea, cyanosis, tachycardia, dyspnoea, cardiovascular failure, disorientation, seizures, and even death. Liquid chromatography (LC) has been mainly utilized for the individual determination of either TCH or OZH; however, there is a need for rapid and efficient methods for simultaneous analysis in pharmaceutical formulations and aqueous samples. This study highlights the use of the fast and efficient separation capabilities of core-shell silica particles in liquid chromatography (LC) for the simultaneous determination of TCH and OZH using UV detection and the enhanced selectivity afforded by electrochemical detection at a boron-doped diamond (BDD) electrode. Rapid reversed-phase (RP) separation and detection of OZH and TCH in nasal spray and eye drops was achieved within 45 s using a poroshell 120 EC-C18 column, by adjusting the ratio of organic solvent, mobile phase pH, detection potential and mobile phase flow rate. Sensitivity was compared using ultraviolet (UV) detection at 280 nm, and ECD at + 1.3 V with detection limits of 40 and 70 nM for TCH and OZH, respectively. The developed rapid method was utilized successfully in the analysis of pharmaceutical formulations, where the estimated levels of TCH and OZH in these formulations are in agreement with the specified values outlined by the manufacturers.

Study of system properties in reversed-phase liquid chromatography for binary and ternary solvent mobile phase compositions using the solvation parameter model

System maps for the individual system constant of the solvation parameter model and five binary solvent containing 20–70% (v/v) acetonitrile, acetone, methanol, 2-propanol, and tetrahydrofuran on a single octadecylsiloxane-bonded silica column (Luna C18) are used to provide insight into the variation of system properties with mobile phase composition and solvent type. Selectivity differences are dominated by variation in solute size and hydrogen-bond basicity with solvent-dependent variation in dipole-type and hydrogen-bond acid interactions. Interactions involving lone pair electrons are important only in the case of the alcohols and to a lesser extent tetrahydrofuran. Selectivity differences are also dependent on solvent strength. To expand the selectivity space four ternary solvent systems (acetonitrile-methanol-water, acetonitrile-2-propanol-water, methanol-tetrahydrofuran-water, and tetrahydrofuran-2-propanol-water) 1:1:2% (v/v) containing 50% (v/v) total organic solvent are compared with the binary solvent systems at the same organic solvent composition. There is no simple model that links the system properties of the ternary solvents to the binary solvent systems, but it is demonstrated that the ternary solvent systems expand the selectivity space available for reversed-phase separations. The solvation properties of the bulk organic solvents provide insufficient information to predict selectivity in RPLC because of the dominant contribution of water and effects related to the selective solvation of the stationary phase and possible changes in the solvent-dependent microstructure of the mobile phase.

Comprehensive analysis of phenolics compounds in citrus fruits peels by UPLC-PDA and UPLC-Q/TOF MS using a fused-core column.

In this work, a method based on ultra-high-performance liquid chromatography with a photodiode array detector (UPLC-PDA) was developed to comprehensively analyze phenolic compounds in peels of lime (Citrus×latifolia), lemon (Citrus limon), and rangpur lime (Citrus×limonia). The reverse-phase separation was achieved with a C18 fused-core column packed with the smallest particles commercially available (1.3 um). The method was successfully coupled with high-resolution mass spectrometry (HRMS), allowing the detection of 24 phenolic compounds and five limonoids in several other citrus peels species: key lime, orange and sweet orange, tangerine, and tangerine ponkan, proving the suitability for comprehensive analysis in citrus peel matrices. Additionally, the developed method was validated according to the Food and drug administration (FDA) and National Institute of Metrology Quality and Technology (INMETRO) criteria, demonstrating specificity, linearity, accuracy, and precision according to these guidelines. System suitability parameters such as resolution, tailoring, plate count were also verified.

Equivalent Carbon Number and Interclass Retention Time Conversion Enhance Lipid Identification in Untargeted Clinical Lipidomics.

Chromatography is often used as a method for reducing sample complexity prior to analysis by mass spectrometry, and the use of retention time (RT) is becoming increasingly popular to add valuable supporting information in lipid identification. The RT of lipids with the same headgroup in reversed-phase separation can be predicted using the equivalent carbon number (ECN) model. This model describes the effects of acyl chain length and degree of saturation on lipid RT. For the first time, we have found a robust correlation in the chromatographic separation of lipids with different headgroups that share the same fatty acid motive. This relationship can be exploited to perform interclass RT conversion (IC-RTC) by building a model from RT measurements from lipid standards that allows the prediction of RT of one lipid subclass based on another. Here, we utilize ECN modeling and IC-RTC to build a glycerophospholipid RT library with 517 entries based on 136 tandem mass spectrometry-characterized lipid RTs from NIST SRM-1950 plasma and lipid standards. The library was tested on a patient cohort undergoing coronary artery bypass grafting surgery (n = 37). A total of 156 unique circulating glycerophospholipids were identified, of which 52 (1 LPG, 24 PE, 5 PG, 18 PI, and 9 PS) were detected with IC-RTC, thereby demonstrating the utility of this technique for the identification of lipid species not found in commercial standards.

Fabrication of monolithic frits and columns for chip-based multidimensional separation devices.

In this work, devices for two-dimensional separations are considered. The device contains a flow distributor, a first-dimension channel, and 17 second-dimension outlets. In the design, all connections between the first-dimension channel, the flow distributor, and the second-dimension outlets were tapered, with a minimal diameter of 20 μm. The use of photo-masking is explored for the fabrication of monolithic frits in all tapered connections. Monolithic frits with optimized permeability and length were successfully fabricated in all 33 tapered channels through light-induced polymerization, photo-masking, and selective exposure. The efficacy of the monolithic frits was demonstrated by creating a packed bed of 15-μm particles, confined within the first-dimension channel. The outlet of the first-dimension channel was successfully connected to a mass spectrometer. Effective flow confinement was demonstrated with a reversed-phase separation of a mixture of five standard peptides.

High sensitivity acidic N-glycan profiling with MS-enhancing derivatization and mixed mode chromatography

Acidic N-linked glycan content is often associated with a protein drug’s stability, efficacy and immune response. It has often been a challenge to analyze these types of glycans, including those that are differentiated by the incorporation of N-acetyl (NANA) and N-glycolyl neuraminic acid (NGNA) residues. In this study, a strategy for rapid N-glycan profiling by mixed mode chromatography is proposed as a complement to established HILIC methodologies. Hybrid silica chromatographic surfaces are used to improve recoveries during a column’s initial use and to eliminate the need for column conditioning. In addition, the loss of labeled acidic glycans, especially phosphorylated glycan species, during SPE purification is addressed through the use of a citrate containing eluent. Yields for both singly and doubly phosphorylated glycan species are markedly improved. Combined with a mixed mode anion exchange reversed phase separation, these advances afford a class separation of glycans derivatized with labels designed to enhance positive ion mode MS detection. These labeled glycan species are separated according to their charge and with an added level of resolution imparted by the reversed phase retention mechanism. The separation technique itself can be accomplished with a low ionic strength gradient running from 0 to 22 mM ammonium formate such that high sensitivity detection can be achieved by both fluorescence and mass spectrometry. Using analytical scale chromatography, features in an N-glycan profile were easily interrogated to well below a 0.1% relative abundance. As such, it became possible to characterize N-glycans from recombinant beta glucuronidase and to quickly identify a number of unique phosphorylated glycan species.

Preparation and chiral resolution properties of bridged bis(cyclodextrin)s hybrid spheres for high performance liquid chromatography.

Selenium-bridged bis(β-cyclodextrin)s organic-inorganic hybrid silica material with regular spherical shape as new type of chiral stationary phase was directly synthesized under the one-pot hydrothermal synthesis method, and the chiral stationary phase was further characterized by infrared spectroscopy, scanning electron microscopy, thermogravimetry, and elemental analysis. The results of chiral separation showed that eight chiral compounds including various types of chiral alcohols and flavanone were successfully separated in the reversed-phase separation mode by high performance liquid chromatography, which showed the better chiral resolution effect than that on the C2 position of single β-cyclodextrin. The mechanism of chiral separation was likely due to multiple interactions such as inclusion, hydrogen bonding, electrostatic interaction, dipole-dipole interaction, and the synergistic effect of two cyclodextrins during the chiral resolution process. The synergy of the two cyclodextrins has great potential for development in chiral resolution.

Ion-pair Reversed-phase Liquid Chromatographic Separation of Oligonucleotides.

Therapeutic oligonucleotides have recently been approved in the United States, the EU, and Japan. Hence, the analysis of oligonucleotides is an important topic in drug development. Liquid chromatographic techniques are commonly used for purity verification and the determination of oligonucleotides. In ion-pair reversed-phase separation, several parameters, such as the pore size of the stationary phase, mobile phase additives, and column temperature, were investigated using three types of oligonucleotides (18, 19, and 20 mer). All of the investigated parameters could influence the separation, and they are expected to be useful for optimizing oligonucleotide separation.