Porous Graphitic Carbon Surface Research Articles

Porous Graphitic Carbon-Based Imprint Mass Spectrometry Imaging with an Ambient Liquid Extraction Technique for Enhancing Coverage of Glycerolipids and Sphingolipids in Brain Tissue.

Localization of lipidomes and tracking their spatial changes by mass spectrometry imaging (MSI) is critical for the mechanism studies on living process, disease, and therapeutic treatment. However, due to the strong ion suppression in complex biotissue, the imaging coverage for lipids with low polarity or low abundances, such as glycerolipids and sphingolipids, is usually limited. To address this issue, we utilized a porous graphitic carbon (PGC) material to imprint brain tissue sections for selective enrichment of neutral lipids with polar phospholipids removed. Then, the tissue imprint was scanned for desorption by the ambient liquid extraction MSI system. It was found that on the PGC surface, hydrophobic interaction dominates in protic solvents, and polar interaction dominates in aprotic solvents. Accordingly, methanol was selected as the spray solvent for tissue imprinting, and 75% acetonitrile-methanol was selected as the desorption solvent for the ambient liquid extraction MSI system. The results showed that glycerides had high recoveries after the imprinting-desorption process (recovery ∼ 70%) with phospholipids eliminated (recovery < 7%). To increase the transferring efficiencies of lipids from tissue onto PGC, electrospray was used for solvent application during imprinting, and the signals of diglycerides (DGs) in the imprint MSI of brain tissue increased by 2-3 times as compared to those via air spray. Finally, the new imprint MSI approach was applied to the imaging of the rat cerebellum and was compared with direct tissue MSI. The results showed that with imprint MSI, the coverage of DGs, sphingomyelins (SMs), and ceramides was enhanced by 4-5-fold (32 vs 6, 4 vs 1, and 5 vs 0). The ion images showed that with imprint MSI, higher signal intensities and clearer spatial distribution of DGs and SMs were obtained without interference from phosphatidylcholine signals compared with tissue MSI. This new method provides a complementary approach for traditional MSI to address the issues in imaging poorly ionizable or low-abundance lipids.

Engineering PtRu bimetallic nanoparticles with adjustable alloying degree for methanol electrooxidation: Enhanced catalytic performance

PtRu bimetal is of particularly attractive in various electrocatalytic reactions owing to its synergistic effect, ligand effect and strain effect. Here, PtRu nanoalloy supported on porous graphitic carbon (PC) has been successfully prepared via a very facile method involving co-reduction the precursors of Pt and Ru at 300 °C by H2 (PtRu/PCL) followed by thermal treatment at high temperature (700 °C, PtRu/PC–H). Specifically, the electrocatalytic performance of PtRu/PC nanoalloy could be dramatically enhanced through high-temperature annealing. This strategy has synthesized smaller Pt and PtRu nanoparticles (ca. < 3 nm); what's more, they are all homogeneous deposited on the surface of PC. PtRu/PC–H nanocatalyst displays higher alloying degree and stronger electronic interaction between Pt and Ru atoms accompanied by the downshift of Pt d-band center. Studies of electrochemical tests indicate that the as-fabricated PtRu/PC–H sample exhibits superior electrocatalytic performance and excellent CO-poisoning tolerance compared with PtRu/PCL and Pt/PC nanocatalysts. The mass activity and specific activity on PtRu/PC–H nanoalloy can be increased to 1674.2 mA mg−1Pt and 4.4 mA cm−2 for MOR, it is 4.08 and 8.80 times higher than that of the Pt/PC nanocatalyst, respectively. From in-situ FTIR spectra, we can discover PtRu/PC–H nanoalloy generates CO2 at a lower potential of −150 mV than those on PtRu/PC–L (0 mV) and Pt/PC (50 mV) nanocatalysts, dramatically improves the ability of cleavage C–H bond and alleviates the COads poisoning on active sites. The PtRu/PCH nanocatalyst exhibits maximum power density of 83.7 mW cm−2 in single methanol fuel cell test, which more than threefold than that of commercial Pt/C as the anode catalyst. Those experimental results open an effective and clean avenue in the development and preparation of high-performance Pt-based nanocatalysts for direct methanol fuel cells.

Sensitive untargeted identification of short hydrophilic peptides by high performance liquid chromatography on porous graphitic carbon coupled to high resolution mass spectrometry

The combination of an efficient chromatographic separation with post-column addition of a supercharging agent was evaluated for the determination of small peptides. The procedure takes advantage of porous graphitic carbon (PGC) ability in retaining very polar and ionic molecules to overstep the poor retention of small peptides on conventional reversed phase (RP) columns. The method was developed specifically for the most hydrophilic di-, tri- and tetrapeptides, which are not identified in ordinary peptidomics experiments. In addition to retention mechanisms acting on conventional RP, the method exploited the charge induced interactions generated by the charges on the peptides with the polarizable surface of PGC. This results in efficient retention of very short and highly polar peptides using classical RP mobile phases. The effects of varying mobile phase composition (organic solvent and ion-pairing additives) as well as column temperature have been thoroughly investigated using short peptide standards. Under optimized conditions (water and acetonitrile/tetrahydrofuran 99:1 (v/v), both with 0.15% trifluoroacetic acid, as phase A and B, respectively, 0.5 mL min−1 flowrate at 50 °C) the effect of post-column addition of 3-nitrobenzylic alcohol was also investigated allowing effective coupling of the chromatographic system with high resolution mass spectrometry. Finally, an untargeted approach for peptide identification was pursued, based on precursor identification in database with all possible combinations of the 20 natural amino acids and fragmentation spectra matching to in silico generated spectra. The method was then applied to investigation of the short endogenous peptides in human serum from healthy individuals resulting in the identification of 30 short peptides.

Simultaneous analysis of heparosan oligosaccharides by isocratic liquid chromatography with charged aerosol detection/mass spectrometry

Uncovering the biological roles of heparosan oligosaccharides requires a simple and robust method for their separation and identification. We reported on systematic investigations of the retention behaviors of synthetic heparosan oligosaccharides on porous graphitic carbon (PGC) column by HPLC with charged aerosol detection. Oligosaccharides were strongly retained by PGC material in water-acetonitrile mobile phase, and eluted by trifluoroacetic acid occurring as narrow peaks. Addition of small fraction of methanol led to better selectivity of PGC to oligosaccharides than acetonitrile modifier alone, presumably, resulting from displacement of methanol to give different chemical environment at the PGC surface. Van’t-Hoff plots demonstrated that retention behaviors highly depended on the column temperature and oligosaccharide moieties. By implementing the optimal MeOH content and temperature, a novel isocratic elution method was successfully developed for baseline resolution and identification of seven heparosan oligosaccharides using PGC-HPLC–CAD/MS. This approach allows for rapid analysis of heparosan oligosaccharides from various sources.

Understanding the Complexity of Porous Graphitic Carbon (PGC) Chromatography: Modulation of Mobile-Stationary Phase Interactions Overcomes Loss of Retention and Reduces Variability

Porous graphitic carbon (PGC) is an important tool in a chromatographer’s armory that retains polar compounds with mass spectrometry (MS)-compatible solvents. However, its applicability is severely limited by an unpredictable loss of retention, which can be attributed to contamination. The solutions offered fail to restore the original retention and our observations of retention time shifts of gemcitabine/metabolites on PGC are not consistent with contamination. The mobile phase affects the ionization state of analytes and the polarizable PGC surface that influences the strength of dispersive forces governing retention on the stationary phase. We hypothesized that failure to maintain the same PGC surface before and after running a gradient is a cause of the observed retention loss/variability on PGC. Herein, we optimize the choice of mobile phase solvent in a gradient program with three parts: a preparatory phase, which allows binding of analytes to column; an elution phase, which gives the required separation/peak shape; and a maintenance phase, to preserve the required retention capacity. Via liquid chromatography/tandem mass spectrometry (LC-MS/MS) analysis of gemcitabine and its metabolites extracted from tumor tissue, we demonstrate reproducible chromatography on three PGC columns of different ages. This approach simplifies use of the PGC to the same level as that of a C-18 column, removes the need for column regeneration, and minimizes run times, thus allowing PGC columns to be used to their full potential.

Aniline-modified porous graphitic carbon for hydrophilic interaction and attenuated reverse phase liquid chromatography

Most stationary phases for hydrophilic interaction liquid chromatography (HILIC) and reversed phase liquid chromatography (RPLC) are based on silica. Porous graphitic carbon (PGC) is an attractive alternative to silica-based phases due to its chemical and thermal stability, and unique selectivity. However, native PGC is strongly hydrophobic and in some instances excessively retentive. PGC particles with covalently attached aniline groups (Dimethylaniline-PGC and Aniline-PGC) were synthesized to alter the surface polarity of PGC. First, the diazonium salt of N,N-dimethyl-p-phenylenediamine or 4-nitroaniline was adsorbed onto the PGC surface. The adsorbed salt was reduced with sodium borohydride and (Aniline-PGC only) the nitro group was further reduced with iron powder to the aniline. X-ray photoelectron spectroscopy confirmed the surface functionalities and that these moieties were introduced to the surface at concentrations of 0.9 and 2.1molecules/nm2, respectively. These modified PGC phases (especially Aniline-PGC) were evaluated as HILIC and reversed phases. The Dimethylaniline-PGC phase displayed only weak HILIC retention of phenolic solutes. In contrast, the Aniline-PGC phase displayed up to nearly a 7-fold increase in HILIC retention vs. an aniline-silica phase and selectivity that differed from 10 other HILIC phases. Introduction of aniline groups to the PGC surface reduced the RPLC retentivity of PGC up to more than 5-fold and improved the separation efficiency up to 6-fold. The chromatographic performance of Aniline-PGC is demonstrated by separations of nucleotides, nucleosides, carboxylic acids, basic pharmaceuticals, and other compounds.

Thermodynamics of adsorption of 1,3,4-oxadiazole and 1,2,4,5-tetrazine derivatives from aqueous-organic solutions on the surface of porous graphitic carbon

The dynamic method was used under conditions close to equilibrium to study thermodynamics of the Henry adsorption of some 1,3,4-oxadiazoles and 1,2,4,5-tetrazines from aqueous-acetonitrile and aqueous-methanol solutions on the surface of porous graphitic carbon (PGC) (Hypercarb™) in the column temperature range of 313 to 333 K. It is shown that dependences of logarithm of retention factor on inverse temperature for all heterocycles are approximated by linear functions with r2 ≥ 0.999. An increase is observed in absolute values of Gibbs energy and enthalpy change at adsorption from aqueous-organic solutions on PGC at an increase in surface area of adsorbate molecules and, on the contrary, a decrease is found in absolute values of entropy change at an increase in the van der Waals volume of molecules. Assumptions are made as to the possible causes of anomalously strong adsorption from solutions under dynamic conditions of planar heterocycles molecules in the range of extremely low PGC surface coverages. Linear correlation is found between the values of variation of Gibbs energy at adsorption from aqueous-acetonitrile and aqueous-methanol solutions.

The adsorption of hetero- and alicyclic thiophene derivatives from water-acetonitrile solutions on the surface of porous graphitic carbon under high-performance liquid chromatography conditions

The adsorption of hetero- and alicyclic thiophene derivatives synthesized for the first time from water-acetonitrile solutions with various compositions on the surface of porous graphitic carbon was studied by the high-performance liquid chromatography method. The retention factor and Henry adsorption constant values and equilibrium constants of quasi-chemical reactions of the adsorption and solvation of the heterocyclic compounds studied were calculated using the Lanin-Nikitin equation. The influence of the structure of hetero- and alicyclic thiophene derivative molecules on their adsorption on the surface of porous graphitic carbon and solvation in water-acetonitrile solutions was discussed. The Lanin-Nikitin model was found to be more informative in the interpretation of the adsorption-chromatographic experiment data compared with the Snyder-Soczewinski and Scott-Kucera models.

Features of the adsorption of aryl- and hetaryl-substituted 1,3,4-oxadiazoles from solutions on the surface of porous graphitic carbon in Henry’s range

Adsorption in Henry’s range of structurally related aryl- and hetaryl-substituted 1,3,4-oxadiazoles from n-hexane-dichloromethane and water-acetonitrile solutions on the surface of porous graphitic carbon (PGC) under the conditions close to equilibrium is studied by high-performance liquid chromatography. The characteristics of sorption of 1,3,4-oxadiazoles from solutions on the surface of this adsorbent, octadecyl silica gel, and on hypercrosslinked polystyrene are compared. It is shown that the structure of the molecules and their Van der Waals surface area and polarity affect adsorption on a PGC surface. A variant of the mechanism of adsorption of the polar molecules on the PGC surface, explaining the anomalously high values of their Henry constants of adsorption and based on the planar location of the 1,3,4-oxadiazoles molecules with respect to the adsorbent surface and the specific adsorbate-adsorbent intermolecular interaction in addition to the background dispersion interactions, is proposed. It is established that the linear the Gibbs energy relationship upon the adsorption of the studied compounds from n-hexane-dichloromethane and water-acetonitrile solutions on the PGC surface holds.

Chromatographic retention behaviour of n-alkylbenzenes and pentylbenzene structural isomers on porous graphitic carbon and octadecyl-bonded silica studied using molecular modelling and QSRR

The retention behaviour of a series of 15 n-alkylbenzenes and pentylbenzene structural isomers and benzene were investigated using porous graphitic carbon (PGC) and octadecyl-bonded silica (ODS) stationary phases. Shorter chain n-alkylbenzenes and benzene ( n = 0–6), and all the pentylbenzene isomers were more strongly retained on ODS, although the selectivity was greater with PGC. For the pentylbenzene analytes the degree of branching in the alkyl chain at the position adjacent to the aromatic ring affects retention on PGC, with higher retention in less branched molecules. Molecular modelling studies have provided new insights into the geometry of aromatic π–π stacking interactions in retention on PGC. For alkylbenzenes with high branching at the position adjacent to the ring, the preferred geometry of association with the surface is with the branched chain directed away from the surface, a geometry not seen in the other alkylbenzenes. The most energetically favoured orientation for interaction between analytes and the PGC surface was found to be cofacial for toluene and ethylbenzene, whereas for other analytes this interaction was in a face-edge orientation. The alternative geometry of association observed with both toluene and ethylbenzene may explain the enhanced retention of these two analytes on PGC compared with their longer chain analogues. Quantitative structure–retention relationships revealed the importance of compactness in analyte structure during retention on PGC, with decreased compactness (associated with longer chain length and reduced chain branching) improving retention.

Solvent effects on the retention of oligosaccharides in porous graphitic carbon liquid chromatography

Porous graphitic carbon (PGC) is known as well suited adsorbent for liquid chromatography of carbohydrates. In this work we report on systematic investigations of solvent effects on the retention mechanism of fluorescence labeled malto-oligosaccharides on PGC. The adsorption mechanism was found to depend on the type of organic modifier used in the mobile phase. Positive adsorption enthalpies and entropies, which have already been reported in the literature, were solely produced using acetonitrile. Both alternative solvents (tetrahydrofuran, 2-propanol) yielded in contrast negative enthalpies. As plausible retention mechanism for oligosaccharides on PGC applying acetonitrile as mobile phase component we propose the formation of a dense and highly ordered solvation layer of the PGC surface with the linear acetonitrile molecules. Adsorption of analyte molecules requires a displacement of numerous acetonitrile molecules, which explains the positive enthalpy and entropy values measured. The interplay of enthalpic and entropic contributions to the overall adsorption phenomena results in strongly temperature dependent chromatographic selectivity values.

Effects of the redox state of porous graphitic carbon on the retention of oligosaccharides

Retention of hydrophilic compounds on porous graphitic carbon (PGC) is afforded by polar interactions with induced dipoles within this polarizable stationary phase. These interactions depend on the redox state of PGC, which can be influenced by application of an electrical field or by chemical means. We explored the impact of oxidizing and reducing agents on the retention of fluorescence labeled neutral oligosaccharides. Malto-oligosaccharides were employed as simple model system. Subsequently, the effects on the retention of glycans typical for immunoglobulin G (IgG) antibodies were investigated. Chemical oxidation of the PGC surface increased the retention of all analytes tested. Selectivities were significantly altered by the redox treatment, emphasizing the need for controlling the redox state of PGC to achieve reproducible conditions. Furthermore a column pre-conditioning protocol is presented, which allowed for reproducible chromatography of neutral IgG glycans.

Separation of substituted aromatic isomers with porous graphitic carbon in subcritical fluid chromatography

The ability of porous graphitic carbon (PGC) to separate structural isomers has been reported in high-performance liquid chromatography (HPLC). This paper presents studies carried out in subcritical fluid chromatography (SubFC). Various polar and nonpolar modifiers were added to the carbon dioxide mobile phase, in proportions ranging from 5 to 40%. The effects of both the nature and the percentage of the modifier on aromatic isomer separations were studied. Two types of selectivity behaviour appear. The first one, related to steric recognition, is due to the number of contact points between the compounds and the flat surface of PGC. In this case, retention orders are often identical to that reported in HPLC. The second is related to the favourable interaction between the polar moieties of the solutes and the stationary phase. In this case, the retention and selectivity strongly depend on the mobile phase composition. Thus, the separations obtained are greatly enhanced, compared to those obtained in HPLC. The retention and selectivity variations observed when the composition of the mobile phase is changed are discussed based on linear solvation energy relationships (LSERs). Practical applications are presented, namely benzene, toluene, ethylbenzene and xylenes (BTEX) and flavour molecules separations.

Saccharose effects on surface association of phenol derivatives with porous graphitic carbon

In this paper, the effect of saccharose on the association of phenol derivatives on both the porous graphitic carbon (PGC) surface and the C18 stationary phase and for two methanol fractions (v/v) in the mobile phase is described. A novel approach based on an extended Langmuir distribution isotherms was used. The results demonstrated that: (i) the saccharose can be adsorbed on the PGC surface; (ii) the phenol derivatives can be associated with saccharose adsorbed on the PGC surface; and (iii) the saccharose do not interact with the C18 stationary phase. This was confirmed by the thermodynamic data and the Wyman equation parameters.

Comparative analysis of enzymatically digested κ-carrageenans, using liquid chromatography on ion-exchange and porous graphitic carbon columns coupled to an evaporative light scattering detector

Enzymatically digested κ (A-G4S)-carrageenans, apart from their biological activities in plants, could be used as ‘model’ molecules to elucidate potential problems in nuclear magnetic resonance spectroscopy of carrageenans. Thus, oligosaccharides obtained from κ-carrageenan by enzymatic digestion using κ-carrageenase have been separated on silica and polymeric based ion-exchange and porous graphitic carbon (PGC) columns, coupled to an evaporative light scattering detector. Oligomers were separated on ion-exchange columns using a gradient of ammonium acetate as a developing ion, while analysis on PGC column presented an additional adjacent peak next to each main one, using a gradient of ammonium acetate in water/acetonitrile as a mobile phase. The phenomenon can be attributed to different retention mechanisms that govern the PGC surface. Furthermore, it has been demonstrated that acetonitrile can regulate the selectivity between the peaks raising hopes for preparative chromatography.

Role of the Na + ion on phenol derivatives/hydroxypropyl-β-cyclodextrin complex formation on porous graphitic carbon phase

The reversed-phase liquid chromatography retention of phenol derivatives was investigated over a concentration range of sodium chloride (0–10 −2 M) and hydroxypropyl-β-cyclodextrin (HP-β-CD) (0–35×10 −3 M) using a porous graphitic carbon (PGC) stationary phase and a methanol/water mixture (50:50 (v/v)) as the mobile phase. A theoretical treatment was developed to investigate the effect of the sodium chloride and hydroxypropyl-β-cyclodextrin on the equilibrium between the solutes with the PGC surface and the aqueous medium, respectively. The thermodynamic parameter variations were calculated using van’t Hoff plots. It was expected that the sodium ion acted on the solute–PGC association process by modifying the surface tension of both the bulk solvent and the PGC surface. The phenol derivative/HP-β-cyclodextrin complexation was shown to be entropically controlled for all the solutes except for the one which contained the –NO 2 group in its structure, i.e. the nitro phenol derivative. A comparison of the compensation temperature of the solute–PGC association process when sodium chloride and HP-β-CD concentration changed in the mobile phase led to the conclusion that these two modifiers acted via a variation in the hydrophobic effect.

Electrosorption-Based Modification of Porous Graphitic Carbon: Use of Electrochemically Modulated Liquid Chromatography To Create a Chiral Stationary Phase for Enantiomeric Separations

A novel approach for the modification of the separation capabilities of electrochemically modulated liquid chromatography is described. The basis of the approach derives from the electrosorption of an organic additive from the mobile phase onto a porous graphitic carbon (PGC) stationary phase, resulting in a new dimension in the ability to alter reversibly the chemical composition at the surface of the stationary phase. The viability of the approach is demonstrated by the enantiomeric separation of racemic mixtures of the pharmaceutical agents hexobarbital and mephenytoin. These separations are achieved by the electrosorption of the organic additive β-cyclodextrin to form a PGC surface that is coated with a chiral selector. Results indicate that changes in applied potential can not only manipulate the effectiveness of these separations but can also reverse the enantiomeric elution order. A general mechanism for these separations is proposed and the potential applicability of the strategy to other types of LC separations is briefly noted.

Role of alkyl and aryl substituents in chiral ligand exchange chromatography of amino acids study using porous graphitic carbon coated with N-substituted- l-proline selectors

Five chiral stationary phases were prepared by coating the surface of porous graphitic carbon with a series of N-substituted l-proline chiral selectors. The N-substituents served as anchor molecules for immobilization of chiral selectors on the support material. The effect of the alkyl (C 7, C 9, C 12) and aryl (naphthylmethyl, anthrylmethyl) anchor molecules on retention and enantioselectivity were studied using thirty-six amino acid enantiomers as probe compounds. The surface concentrations of the chiral selectors, determined using the breakthrough method, were found to be in the range 0.39–0.68 μmol m −2. The coated PGC phases all showed appreciable enantioselectivity for both non-polar and acidic amino acids with the naphthylmethyl- l-proline showing the greatest overall values. The N-substituents were shown to have strong influence on retention and enantioselectivity. An increase in the chain length of the alkyl N-substituents resulted in improved enantioselectivity whereas the retention decreased. However, with the aryl N-substituents, both the retention and enantioselectivity decreased with an increase in the ring number of the aryl substituent. The retention order for the majority of the amino acids examined was the same on all the columns, that is d > l . These behaviours were interpreted in terms of the involvement of the N-substituent in intramolecular hydrophobic interactions responsible for the observed chiral recognition.

Chiral chromatography of amino acids on porous graphitic carbon coated with a series of N-substituted l-phenylalanine selectors effect of the anchor molecule on enantioselectivity

Six chiral stationary phases were prepared by coating the surface of porous graphitic carbon (PGC) with a series of N-substituted l-phenylalanine chiral selectors. The N-substituents served as anchor molecules for immobilization of chiral selectors on the support material. The effect of the alkyl (C 7, C 9, C 12) and aryl (methoxybenzyl, naphthylmethyl, anthrylmethyl) anchor molecules on retention and enantioselectivity was studied using 36 amino acid enantiomers as probe compounds. The surface concentrations of the chiral selectors, determined using the breakthrough method, were found to be in the range 0.55–1.26 μmol/m 2. The coated PGC phases all showed appreciable enantioselectivity for both non-polar and acidic amino acids, but basic amino acids were predominantly unretained. The order of retention of a pair of amino acid enantiomers was l > d on the alkyl- l-phenylalanine phases but a reversed retention order ( d > l ) was observed aryl-substituted l-phenylalanine phases. The chromatographic properties of each of the chiral stationary phases were compared in terms of retention order, enantioselectivity and diastereomeric complex stability. The reversal of the elution order on these two types of phases is discussed in relation to competitive intramolecular interactions involved in transient diasteriomeric complexes.

Chromatographic resolution of enantiomers on a chiral stationary phase physically anchored to porous graphitic carbon

A chrysene–tartramide derived chiral stationary phase (CSP) was prepared and adsorbed onto the surface of porous graphitic carbon (Hypercarb). The stereoselective properties of the carbon-based chiral stationary phase was evaluated in a pre-packed high-performance liquid chromatography column. The carbon-based CSP was found to be capable of separating the enantiomers of a range of compounds including 1,1′-bi(2-naphthol),2,2-diaminobinaphthalene, 1,5-hexadiene-3,4-diol, benzoin, 1,4-di-O-benzylthreitol, 1,1′-binaphthalene-2,2′-diylhydrogenphosphate, propranolol, nadolol and labetolol using a non-aqueous mobile phase. The addition of ammonium acetate to the mobile phase was found to improve the resolution of enantiomers for 1,1′-binaphthalene-2,2′-diylhydrogenphosphate, propranolol, nadolol and labetolol. The amount of chrysene–tartramide adsorbed on porous graphitic carbon was calculated from the cycling process and the value corresponded to 2.6% by mass of porous graphitic carbon.