Carboxylate modified porous graphitic carbon: a new class of hydrophilic interaction liquid chromatography phases

Further Evaluation of the Base Stability of Hydrophilic Interaction Chromatography Columns Packed with Silica or Ethylene-Bridged Hybrid Particles

One of the fundamental attributes of a liquid chromatography column is its stability when exposed to acidic and basic mobile phases. However, there have been relatively few reports to date on the stability of hydrophilic interaction chromatography (HILIC) columns. Here, we report the results of stability evaluations carried out for HILIC columns packed with ethylene-bridged hybrid or silica particles using accelerated conditions, employing a 100% aqueous pH 11.3 ammonium bicarbonate mobile phase at 70 °C. Under these conditions, the primary mode of column failure was a loss of efficiency due to the formation of voids resulting from the hydrolysis of the particles. We investigated the dependence of stability on the surface area of both unbonded and sulfobetaine-bonded ethylene-bridged hybrid stationary phases. The results show a clear trend of stability increasing as the surface area decreases. Several commercially available HILIC columns that are recommended for use with high-pH mobile phases were also evaluated. The results show times to 50% loss of the initial efficiency ranging from 0.3 to 9.9 h. Columns containing unbonded, sulfobetaine-bonded or diol-bonded ethylene-bridged hybrid stationary phases had longer lifetimes than amino-bonded silica or sulfobetaine-bonded, hybrid-coated, superficially porous silica columns.

Evaluation of the Base Stability of Hydrophilic Interaction Chromatography Columns Packed with Silica or Ethylene-Bridged Hybrid Particles

Stability as a function of mobile phase pH is an important consideration when selecting a chromatographic column. While the pH stability of reversed-phase columns is widely studied, there are relatively few reports of the stability of hydrophilic interaction chromatography (HILIC) columns. We evaluated the stability of silica and ethylene-bridged hybrid HILIC columns when used with mobile phases containing basic buffers. The predominant mode of column degradation observed in our studies was a decrease in efficiency due to voiding, resulting from the hydrolysis of the silica particles. Associated with this were increases in tailing factors. Retention factor changes were also noted but were smaller than the efficiency losses. The dependence of the rate of efficiency decrease on the key variables of temperature, mobile phase pH and water content were studied for an unbonded silica column. The effect of the acetonitrile concentration on the pH of the mixed aqueous/acetonitrile mobile phases was also investigated. Using conditions found to cause a 50% decrease in efficiency after approximately five hours of exposure to the basic solution, we evaluated eight different commercially available HILIC columns containing silica or ethylene-bridged hybrid particles. The results show large differences between the stability of the silica and ethylene-bridged hybrid particle stationary phases, with the latter exhibiting greater stability.

Grafting copolymer brushes on polyhedral oligomeric silsesquioxanes silsesquioxane-decorated silica stationary phase for hydrophilic interaction liquid chromatography

A strategy is proposed to develop a stationary phase for hydrophilic interaction liquid chromatography (HILIC) using the synergistic effect of polyhedral oligomeric silsesquioxane (POSS) and copolymer brushes. Octahedral octa-aminopropylsisesquioxane (8NH₂-POSS) was first bound to silica gel, followed by bromination to form a cubic initiator. Then, using acrylamide (AM) and dihydroxypropyl methacrylate (DPMA) as mixed monomers, surface initiated-atom transfer radical polymerization was conducted to prepare a stationary phase comprising cubic copolymer brushes with amide and diol groups. The characterization of the stationary phase confirmed the successful synthesis of Sil-NH₂-POSS/Poly(AM-co-DPMA). The chromatographic properties were investigated using nucleosides, organic acids and β-agonists to find that our designed column has superior hydrophilic property, better separation performance compared with classical HILIC columns consisting of diol- or amino-modified silica. The systematic investigation of the retention mechanism and separation selectivity using various types of polar compounds revealed that Sil-NH₂-POSS/Poly(AM-co-DPMA) follows a mixed-mode retention composed of HILIC and electrostatic interactions. Besides, it exhibits good column efficiency and stability. The role of 8NH₂-POSS in the separation was evaluated by comparing the performance of Sil-NH₂-POSS/Poly(AM-co-DPMA) and poly(AM-co-DPMA)-modified silica without 8NH₂-POSS. In conclusion, our designed based on POSS and hydrophilic copolymer brushes can contribute to the development of HILIC separation materials with enhanced performance.

High efficiency functionalized hydrophilic cyclofructans as stationary phases in sub/supercritical fluid chromatography

Packed column SFC has become very popular for preparative and analytical separations due to the low cost of CO₂, its accessible critical temperature, and pressure, with the additional benefit of a low environmental burden. Currently, there is a shortage of new polar stationary phase chemistries for SFC. In this work, two new functionalized cyclofructan columns are introduced and evaluated for their performance in achiral SFC separations for the first time. Cyclofructan (CF6), a macrocyclic oligosaccharide, was covalently linked with benzoic acid (BCF6) and propyl sulfonic acid (SCF6) groups by ether bonds. Superficially porous particles (2.7 μm) bonded with modified CF6 showed markedly different selectivity than native CF6. In SFC, peak shapes of amines and basic compounds are often compromised. We show that small quantities (~5.7% v/v) of water added to the methanol modifier in CO₂ improves peak symmetries of primary, secondary, and tertiary amines. Efficiencies as high as 200,000 plates/m (reduced plate height ~ 1.8) were observed for benzamide and amitriptyline on the BCF6 column. The relative standard deviations (RSDs) of retention times on BCF6 were about 1.4%, and on SCF6 were less than 1%. Amines on the SCF6 column showed plate counts as high as 170,000 plates/m. Tetramethylammonium acetate is examined as an alternative to water in MeOH. A run time of 36 min with methanol, trifluoroacetic acid, triethylamine mobile phase was reduced to <5 min with complete baseline resolution for a set of amines. The new stationary phases allow greener approaches towards solving separation problems.

Enhancing the selectivity of polar hydrophilic analytes with a low concentration of barium ions in the mobile phase using geopolymers and silica supports

Charged analytes such as organic sulfonic acids, sulfates, carboxylates, and phosphates are often analyzed by hydrophilic interaction liquid chromatography (HILIC). In many cases, these analytes do not show any selectivity and elute near the dead time using the conventional acetonitrile-ammonium acetate buffers. In this work, we introduce a powerful selectivity enhancing technique by using a trace amount of Ba²⁺ ion in the mobile phase as a general approach for HILIC with UV-Vis detection. Silica and a newly developed material called geopolymers are used as stationary phases. Geopolymers are X-ray amorphous aluminosilicate inorganic polymers with cation exchange properties. Barium exchanged geopolymers (Ba-NM-GP) are synthesized from metakaolin based geopolymer. Thorough characterization of Ba-NM-GP is reported using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), Brunauer-Emmett-Teller (BET) surface area analyzer and laser diffraction particle size distribution analyzer for the determination of their shape, size, porosity, surface area and particle size distribution respectively. It is demonstrated that in the absence of Ba²⁺, baseline separations of sulfonates, carboxylates, and phosphates is not possible, whereas, in the presence of Ba²⁺ in the mobile phase, these analytes are easily separated. Barium perchlorate is suggested as an additive for it is UV transparent, and it has practically an unlimited solubility in acetonitrile.

High efficiency and simple preparation of polyacrylamide coated silica stationary phase for hydrophilic interaction liquid chromatography

A polyacrylamide (PAM)-coated silica packing as a novel stationary phase for hydrophilic interaction liquid chromatography (HILIC) was produced firstly by thermal immobilization. The coated layer was formed by a simple and facile physical method that dipping silica spheres into a PAM solution and then stirring the mixture until the solution was evaporated absolutely, leaving a certain amount of PAM on the silica particles. Although there are no chemical bonds involved, the bleeding measurement and the background noise are acceptable. Polyacrylamide coated silica (PAM-Sil, mean diameter: 5 μm) demonstrated a good hydrophilic property and excellent separation efficiency, which was observed especially for several high polar analytes such as saccharides. It is much higher than bare silica or amino columns under the same conditions. Moreover the obtained stationary phase demonstrated good stability under our operation conditions. The specific preparing method offers an easy and economical way to manipulate the thickness of PAM coating and can be extended as a universal way to produce various polymer coated stationary phases with dipping-heating/stirring-evaporating procedure.

Surface-bonded amide-functionalized imidazolium ionic liquid as stationary phase for hydrophilic interaction liquid chromatography

The amide group modified silica materials are popular stationary phases for hydrophilic interaction liquid chromatography (HILIC). Meanwhile, surface-confined imidazolium ionic liquids (ILs) have been proved to be useful HILIC stationary phases and possess many unique properties. In this study, the synthesis of an amide-functionalized imidazolium IL was conducted which was then bonded onto silica surface to obtain a novel imidazolium-embedded amide stationary phase for HILIC. The combination of the amide group and imidazolium IL moiety might bring some advantages in selectivity or retention and therefore extended its applications. After characterizing the prepared IL and the resulting modified silica materials, the chromatographic performance and separation selectivity of the packed column were evaluated and compared with a commercial amide column. Then, the retention behavior was investigated through observing the retention factors at different chromatographic conditions using a wide range of compounds. Exceptionally, the prepared amide IL column exhibited superior separation performance towards complex samples such as flavonoids mixture, soybean flavonoids and human urine. All the results indicated that the novel amide IL column possessed an anion-exchange/HILIC mixed-mode retention mechanism and could be useful in the sample analysis as a promising candidate for HILIC stationary phase.

Diazonium modification of porous graphitic carbon with catechol and amide groups for hydrophilic interaction and attenuated reversed phase liquid chromatography

Porous graphitic carbon (PGC) is an increasingly popular and attractive phase for HPLC on account of its chemical and thermal stability, and its unique separation mechanism. However, native PGC is strongly hydrophobic and in some instances excessively retentive. As part of our effort to build a library of hydrophilic covalently modified PGC phases, we functionalized PGC with catechol and amide groups by means of aryl diazonium chemistry to produce two new phases. Successful grafting was confirmed by X-ray photoelectron spectroscopy (XPS). Under HILIC conditions, the Catechol-PGC showed up to 5-fold increased retention relative to unmodified PGC and selectivity that differed from four other HILIC phases. Under reversed phase conditions, the Amide-PGC reduced the retentivity of PGC by almost 90%. The chromatographic performance of Catechol-PGC and Amide-PGC is demonstrated by separations of nucleobases, nucleosides, phenols, alkaline pharmaceuticals, and performance enhancing stimulants. These compounds had retention factors (k) ranging from 0.5 to 13.

Homogeneous edge-plane carbon as stationary phase for reversed-phase liquid chromatography

Carbon stationary phases have been widely used in HPLC due to their unique selectivity and high stability. Amorphous carbon as a stationary phase has at least two sites of interaction with analytes: basal-plane and edge-plane carbon sites. The polarity and adsorptivity of the two sites are different. In this work, the edge-plane carbon stationary phase is prepared by surface-directed liquid crystal assembly. Specific precursor polymers form discotic liquid crystal phases during the pyrolysis process. By using silica as the substrate to align the discotic liquid crystal, edge-plane carbon surfaces were formed. Similar efficiencies as observed for Hypercarb were observed in chromatograms. The column efficiency was studied as a function of linear flow rate. A minimum reduced plate height of 6 was observed in these studies. To evaluate the performance of the homogeneous edge-plane carbon stationary phase, linear solvation energy relationships were used to compare these ordered carbon surfaces to commercially available carbon stationary phases, including Hypercarb. Reversed-phase separations of nucleosides, nucleotides, and amino acids and derivatives were demonstrated using the ordered carbon surfaces, respectively. The column batch-to-batch reproducibility was also evaluated. The retention times for the analytes were reproducible within 1-6% depending on the analyte.

Separation of therapeutic peptides with cyclofructan and glycopeptide based columns in hydrophilic interaction liquid chromatography

Three cyclofructan-based, two glycopeptide-based, and one zwitterionic column used in the HILIC mode were assessed within a graphical framework based on different functional characteristics contributing to selectivity. The characteristics of these six HILIC columns are put in the perspective of 33 columns evaluated previously. The isopropyl carbamate modified cyclofructan 6 (CF6) stationary phase, Larihc P, showed reduced component contributions for hydrophilicity and hydrogen bonding relative to the native cyclofructan 6 column (Frulic N). Both Frulic N and Larihc P exhibited cation exchange attributed primarily to deprotonation of residual unsubstituted silica with the greater exchange ascribed to the reduced loading of CF6 observed for Larihc P. The cyclofructan 6 column with a polymeric styrene divinylbenzene support (MCI GEL™ CRS100) showed distinct selectivities consistent with its decreased cation exchange attributable to its nonionic core. The Chirobiotic T, Chirobiotic V, and ZI-DPPS columns displayed hydrophilicity and ion exchange selectivities similar to other zwitterionic stationary phases. All of the more hydrophilic columns showed excellent separation for the four classes of therapeutic peptides investigated: microbial secondary metabolites used as immune suppressants, synthetic gonadotropin hormones, synthetic cyclic disulfide-linked hormone-regulating hormones, and non-ribosomally derived polycyclic antibiotics. Resolution provided by these columns and ZIC-HILIC is compared for each class of peptide. Frulic N is primarily suitable for use in the HILIC mode whereas Chirobiotic T, because of its increased efficiency and selectivity, can be useful in both HILIC and reverse phase modes. In some Chirobiotic T applications, addition of low levels of a strong additive (trifluoroacetic acid, formic acid, etc.) to the mobile phase can be beneficial. In these peptide analyses, a relative weakening of the often-dominant ionic interaction between analyte and residual charge on the stationary phase improved resolution and selectivity.

A polyvinyl alcohol-coated silica gel stationary phase for hydrophilic interaction chromatography

A facile and green way is proposed to prepare a polyvinyl alcohol coated silica gel stationary phase for HILIC. It demonstrated high separation efficiency and high chemical stability.

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/nm(2), 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.

Facile preparation of polyvinyl alcohol coated SiO2 stationary phases for high performance liquid chromatography

A facile physical method to prepare polyvinyl alcohol coating-based silica stationary phase for HPLC was proposed.