Packing Materials For Liquid Chromatography Research Articles

Monodisperse core–shell silica particles as a high-performance liquid chromatography packing material: Facile in situ silica sol-gel synthesis

In recent years, core–shell silica particles (CSSPs) have been increasingly used for highly efficient separation at fast flow rates and relatively low back pressures in high-performance liquid chromatography (HPLC). However, material synthesis techniques for producing CSSPs economically in batch processes remain elusive. In this report, a practical and straightforward method for the preparation of CSSPs is presented. By refluxing freshly prepared nonporous silica particles in ammonia–water solution in the presence of poly(diallyldimethylammonium chloride) at 70–100 °C, CSSPs with shell thicknesses of up to 300 nm and pore sizes from 8 to 25 nm were easily prepared. The effects of the synthetic conditions on the shell thickness, surface area, and pore size were investigated in detail, and the method reproducibility was evaluated in scale-up experiments. A mechanism of CSSP formation is also proposed. The CSSPs were characterized via scanning electron microscopy, transmission electron microscopy, laser particle size (dynamic light scattering) analysis, and nitrogen adsorption and desorption experiments. The synthesized 3.4-μm CSSPs were functionalized with dimethyloctadecylchlorosilane and used as an HPLC packing material, exhibiting excellent separation performance for both small molecules and large biomolecules. In summary, we report the simplest method developed thus far for the preparation of monodisperse core–shell silica particles suitable for HPLC column packing.

Fundamental Properties of Packing Materials for Liquid Chromatography

The high performance of chemically-modified silica gel packing materials is based on the utilization of pure silica gels. Earlier silica gels used to be made from inorganic silica; however, nowadays, silica gels are made from organic silanes. The surface smoothness and lack of trace metals of new silica gels permits easy surface modifications (chemical reactions) and improves the reproducibility and stability. Sharpening peak symmetry is based on developing better surface modification methods (silylation). Typical examples can be found in the chromatography of amitriptyline for silanol testing and that of quinizarin for trace metal testing. These test compounds were selected and demonstrated sensitive results in the measurement of trace amounts of either silanol or trace metals. Here, we demonstrate the three-dimensional model chemical structures of bonded-phase silica gels with surface electron density for easy understanding of the molecular interaction sites with analytes. Furthermore, a quantitative explanation of hydrophilic and hydrophobic liquid chromatographies was provided. The synthesis methods of superficially porous silica gels and their modified products were introduced.

Electrochromatographic behavior of core-shell particles: A comparison study

Core-shell particle is a new generation high performance packing material for liquid chromatography. Through comparison with a classical totally porous silica phase of the same particle size, we studied Ascentis Express 5 μm core-shell particle's electrochromatographic behavior, in terms of voltage-current property, electroosmotic flow (EOF) and van Deemter curve. It was found, due to the nonpermeable solid core, the core-shell particle presented a diminished EOF and efficiency than the totally porous paricle. This on the other hand proved that the intra-particle pore flow extensively exists and plays an important role in electrochromatography on totally porous material. The core-shell particle's high retentivity led to an enhanced resolution for weakly retained hydrophilic peptides, which were poorly retained and co-eluted on totally porous particles. Further exploration has shown the core-shell material can achieve efficient electrochromatography of protein digests, excellent performance in terms of resolution, reproducibility and long term stability have been observed. The results indicate that the core-shell structure may suggest a reasonable design of stationary phase for bioelectrochromatography of peptides and proteins.

Online measurement of phthalate–particulate matter interactions by membrane introduction mass spectrometry (MIMS)

ABSTRACTTo enable further study and assessment of indoor inhalation exposure risk, an online apparatus enabling measurement of semi-volatile compound partitioning on household particulates was developed. An example for use of the apparatus is described using dimethyl phthalate (DMP). The system employs direct measurement by membrane introduction mass spectrometry (MIMS). The MIMS system was calibrated using known gas phase DMP concentrations produced by gravimetrically calibrated permeation devices. The quantity of DMP sorbed by particles is described first using a model particle type, a reverse-phase liquid chromatography packing material, and then with a household dust sample. In addition, the desorption of semi-volatile compounds from a household dust sample was monitored using the apparatus, and characteristic fragment ion signals for phthalate compounds were observed.

Structural dependence on the property of chiral stationary phases derived from chitosan bis(arylcarbamate)-(amide)s

The goal of present study was to investigate the structural dependence of chitosan derivatives on enantioseparation and mobile phase tolerance of the corresponding chiral packing materials for liquid chromatography. Hence, a series of chitosan bis(arylcarbamate)-(n-pentyl amide)s and the related chiral stationary phases (CSPs) were prepared from chitosans with different molecular weights. Because of the H-bond formed via CH3-π interaction, the CSP bearing methyl substituent exhibited high tolerance than the ones bearing dichloro substituents. The CSP derived from the chitosan bis(3,5-dichlorophenylcarbamate)-(n-pentyl amide) with a higher molecular weight possessed high tolerance to mobile phases, whereas the enantioseparation capability of this CSP was not as good as that of the one prepared from the chitosan derivative with a lower molecular weight. Therefore, enantioseparation capability and mobile phase tolerance have to be counterbalanced in designing chiral selectors for the CSPs derived from chitosan bis(arylcarbamate)-(amide)s.

Investigation on synthesis of spheres-on-sphere silica particles and their assessment for high performance liquid chromatography applications

There is a continuing challenge in improving the separation speed while keeping both the resolution high and the back pressure low in high performance liquid chromatography (HPLC). The recent renaissance of core–shell (or fused-core) silica particles has shown great promise in this respect. However, the fused-core silica particles are typically synthesized by the time-consuming multiple-step layer-by-layer technique. An one-pot synthesis of spheres-on-sphere (SOS) silica microspheres is presented here. The preparation parameters including Si precursors, mixing methods (magnetic stirring, mechanical stirring and homogenization), heating (microwave heating and conventional heating), and reaction temperature are investigated in order to control the morphology and improve the size distribution of the SOS particles. The improvement and influence on SOS morphology, particle size and particle size distribution are discussed. Furthermore, the calcined and modified SOS particles are packed into stainless steel columns, which are then assessed for the separation of various test mixtures containing small molecules and proteins under normal phase, reversed phase, and HILIC conditions. The HPLC tests demonstrate fast and efficient separation with very low back pressure, suggesting that SOS particles are a type of new and highly promising packing materials for liquid chromatography.

On the relationship between band broadening and the particle-size distribution of the packing material in liquid chromatography: Theory and practice.

The influence of the particle size distribution (PSD) on the band broadening and the efficiency of packed columns is investigated on both theoretical and practical viewpoints. Each of the classical contributions to mass transfer kinetics, those due to longitudinal diffusion, eddy dispersion, and solid–liquid mass transfer resistance are measured and analyzed in terms of their expected and observed intensity as a function of the PSD of mixtures of the commercially available packing materials, 5 and 3 μm Luna-C 18(2) particles (Phenomenex, Torrance, CA, USA). Six 4.6 mm × 150 mm columns were packed with different mixtures of these two materials. The efficiencies of these columns were measured for a non-retained and a retained analytes in a mixture of acetonitrile and water. The longitudinal diffusion coefficient was directly measured by the peak parking method. The solid–liquid mass transfer coefficient was measured from the combination of the peak parking method, the best model of effective diffusion coefficient and the actual PSDs of the different particle mixtures measured by Coulter counter experiments. The eddy diffusion term was measured according to a recently developed protocol, by numerical integration of the peak profiles. Our results clearly show that the PSD has no measurable impact on any of the coefficients of the van Deemter equation. On the contrary and surprisingly, adding a small fraction of large particles to a batch of small particles can improve the quality of the packing of the fine particles. Our results indirectly confirm that the success of sub-3 μm shell particles is due to the roughness of their external surface, which contributes to eliminate most of the nefarious wall effects.

Preparation and Characterization of SiO2/SiO2 Core–Shell Microspheres as RP-HPLC Stationary Phase

Novel SiO2/SiO2 core–shell monodisperse silica spheres for high-performance liquid chromatography packing materials are prepared by the layer-by-layer self-assembly technique. The core–shell silica spheres consist of micrometer-sized porous silica spheres as the core and a thin mesoporous silica shell formed from multilayer nanometer-sized silica particles. In addition, a reversed-phase packing by bonding octadecyltrichlorosilane on SiO2/SiO2 particles through alkyl-modified method is prepared and characterized. The results show that the carbon content of the new reversed-phase stationary phase increases by approximately 45% compared with the uncoated octadecyl-bonded SiO2 stationary phase. Eight kinds of tested aromatic compounds are well-separated on the packing and the peaks are symmetrical, which demonstrates that the packing acts as an excellent reversed-phase chromatographic stationary phase.

Reversed‐phase HPLC of peptides: Assessing column and solvent selectivity on standard, polar‐embedded and polar endcapped columns

We desired to evaluate the chromatographic selectivity for peptides of silica-based RP high-performance liquid chromatography stationary phases with various modifications (polar embedding and polar endcapping on C(18) columns; ether-linked phenyl column with polar endcapping) compared with n-alkyl (C(18), C(8)) and aromatic phenylhexyl columns. Thus, we have designed and synthesized two series of synthetic peptide standards with the sequence Gly-Gly-Leu-Gly-Gly-Ala-Leu-Gly-X-Leu-Lys-Lys-amide, where the N-terminal either contains a free α-amino group (AmC series) or is N(α)-acetylated (AcC series) and where position X is substituted by Gly, Ala, Val, Ile, Phe or Tyr. These represent series of peptides with single substitutions of n-alkyl (Gly<Ala<Val<Ile in order of increasing hydrophobicity), alkyl versus aromatic side-chains (Phe, Tyr) and a side-chain also with polar character (Tyr). Peptide mixtures were run on all columns at pH 2 (linear gradients of ACN or methanol in aqueous mobile phases). Our initial results clearly demonstrate the useful complementarity of different RP high-performance liquid chromatography packing materials, with peptide mixtures showing profound selectivity differences between n-alkyl versus polar-embedded and polar endcapped C(18) stationary phases. In addition, changing the polarity of the RP matrix in combination with changing the polarity of the mobile phase (ACN versus methanol) is shown to be synergistic to maximize selectivity of peptide separations.

ChemInform Abstract: Silica-Containing Mixed-Oxide Gels Prepared by a Coprecipitation Method as Novel Packing Materials for Liquid Chromatography.

The separation characteristics of silica-zirconia. silica-alumina, Silica-titania, and silica-magnesia gels for benzene, dimethyl phthalate, and pyridine have been evaluated as column packing materials for liquid chromatography. Of these mixed-oxide gels, silica-magnesia had the best separation ability particularly for a basic compound, pyridine depending presumably on the reduction of the activity of silanol group on silica surface.

Cardiac Muscle Sarcolemma Chromatographic Stationary Phase and Its Potential Application in Drug Screening

A new bioactive packing material for liquid chromatography, sarcolemma chromatography stationary phase (SCSP), is presented. Its surface characteristics are investigated, and it is found that the acceptors embedded in sarcolemma remained bioactive for more than a week. The retention behavior of antagonists and activators related to cardiac muscle sarcolemma on the SCSP chromatographic column shows the screening function of the SCSP column, and the retention behavior of the active components in the aether extract from the Chinese herb Ligusticum chuanxiong Hort. on the SCSP column reveals, to a certain extent, the separation function of the SCSP column. These suggest that SCSP is a potentially useful material in drug screening.

A chromatographic estimate of the degree of surface heterogeneity of reversed-phase liquid chromatography packing materials: II-Endcapped monomeric C 18-bonded stationary phase

In a previous report, the heterogeneity of a non-endcapped C 30-bonded stationary phase was investigated, based on the results of the measurements of the adsorption isotherms of two neutral compounds (phenol and caffeine) and two ionizable compounds (sodium naphthalene sulfonate and propranololium chloride) by frontal analysis (FA). The same method is applied here for the characterization of the surface heterogeneity of two new brands of endcapped C 18-bonded stationary phases (Gemini and Sunfire). The adsorption isotherms of the same four chemicals were measured by FA and the results confirmed by the independent calculation of the adsorption energy distribution (AED), using the expectation-maximization (EM) method. The effect of the length of the bonded alkyl chain was investigated. Shorter alkyl-bonded-chains (C 18 versus C 30) and the end-capping of the silica surface contribute to decrease the surface heterogeneity under the same experimental conditions (30% methanol, 25 mM NaCl). The AEDs of phenol and caffeine are bimodal with the C 18-bonded columns while they are trimodal and quadrimodal, respectively, with a non-endcapped C 30-bonded column. The “supersites” (adsorption energy >20 kJ/mol) found on the C 30-Prontosil column and attributed to a cation exchange mechanism completely disappear on the C 18-Gemini and C 18-Sunfire, probably because the end-capping of the silica surface eliminates most if not all the ionic interactions.

Coadsorption Studies of Tris(1,10-phenanthroline)ruthenium(II) and N-Methylated Alkaloid Cation by Laponite with an Application for a Chiral Column Packing Material

Coadsorption studies of chiral tris(1,10-phenanthroline)ruthenium(II) ([Ru(phen)3]2+) and N-methylated cinchona alkaloid cation (MQN+) were conducted on a smectite clay by ion exchange reactions. Photoinduced energy transfer between these cations was confirmed by the photoluminescence spectra, indicating that the cations were co-intercalated in the interlayer space of a smectite. When a ternary ion-exchanged adduct of [Ru(phen)3]2+/MQN+/smectite was employed as a packing material for liquid chromatography, the column proved to resolve a racemic mixture of 1,1‘-binaphthol more effectively than the column packed with a binary adduct of [Ru(phen)3]2+/smectite. The results reflected the enhancement of the chiral discrimination ability through the cooperative interactions among two kinds of preadsorbed chiral cations.

Selectivity enhancement of diastereomer separation in RPLC using crystalline-organic phase-bonded silica

Poly(octadecyl acrylate) exists in a crystalline state at room temperature. This crystalline state was formed even after being grafted onto silica through the terminal group of the polymer. When this polymer-grafted silica (Sil-ODAn) was used as a reversed phase liquid chromatography packing material, better selectivity for diastereomerized phenylethylamines derivatized by (S)-(−)-(2,3-naphthalenedicarboximidyl)propionyl fluoride was observed at room temperature than was observed for simple octadecylated silica (ODS). On the other hand, increasing temperature reduced the selectivity to be close to that of ODS. Additional experiments showed the better selectivity derived from Sil-ODAn was due to both the highly-oriented structure of the polymer and the carbonyl-π interaction with the diastereomer. A theoretical investigation of this carbonyl-π interaction is also described briefly.

Silicon dioxide surface modified with cholesterol derivatives

The major aim of the presented work was focused on the preparation and characterization of surface architecture of home-made packing materials for liquid chromatography and related techniques. Four stationary phases were synthesized on the basis of the same batch of silica gel. Two of them were obtained using compounds possessing liquid crystalline properties, one consisted of alkylamide ligands and one was conventionally prepared hydrocarbonaceous material. Properties of the silica matrix as well as the preparation of phases were discussed. Various physicochemical techniques were applied for the characterization of the surface of obtained packings. The progress of the modification reactions was confirmed by elemental analysis, which allowed calculation of the surface coverage density with bonded ligands. Created undersurface structure was examined by means of the solid-state NMR and FT-IR spectroscopy. Thermal analysis revealed the presence of phase transitions. The chromatographic experiments were designed and performed to prove the evidence of the non-homogenous nature of the surface of the packing materials and to create a relative hydrophobicity scale.

Continuous-bed columns containing sol-gel bonded octadecylsilica for capillary liquid chromatography

Continuous-bed columns of 200 μm inner diameter containing sol–gel bonded small-pore (5 μm, 90 A) and large-pore (5 μm, 1400 A) octadecylsilica particles were prepared for capillary liquid chromatography using a sol–gel process. These columns were found to be mechanically strong and permeable. The back pressures per centimeter column bed for columns packed with the same particle size were nearly the same, regardless of the pore size. The large-pore and small-pore continuous-bed columns generated efficiencies of 1.16×105 and 0.57×105 plates per meter, respectively. Since conventional liquid chromatography packing materials were utilized to form the continuous-bed columns, selectivities similar to conventional liquid chromatography were obtained. The small-pore continuous-bed columns exhibited stronger retention than the large-pore continuous-bed columns because the small-pore particles possessed higher surface area and carbon load than the large-pore particles. The sol–gel bonded continuous-bed columns were inert to aromatic amines primarily due to the incorporation of a deactivation precursor in the sol–gel matrix. ©2000 John Wiley & Sons, Inc. J Micro Sep 12: 6–12, 2000

Measurement of the degree of internal friction of two native silica packing materials

The degree of internal friction of two different packing materials for liquid chromatography (Kromasil NP10 and DuPont Pro Sil) has been measured using conventional methods of the mechanics of continuous solids applied in the study of the storage, handling and processing of powders. The former material was found to have a lower angle of internal friction and to flow more freely than the latter one. Possible relationships between these properties and the qualities of the column beds obtained are discussed.

Consolidation and column performance of several packing materials for liquid chromatography in a dynamic axial compression column

The mechanical properties of four different packing materials for preparative liquid chromatography were investigated. Their compression kinetics and consolidation behavior were measured under dynamic compression stress in a 5-cm I.D. axial compression column. The internal and external porosities, the packing density, the permeability, and the efficiency of these columns were also measured. The materials studied included two spherical silica-based materials for preparative chromatography, NovaPak C 8 1 (6 μm) and Hyperprep HS BDS C 1 8 (12 μm), another octadecyl bonded silica material made of irregular shaped particles, Baker 1-130 C 1 8 (130 μm), and an unmodified alumina (50 μm). The first two packing materials showed much higher mechanical stability than the third one which experienced extensive particle breakage under high compression stress. The material with the smallest spherical particles (average size, 6 μm) gave a stable, rigid column, but, either because it contains fine particles or because of a small degree of particle breakage, it caused problems. Maintenance after unpacking a column and cleaning the piston frit was difficult. This could lead to a significant decrease of the productivity. The behavior of the fourth column material, unmodified alumina, is very different from the ones of all silica-based materials so far studied; it is not superior to that of the three silica-based materials presented here.

Characterization of Pitches by Liquid Chromatography Using Cellulose 3, 5-Dinitrobenzoate as the Packing Material

3,5-ジニトロベンゾイルセルロース(DNBセルロース)を液体クロマトグラフイーの充填剤として用いて,石炭ピッチ,石油ピッチおよびPVCピッチの特徴づけを試みた.ジニトロベンゾイル基と多環芳香族化合物との間の電荷移動相互作用に基づく分離機構がピッチ成分の分離に役立つと期待した.まず,26種のモデル多環芳香族化合物を試験して,液体クロマトグラフィーによるこの充填剤の特性を調べた.その結果,これらの化合物は主として縮合環の環数に従つて保持容量の異なる四つの群に大別できることを認めた.多環芳香族化合物の非平面構造や脂肪族側鎖の有無もその分離挙動に影響を及ぼした.ついで3種のピッチのペンゼン可溶-ヘキサン可溶部およびペンゼン可溶-ヘキサン不溶部の両方をDNB_セルロースで分離した.石炭ピッチは縮合環数4-5の高度に縮合したいくつかの多環芳香族化合物を比較的多量に含み,また石湘ピッチではこのような特定の多環芳香族化合物の数および量がともに少なく,他方PVCピッチは縮合度の低い多環芳香族化合物を多く含み,高度に縮合した化合物をほとんど含まないことを見いだした.このようにDNB-セルロースは炭素材料用ピッチを特徴づけるための液体クロマトグラフィーの簡便な充填剤として有用であつた.

Use of cerium oxide (CeO 2) as a packing material for the chromatographic separation of C 60 and C 70 fullerenes

Cerium oxide (CeO 2) was tested as a packing material in liquid chromatography for the separation of C 60 and C 70 fullerenes. The separation of C 60 and C 70 fullerenes could be achieved within 20 min by using pure n-hexane as the mobile phase. Furthermore, some higher fullerenes could also be separated in less than 40 min. The peak area was reproducible to a large extent. The separation of fullerenes by liquid chromatography on CeO 2 is shown to be an effective method for their isolation in large amounts. The column efficiency of the CeO 2 column was compared with commercial silica gel and ODS columns. The main advantage of the CeO 2 column is its ability to separate large amounts of fullerenes (C 60 and C 70) in toluene.