Excess adsorption of binary aqueous organic mixtures on various reversed-phase packing materials.

Excess adsorption isotherms of acetonitrile and methanol from water were measured on eight commercial columns. Columns used in this study represent latest examples in column development and include three different poroshell columns (Kinetex-C18, Acsentis-C18 and Halo-C18) as well as conventional columns with significantly different adsorbent geometry (Allure-C18, YMC-C18) and various hybrid-silica columns (Gemini-C18, Xterra-C18 and XBridge-C18). Comparison of the excess adsorption isotherms measured on all these columns and expressed in surface specific form demonstrated significant similarity of the adsorption properties of all columns, which allows us to introduce the "standard adsorption isotherm" for reversed-phase C18-type columns. The methodology of the evaluation of the total amount of adsorbent in the column and effective surface area of the C18 modified adsorbent is also discussed. These terms are critical for successful evaluation of surface specific parameters.

Effect of the surface coverage of endcapped C18-silica on the excess adsorption isotherms of commonly used organic solvents from water in reversed phase liquid chromatography.

The excess adsorption isotherms of methanol, ethanol, 2-propanol, acetonitrile, and tetrahydrofuran from water were measured on five different silica-based packing materials by the minor disturbance method. These materials were prepared with the same lot of 5-microm particles (average pore size 90 A), all endcapped with trimethylchlorosilane (TMS), and bonded to octadecyl chains with different surface coverages (0, 0.42, 1.01, 2.03, and 3.15 micromol/m2). The relative adsorption of one eluent by respect to a second one informs on the heterogeneity of the material (alkyl-bonded and bare silica regions) and on the accessibility of the unreacted silanol groups to the mobile phase. It is shown that the total surface area of the adsorbent decreases with increasing degree of surface coverage with octadecyl chains and that the relative surface area of the regions occupied by accessible silanol groups to the regions occupied by alkyl-bonded groups decreases. For the five columns, an average of 10% of the adsorbent surface area is covered of bare silica accessible to the liquid phase, with a minimum of 5% with tetrahydrofuran and a maximum of 12% with ethanol or 2-propanol. Increasing the surface coverage by the C18 chains causes a significant increase of the attraction potential of the hydrophobic surface toward the organic solvent. This result is confirmed by the increase of the number of adsorbate monolayers with increasing bonding density of the octadecyl chains. This number is twice larger for the 315C18 column than for the C1 column.

High-performance liquid chromatography retention mechanisms and their mathematical descriptions.

Different mechanisms of analyte behavior in HPLC columns are considered from the point of view of analyte distribution between the adsorbent and the bulk liquid phase. Expressions describing the analyte retention volume are derived for each selected retention mechanism. Methodology of the incorporation of multi-equilibrium processes in the analyte retention expression is suggested for ideal linear chromatography conditions. The description of eluent components' adsorption is discussed on the basis of the excess adsorption process. The applicability of the excess adsorption description without introduction of the adsorbed layer model or any other convention to the chromatographic process is shown.

Interpretation of the excess adsorption isotherms of organic eluent components on the surface of reversed-phase phenyl modified adsorbents.

The adsorption of three organic eluent components (acetonitrile, methanol, and tetrahydrofuran) from water were measured on four phenyl-type bonded phases using the minor disturbance method. The thicknesses of organic layer enriched above the phenyl-type bonded ligands were assessed and interpreted. Acetonitrile and tetrahydrofuran showed multilayer formation while methanol showed monomolecular adsorption. These results were compared to those obtained on alkyl bonded phases.

Characterization of phenyl-type HPLC adsorbents.

A set of different phenyl-modified HPLC adsorbents were characterized in terms of their surface area, pore volume, and bonded phase volume using low temperature nitrogen adsorption (LTNA). Adsorbents pore volume and interparticle volume were also measured using HPLC. Comparison of the pore volumes assessed with LTNA and HPLC suggests a compact molecular arrangement for all bonded phases studied. Simple and effective method for determination of the exact mass of adsorbent and total surface area in the column is suggested.

Reversed-phase high-performance liquid chromatography behavior of chaotropic counteranions.

The retention behavior of inorganic liophilic anions in reversed-phase HPLC columns was studied. Usually, the addition of these ions to the mobile phase influences the retention of protonated basic analytes similar to the effect of amphiphilic ions (ion-pairing agents). The nature of this influence is the subject of this paper. HPLC retention of perchlorate (ClO4-), tetrafluoroborate (BF4-), and hexafluorophosphate (PF6-) ions was studied on six columns with different bonded phases including alkyl, phenyl and perfluorophenyl phases. The effect of the mobile phase ionic strength on the retention of liophilic ions was investigated. The influence of the type of organic modifier, acetonitrile and methanol, on the retention of inorganic ions was also studied and interpreted on the basis of adsorption from solutions. Semi-empirical expression is suggested for the description of the retention profile of studied liophilic ions versus the eluent composition. Significant retention of these ions is observed in acetonitrile-water eluents. Multilayer-type adsorption of the acetonitrile on the reversed-phase surface and its strong dispersive (or pi-pi) interactions with liophilic ions are responsible for significant retention of these ions. This accumulation of liophilic ions in the adsorbed layer on the surface of reversed-phase material introduces an electrostatic component in the retention of protonated basic analytes.

Elution behavior of polyethylene in polar mobile phases on a non-polar sorbent.

Linear polyethylene standards in the range of 1-500 kg/mol, dissolved in 1,2,4-trichlorobenzene, were injected into a column packed with oligo(dimethylsiloxane) modified silica gel. Fifteen polar solvents (cyclohexanone, cyclohexylacetate, cyclohexanol, nonylalcohol, dimethylformamide, dimethyl sulfoxide, ethylene- and diethylene glycol monobutyl ether, benzylalcohol, hexylacetate, bis(2-ethyl-hexyl)phthalate, N,N-dimethylacetamide, propylene carbonate, dipropylene glycol and N-methyl-pyrrolidone) were evaluated as mobile phases. Depending on the type of mobile phase evaluated, different elution behaviors are observed for polyethylene: (1) polyethylene was eluted in the size exclusion mode, (2) polyethylene was eluted together with the sample solvent peak at constant elution volume, (3) polyethylene was partially or fully retained on the column. The retained polymer was easily removed from the column by injecting a small volume of trichlorobenzene. The use of ethylene glycol monobutyl ether as the mobile phase enabled separation of the polyethylene from polypropylene. In this case polypropylene is eluted in the size exclusion mode, while polyethylene is eluted at a constant elution volume or remains in the column.

Effect of counter-anion concentration on retention in high-performance liquid chromatography of protonated basic analytes.

The influence of acid and salt concentration in the mobile phase on the retention of basic analytes has been studied. An increase in the retention of fully protonated analytes with increasing the concentration of inorganic additives was found. The addition of salt, such as perchlorate, trifluoroacetate, and phosphate, leads to the increase of retention for fully protonated analytes while mobile phase pH remains constant. The observed effect was attributed to the interaction of protonated analytes with the counter-anion of acid or salt, which leads to the disruption of the analyte solvation shell and the increase of its hydrophobicity and corresponding increase of retention. A mathematical model for the description of the influence of counter-anion concentration on analyte retention is proposed.

Effect of the eluent pH and acidic modifiers in high-performance liquid chromatography retention of basic analytes.

The retention of ionogenic bases in liquid chromatography is strongly dependent upon the pH of the mobile phase. Chromatographic behavior of a series of substituted aniline and pyridine basic compounds has been studied on C18 bonded silica using acetonitrile-water (10:90) as the eluent with different pHs and at various concentrations of the acidic modifier counter anions. The effect of different acidic modifiers on solute retention over a pH range from 1.3 to 8.6 was studied. Ionized basic compounds showed increased retention with a decrease of the mobile phase pH. This increase in retention was attributed to the interaction with counter anions of the acidic modifiers. The increase in retention is dependent on the nature of the counter anion and its concentration in the mobile phase. It was shown that altering the concentration of counter anion of the acidic modifier allows the optimization of the selectivity between basic compounds as well as for neutral and acidic compounds.

Geometry of chemically modified silica.

The effect of alkyl chain length on adsorbent pore volume and void volume of HPLC columns is described. The results provide evidence that alkyl chains attached on silica surface are densely packed. A correlation of a decrease of pore volume with an increase of the alkyl modifier chain length was found. Effective molecular volume of bonded chains was found to be similar to the molecular volume of corresponding liquid alkanes. An absence of noticeable penetration of acetonitrile, methanol, or tetrahydrofuran molecules between bonded chains at any water-organic eluent composition was found.

Interpretation of the excess adsorption isotherms of organic eluent components on the surface of reversed-phase adsorbents. Effect on the analyte retention.

The excess adsorption isotherms of acetonitrile, methanol and tetrahydrofuran from water on reversed-phase packings were studied, using 10 different columns packed with C1-C6, C8, C10, C12, and C18 monomeric phases, bonded on the same type of silica. The interpretation of isotherms on the basis of the theory of excess adsorption shows significant accumulation of the organic eluent component on the adsorbent surface on the top of "collapsed" bonded layer. The accumulated amount was shown to be practically independent of the length of alkyl chains bonded to the silica surface. A model that describes analyte retention on a reversed-phase column from a binary mobile phase is developed. The retention mechanism involves a combination of analyte distribution between the eluent and organic adsorbed layer, followed by analyte adsorption on the surface of the bonded phase. A general retention equation for the model is derived and methods for independent measurements of the involved parameters are suggested. The theory was tested by direct measurement of analyte retention from the eluents of varied composition and comparison of the values obtained with those theoretically calculated values. Experimental and theoretically calculated values are in good agreement.

Low-energy interactions in high-performance liquid chromatography.

Liquid chromatographic systems with very weak excessive analyte-adsorbent interactions have been studied. These systems consisted of a homologous series of n-alkanes as both analytes and mobile phases with a C18 reversed-phase adsorbent. A linear decrease of the analyte retention volume with an increase of the number of analyte carbon atoms was found. Corresponding increases of analyte retention with an increase in the number of eluent carbon atoms was also discovered. An explanation of these two effects on the basis of adsorption theory is proposed. A good correlation of column hold-up volume calculated by interpolation of the retention dependencies for above mentioned systems with that measured by the minor disturbance method has been shown. A study of the temperature dependencies of these alkane systems has shown entropy-governed retention dependencies.