Comprehensive two-dimensional gas chromatography for fast screening of wash oils.

Fast screening of wash oils is demonstrated using comprehensive two-dimensional gas chromatography (GCxGC). Wash oils are used in ethylene production plants to minimize compressor fouling. The composition of a wash oil determines its effectiveness in solubilizing heavy hydrocarbons. In particular, the relative amount of 1- and 2-ring aromatics is important. The presence of oxygenates is undesirable because of adverse effects to the process. It is shown that GCxGC is well suited for this application. Species in wash oils are separated and grouped into three bands: a nonpolar aliphatics band, 1- and 2-ring aromatics band, and polyaromatics band. For a given polar secondary column, the spacing between bands in the second dimension can be adjusted in a broad range by selecting a primary column and an oven-temperature-programming rate. Integration of GCxGC peaks is evaluated using a standard GC integration program and a new GCxGC integration program. Consistent results are obtained using both programs for well-separated GCxGC peaks with relative differences for individual peak ranging from 0.04% to 1.6%. Peak responses are integrated by the GCxGC software, and the relative amounts of aromatics content and aliphatics content are estimated by peak response percent with relative standard deviations ranging from 0.15% to 2.8% (n = 3).

Analysis of silica hydride and surface hydrosilation reactions by solid-state NMR in the preparation of p-chlorobenzamide bonded silica phase.

29Si and (13)C CP-MAS NMR spectroscopy was used to follow the conversion of native silica to a p-chlorobenzamide bonded silica material. The benzamide bonded phase was prepared via a hydrosilation reaction of a hydride silica intermediate with p-chloro- N-allylbenzamide. Solid-state NMR was used to show the disappearance of reactive surface hydride species (M(H)) and to identify newly formed bonded chemical species on the silica surface. DRIFT spectroscopy, elemental analysis, and specific surface-area determinations (BET) of the prepared phases are also reported.

Poly(ethylene-co-acrylic acid) stationary phases for the separation of shape-constrained isomers.

A new approach for the synthesis of long alkyl chain length stationary phases for use in reversed-phase liquid chromatography is described. Poly(ethylene-co-acrylic acid) copolymers (i.e., (-CH2CH2-)x[CH2CH(CO2H)-]y) with different levels of acrylic acid were covalently bonded to silica via glycidoxypropyl or aminopropyl linkages. 13C cross polarization magic angle spinning (CP/MAS) nuclear magnetic resonance (NMR) spectroscopy was used to characterize the new reversed-phase materials. Aspects of shape selectivity were evaluated for six different columns with Standard Reference Material (SRM) 869a, Column Selectivity Test Mixture for Liquid Chromatography. Selectivity for isomer separations was enhanced for stationary phases prepared with poly(ethylene-co-acrylic acid) containing a mass fraction of 5% acrylic acid. The relationship between alkyl conformation and chromatographic properties was studied by 13C magic angle spinning (MAS) NMR measurements, and correlations were made with the composition of the polymer. Finally, the effectiveness of this phase is demonstrated by the separation of several beta-carotene isomers.

Stationary phases for capillary electrochromatography.

This review summarizes the variety of stationary phases that have been employed for capillary electrochromatography (CEC) separations. Currently, about 70% of reported CEC research utilizes C18 stationary phases designed for liquid chromatography, but an increasing number of new materials (e.g., ion-exchange phases, sol-gel approaches, organic polymer continuous beds) are under development for use in CEC. Novel aspects of these different materials are discussed including the ability to promote electroosmotic flow, phase selectivity and activity for basic solutes. In addition, new column designs (polymer continuous beds and silica-sol-gel monoliths) are described.

C30 stationary phases for the analysis of food by liquid chromatography.

The introduction of a polymeric C30 liquid chromatographic column by Sander et al. [Anal. Chem., 66 (1994) 1667] designed for the separation of carotenoid isomers, has led to the development of improved analytical methods for these compounds. Subsequent commercial availability of polymerically bonded C30 columns has facilitated these advances, and applications to a wide variety of separation problems with biological samples have been described. This report provides a comprehensive review of applications of polymeric C30 columns, utilized in the determination of carotenoids, retinoids, and other nutrients and related compounds in complex, natural-matrix samples.

Hyphenation of high performance liquid chromatography with nuclear magnetic resonance spectroscopy for the characterization of beta-carotene isomers employing a C30 stationary phase.

The hyphenation of HPLC together with NMR spectroscopy proves advantageous for the structure elucidation of oxidation- and UV-sensitive compounds such as beta-carotene isomers. In the closed-loop HPLC-NMR system, degradation or isomerization of separated compounds is largely hindered. With the help of 3-microm C30 stationary phases a better separation efficiency towards the different beta-carotene cis/trans isomers could be obtained in comparison to a 5-microm material, resulting in sharper peaks and a better resolution of all compounds. This effect greatly facilitated the structure determination of the isomers by HPLC-NMR coupling. Due to the introduction of a superior stationary phase, the structure of seven cis-isomers of beta-carotene could thereby be determined employing the stopped-flow HPLC-1H-NMR mode.

Peer Reviewed: Understanding Reversed-Phase LC with solid-state NMR.

NMR techniques offer a variety of tools for studying bonded-phase structure, surface chemistry and stability, and dynamic behavior.

Shape selectivity for constrained solutes in reversed-phase liquid chromatography.

In reversed-phase liquid chromatography (RPLC), the separation of compound mixtures of similar polarity can present a significant challenge for the analyst. Examples of such compounds include geometric isomers present in environmental samples (e.g., polycyclic aromatic hydrocarbons, polycyclic aromatic sulfur heterocycles, and polychlorinated biphenyl congeners) and compounds of biological significance (e.g., carotenoids and steroids). In general, compounds with rigid, well-defined molecular shape are best separated using a column with enhanced shape selectivity characteristics. This perspective presents an overview of column properties that influence shape selectivity for constrained solutes. Approaches to the characterization of stationary-phase structure are described, and the findings are correlated with chromatographic performance. Finally, retention models of shape discrimination are presented that are consistent with observed retention behavior. An appreciation for shape recognition effects in RPLC will facilitate method development for certain classes of difficult to resolve compounds.

Shape selectivity of C30 phases for RP-HPLC separation of tocopherol isomers and correlation with MAS NMR data from suspended stationary phases.

Vitamin E (tocopherol) acts in various organisms as the main free radical scavenger. This capacity, which is enhanced by the synergetic effect of vitamin C and carotenes, points to a possible application as anti-tumor agent in chemotherapy. There are several isomeric forms, namely alpha-, beta-, gamma-, and delta-tocopherol, having different antioxidative abilities, with alpha-tocopherol being the most biologically active. Using methanol as eluent and a C30 stationary phase, we achieved complete separation of alpha-, beta-, gamma-, and delta-tocopherol and alpha-tocopherol acetate by RP-HPLC within 14 min. Detection was performed by UV and 1H NMR spectroscopy. The advantage of NMR is the possibility of structural identification of chromatographic peaks. Also, coeluting peaks are easily recognized. The enhanced shape recognition of the C30 phase has been attributed to the high order of the alkyl chains of the stationary phase. This has, so far, been proven by solid-state NMR spectroscopy. We now introduce the technique of 13C MAS NMR spectroscopy of suspended stationary phases. The resulting NMR spectra reveal that, in the presence of weak eluents, like methanol, the overall high order of the C30 chains is slightly altered, whereas in stronger eluents, like MTBE, the alkyl chains possess a higher mobility.

A (-)-menthyl bonded silica phase for chiral separations: synthesis and solid state NMR characterization.

A new (-)-menthyl bonded silica phase has been prepared by hydrosilation of a hydride silica intermediate. The hydride silica intermediate was synthesized by the reaction of a monoalkoxysilane (CH(3))(2)SiH(OEt) with silica gel, yielding a relatively high surface coverage (4.4 µmol/m(2)) of SiH groups. This intermediate was then used successfully in the preparation of a monomeric (-)-menthyl bonded silica phase. The bonded phase produced has been used for the chromatographic separation of enantiomers in a reversed phase mode (chiral separations). Solid state (13)C and (29)Si CP-MAS NMR spectroscopy and DRIFT spectroscopy provides valuable information on the structure of the different surface species formed on silica after modification. The surface coverage of the hydride silica intermediate and of the final bonded silica phase produced are also determined. It is found that this modification procedure can exclusively produce a monomeric coverage of SiH groups on the silica surface and can further produce a final monomeric bonded organic silica phase for the separation of enantiomers.

Stationary interphases with extended alkyl chains: a comparative study on chain order by solid-state NMR spectroscopy.

Stationary interphases with long n-alkyl chains (n = 18, 22, 30, 34) have been examined by solid-state NMR spectroscopy. The determination of the silane functionality and the degree of cross-linking of silane ligands on the silica surface was performed by 29Si CP/MAS NMR spectroscopy. High-speed 1H MAS and 13C CP/MAS NMR spectroscopy were utilized to assess alkyl chain order and mobility of the different bonded phases. For this purpose, 1H NMR line widths and 13C chemical shifts have been evaluated. It is shown that stationary phase order and rigidity increase with alkyl chain length. In addition, the temperature-dependent trans/gauche conformational change occurs at higher temperatures for a polymeric C34 phase compared with a C30 sorbent. This behaviour is discussed in the context of previously reported chromatographic (HPLC) shape selectivity differences.

1H MAS NMR spectroscopy of chemically modified silica gels: a fast method to characterize stationary interphases for chromatography.

Chemically modified silica gels used as stationary phases in chromatography have been investigated by means of solid-state 1H magic angle spinning (MAS) NMR spectroscopy. Since the organosilanes are bonded to the surface of the silica gel, their protons are diluted and possess a higher mobility in comparison to protons in pure organic solids. Thereby the usually strong homonuclear dipole-dipole interactions among the protons are reduced and it is possible to obtain well-resolved 1H NMR spectra of the organic interphases with MAS-only techniques. Effects of temperature and magnetic field strength on the resolution of the spectra are examined as well as the dependence of T1 and T1pH relaxation times on temperature and spinning speed.

Chain order and mobility of high-density c(18) phases by solid-state NMR spectroscopy and liquid chromatography.

C(18) phases prepared by different synthetic pathways are examined by solid-state NMR spectroscopy. Silane functionality is clearly indicated by (29)Si CP/MAS NMR spectroscopy. Order and mobility of the alkyl chains are investigated with high-speed (1)H MAS and (13)C CP/MAS NMR spectroscopy. Differences in coverage are monitored by (1)H line widths,( 13)C chemical shifts, (13)C cross-polarization constants, and (1)H relaxation times in the rotating frame. It is shown that C(18) phases prepared by the surface polymerization technique exhibit a more regular surface coverage than sorbents prepared by conventional polymeric synthesis. The findings from solid-state NMR investigations are discussed in the context of liquid chromatography (LC) separations of linear and bulky polycyclic aromatic hydrocarbon (PAH) solutes.