Stephen A. Wise

Determination of polycyclic aromatic hydrocarbons by liquid chromatography

Abstract Reversed-phase liquid chromatography (LC) using fluorescence detection is a powerful analytical technique for the measurement of polycyclic aromatic hydrocarbons (PAHs) in environmental samples. The National Institute of Standards and Technology (NIST) has been involved in the development of LC methods for the measurement of PAHs since the mid- 1970s particularly for the development of standard reference materials (SRMs) for PAH measurements in environmental samples. The NIST experience in the use of LC for the determination of PAHs in environmental samples is summarized in this paper including: selection of the appropriate column, approaches to analyzing complex PAH mixtures, and the accurate quantitation of PAHs in environmental samples.

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.

Distinguishing the Contributions of Residential Wood Combustion and Mobile Source Emissions Using Relative Concentrations of Dimethylphenanthrene Isomers

As part of the United States Environmental Protection Agencys Integrated Air Cancer Project, air particulate matter samples collected in Boise, ID, were analyzed by gas chromatography with mass spectrometric detection (GC-MS) and apportioned between their two main sources : residential wood combustion (RWC) and motor vehicle (MV) emissions. The technique used for distinguishing the source contributions involved comparison of the concentration of 1,7-dimethylphenanthrene (1,7-DMP), a polycyclic aromatic hydrocarbon (PAH) emitted primarily by burning soft woods (e.g., pines), with that of a PAH emitted in modest concentrations by both RWC and MV sources, 2,6-dimethylphenanthrene (2,6-DMP). These results were then compared with the mean 1,7-DMP/2,6-DMP ratio of 48 samples collected in a roadway tunnel, with any enrichment in the Boise sample ratios over the mean tunnel ratio attributable to the RWC source. These resulting RWC contributions were compared with fraction RWC results obtained by radiocarbon measurements ( 14 C/ 13 C) of the same extracts from Boise, with generally good correlations between the two techniques observed, suggesting that the methods are comparable when used to distinguish emissions of MVs from RWC of soft woods.

Shape selectivity in reversed-phase liquid chromatography for the separation of planar and non-planar solutes

Abstract Solute retention in reversed-phase liquid chromatography is the result of a variety of complex interactions between solute, mobile phase and stationary phase species. An understanding of the parameters that influence retention is useful in the development of separation methods with existing columns. Such knowledge is even more important for the design of new bonded stationary phases with engineered chromatographic properties. This review will examine some of the factors that affect retention and selectivity with alkyl-modified sorbents, particularly for the separation of solutes with well defined, rigid structure (e.g., polycyclic aromatic hydrocarbons). The chromatographic discrimination of compounds on the basis of molecular structure, namely “shape selectivity”, will be studied in terms of contributions from bonded phase morphology, and in terms of operational conditions. An emphasis is placed on practical choices that are available to control selectivity and optimize separations for isomers and related mixtures.

Influence of substrate parameters on column selectivity with alkyl bonded-phase sorbents

Abstract Differences in bonded-phase properties were studied for monomeric and polymeric C18 phases prepared on a variety of silica substrate materials. A total of 22 silicas with pore diameters ranging from 50-1000 A were used in syntheses. Phase loadings for the resulting bonded phases ranged from 1.3–5.4 μmol/m2. Physical properties of the substrates including surface area, pore volume, packing density, and background carbon were measured prior to bonding. Large differences were observed in the properties of the silica substrates and in the chromatographic behavior of the resulting phases. Differences in selectivity as well as absolute retention were observed as a function of pore size, with the greatest changes in selectivity occurring for the polymeric phases. The effect of silica pretreatment on phase synthesis and column selectivity was also examined for wide- and narrow-pore substrates. Phases prepared from silica pretreated with acid had greater polymeric character than those prepared from base-pretreated silica. Variation in phase loading and column selectivity is thought to be a function of both the reactivity of the silica surface and pore size. A model for polymeric phase synthesis is proposed where the extent of reaction is limited by a size-exclusion mechanism.

Optimization of an Isocratic High-Performance Liquid Chromatographic Separation of Carotenoids

Abstract Using a polymeric C 18 high-performance liquid chromatographic (HPLC) column, which demonstrated excellent separation selectivity toward carotenoid compounds in an earlier column evaluation, the effects of mobile phase modifier, modifier concentration, and column temperature were investigated. A seven-component carotenoid mixture was used to monitor changes in separation selectivity in response to variations in HPLC conditions. Both acetonitrile and tetrahydrofuran (THF) improved the resolution of echinenone and α-carotene; THF was selected for use as a modifier due to its solvating properties. At concentrations greater than 6% THF, the resolution of lutein and zeaxanthin deteriorated significantly. Temperature was varied from 15 to 35°C in 5°C increments. Resolution of lutein/zeaxanthin and β-cartone/lycopene were better at lower temperatures while echinenone/α-carotene separation improved as temperature increased. An acceptable separation of all seven carotenoids was achieved at 20°C using 5% THF as a mobile phase modifier. Method applicability is demonstrated for serum and food carotenoids.

Evaluation of reversed-phase liquid chromatographic columns for recovery and selectivity of selected carotenoids

Sixty commercially available and five experimental liquid chromatography columns were evaluated for the separation and recovery of seven carotenoid compounds. Methanol- and acetonitrile-based solvents (either straight or modified with ethyl acetate or tetrahydrofuran) were compared to determine which solvent systems and which columns provided better selectivity and recovery. Methanol-based solvents typically provided higher recoveries than did acetonitrile-based solvents. Polymeric C18 phases generally provided better selectivity for the difficult separation of lutein and zeaxanthin than did monomeric C18 phases.

Pyrolytic and Petrogenic Inputs in Recent Sediments: A Definitive Signature Through Phenanthrene and Chrysene Compound Distribution

Abstract Methylphenanthrene and methylchrysene distributions have been determined by Shpolskii Spectroscopy in chromatographic fractions of various environmental samples, including Standard Reference Materials (SRM). This distribution demonstrated to be an indicator of the temperature of the formation of aromatic material in natural matrices. Specific isomer concentration ratios were calculated in each series and were proven to assess pyrolytic or petrogenic contamination in recent sediments.

RECENT ADVANCES IN BONDED PHASES FOR LIQUID CHROMATOGRAPHY

I. INTRODUCTION Over the past 10 years high performance liquid chromatography (LC) has evolved into a mature analytical technique. Most LC separations are currently carried out in the reversed-phase mode using 25-cm columns filled with 5 or 10 μm nonpolar bonded substrates. A smaller fraction of separations are carried out in the normal-phase mode with polar bonded phases. Although major changes in column technology have become less commonplace in recent times, refinements continue to be made. New bonded phases have been developed for the separation of specific classes of compounds and new substrates have been introduced that extend column lifetimes and permit use over wide pH intervals. Even though bonded phase usage is now common, solute retention mechanisms with these materials are not fully understood (but considerable progress has been made). To aid the study of retention mechanisms, novel approaches have been taken in the physical and chemical characterization of bonded phase sorbents.

Determination of polycyclic aromatic sulfur heterocycles in fossil fuel-related samples

An analytical method is described for the separation, identification, and quantification of a number of polycyclic aromatic sulfur heterocycles (PASHs) in three fossil fuel-related samples including two Standard Reference Materials (SRMs), SRM 1597 (coal tar) and SRM 1582 (petroleum crude oil), and a decant oil. The compounds measured include the 3 possible naphtho[b]thiophenes; dibenzothiophene and selected methyl-, ethyl-, dimethyl-, and trimethyl-substituted isomers; the 3 possible benzo[b]naphthothiophenes; and the 30 methylbenzo[b]naphthothiophenes isomers. Because of the occurrence of polycyclic aromatic hydrocarbons and PASHs together with their large number of possible alkyl-substituted isomers, the analytical method described requires a number of prerequisites:  effective sample cleanup, selective stationary phases, and selective methods of detection. The sample cleanup involves solid-phase extraction using aminopropylsilane cartridges with different solvent mixtures followed by normal-phase liquid chromatographic isolation of the PASHs based on the number of aromatic carbons. These aromatic ring fractions are then separated by capillary gas chromatography using two stationary phases with different selectivities, 5% phenyl-substituted methylpolysiloxane stationary phase and 50% phenyl-substituted methylpolysiloxane stationary phase, and analyzed with mass-selective detection and atomic emission detection. A liquid crystalline stationary phase was also used to separate the methylbenzo[b]naphthothiophene isomers in the crude oil sample. Advantages and limitations of each chromatographic and detection technique are discussed.