Matthias Pursch

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.

Comprehensive two dimensional gas chromatography review

Advances, developments and applications in 2-D comprehensive GC (GC x GC) from 2007 through October 2008 are reviewed, with emphasis on modulation, and data handling, and applications of current relevance. Industrial perspectives as well as suggestions where further developments would be beneficial are summarized in the concluding remarks.

Chain Order and Mobility of High-Density C18 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.

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.

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.

Comparison of the separation of cis/trans isomers of tretinoin with different stationary phases by liquid chromatography-nuclear magnetic resonance coupling

Abstract The comparison of two different types of RP stationary phases in their ability to separate cis/trans isomers of retinoic acid (tretinoin) has been investigated by LC-NMR coupling. Only by recording of 1 H NMR spectra the structural identification of the separated compounds was possible, since their absorption coefficients are very similar and their mass is identical, and therefore identification by UV-Vis is not unambiguous and identification with LC-MS fails due to identical fragmentation patterns. A commonly used C 18 phase and a recently developed C 30 phase have been used for the separation of a mixture of thermal isomerized retinoic acids. Three isomers could be separated and identified with the separation on a C 18 column, whereas five cis/trans isomers could be identified by the use of a C 30 column. It could be shown that even under optimized chromatographic conditions the separation efficiency of the two phases varies and that these differences are probably due to differences in the alkyl chain organization of the stationary phases.

INVESTIGATING THE SELECTIVITY OF TRIACONTYL INTERPHASES

What causes the shape selectivity of C30 phases? This question can be answered by combining NMR and fluorescence spectroscopies with HPLC separations at various temperatures. The selectivities depend on the ratio of trans to gauche conformations of the alkyl chains, whose dynamic behavior was characterized with a two-dimensional solid-state NMR spectrum (shown on the right).

Loop-based multiple heart-cutting two-dimensional liquid chromatography for target analysis in complex matrices.

Loop-based multiple heart-cutting (MHC) two-dimensional liquid chromatography (2D-LC) is presented as a solution to quantify target components in complex matrices, such as additives in polymers, at very high chromatographic resolution. The determination of hexabromocyclododecane (HBCD) in polystyrene (PS) is described. One dimensional ((1)D) LC analysis with UV detection did not allow quantitation of the main isomers of HBCD due to peak overlap with polymer components. MHC 2D-LC analysis provided the separation power, accuracy, and repeatability needed for quantitative analysis of the additives of interest. Heart-cuts from peaks of the (1)D-chromatogram or entire regions of interest are sampled into loops, where they remain parked until their sequential reinjection onto the second dimension ((2)D) column. A column set consisting of phenyl ((1)D) and C18 ((2)D) stationary phases gave baseline separation in (2)D between HBCD and PS background. Linearity for spiked polymer samples was achieved over a range of 0.02-1.00 wt % HBCD relative to the amount of polymer. The limit of quantitation was estimated at 0.01 wt % HBCD in PS. A peak area RSD of 0.7% obtained for ten replicates of a real sample demonstrated excellent repeatability of the analysis. MHC 2D-LC is an elegant solution for quantitative analyses of difficult-to-separate samples when conventional (1)D separation fails.

Hyphenation of high performance liquid chromatography with nuclear magnetic resonance spectroscopy for the characterization of β-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.