Investigation and Control of Alkylsilane Stationary Phase Structure in Reversed Phase Liquid Chromatography PDF Download
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Languages : en
Pages : 726
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Investigation and control of alkylsilane stationary phase structure in reversed phase liquid chromatography is presented. Raman spectroscopy is used to probe thealkyl chain conformational order and interchain coupling as a function of various chromatographic conditions. A new method is further developed to fabricate alkylsilane stationary phases with controlled surface coverage. The alkyl chain conformational order and interchain coupling of a series of high-density docosylsilane (C22) bonded stationary phases is shown as a function of temperature, surface coverage, polymerization method, common solvents and solutes. The conformational order of C22 stationary phases is compared to that of octadecylsilane (C18) stationary phases to understand the chain length effect on stationary phase structure. The conformational order information as indicated by Raman spectral order indicators for a C22 phase are correlated with the capacity factor and separation efficiency for each solute studied to gain insight into the retention mechanism. These studies help to understand the origin of stationary phase shape selectivity and the separation process in general. Based on these results, the molecular pictures at the stationary phase/solvent interface are proposed. The effect of pressurized solvent environments on two C18 phases is studied to obtain direct evidence for changes in stationary phase structure due to pressure. These changes are compared to effects of solvation relative to air in the same solvents. In addition, Raman spectral order indicators are identified for perdeuterated alkyl-containing system. This study provides a foundation for studying stationary phase structure in complex systems comprised of long alkyl-containing solutes. A further development of a new method is presented as well for synthesizing alkylsilane stationary phases with precisely controlled surface coverage by using a displaceable surface template monolayer of n-alcohol. A mechanism for this process is proposed based on the studies of n-alcohol concentration and chain length effect on the stationary phase surface coverage. The utility of these new stationary phases as chromatographic support is demonstrated. The shape selectivity for these new phases is comparable to or better than similar phases prepared by conventional methods.
Author:
Publisher:
ISBN:
Category :
Languages : en
Pages : 726
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Book Description
Investigation and control of alkylsilane stationary phase structure in reversed phase liquid chromatography is presented. Raman spectroscopy is used to probe thealkyl chain conformational order and interchain coupling as a function of various chromatographic conditions. A new method is further developed to fabricate alkylsilane stationary phases with controlled surface coverage. The alkyl chain conformational order and interchain coupling of a series of high-density docosylsilane (C22) bonded stationary phases is shown as a function of temperature, surface coverage, polymerization method, common solvents and solutes. The conformational order of C22 stationary phases is compared to that of octadecylsilane (C18) stationary phases to understand the chain length effect on stationary phase structure. The conformational order information as indicated by Raman spectral order indicators for a C22 phase are correlated with the capacity factor and separation efficiency for each solute studied to gain insight into the retention mechanism. These studies help to understand the origin of stationary phase shape selectivity and the separation process in general. Based on these results, the molecular pictures at the stationary phase/solvent interface are proposed. The effect of pressurized solvent environments on two C18 phases is studied to obtain direct evidence for changes in stationary phase structure due to pressure. These changes are compared to effects of solvation relative to air in the same solvents. In addition, Raman spectral order indicators are identified for perdeuterated alkyl-containing system. This study provides a foundation for studying stationary phase structure in complex systems comprised of long alkyl-containing solutes. A further development of a new method is presented as well for synthesizing alkylsilane stationary phases with precisely controlled surface coverage by using a displaceable surface template monolayer of n-alcohol. A mechanism for this process is proposed based on the studies of n-alcohol concentration and chain length effect on the stationary phase surface coverage. The utility of these new stationary phases as chromatographic support is demonstrated. The shape selectivity for these new phases is comparable to or better than similar phases prepared by conventional methods.
Author: Arthur D. Quast
Publisher:
ISBN:
Category :
Languages : en
Pages : 102
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Reversed-phase liquid chromatography (RPLC) is a widely used technique for analytical separations but routinely requires empirical optimization. Gaining a better understanding of the molecular reasons for retention may mean more efficient separations with fewer trial and error runs to obtain optimized separations. Vibrationally resonant sum frequency generation (VR-SFG) is a surface specific technique that has allowed for in situ examination of model RPLC stationary phases under various solvent and pressure conditions. In order to improve on past work with model RPLC stationary phases two challenges had to be overcome. First, improved vibrational mode assignments of the C18 stationary phase were needed for proper understanding of this model system. Second, the synthesis of back-surface reference mirrors used in these VR-SFG experiments allowed us to better correct the relative intensities of the various spectral peaks present in typical spectra. After examination of model RPLC systems under various conditions, we have found that these model substrates have a significant amount of interference from nonresonant signal. This interference of resonant and nonresonant signals on fused silica surfaces has not been previously examined and further studies of the model RPLC stationary phase must properly deal with the non-negligible nonresonant interference that is present. We have seen changes in the VR-SFG spectra of these model systems under a variety of conditions including elevated pressure, however the changes are mostly due to nonresonant interference. These spectral changes, although apparently not solely from structural changes, need to be investigated further to better understand the molecular basis of retention in model RPLC systems.
Author: Jun-Liang Lin
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Category :
Languages : en
Pages :
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Author: Michael David Bair
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Category : Chemistry
Languages : en
Pages :
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ABSTRACT: Central to the advancement of reversed-phase high-performance liquid chromatography (RPLC) is to develop new synthetic strategies for manufacturing stationary phase materials. Methods to improve the efficiency, retention properties, and chemical stability of stationary phases are always being investigated. The work presented focuses on two new synthesis schemes aimed at improving monomeric silica-based stationary phases in these respects. The first study involves "pre-capping" Type-B silica. Previous work showed that monomeric stationary phases made by pre-treating the silica surface with small amounts of trimethylsilane (TMS) reagents prior to C18 silanization showed vast improvements in the chromatographic efficiency, phase loading, and retention with a maximum at approx. 5% pre-capping. It was concluded that this pre-capping step improved efficiency by selectively neutralizing the most reactive highly-acidic silanol sites, so-called silanol "hot spots," producing a more energetically-homogenous surface prior to exhaustive C18 derivatization that subsequently yielded a more evenly-distributed alkyl bonding arrangement. These previous studies were performed on Type-A silica, an older variety of silica gel material containing higher levels of metal impurities than the Type-B silica used today. It has since been argued that metallic impurities are the primary cause of silanol hot-spots, and that pre-capping Type-B silica would have little or no effect, however the experimental evidence has yet to be produced, and there exists the potential for heterogeneous silanol reactivity inherent in the amorphous silica gel regardless of purity. The purpose of the work presented here is to determine the effects of TMS pre-capping on Type-B silica as compared to the previous Type-A results, with the goal of establishing pre-capping protocol for Type-B silica and to form a better understanding of its chemistry. The current work performed on three Type-B silica substrates of various physical and chemical properties demonstrated optimal TMS pre-capping at approximately 2.5%. The results at this level show only a slight improvement in efficiency for non-polar compounds ( 25%) was observed for some drug compounds and bases under buffered conditions, with the magnitude of the improved efficiencies correlating with metal impurity content and physical parameters of the silica substrate. Pre-capping also resulted in a slight decrease in retention and hydrolytic stability due to a decrease in bonded phase density. The results lend supporting evidence that metal impurities are the primary source of highly acidic silanols, but they also suggest a means to improve efficiency of basic analytes on certain Type-B silica substrates. It was concluded that TMS pre-capping Type-B silica is best done at low levels (
Author: Shih Tak Khew
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ISBN:
Category : Chemical engineering
Languages : en
Pages :
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Author: Shih Hsien Hsu
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ISBN:
Category : Liquid chromatography
Languages : en
Pages : 444
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Category : Dissertations, Academic
Languages : en
Pages : 862
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Author: Brian Scott Ludolph
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Category :
Languages : en
Pages : 366
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Author: Tina M. Mannschreck
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Category : Chromatographic analysis
Languages : en
Pages : 136
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Author: Dale Allen Shoemaker
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ISBN:
Category :
Languages : en
Pages : 210
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