Chromatographic retention, proposed

With just a few exceptions, there is a dearth of published information providing systematic studies of retention volumes as a function of composition of the eluent over the whole composition range of binary solvents. To rectify this situation, a general equation for HPLC binary solvent retention behavior has been proposed [59] that should help generate a chromatographic retention model to fit Eq. (15.20) ... 

To characterize the relative gas-chromatographic retentions of condensed aromatics and heteroaromatics, inclu g thienothiophenes, benzo[b]thiophene, dibenzothiophene, naphthobenzothiophenes, and anthrabenzothiophenes, a system of indices. In, was proposed, In this system a series of similar linearly condensed hydrocarbons (such as benzene, naphthalene, anthracene, tetracene, pentacene,...) was used as a reference scale. The logarithm of the corrected retention volume (adjusted to 0°), log Ft, depends linearly upon the number of condensed benzene rings (z) in the molecule, both in the polar and nonpolar phases. In is expressed by Eq. (58) ... 

A reasonable match of the experimental spectrum to one in the computer library is not proof of molecular structure—it is just a clue.7 You must be able to explain all major peaks (and even minor peaks at high m/z) in the spectrum in terms of the proposed structure, and you should obtain a matching spectrum from an authentic sample before reaching a conclusion. The authentic sample must have the same chromatographic retention time as the proposed unknown. Many isomers produce nearly identical mass spectra. 

Many scales, either empirical or measured, have been proposed for the hydrophobicity of amino acid residues in proteins (Nakai and Li-Chan, 1988). The most extensive study on tlje hydrophobicity index of amino acids was published by Wilce et al. (1995). The authors derived four new scales of coefficients from the reversed-phase high-performance liquid chromatographic retention data of 1738 peptides and compared them with 12 previously published scales. 

Supercritical fluid extraction conditions were investigated in terms of mobile phase modifier, pressure, temperature and flow rate to improve extraction efficiency (104). High extraction efficiencies, up to 100%, in short times were reported. Relationships between extraction efficiency in supercritical fluid extraction and chromatographic retention in SFC were proposed. The effects of pressure and temperature as well as the advantages of static versus dynamic extraction were explored for PCB extraction in environmental analysis (105). High resolution GC was coupled with SFE in these experiments. 

In this work correlations between mobile phase solvent strength and chromatographic retention of a number of different solute families will be presented. The first solvent strength measurements on ternary mobile phases will also be presented. Finally, a retention mechanism for packed column SFC is proposed. 

The expected products of a hexosaminidase digest of structure 2 would be two isomers in which either the Man 1-3 or Man 1-6 antennae could be terminated with Gal-GlcNAc-Man (no terminal GlcNAc, and thus not susceptible to hexosaminidase), while the second antennae would be newly terminated with Man after release of the exposed terminal GlcNAc by the hexosaminidase. These two isomers would likely have retention times shorter than structure 2 (Table 2). Because there are no commercially available standards corresponding to these structures, identification by chromatographic retention time alone would not be possible. Use of a mannosidase that could distinguish terminal mannose on the Man 1-3 or Man 1-6 anteimae (29) could confirm the proposed structures. 

An alternative approach uses the polarity index, P proposed by Snyder. This is based upon experimentally determined gas chromatographic retention of three test solvents on a large number of stationary phases. The test solvents selected are ethanol, 1,4-dioxane and nitromethane. As well as an overall polarity index (P), three other parameters are calculated, Xe (a proton acceptor parameter), xproton donor parameter) and x (a strong dipole parameter). 

Additional information may come from a variety of sources. Certainly a proposed structure based on plausible chemistry and the total molecular mass is helpful. However, one must be careful to keep in mind that proposed structures are based on preliminary data only and thus may not be consistent with subsequently collected data. If LC-NMR is to be performed, then it is essential to obtain details of the chromatographic method to be used. Other information may include color to suggest conjugation, IR absorption to detect carbonyl stretches, and relative chromatographic retention time to evaluate polarity compared to the parent and other known compounds. 

Peracid Classification. Peracids can be broadly classified into organic and inorganic peracids, based on standard nomenclature. The limited number of inorganic peracids has required no subclassification scheme (4). However, the tremendous number of new organic peracids developed (85) has resulted in proposals for classification. Eor example, a classification scheme based on Hquid chromatography retention times and critical miceUization constants (CMC) of the parent acids has been proposed (89). The parent acids are used because of the instabiHty of the peracids under chromatographic and miceUization measurement conditions. This classification scheme is shown in Table 1. 

So far the models proposed to explain retention in RPC have largely remained the province of the physical chemist. The mathematical difficulty of using these models and their lack of a simple conceptual picture of the retention process in familiar chromatographic terms has diminished Interest in their use compared to simple empirical rules for trial and error optimization of separations. 

The quantitation of products that form in low yields requires special care with HPLC analyses. In cases where the product yield is <1%, it is generally not feasible to obtain sufficient material for a detailed physical characterization of the product. Therefore, the product identification is restricted to a comparison of the UV-vis spectrum and HPLC retention time with those for an authentic standard. However, if a minor reaction product forms with a UV spectrum and HPLC chromatographic properties similar to those for the putative substitution or elimination reaction, this may lead to errors in structural assignments. Our practice is to treat rate constant ratios determined from very low product yields as limits, until additional evidence can be obtained that our experimental value for this ratio provides a chemically reasonable description of the partitioning of the carbocation intermediate. For example, verification of the structure of an alkene that is proposed to form in low yields by deprotonation of the carbocation by solvent can be obtained from a detailed analysis of the increase in the yield of this product due to general base catalysis of carbocation deprotonation.14,16... 

A mixture of acetyl acetone, 1-nitronaphthalene, and naphthalene has been proposed for evaluating reversed-phase packing material [102]. This reveals the usual optimum kinetic chromatographic parameters (the naphthalene peak), the degree of activity or end-capping status of the column (the ratio of the 1-nitronaphthalene and naphthalene retention times) and trace metal activity (the shape and intensity of the acetylacetone peak). 

Several different physicochemical models have been proposed to predict and explain the retention behavior in liquid-solid chromatography. The models can be divided into two groups depending on the assumptions made concerning the fundamental mechanism of the chromatographic process. The two assumptions are as follows ... 

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