Chromatography selection

Selectivity In chromatography, selectivity is defined as the ratio of the capacity factors for two solutes (equation 12.11). In capillary electrophoresis, the analogous expression for selectivity is... 

W. Beitsch, Two-dimensional gas chromatography concept, instmmentation and appli-cations-Pait 1 fundamentals., conventional two-dimensional gas chromatography, selected applications , ]. High. Resolut. Chromatogr. 22 647 (1999). 

Snyder L. R., Changing reversed-phase high performance liquid chromatography selectivity which variables should be tried first , ]. Chromatogr. B, 689(1), 105, 1997. 

Stability of Cyclic Nitronates For steric reasons, fragmentation of five-and six-membered cyclic nitronates cannot follow pathways presented in Scheme 3.72. Hence, stability of these compounds can be substantially higher than that of alkyl nitronates. These compounds generally exist in the crystalline state and can be purified by recrystallization or liquid chromatography. Selected melting points of nitronates are given in Table 3.8. 

In preparative chromatography, selectivity and efficiency no longer have the same importance they do in analytical chromatography. A certain selectivity is required in preparative chromatography as everywhere else in order to achieve the separation, but other parameters are at least as important if not more so. These include the loading capacity of the stationary phase and the maximum speed (throughput) of the process. The three main economic criteria for a large scale separation process are... 

The three main modes of chromatographic operation are elution chromatography, selective adsorption/desorption, and simulated countercurrent chromatography. Of these, elution chromatography, used as a cyclic batch process, was the first to be developed for large-scale separations. 

Holmes, M., Teo, S.L. and Khoo, H.W., Detection of diarrheic shellfish poisoning toxins from tropical shellfish using liquid chromatography-selected reaction monitoring mass spectrometry, Nat. Toxins., 1, 6, 361, 1999. 

The cross aldol condensation of citral (Millennium Chemicals, 40 % cis-isomer + 55 % trans-isomer) with acetone (Merck, PA) was carried out at 353 K in N2 atmosphere under autogenous pressure ( 250 kPa) in a batch PARR reactor, using an acetone/citral = 49 (molar ratio) and a catalyst/(citral+acetone) = 1 wt.% ratio. Catalysts were pre-treated ex-situ in flowing N2 at 773 K for 2 h to remove adsorbed water and carbon dioxide and then quickly transferred to the reactor without exposing them to air. Reaction products were analyzed by gas chromatography. Selectivities (Sj, mol of producty /mol of citral reacted) were calculated as (%) = Cj X 100/ TCj where Cj is the concentration of product j. Product yields rjj, mol of product y/mol of citral fed) were calculated as Tfj = SjXat- Thirteen samples of the reaction mixture were extracted and analyzed during the 6-hour reaction. The main reaction product of citral conversion was pseudoionone, PS (cis- and trans- isomers). 

Wilson, A.E., and Domino, E.F. Plasma phencyclidine pharmacokinetics in dog and monkey using a gas chromatography selected ion monitoring assay. Blomed. Mass Spectrom. 5 112-116, 1978. 

To a solution of tile alkenc (1 mmol) in anhyd C112C12 (4mL) was added. V-(phenylselanyl)phthalimide (1.5 cquiv) and Et3N 3HF (6 equiv) and the mixture was stirred at 25 C (for reaction time sec Tabic 14). The mixture was then poured onto Et20/ aq NaHCOj, the organic products were extracted into the ethereal layer, dried, and the solvents and Et3N evaporated. The residue was analyzed by 19F NMR spectroscopy and purified by chromatography. Selected results arc given in Table 14, for further examples, see ref 216. 

RT Wilson, JM Groneck, AC Henry, LR Rowe. Multiresidue assay for benzimidazole anthelmintics by liquid chromatography and confirmation by gas chromatography/selected-ion monitoring electron impact mass spectrometry. J Assoc Off Anal Chem 74 56-57, 1991. 

If there is no or little information on the method s performance characteristics, it is recommended that the method s suitability for its intended use in initial experiments be proven. These studies should include the approximate precision, working range, and detection limits. If the preliminary validation data appear to be inappropriate, the method itself, the equipment, the analysis technique, or the acceptance limits should be changed. In this way method development and validation is an iterative process. For example, in liquid chromatography selectivity is achieved through selection of mobile-phase composition. For quantitative measurements the resolution factor between two peaks should be 2.5 or higher. If this value is not achieved, the mobile phase composition needs further optimization. 

Schurig V., Weber R. A., Nicholsen G. J., Oehlschlager A. C., Pierce H. D., Jr, Borden J. H. and Ryker L. C. (1983) Enantiomer composition of natural exo- and endo-brevicomin by complexation gas chromatography/selected ion mass spectrometry. Naturwissenschaften 70, 92-93. 

Gas chromatography-selected ion monitoring (GC-SIM) is often used for the quantification of GAs by GC-MS, an internal standard labeled with stable isotopes (usually with deuterium) being used as the most reliable and sensitive method. A mass chromatogram reconstructed from full-scan GC-MS is also used for semiquantification. The amounts of GAs are determined by measuring the peak areas of ions characteristic for each GA and comparing these areas to those of authentic samples. When an internal standard labeled with a stable isotope is used, the ratio of the area of an ion peak characteristic for a sample and the area of the corresponding peak of the labeled internal standard is used for quantification. The analysis of GAs by GC-MS has been discussed in numerous publications and review articles.254-256... 

Synthetic Library Method that Requires Chromatography Selection... 

For the synthetic library method that uses the affinity chromatography selection approach, the bound peptides can be eluted and microsequenced by Edman degradation. Concurrent microsequencing of the retrieved peptide mixture can be performed rather than sequencing individual peptides. Sequeuce motifs then can be defined in a fast and efficient way. However, the amino acid sequence obtained wiU be the result of the summation of the peptide mixture. Uuless a predomiuaut, distinct motif and an alignment of one or more of the critical residues exists within the peptide sequence of the library (e.g., with a fixed residue at a specific position), the result could be very difficult if not impossible to interpret. 

Synthetic library method that requires affinity chromatography selection... 

For sample preparation before chromatography select filter size according to the bead size of the chromatographic medium. 

Selection of adsorbent is based on the same principles that are used for chromatography. Select the medium showing the strongest binding to the target protein which binds as few of the contaminants as possible i.e. the medium with the highest selectivity and/or capacity for the protein of interest. 

Poch GK, Klette KL, Anderson C. The quantitation of 2-0X0-3-hydroxy lysergic acid diethylamide (0-H-LSD) in human urine specimens, a metabolite of LSD comparative analysis using liquid chromatography-selected ion monitoring mass spectrometry and liquid chromatography-ion trap mass spectrometry. 

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