Selectivity chromatographic studies

Silyl Ethers. The preparation of per- O-trimethyl silyl ethers of sucrose is generally achieved by reaction with chi orotrimethyl sil ane and/or hexamethyldisila2ane in pyridine (25,26). However, this reaction is not selective and in general per-trimethyl silyl ethers are only used as derivatives for gas chromatographic studies. 

Sellergren B, Lespisto M, Mosbach K. Highly enantioselective and substrate-selective polymers obtained by molecular imprinting utilizing noncovalent interactions. NMR and chromatographic studies on the nature of recognition. J Am Chem Soc 1988 110 5853-5860. 

Product characterization provides further insight on the course of diene carboxylation. The monocarboxy acids were identified as methyl carboxyoctadecenoate (1) from chromatographic, IR, mass spectral, and selective hydrogenation studies. The double bond of 1 from carboxylated linoleate is 40% trans in configuration (IR), and its carboxy group is located mainly on carbon-10 and -12 positions (Table II). In contrast,... 

The most convenient and economic techniques of choice for the rapid analysis of starting materials and for the assessment of purity of a crude reaction product are t.l.c. and g.l.c. These techniques may also be used to monitor the progress of a reaction for which optimum conditions are uncertain, as may be the case when an established published procedure is used as the basis for carrying out other preparations of a similar nature. In these cases the reaction is monitored by the periodic removal from the reaction mixture of test portions for suitable chromatographic study. Clearly the chromatographic behaviour of starting materials and, if possible, expected products, needs to be established prior to the commencement of the reaction. For t.l.c. this would include solvent and thin layer selection, a detection method, and an appraisal of sensitivity of detection with respect to the concentration of components in the reaction medium. For g.l.c. preliminary experiments would be required to select a suitable column and the appropriate operating conditions. 

Addition of surfactants, such as cetyltrimethylammonium bromide (CTMA), causes silica to mimic separations obtained with reversed-phase sorbents. The impurities of propranolol and pharmaceutical preparations of catecholamines have been chromatographically studied using this surfactant [310,316]. The one apparent advantage of using these surfactants is that the brand-to-brand variations in selectivity commonly seen for bonded phases is avoided [310]. For basic analytes, the addition of either methanol or acetonitrile changes the sorbent selectivity. Commonly, a retention minimum occurs at about 50% organic solvent content with increases in retention at either increased or decreased organic content [312]. 

For exanple, it can be seen in Fig. 3, that the highest possible k values for the phenolic solutes are lower than desirable, even in pure water as eluent. The solubility of these phenolic compounds is also low in pure water. Therefore, as a oenpremise between the opposing requirements of sufficient retention and high solubility, a carrier solution oenpositien of 10 % methanol water was selected for the further studies. Similarly, it can be seen from Fig. 5, that the k values of the Ibuprofen enantiomers are around 10 in the 30 % acetonitrile buffer eluent, therefore this conposition was selected for the carrier solution for the displacement chromatographic studies. 

A. K. Chatjigakis, I. Clarot, P. J. P. Cardot, R. Nowakowski, and A. Coleman, Reversed phase chromatographic study of the inclusion selectivity of terpene derivatives with P-cyclodextrines in water/cosolvent mixtures, J. Liq. Chrom Rel. Technol. 22 (1999), 1267. 

Thermospray LC/MS has been extensively used for the study of sulfonylurea herbicides (1-2). These compounds are thermally labile and can not be successfully analyzed by conventional GC/MS. Early applications of thermospray LC/MS included metabolite identification and product chemistry studies. We have recently evaluated the use of thermospray LC/MS for multi-sulfonylurea residue analysis in crops and have found the technique to meet the criteria for multiresidue methods. LC/MS offers both chromatographic separation and universal mass selectivity. Our study included optimization of the thermospray ionization and LC conditions to eliminate interferences and maximize sensitivity for trace level analysis. The target detection levels were SO ppb in crops. Selectivity of the LC/MS technique simplified sample extraction and minimized sample clean up, which saved time and optimized recovery. Average recovery for these compounds in crop was above 85%. 

The properties of Fm metal and of its solid compounds are for the most part unknown because there are insufficient quantities to prepare even microsamples. In the numerous thermo-chromatographic studies by Zvara and coworkers, the evaporation of Fm and Md tracer from molten La at 1150°C was compared with the behavior of other selected lanthanides and actinides (7). 

D-Glucitol was selected for study because the absorbing system is composed entirely of hydroxyl groups and is not complicated by the presence of the proposed acetal chromophore. The chemical effects of ultraviolet irradiation of 5 X 10 M oxygenated solutions were examined, using paper-chromatographic and isotope-dilution methods. Using un-... 

For some years the potential of the MIP He source for selective chromatographic detection has been recognized. To date most studies have involved element selective detection for gas chromatography using... 

Sevenich and Fritz [2] used eluents containing the ethylenediammo-nium cation and either tartrate or a-hydroxyisobuiyrate as the com-plexing anion to separate polyvalent metal cations on cation-exchange columns. The emphasis of this study was to achieve more selective chromatographic separation of metal ions, and also to show that the metal ions could be sensitively detected using conductimetry. 

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