Acetonitrile buffer, alkaline

Figure 9 serves to demonstrate this equalizing of the stationary phases in the presence of buffers even for non-ionic analytes. In Fig. 9a, the separation of the isomers of nitroaniline on four rather different stationary phases with the help of an alkaline acetonitrile buffer is shown. Apart from small differences in the retention time, the separation of the three peaks looks rather similar on each of the four columns. Fig. 9b shows the separation of the nitroanilines on Symmetry Shield and on Zorbax Bonus in a methanol/water mixture. The chromatograms look absolutely different even an inversion of the elution order is observed. This means that to exploit the individual properties of the stationary phases in the realm of ultimate selectivity, one should dispense with buffers, which is not easy to realize in routine work, where reproducible retention times are required. Nevertheless, one should remember this in the case of orthogonal tests see below. These phenomena are observed even with simple, polar, non-ionizable analytes such as ketones (see Fig. 10). 

Paquette and co-workers synthesized the 5,11-dinitro isomer of 1,3-bishomopentaprismane (95) by treating the dioxime (94) with a buffered solution of m-CPBA in refluxing acetonitrile. A significant amount of lactone by-product (96) is formed during this step and may account for the low isolated yield of (95). Oxidative nitration of (95) with sodium nitrite and potassium ferricyanide in alkaline solution yields a mixture of isomeric trinitro derivatives, (97) and (98), in addition to the expected 5,5,11,11-tetranitro derivative (99), albeit in low yield. Incomplete reactant to product conversion in this reaction may result from the low solubility of either (97) or (98) in the reaction medium, and hence, incomplete formation of the intermediate nitronate anions. 

Eight BDZs among the most frequently encountered in forensic toxicology (clonazepam, desal-kylflurazepam, diazepam, flunitrazepam, lorazepam, midazolam, nordiazepam and oxazepam) were determined in whole blood after solvent extraction with butyl chloride and fast isocratic separation using a C18 (100 x 4.6 mm x 5 (tm) column [61]. The mobile phase was composed of phosphate buffer (35mM, pH 2.1) and acetonitrile (70 30, v/v) and the flow rate was 2mL/min. Within less than 4 min of analysis time, the analytes could be successfully determined starting from therapeutic concentrations. Using HPLC coupled with APCI-MS-MS, Rivera et al. [62] set up a method for the detection of 18 BDZ and metabolites after butyl chloride extraction at alkaline pH in 0.5mL... 

For efficient extraction of macrolide and lincosamide residues from edible animal products, bound residues should be rendered soluble, most if not all of the proteins should be removed, and high recoveries for all analytes should be provided. Since tliese antibiotics do not strongly bind to proteins, many effective extraction methods have been reported. Sample extraction/deproteinization is usually accomplished by vortexing liquid samples or homogenizing semisolid samples with acetonitrile (136—139), acidified (136,140-142) orbasified acetonitrile (143), methanol (14, 144, 145), acidified (145-147) or basified methanol (148), chloroform (149-151), or dichloromethane under alkaline conditions (152). However, for extraction of sedecamycin, a neutral macrolide antibiotic, from swine tissues, use of ethyl acetate at acidic conditions has been suggested (153), while for lincomycin analysis in fish tissues, acidic buffer extraction followed by sodium tungstate deproteinization has been proposed (154). 

Shimizu et al. [75] described a column-switching HPLC method for the simultaneous determination of omeprazole and its two main metabolites, 5-hydroxyomeprazole and omeprazole sulfone, in human plasma. Omeprazole and its two metabolites and lansoprazole as an internal standard were extracted from 1 ml of alkalinized plasma sample using diethyl ether-dichloromethane (45 55). The extract was injected into a column I (TSK-PW precolumn, 10 /im, 35 mm x 4.6 mm) for cleanup and column II (Inertsil ODS-80A column, 5 /an, 150 mm x 4.6 mm) for separation. The mobile phase consisted of phosphate buffer-acetonitrile (92 8, pH 7) for cleanup and phosphate buffer-acetonitrile-methanol (65 30 5, pH 6.5) for separation, respectively. The peak was detected with a UV detector set at a wavelength of 302 nm, and total time for chromatographic separation was approximately 25 min. The validated concentration ranges of this method were 3-2000 ng/ml for omeprazole, 3-50 ng/ml for 5-hydroxyomeprazole, and 3-1000 ng/ml for omeprazole sulfone. Mean recoveries were 84.3% for omeprazole, 64.3% for 5-hydroxyomeprazole, and 86.1%... 

Reaction kinetics using radiolabeled DKP were carried out as follows Crude enzyme was incubated at 30°C with 25 mM DKP (1.2 )iCi [ C-Phe]-DKP) in 25 mM phosphate buffer, pH 8.0, with and without 5.0 mM EGTA in a total volume of 0.1 mL. Aliquots (10 pL) were removed over time, spotted on silica TLC plates and developed with the following solvent system chloroform methanol ethanol water formic acid (10 4 4 2 1) with visualization using alkaline fluorescamine (0.5 mg/mL in 50 50 acetonitrile water) and Kl/starch after exposure to calcium hypochlorite vapor. The TLC plates were scraped in 1 cm bands and radioactivity determined by scintillation counting using a Beckman LS-3801 counter. 

The chromatography of leukotriene A 4 presents certain problems since it contains an allyhc 5,6-epoxide which breaks down almost instantaneously at neutral or acidic pH. To overcome this chemical instability, alkaline chromatographic conditions are required which use a mobile phase of acetonitrile-0.01 M borate buffer, pH 10 (30 70) in combination with either an octyl or ODS colunm (Wynalda et al., 1982). As silica-based columns are somewhat labile at alkaline... 

Figure 7c shows the change in pH value of a 20 mMol Na buffer after the addition of methanol. This explains why a column that is run with an eluent at a nominal pH of 6 or 7 has a comparatively low lifespan. After the addition of methanol or acetonitrile, the pH value shifts towards the alkaline. Most silica gels detach and partially dissolve above a pH of 8. 

If one decides to opt for a buffer because of the robustness and the better peak symmetry and for the alkaline range because of the better selectivity, then acetonitrile seems more suitable than methanol. Here, non-endcapped phases show a better peak form compared to an unbuffered eluent, although not a satisfying one. 

Chem. 1 Comparison according to all retention factors in acidic/alkaline methanol/acetonitrile phosphate buffers, S2. 

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