Acidic modifiers/buffers acetate buffer

The separations of nitrofurans have been carried out, in most instances, with a reversed phase (RP) column. As summarized in Table 1, Cig columns were selected as the analytical column for LC in most reports however, the use of a Cg, CN, and cyanopropyl-modified silica analytical column has also been described. The parent compounds, as well as their metabolites, are satisfactorily separated by Cig columns with commonly used mobile phases, such as mixtures of acetonitrile with phosphate buffer, acetate buffer, or water. The mobile phase elution for HPLC analysis can be isocratic or as a gradient system. Acetonitrile-0.1 M aqueous solution of sodium perchlorate (28 72), with 0.5% glacial acetic acid, was reported by Galeano Diaz " as an optimum mobile phase for the separation of the three nitrofuran derivatives nitrofurantoin, furazolidone, and furaltadone in milk. Lin et al." used an acetonitrile gradient with an initial hold time of 1 min at 0% acetonitrile. 

Eluent components should be volatile. Solvents such as ethyl acetate, isopropyl ether, diethylketone, chloroform, dichloromethane, and toluene as modifiers and n-hexane as diluent are recommended for normal phase chromatography. For reversed-phase systems, methanol or acetonitrile are used as modifiers. Such components as acetic acid or buffers, as well as ion association reagents, should be avoided. 

Some commonly used buffers, such as sodium and potassium phosphate, are incompatible with ELSD, but there are ready alternatives. For example, ammonium acetate has similar buffering properties to potassium phosphate, and ammonium carbonate, ammonium formate, pyridinium acetate, and pyridinium formate are options for different pH ranges. Typical mobile phase modifiers that do not meet the volatility criteria can be replaced by a wide variety of more volatile alternates. For example, phosphoric acid, commonly used as an acid modifier fo control pH and ionization, can be replaced by trifluoroacetic acid other acids that are sufficiently volatile for use with FLSD include, acetic, carbonic, and formic acids. Triethylamine, commonly used as a base modifier, is compatible with FLSD other base modifiers that can be used are ethylamine, methylamine, and ammonium hydroxide [78]. 

M.I. Fauth H. Stalcup, AnalChem 30,1670— 72(1958) Be CA 53, 2929(1959) [Evaluation of six methods for detn of nitrogen in NGu. (Nitro N methods tested are modified nitrometer — greatest precision of the six acetate-buffered titanous chloride transnitration with sulfuric acid and Bowman-Scott). Total N methods are modified Kjeldahl and micro-Dumas — better of these two. The titanous chloride and trans-nitration methods are satisfactory]... 

To achieve the optimum reversed-phase LC separation, one needs to explore variables such as the analyte chemistry, mobile-phase composition (solvent type, solvent composition, pH, and additives), column composition, column particle size, and column temperature. For pharmaceutical analysis using mass spectrometry, the chemistry of an analyte is rarely changed beyond manipulation of the mobile phase pH, and even there options are limited. Volatile pH modifiers (buffers) are still preferred for LC-MS, and concentrations of these modifiers are kept low. Relatively simply mobile phases consisting of water, acetonitrile, and either formic acid (0.1% v/v), ammonium acetate (1-20 mM), or both have been common. 

The ssDNA was prepared by treating a sample of calf thymus DNA of approximately 4 mg of dsDNA with 0.5 mL of 60% pure perchloric acid after dissolution, 0.5 mL of 9 M NaOH were immediately added to neutralize the solution followed by 9 mL of pH 4.5 acetate buffer. The DNA-modified glassy carbon electrode was prepared by covering a glassy carbon electrode with 3 mg of dsDNA dissolved in 80 jjlL of pH 4.5 acetate buffer and leaving the electrode to dry. The DNA layer was 0.5 mm thick and... 

Titanium trichloride fimctions as an excellent reductive Nef alternative reagent. This aqueous reagent is very acidic, so that acid sensitive groups such as ketals and esters do not survive unless an acetate buffer is used. Systems prcme to acid-catalyzed rearrangements may then successfully undeigo the reaction (equation 10).Some veiy sensitive multifunctional compounds have been obtained using this modified Nef procedure (equation 11). A related process is the formation of 1,4-diketones via in situ generation of a nitronatc anion by the Lewis acid catalyzed addition of an enol silyl ether to a nitroalkene (equation 12). ... 

Two reactions used in steroid chemistry were modified by Bennett for histochemical use. Frozen sections of either unfixed or formalin-fixed tissue were used, with no differences reported in their reactivity (see, however, Section V.2) the sections were 80 to 100 microns in thickness. In the first method, sections were treated with phenylhydrazine hydrochloride (1 %) in acetate buffer, pH 6 to 6.5, overnight. The formation of yellow phenylhydrazones indicated the presence of carbonyl groups. The pH of the solution was kept low enough to prevent extensive accumulation of the decomposition products of phenylhydrazine, which are yellow and soluble in lipid. In order to avoid reaction with ascorbic acid the sections were first oxidized briefly with iodine or indophenol. Since dehydro-ascorbic acid, which is formed by the oxidation of ascorbate, also forms phenylhydrazones, it is doubtful that this procedure had any value. However, since ascorbic acid and its oxidation product are soluble in most aqueous mixtures, they probably would not remain in sections as ordinarily treated. 

Fig. 2.11 Differential pulse voltanunetric responses recorded at polypyrrole/Congo red modified electrodes in solutions containing (a) 10.0 pM dopamine (constant) and various concentrations of ascorbic acid down to up) 4.0, 10.0, 20.0, 40.0 and 80.0 pM (b) 10.0 pM solutions of ascorbic acid (constant) and various concentrations of dopamine down to up) 1.0,7.0,15,35 and 70.0 pM. 0.1 M acetate buffer (pH 5.0) was the supporting electrolyte (Reproduced from Ref. [60] with the permission of Wiley)...

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