Organic phase buffer agents

Mobile phase options are quite restricted, as only volatile buffers are suitable for LC-MS. In addition, ion-pairing agents traditionally used in LC to improve peak shape and retention time such as trifluoroacetic acid (TFA), have shown to produce ion suppression, and they are not recommended for LC-MS analysis [21]. Therefore, although few applications for specific antidepressants employ TFA or its ammonium salt due to sensitivity enhancement [48, 81-85], aqueous phases in most LC-MS analytical methods are composed by formic or acetic acid in water, or its ammonium buffers. Although acid mobile phases are by far the most common, basic aqueous mobile phases (pH ranging from 8 to 10) have been used for specific applications in order to increase antidepressant retention time or to couple the online SPE elution to chromatographic analysis [57, 72, 86, 87], Organic phase composition was typically ACN and/or MeOH. 

The microemulsion of the MCBD system consists of (by weight) tetrachloroethane (7%), cetyltrimethylammonium chloride (28%), water (60%) and a small amount of tetrabuty-lammonium hydroxide as co-surfactant. The reactive decontaminants are FiClor (4%) and sodium 2-nitro-4-iodoxybenzoate (IBX, 0.1%) in sodium borate buffer. IBX is added as a nucleophilic catalyst for the hydrolysis of G-agents The borate buffer (pH 10) is essential for the maintenance of the catalytic activity of the IBX (see above and Scheme 15). IBX has little catalytic effect on the hydrolysis of VX In the phase-transfer system, hypochlorite ion is transferred into the organic phase by the phase-transfer catalyst, tetra-butylammonium chloride. 

Radiometric Assays Traditionally, sulfonated metabolites for many small chemicals have been detected with a radiometric assay that utilizes S -labeled PAPS (Anderson and Weinshilboum, 1980 Foldes and Meek, 1973). The reaction involves incubation of the substrate, cosubstrate, and enzyme in an appropriate buffer. The incubation is terminated by the addition of barium hydroxide, barium acetate, and zinc sulfate, which cause the unreacted PAPS to precipitate out. Thus, the unprecipitated radioactivity is associated with the sulfonated product and can be quantitated with liquid scintillation counting. A variation of this assay has been developed for larger molecules such as flavonoids, where the incubation is terminated by the addition of ethyl acetate under acidic pH conditions and in the presence of an ion-pairing agent, whereby the sulfonated product can then be detected in the organic phase upon liquid-liquid phase separation (Varin et al., 1987). 

The effects of various factors, such as the organic modifier, different ODS C18 columns and the concentration of the hydrox5rpropyl-j3-cyclodextrin, were investigated. The chiral mobile phase was composed of methanol or acetonitrile and 0.5% tilethylammonium acetate buffer at pH 3.0 added with 25 mmol l of hydrox5rpropyl-j3-cyclodextrin. Triethylammonium acetate is a volatile buffering agent. It is prepared from equimolar quantities of triethylamine and acetic acid. Baseline separations could be reached for all race-mates. 

Sample preparation To 1 ml of urine in a stoppered test tube 1.0 ml of 0.5 mol/1 carbonate buffer at pH 9.9 was added and shaken with 6.0 ml of toluene for 10 min. After centrifugation for 15 min, 4.0 ml of organic phase were transferred into another tube and evaporated to dryness in vacuum. The residue was dissolved in 1.0 ml of methylene chloride, then 100 mg of anhydrous sodium carbonate and 100 /ul of derivatizing agent solution were added. The samples were left to react overnight at room temperature. Then 50 /rl of methanol was added to stop the reaction and the solution was evaporated as described above. The residue was dissolved in 500 /ul of cyclohexane by shaking vigorously. 

To a solution of hexamethyldisilane (2.5 mmol) in HMPA (CAUTION— CANCER SUSPECT AGENT) (3 ml) at 0-5 °C was added methyl lithium (2.5 mmol, 1.5 m MeLi.LiBr complex in ether) dropwise. After being stirred for 3 min, the red solution was treated with Cul (2.5 mmol) in Me2S (1 ml), the resulting black reaction mixture was stirred for 3 min. and 2,3-dibromo-propene (1 mmol) was added rapidly via a syringe. The reaction mixture was allowed to warm to room temperature, and was stirred for 1.5 h. It was then poured into pentane (25 ml) and saturated ammonium chloride solution (25 ml, buffered to pH 8 by the addition of ammonium hydroxide), and the mixture was stirred vigorously for 1 h. The aqueous phase was re-extracted with pentane, and the combined organic extracts were dried. Removal of... 

Common surfactants that have been used in MEKC, are listed in Table 3.1 with the respective critical micelle concentrations the most popular are SDS, bile salts, and hydrophobic chain quaternary ammonium salts. Selectivity can also be modulated by the addition to the aqueous buffer of organic solvents (methanol, isopropanol, acetonitrile, tetrahydrofuran, up to a concentration of 50%). These agents will reduce the hydrophobic interactions between analytes and micelles in a way similar to reversed-phase chromatography. Organic modifiers also reduce the cohesion of the hydrophobic core of the micelles, increasing the mass transfer kinetics and, consequently, efficiency. Nonionic... 

---