ascorbic-acid and 18-crown-6

Double-barrel wire-in-a-capillary electrospray emitters prepared from theta-glass capillaries were used to mix solutions during the electrospray process. The relative flow rate of each barrel was continuously monitored with internal standards. The complexation reaction of 18-crown-6 and K(+), introduced from opposite barrels, reaches equilibrium during the electrospray process, suggesting that complete mixing also occurs. A simplified diffusion model suggests that mixing occurs in less than a millisecond, and contributions of turbulence, estimated from times of coalescing ballistic microdroplets, suggest that complete mixing occurs within a few microseconds. This mixing time is 2 orders of magnitude less than in any mixer previously coupled to a mass spectrometer. The reduction of 2,6-dichloroindophenol by l-ascorbic acid was performed using the theta-glass emitters and monitored using mass spectrometry. On the basis of the rate constant of this reaction in bulk solution, an apparent reaction time of 274 ± 60 μs was obtained. This reaction time is an upper limit to the droplet lifetime because the surface area to volume ratio and the concentration of reagents increase as the droplet evaporates and some product formation occurs in the Taylor cone prior to droplet formation. On the basis of increases in reaction rates measured by others in droplets compared to rates in bulk solution, the true droplet lifetime is likely 1-3 orders of magnitude less than the upper limit, i.e., between 27 μs and 270 ns. The rapid mixing and short droplet lifetime achieved in these experiments should enable the monitoring of many different fast reactions using mass spectrometry.

Topics: 2,6-Dichloroindophenol; Ascorbic Acid; Crown Ethers; Enkephalins; Glass; Oxidation-Reduction; Potassium; Spectrometry, Mass, Electrospray Ionization

A piezoelectric microgravimetry (PM) chemosensor, featuring a film of molecularly imprinted polymer (MIP) of poly[bis(2,2'-bithienyl)methane] bearing either a 3,4-dihydroxyphenyl or benzo-18-crown-6 substituent, for selective determination of dopamine was devised and tested. A Pt/quartz resonator and a dopamine-templated MIP film, deposited by electropolymerization onto an underlayer of poly(bithiophene), served as the transducer and recognition element of the chemosensor, respectively. The UV-vis spectroscopic and XPS as well as electrochemical measurements verified completeness of the dopamine template extraction with a strong base solution. The extraction-generated molecular cavities featured recognition sites that served selective dopamine analyte binding. The SECM imaging substantiated the permeability characteristics of the template-free MIP film. The dopamine analyte was determined under FIA conditions with the PM detection. The lower limit of detection was 10nM dopamine at favorable conditions involving the 35 μL/min carrier solution flow rate and the injected sample volume of 1 mL. The sensitivity of the chemosensor increased almost fivefold when the poly(bithiophene) film coated Pt/quartz electrode was used instead of the bare Pt/quartz electrode as the substrate for deposition of the MIP film. The chemosensor successfully discriminated dopamine from structural and functional analogues, such as 2-phenylethylamine, histamine, and ascorbic acid. The optimum mean thickness of the MIP film was ∼220 nm.

Topics: Ascorbic Acid; Biosensing Techniques; Crown Ethers; Dopamine; Electrochemistry; Electrodes; Histamine; Methane; Molecular Imprinting; Phenethylamines; Photoelectron Spectroscopy; Platinum; Polymers; Quartz; Spectrophotometry, Ultraviolet; Thiophenes; Transducers

The superoxide-mediated base catalyzed autoxidation of alpha-oxo enols is initiated by the deprotonation of the labile hydroxyl group. Thus, the reaction of O2-. (generated from KO2/crown ether in aprotic media) with 3-hydroxycoumarin (1), followed by a CH3I-workup, generates products 2-4 via a deprotonation-oxidation sequence complicated by a competing saponification of the lactone linkage. The related coumarin reductone (alpha-oxo enediol) 8 is rapidly oxidized by O2-., HO- and t-butoxide to the corresponding triketone, which in turn undergoes further oxidation and rearrangement ultimately yielding (upon methyl iodide workup) products 9-14. When the O2-. mediated oxidation is carried out under argon in completely degassed solutions, large amounts (greater than 20%) of monodeprotonation product (detected as 9) accumulate. These results are discussed in light of the differing mechanisms proposed by Sawyer and Afanas'ev for the interaction of O2-. with the reductone ascorbic acid.

Topics: Alcohols; Ascorbic Acid; Coumarins; Crown Ethers; Ethers, Cyclic; Free Radicals; Ketones; Models, Chemical; Oxidation-Reduction; Superoxides