RESUMO
Chromium oxyanions, Cr(x)O(y)H(z)(-), were generated in the gas-phase using a quadrupole ion trap secondary ion mass spectrometer (IT-SIMS), where they were reacted with O(2). Only CrO(2)(-) of the Cr(1)O(y)H(z)(-) envelope was observed to react with oxygen, producing primarily CrO(3)(-). The rate constant for the reaction of CrO(2)(-) with O(2) was approximately 38% of the Langevin collision constant at 310 K. CrO(3)(-), CrO(4)(-), and CrO(4)H(-) were unreactive with O(2) in the ion trap. In contrast, Cr(2)O(4)(-) was observed to react with O(2) producing CrO(3)(-) + CrO(3) via oxidative degradation at a rate that was approximately 15% efficient. The presence of background water facilitated the reaction of Cr(2)O(4)(-) + H(2)O to form Cr(2)O(5)H(2)(-); the hydrated product ion Cr(2)O(5)H(2)(-) reacted with O(2) to form Cr(2)O(6)(-) (with concurrent elimination of H(2)O) at a rate that was 6% efficient. Cr(2)O(5)(-) also reacted with O(2) to form Cr(2)O(7)(-) (4% efficient) and Cr(2)O(6)(-) + O (2% efficient); these reactions proceeded in parallel. By comparison, Cr(2)O(6)(-) was unreactive with O(2), and in fact, no further O(2) addition could be observed for any of the Cr(2)O(6)H(z)(-) anions. Generalizing, Cr(x)O(y)H(z)(-) species that have low coordinate, low oxidation state metal centers are susceptible to O(2) oxidation. However, when the metal coordination is >3, or when the formal oxidation state is > or =5, reactivity stops.
RESUMO
A tubular silicone membrane interface has been developed for trace detection of benzene, toluene, ethyl benzene, and xylene (BTEX) compounds in water with a portable ion mobility spectrometer. Effects of flow rate, membrane length and stirring conditions on the IMS signals have been systematically investigated. Besides conventional dynamic mode operation, static mode sampling has been demonstrated for the first time and high sensitivities were achieved by sampling of BTEX contaminated water with static mode operation. A toluene concentration of 0.101 mg l(-1) in purified water, corresponding to a headspace concentration of 2.75 (mug m(-3)), was determined by static mode sampling. Headspace sampling without the membrane interface could not detect toluene at this concentration. This method has high sensitivity for trace concentrations of gasoline components in river water with a response time of several seconds. The apparatus developed is portable and can be used for sensitive detection of organic contaminants in water, with improved performance compared to conventional modes of IMS sampling.
RESUMO
A novel neural network has been devised that combines the advantages of cascade correlation and computational temperature constraints. The combination of advantages yields a nonlinear calibration method that is easier to use, stable, and faster than back-propagation networks. Cascade correlation networks adjust only a single unit at a time, so they train very rapidly when compared to back-propagation networks. Cascade correlation networks determine their topology during training. In addition, the hidden units are not readjusted once they have been trained, so these networks are capable of incremental learning and caching. With the cascade architecture, temperature may be optimized for each hidden unit. Computational temperature is a parameter that controls the fuzziness of a hidden unit's output. The magnitude of the change in covariance with respect to temperature is maximized. This criterion avoids local minima, forces the hidden units to model larger variances in the data, and generates hidden units that furnish fuzzy logic. As a result, models built using temperature-constrained cascade correlation networks are better at interpolation or generalization of the design points. These properties are demonstrated for exemplary linear interpolations, a nonlinear interpolation, and chemical data sets for which the numbers of chlorine atoms in polychlorinated biphenyl molecules are predicted from mass spectra.
