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At least 955 records · Page 53Linked to original sources
PubMed2004

Retention behaviour of unsaturated fatty acid methyl esters on porous graphitic carbon.

The eluotropic strength of binary mobile phases was calculated for three homologous series of cis, trans, and cis-cis unsaturated fatty acid methyl esters (FAMEs). Binary mobile phases with chloroform, dichloromethane, or tetrahydrofuran as strong solvent and methanol or acetonitrile as weak solvent were tested. The volume fraction of strong solvent in the binary phases was between 0.3 and 0.8. Curves of eluotropic strength versus volume fraction of strong solvents showed similar trends to previously published results for saturated homologues. Correlation coefficients of the plots of eluotropic strength values for saturated versus unsaturated FAMEs were close to 1.0. Therefore these similarities validate the model of eluotropic strength previously established with saturated FAMEs as relevant for unsaturated FAMEs. The separation factors between cis and trans homologues always showed elution of the cis before the trans homologue. The difference in retention is due primarily to the geometry of the molecule. The retention is lowered more by the addition of a first carbon double bond than by the addition of a second one, independently of the mobile phase composition.

Journal Article↗
PubMed1995

Unrecognized retention of intraorbital graphite pencil fragments: the role of computerized tomography.

Children with unrecognized intraorbital pencil fragments may come to attention because of surrounding abscess and granuloma formation, after a long delay, and may be suspected to have an intraorbital neoplasm. Two such patients are reported, with emphasis on CT findings. Recognition of the CT manifestations of intraorbital pencil fragments may allow a more conservative surgical intervention than that for presumed intraorbital neoplasm.

Child↗
PubMed2002

Separation and preconcentration of Se(IV)/Se(VI) species by selective adsorption onto nanometer-sized titanium dioxide and determination by graphite furnace atomic absorption spectrometry.

A simple and sensitive method for the selective determination of Se(IV) and Se(VI) in natural water and sludge samples through an adsorptive process on a nanometer-sized TiO(2) (anatase) was developed. The conditions for quantitative and reproducible preconcentration, elution, and subsequent GFAAS determination were established. The proposed method gave a concentration factor of 50 for a 100 mL sample volume, characterized by high precision, high reproducibility, and direct determination of Se(IV)/Se(VI). The detection limits (3 sigma, n=11) were 4.7ng L(-1) for Se(IV) and 6.3ng L(-1) for Se(VI); the precision (relative standard deviation) was 0.7% for Se(IV) and 0.9% for Se(VI) at the 0.5microg L(-1) level.

Adsorption↗
PubMed2005

Evaluation of a generalized regression artificial neural network for extending cadmium's working calibration range in graphite furnace atomic absorption spectrometry.

A generalized regression artificial neural network (GRANN) was developed and evaluated for modeling cadmium's nonlinear calibration curve in order to extend its upper concentration limit from 4.0 microg L-1 up to 22.0 microg L-1. This type of neural network presents important advantages over the more popular backpropagation counterpart which are worth exploiting in analytical applications, namely, (1) a smaller number of variables have to be optimized, with the subsequent reduction in "development hassle"; and, (2) shorter development times, thanks to the fact that the adjustment of the weights (the artificial synapses) is a non-iterative, one-pass process. A backpropagation artificial neural network (BPANN), a second-order polynomial, and some less frequently employed polynomial and exponential functions (e.g., Gaussian, Lorentzian, and Boltzmann), were also evaluated for comparison purposes. The quality of the fit of the various models, assessed by calculating the root mean square of the percentage deviations, was as follows: GRANN>Boltzmann>second-order polynomial>BPANN>Gauss>Lorentz. The accuracy and precision of the models were further estimated through the determination of cadmium in the certified reference material "Trace Metals in Drinking Water" (High Purity Standards, Lot No. 490915), which has a cadmium certified concentration (12.00+/-0.06 microg L-1) that lies in the nonlinear regime of the calibration curve. Only the models generated by the GRANN and BPANN accurately predicted the concentrations of a series of solutions, prepared by serial dilution of the CRM, with cadmium concentrations below and above the maximum linear calibration limit (4.0 microg L-1). Extension of the working range by using the proposed methodology represents an attractive alternative from the analytical point of view, since it results in less specimen manipulation and consequently reduced contamination risks without compromising either the accuracy or the precision of the analyses. The implementation of artificial neural networks also helps to reduce the trial-and-error task of looking for the right mathematical model from among the many possibilities currently available in the various scientific and statistic software packages.

Journal Article↗
PubMed2006

Determination of arsenic in diesel, gasoline and naphtha by graphite furnace atomic absorption spectrometry using microemulsion medium for sample stabilization.

A procedure for the determination of As in diesel, gasoline and naphtha at microg L(-1) levels by GFAAS is proposed. Sample stabilization was achieved by the formation of three component solutions prepared by mixing appropriate volumes of the samples propan-1-ol and nitric acid aqueous solution. This mixture resulted in a one-phase medium, which was indefinitely stable. No changes in the analyte signals were observed over several days in spiked samples, proving long-term stabilization ability. The use of conventional (Pd) and permanent (Ir) modification was investigated and the former was preferred. Central composite design multivariate optimization defined the optimum microemulsion composition as well as the temperature program. In this way, calibration using aqueous analytical solutions was possible, since the same sensitivity was observed in the investigated microemulsion media and in 0.2% v/v HNO(3). Coefficients of correlation larger than 0.999 and an As characteristic mass of 22 pg were observed. Recoveries (n=4) obtained from spiked samples were 98+/-4, 99+/-3 and 103+/-5%, and the limits of detection in the original samples were 1.8, 1.2 and 1.5 microg L(-1) for diesel, gasoline and naphtha, respectively. Validation was performed by the analysis of a set of commercial samples by independent comparative procedures. No significant difference (Student's t-test, p<0.05) was observed between comparative and proposed procedure results. The total determination cycle lasted 4 min for diesel and 3 min for gasoline and naphtha, equivalent to a sample throughput of 7 h(-1) for diesel and 10 h(-1) for gasoline and naphtha.

Journal Article↗