Characterization of reaction products formed during thin-layer chromatographic detection of thiocarbamate herbicides

The reaction between thiocarbamate herbicides and 2,6-dichlorobenzoquinone-N-chloroimine or 2,6-dibromobenzoquinone-N-chloroimine is suitable for the detection of these herbicides on thin-layer plates with high sensitivity. The reactions were followed by infrared, nuclear magnetic resonance and mass spectrometry. We have established the formation of 2,6-dichlorobenzoquinone-S-alkyl sulphenylimines. In the case of the bromo-derivative, halogen exchange and substitution on the quinone ring took place simultaneously leading to the formation of mixed halogenated 2,6-dihalo- and, in addition, 2,3,6-trihalobenzoquinone-S-alkyl sulphenylimines. The final product of the detection reaction, i.e. 2,6-dichlorobenzoquinone-S-alkyl sulphenylimine was reacted with 2,6-dibromobenzoquinone-N-chloroimine where 2,6-dichloro-3-bromobenzoquinone-S-alkyl sulphenylimine formed as a consequence of the looser bromine-carbon linkage on the 2 and 6 positions of the quinone ring.     

Determination of gallium by anodic stripping square-wave voltammetry Detection of a GaZn intermetallic compound and its destruction by Sb in an NaSCNNaClO(4) supporting electrolyte

Reproducibility for successive determinations with a hanging mercury drop electrode is assessed in relation to solution stirring, drop size and back-diffusion to the mercury thread. The effect of experimental parameters such as drop size, deposition time and gallium concentration on the observed stripping current is investigated. The interference of zinc present in a 0.5M NaSCN + 4.2M NaClO(4) supporting electrolyte on the gallium detection limit and calibration plots is described. Formation of an intermetallic compound with a Zn:Ga ratio of 2:3 and its destruction by co-deposition of zinc with Sb are reported. A detection limit of 10(-8)M gallium was obtained in the presence of 10(-5)M Sb(III).     

Correction of T2 distortion in multi-excitation RARE sequence

Correction schemes have been implemented to correct for T2 distortions in a multiexcitation RARE (rapid acquisition with relaxation enhancement) sequence where data from multiple echoes and multiple excitations are combined. Computer simulation studies and human imaging studies have been conducted to develop and test the correction procedures. A direct method and an iterative technique have been investigated. The direct technique utilizes Hermitian symmetry of the T2 weighted data and is shown to reduce distortions in T2 weighted images. The iterative scheme begins with an estimation of T2, wherefrom k-space data are computed and compared to the true data to provide error images. The error images are then used to refine iteratively the reconstructed images at a specified echo time. The iterative procedure has been used to improve T1 weighted images acquired through a sequence based on acquisition of two half-plane Fourier samples. These correction techniques should enable a practical implementation of RARE for producing T1 and T2 weighted images comparable to standard spin echo images.     

Vector-extrapolated fast maximum likelihood estimation algorithms for emission tomography

A new class of fast maximum-likelihood estimation (MLE) algorithms for emission computed tomography (ECT) is developed. In these cyclic iterative algorithms, vector extrapolation techniques are integrated with the iterations in gradient-based MLE algorithms, with the objective of accelerating the convergence of the base iterations. This results in a substantial reduction in the effective number of base iterations required for obtaining an emission density estimate of specified quality. The mathematical theory behind the minimal polynomial and reduced rank vector extrapolation techniques, in the context of emission tomography, is presented. These extrapolation techniques are implemented in a positron emission tomography system. The new algorithms are evaluated using computer experiments, with measurements taken from simulated phantoms. It is shown that, with minimal additional computations, the proposed approach results in substantial improvement in reconstruction.