Matrix-induced peak enhancement of pesticides in gas chromatogrtaphy: Is there a solution?

In this study, it is shown that calibration solution prepared in control matrix extrix extract can be used to compensate for matrix-induced chromatographic response enhancement observed for certain pesticedes. This phenomenon is characterized by enhanced chromatographic response for certain pesticides in the presence of matrix du to reduced analyte loss during injection. Unacceptably high recoveries are seen for affected pesticides when maatrix-free solutions are used for reference and calibration. The effects of matrix concentration on overall enhancement and linear response were determined. When used as reference standards for the determination of recovery, matrix-standard solutions were found to provide acceptable recoveries for pesticides subject to matrix-induced chromatographic response enhancement along with other pesticides. General guidelines are presented for using matrix-standard calibration solutions in pesticide residue analysis.     

Preparation of crown compounds containing allyloxymethyl or butenyl groups for attachment to silica gel or containing long chain lipophilic groups for use in liquid membrane systems

Several new macrocyclic polyether ligands have been prepared for use in the separation of metal ions from aqueous solutions. Four of the crown ethers reported contain 1,2,4-triazole or 4-pyridone protonionizable subcyclic units and lipophilic groups. The remaining crown ethers are not proton-ionizable but contain alkene groups and were prepared for attachment to silica gel. The crowns were prepared by reacting the appropriate glycols with the appropriate ditosylates or dichloride in the case of the 1,2,4-triazole subcyclic unit. The crowns with proton-ionizable and lipophilic substituents were tested in liquid membrane transport systems and some of the crowns with alloxymethyl or butenyl substituents were attached to silica gel. The log K values for the interaction of these silica gel-bound macrocycles with certain metal ions were nearly the same (± 10%) as those for the association of the unbound macrocycles with the same metal ions.     

Selective determination of iron by fluorescence quenching of a naturally occurring pigment

A fluorescent pigment produced by Pseudomonas fluorescens is isolated and partially characterized. It is verified that the pigment reacts with iron(III) and iron(II0 with respective conditional formation constants of 1.1 × 10²⁴ and 5.3 × 10²³ at pH 8 and that the reaction results in quenching of the pigment fluorescence. The quenching effect permits the measurement of iron with a detection limit of 5 × 10^?9 M (0.3 μg 1^?1). Examination of 21 common constituents in water as potential interferents for the iron measurement indicates that a combination buffer solution can be used to mask those that deleteriously affect the measurement. The general accuracy of the procedure is confirmed with reference water samples.     

Assessment of ion size effects in the diffuse double layer with use of an integral equation approach

An analytical expression is developed for the potential drop across the diffuse layer phi(d) in terms of a cubic polynomial in the corresponding estimate in the Gouy-Chapman approximation, phi(d)(GC). The coefficients of this polynomial are defined in terms of the MSA volume fraction eta and the reciprocal distance parameter Gamma. The resulting expression is shown to describe the Monte Carlo estimates of phi(d) obtained in a primitive level simulation of diffuse layer properties.     

Empirical regularities of atomic ionization potentials

Regularities of the averaged ionization potentials for atoms and ions containing up to 18 electrons are studied in detail. It is shown that a two-variable function constructed from the averaged ionization potentials for each subshell is linear with respect to the degree of ionization q and the occupancy k of the s^k or p^k subshell. One linearity includes previous findings as a special case, and the other introduces a new regularity for atomic ionization potentials. Existing atomic ionization potentials and electron affinities are analyzed employing the regularities, and improved values of these quantities as well as term and fine structure separations in negative ions are derived.     

X=Y-ZH systems as potential 1,3-dipoles. Part 61: Metal exchanged zeolites, silver(I) oxide, Ni(II) and Cu(I) complexes as catalysts for 1,3-dipolar cycloaddition reactions of imines generating proline derivatives

A range of regio- and stereo-selective 1,3-dipolar reactions of imines of α-amino esters, generating polysubstituted prolines, catalysed by silver(I) exchanged zeolites or silver(I) supported on titania, both in combination with DBU, are described. The use of a catalytic amount of silver(I) oxide, Ni(II) complexes and cuprous iodide as catalysts for the cycloaddition reactions are also disclosed.     

Spectroscopic and computational studies of the de novo designed protein DF2t: correlation to the biferrous active site of ribonucleotide reductase and factors that affect O2 reactivity

DF2t, a de novo designed protein that mimics the active-site structure of many non-heme biferrous enzymes, has been studied using a combination of circular dichroism (CD), magnetic circular dichroism (MCD), and variable-temperature variable-field (VTVH) MCD. The active site of DF2t is found to have one five-coordinate iron and one four-coordinate iron, which are weakly antiferromagnetically coupled through a mu-1,3 carboxylate bridge. These results bear a strong resemblance to the spectra of Escherichia coli ribonucleotide reductase (R2), and density functional theory calculations were conducted on the W48F/D84E R2 mutant in order to determine the energetics of formation of a monodentate end-on-bound O2 to one iron in the binuclear site. The mu-1,3 carboxylate bridges found in O2-activating enzymes lack efficient superexchange pathways for the second electron transfer (i.e., the OH/oxo bridge in hemerythrin), and simulations of the binding of O2 in a monodentate end-on manner revealed that the bridging carboxylate ligands do not appear capable of transferring an electron to O2 from the remote Fe. Comparison of the results from previous studies of the mu-1,2 biferric-peroxo structure, which bridges both irons, finds that the end-on superoxide mixed-valent species is considerably higher in energy than the bridging peroxo-diferric species. Thus, one of the differences between O2-activating and O2-binding proteins appears to be the ability of O2 to bridge both Fe centers to generate a peroxo intermediate capable of further reactivity.     

Spectroscopic and electronic structure studies of protocatechuate 3,4-dioxygenase: nature of tyrosinate-Fe(III) bonds and their contribution to reactivity.

The geometric and electronic structure of the high-spin ferric active site of protocatechuate 3,4-dioxygenase (3,4-PCD) has been examined by absorption (Abs), circular dichroism (CD), magnetic CD (MCD), and variable-temperature-variable-field (VTVH) MCD spectroscopies. Density functional (DFT) and INDO/S-CI molecular orbital calculations provide complementary insight into the electronic structure of 3,4-PCD and allow an experimentally calibrated bonding scheme to be developed. Abs, CD, and MCD indicate that there are at least seven transitions below 35 000 cm(-1) which arise from tyrosinate ligand-to-metal-charge transfer (LMCT) transitions. VTVH MCD spectroscopy gives the polarizations of these LMCT bands in the principal axis system of the D-tensor, which is oriented relative to the molecular structure from the INDO/S-CI calculations. Three transitions are associated with the equatorial tyrosinate and four with the axial tyrosinate. This large number of transitions per tyrosinate is due to the pi and importantly the sigma overlap of the two tyrosinate valence orbitals with the metal d orbitals and is governed by the Fe-O-C angle and the Fe-O-C-C dihedral angles. The previously reported crystal structure indicates that the Fe-O-C angles are 133 degrees and 148 degrees for the equatorial and axial tyrosinate, respectively. Each tyrosinate has transitions at different energies with different intensities, which correlate with differences in geometry that reflect pseudo-sigma bonding to the Fe(III) and relate to reactivity. These factors reflect the metal-ligand bond strength and indicate that the axial tyrosinate-Fe(III) bond is weaker than the equatorial tyrosinate-Fe(III) bond. Furthermore, it is found that the differences in geometry, and hence electronic structure, are imposed by the protein. The consequences to catalysis are significant because the axial tyrosinate has been shown to dissociate upon substrate binding and the equatorial tyrosinate in the enzyme-substrate complex is thought to influence asymmetric binding of the chelated substrate moiety via a strong trans influence which activates the substrate for reaction with O2.     

Determination of vanadium in biological tissue by anion exchange chromatography and neutron activation analysis

A method is described for determination of vanadium in biological tissues. This method consists of a wet digestion of the tissue with nitric acid followed by anion exchange chromatography, neutron irradiation, and radioassay. The chromatographic separation will allow the decontamination of vanadium from radioactivatable sodium and chlorine which are present in large quantities in biological tissues. The validity of the method is evaluated by employing NBS-SRM 157 and 157a Bovine Liver employing the method of standard additions. The method is successfully applied to human, cow and rat liver specimens. The detection limit of the method is one ppb.     

Data evaluation in combined gas chromatography—infrared spectroscopy—mass spectrometry

The technique of gas chromatography—infrared spectroscopy—mass spectrometry (g.c./i.r./m.s.) has great potential m qualitative and quantitative analysis. The utility of g.c./i.r./m.s. is presently limited by the lack of methods for reduction of the large amounts of data produced from complex mixtures. Approaches to qualitative and quantitative data evaluation which take into account differences in instrumental configuration are presented.