Amperometric probe for 3-hydroxybutyrate with immobilized 3-hydroxybutyrate dehydrogenase

The enzyme 3-hydroxybutyrate dehydrogenase is immobilized on a graphite electrode suitable for determination of NADH. Of the several procedures tested for the immobilization, direct adsorption on the electrode surface was most satisfactory, with useful lifetimes of up to 3 days. The best calibration graphs for the NADH, and for 3-hydroxybutyrate were obtained at pH 7.5 (phosphate buffer). 3-Hydroxybutyrate was determined in the range 5–100 μmol l^?1 at pH 7.5 with good precision. Interferences are discussed.     

Chromatographic and electrophoretic migration parameters in capillary electrochromatography

Chromatographic retention factor, k', as defined in high-performance liquid chromatography (HPLC) in terms of the migration times of the separand and the inert tracer, has limited applicability to capillary electrochromatography (CEC) when both chromatographic and electrophoretic processes determine the magnitude of the overall migration rates of the separands. This situation is unlike that in HPLC, where k' serves as a useful peak locator for the various sample components, as well as, provides thermodynamic insights into the interactions between the components and the stationary phase. Most publications have borrowed the definition of k' from HPLC and applied it on CEC. However, due to the dual separation mechanisms that are in action in CEC, the system is significantly complicated in comparison to that of HPLC. This paper discusses the impossibility of defining with a k' which would have all the attributes that it has in regular chromatography. The interplay of the two separation mechanisms in determining the overall migration process in CEC is discussed and the various definitions of the electrochromatographic retention factor are presented.     

Migration behavior of weakly retained, charged analytes in voltage-assisted micro-high performance liquid chromatography

The application of voltage in micro-high performance liquid chromatography (micro-HPLC) creates a system where separation is governed by a hybrid differential migration process, which entails the features of both HPLC and capillary zone electrophoresis (CZE), i.e., chromatographic retention and electrophoretic migration. In this paper, we use our previously published approach to decouple these two mechanisms via analysis of the input data for estimation of electrokinetic parameters, such as conductivity, equivalent lengths, mobilities and velocities. Separation of weakly retained, charged analytes was performed via voltage-assisted micro-HPLC. Contrary to conclusions from data analysis using the conventional definitions of the retention factor, it is shown that our approach allows us to isolate the "chromatographic retention" component and thus, investigate the "modification" of the retention process upon application of voltage in micro-HPLC. It is shown that the traditional approaches of calculating retention factor would erroneously lead to conclusion that the retention behavior of these analytes changes upon application of voltage. However, the approach suggested here demonstrates that under the conditions investigated, most of the charged analytes do not show any significant retention on the columns and that all the changes in their retention times can be attributed to their electrophoretic migration.     

Use of electrokinetic measurements for characterization of columns used in capillary electrochromatography

The separation mechanism in capillary electrochromatography (CEC) is a hybrid differential migration process, which entails the features of both high-performance liquid chromatography (HPLC) and capillary zone electrophoresis (CZE), i.e., chromatographic retention and electrophoretic migration. The focus of this paper is on the use of electrokinetic data, such as current, electroosmotic flow (EOF) and column efficiency measurements, that are readily available, for an improved understanding of CEC separations. A framework is presented here for the use of this data for evaluation of a variety of performance parameters including, conductivity ratio, interstitial EOF mobility, porosity, and zeta potential. This framework is applied for characterization of two monolithic columns with different chemistry that were manufactured in-house. The above-mentioned performance parameters were calculated for the two columns and it is found that the poly(VBC-EGDMA-SWNT) monolithic column with the GPTMS-PEI coating offers a significantly improved flow distribution in comparison to the poly(VBC-EGDMA) monolithic column. This observation is confirmed by performing separation of peptides on the two columns and height equivalent of a theoretical plate (HETP) measurements on the resulting peaks. It is shown that following our approach leads to an improved understanding of the separations achieved with the columns and to better column design.     

Joule heating and determination of temperature in capillary electrophoresis and capillary electrochromatography columns

This article reviews the progress that has taken place in the past decade on the topic of estimation of Joule heating and temperature inside an open or packed capillary in electro-driven separation techniques of capillary electrophoresis (CE) and capillary electrochromatography (CEC), respectively. Developments in theoretical modeling of the heat transfer in the capillary systems have focused on attempts to apply the existing models on newer techniques such as CEC and chip-based CE. However, the advent of novel analytical tools such as pulsed magnetic field gradient nuclear magnetic resonance (NMR), NMR thermometry, and Raman spectroscopy, have led to a revolution in the area of experimental estimation of Joule heating and temperature inside the capillary via the various noninvasive techniques. This review attempts to capture the major findings that have been reported in the past decade.     

Influence of unsaturated aliphatic and aromatic volatile organic compounds on the oxidation of aqueous sulfur dioxide by oxygen in aqueous medium

The trace atmospheric volatile organic compounds(VOCs) are reported to influence the oxidation of dissolved sulfur dioxide, S(IV), by oxygen, which is an important reaction in atmospheric acid formation. In this work, the influence of 15 VOCs on uncatalyzed oxidation of S(IV) by oxygen has been investigated by following the disappearance of [O₂]. The inhibition of oxidation reaction by toluene, aniline, benzene, cinnamic acid, hydroquinone, acrylamide, acrylonitrile and allyl alcohol was defined by the rate law(A):k_VOC = k₀/(1+B [Inh]) (A)where, k_VOC, and k₀ are the first order rate constants in the presence and the absence of VOCs respectively. B is the inhibition parameter and [Inh] is the concentration of inhibitor.On the other hand, fumaric, maleic and crotonic acids, which have conjugated CC bonds, catalyze the reaction defined by the rate law(B):k_VOC = k₀ + k_cat [VOC] (B)where k_cat is catalytic constant and [VOC] is the concentration of catalytic VOC.Interestingly, presence of 1-hexene, cyclohexene, methacrylic acid, methyl acrylate was without any significant effect on the rate of reaction.     

A new spot test for the detection of traces of citric and tartaric acids in vinegar

A sensitive, new color reaction for the detection of citric and tartaric acids in vinegar has been developed. Acetic anhydride in the presence of trimethylamine has been used as the reagent. Traces of citric (2 μg) and tartaric acid (5 μg) can be detected in concentrated acetic and formic acids.     

Intramolecular C-H/C-D exchange in cofacially stacked polyfluorenes via electron-induced bond activation

EPR studies in hexamethylphosphoramide, along with DFT studies, reveal that the one-electron reduction of pi-stacked polyfluorenes containing two, three, and four fluorene units leads to the corresponding anion radicals where the odd electron is located exclusively on an outside fluorene moiety. These anion radicals disproportionate to form small concentrations of diamagnetic dianions, wherein there is a nonclassical pz overlap that crosses the C2 axis. The presence of the two extra electrons activates the C-H bonds in the ortho positions, and concomitant overlap of the p-orbitals involving adjacent fluorene moieties results in intramolecular hydrogen exchange at temperatures as low as 90 K.     

Studies on chromium(III) hydroxide, arsenate, antimonate, molybdate and tungstate

Five water-insoluble compounds of chromium(III) have been synthesized by mixing 0.10M solutions of chromic chloride and the appropriate sodium salts in the volume ratio of 1:2, at pH 6-7. Their ion-exchange characteristics have been compared. The tungstate has the highest chemical stability and the arsenate has the highest ion-exchange capacity. Chromium(III) molybdate columns have been used to separate Pb(2+) from numerous metal ions.     

Stable dimeric aromatic cation-radicals. Structural and spectral characterization of through-space charge delocalization

The spontaneous assembly of aromatic cation-radicals (D(+)(*)()) with the parent donor (D) to afford the paramagnetic dimer (D)(2)(+)(*)() is accompanied by a dramatic color change. For example, spectral (UV-vis and ESR) and X-ray crystal structure analyses establish the molecular association of octamethylbiphenylene cation-radical with its neutral counterpart to produce the mixed-valence or dimeric cation-radical in which the positive charge is completely delocalized over both aromatic moieties. The use of the sterically hindered cation-radicals confirms the new spectral or charge-resonance (CR) band to result in dimeric cation-radicals in which the intermolecular separation occurs at an optimum distance allowed by van der Waals contacts. The striking similarities between the classical donor/acceptor (EDA) complexes and the dimeric cation-radicals (D)(2)(+)(*)() (both in terms of the geometrical requirement as well as the appearance of new absorption bands) suggest that the latter can be considered as particular examples of Mulliken's charge-transfer complexes in which the positive charge is completely (equally) delocalized over both donor (D) and acceptor (D(+)(*)()).