Heating caused by radiofrequency irradiation and sample rotation in 13C magic angle spinning NMR studies of lipid membranes

Application of rapid sample rotation and radiofrequency irradiation in magic angle spinning (MAS) NMR of lipid bilayers can significantly increase the sample temperature. In this work, we studied the extent of heating during the acquisition of 1H-decoupled 13C MAS spectra of hydrated dimyristoylphosphatidylcholine (DMPC) in the L(alpha) phase. First, we describe a simple procedure for determining the increase in temperature by observing the shift of the 1H water signal. The method is then used to identify and assess the various factors that contribute to the sample heating. The important factors discussed in this paper include: (i) the spinning speed, (ii) the variable-temperature gas pressure, (iii) the rotor geometry, (iv) the power, duration and frequency of the radiofrequency irradiation and (v) the hydration level. A comparison of different heteronuclear decoupling schemes in terms of their ability to produce highly resolved 13C spectra of DMPC is also reported.     

Determination of creatine kinase activity using a co-immobilized auxiliary enzyme reactor coupled on-line with a flow injection system

Two flow injection methods (based on spectrophotometric and spectrofluorimetric detection) were developed for the determination of over-all creatine kinase activity. Despite the complexity of the reactions involved (both include three enzyme-catalysed steps), the manifold is very simple because the two auxiliary enzymes which catalyse the two-step indicator reaction are co-immobilized on controlled-pore glass. The features of the proposed methods (calibration ranges between 0.1 and 2.0 and 0.01 and 1.0 U l-1, relative standard deviation 0.93 and 0.53% for the spectrophotometric and spectrofluorimetric methods, respectively) allow the successful determination of the analyte activity in serum samples (recoveries better than 95-105% for both methods).     

Critical parameters from power series expansions

A simple and quite general method is developed for calculating critical para meters from power series expansions. The series coefficients of the problem function are introduced into a properly parametrized recurrence relationship. As a result, sequences are obtained that converge towards the critical parameters characterizing the closest singularity to the origin. The low-temperature series for the thermodynamic functions of spin-1/2 Ising models are discussed. Results are shown for the interfering, nonphysical singularities of the face-centred cubic and body-centred cubic lattices. The determination of these singularities would allow their factorization previous to the study of the physically important ones.     

Simultaneous flow-injection flourimetric determination of ammonia and hydrazine with a novel mode of forming pH gradients

A flow injection configuration is suggested for the simultaneous determination of ammonia and hydrazine at the μg ml^?1 level by formation of zones of different pH. The analytes react with o-phthalaldehyde and mercaptoethanol to form fluorescent derivatives at different pH values. In addition to the normal flow method, a stopped flow method is proposed to increase the ranges that can be quantified. The analysis of samples containing hydrazine and ammonia in ratios between 0.3 and 70.0 is described.     

New methods for acceleration of meat sample preparation prior to determination of the metal content by atomic absorption spectrometry

Focused microwave-assisted digestion and ultrasound leaching have been applied for the extraction of Pb, Cd, Cr, Cu, Fe, Zn, Ca, and Mg from raw meat. Semimembranous muscle (SM) of raw pig ham was used for optimizing both the digestion and extraction steps by multivariate approaches. The detection and quantification limits were 0.5 and 0.9 g kg^–1 for Pb, 0.06 and 0.1 g kg^–1 for Cd, 0.2 and 1.2 g kg^–1 for Cr, 0.4 and 3 g kg^–1 for Cu, 0.04 and 0.1 mg kg^–1 for Fe, 0.012 and 0.017 mg kg^–1 for Zn, 0.3 and 0.4 mg kg^–1 for Ca, and 0.01 and 0.03 mg kg^–1 for Mg. The precision, expressed as relative standard deviation (RSD), ranged between 2.5 and 9.6% for focused microwave-assisted digestion and between 3.5 and 10.6% for ultrasound leaching. The methods were then compared with a reference method and applied to a certified reference material (bovine muscle 184, from the BCR). The t-test, applied to the results obtained from focused microwave-assisted digestion, revealed that they are in agreement (p>0.01) with the certified and estimated values in the case of Pb, Cd, Cr, Cu, Fe, Ca, Mg, and Zn but not in that of Fe. In the case of ultrasound leaching, only the extraction of Pb, Cu, and Ca was quantitative. The method based on microwave digestion provides more accurate and precise results than ultrasound leaching. These new procedures have many advantages with regards to conventional methods, namely, reduction of the extraction time, simplification of the process, avoidance of chemical emissions to the atmosphere, and no losses of metals by volatilization.     

Identification of drying oils used in pictorial works of art by liquid chromatography of the 2-nitrophenylhydrazides derivatives of fatty acids

A new HPLC-UV-Vis method for identification of drying oils from binding media or protective film used in pictorial works of art prior to conservation or restoration is proposed. Chromophore derivatization of fatty acids released by hydrolysis of structural drying oils is studied. The derivatization reagent selected was 2-nitrophenylhydrazine with 1-ethyl-3-(3-dimethyl animopropyl)carbodiimide hydrochloride/pyridine as catalyst. This reaction was carried out using microwave heating. Mobile phase was methanol/water/n-propanol/acetic acid (80:14:5:1) running in isocratic mode. Absorbance was measured at 400 nm. In these conditions, hydrazides of myristic, palmitic, oleic, and stearic acids were satisfactorily resolved. Method shows good sensitivity, with a detection limit of 15 μmol l⁻¹, and good linearity between 0.03 and 3 mmol l⁻¹. Peak area ratios among fatty acids derivatives allows identification of the drying oils. The stearic/palmitic ratio is the most important, because it allows to differentiate among the different drying oils. The proposed method has been successfully applied to real samples from items of the cultural heritage of Valencia (Spain).     

Arylpyrrolylmethenes

The synthesis of 2-p-hydroxyphenylmethylene-2H-pyrolenine hydrobromides and their properties are described. The results of attempted synthesis of other “mixed” arylpyrrolylmethenes are discussed and an improved procedure for the synthesis of 2-ethoxycarbonyl-4-methoxypyrrole is reported. Deformylation and apparent self-condensation of pyrrole aldehydes were observed in a number of methene syntheses tried and a mechanism to account for this is presented.     

Microwave-assisted water extraction of acid herbicides from soils coupled to continuous filtration, pre-concentration, chromatographic separation and UV detection

A microwave-assisted extraction procedure using water as extractant has been performed for the extraction of acid herbicides (namely, bentazone, 2,4-D, trichlopyr, 2,4,5-T and 2,4,5-Tp) from different types of soil. Two experimental designs were used for the optimization of the leaching step. The selection of water as extractant provided a clean approach by avoiding the use of organic solvents. The use of water also enhanced the extraction efficiency due to the high interaction of the microwave irradiation with polar solvents. The time required for total removal of the target compounds was 35 min. The recoveries yield were from 87.64 to 106.14% with a repeatability (expressed as relative standard deviation) ranging between 1.34 and 9.24%. A within-laboratory reproducibility, also expressed as relative standard deviation, varied from 3.97 to 10.41%. A flow-injection manifold allowed automation of the whole process by hyphenating the steps subsequent to extraction (namely, filtration, preconcentration, chromatographic separation and UV detection) for the determination of the analytes.     

Classification of ancient Roman glazed ceramics using the neural network of Self-Organizing Maps

Artificial neural networks with unsupervised learning strategy known as Self-Organizing Maps were applied to classify ancient Roman glazed ceramics. Their clay ceramic bodies were analyzed by Inductively Coupled Plasma-Atomic Emission Spectroscopy and the chemical composition obtained was processed by this neural algorithm. The results obtained provide two types of information: firstly, classification of ceramic samples with identification of several groups and secondly, differentiation between the elemental chemical information. It was found that there are certain chemical elements which can be considered as principal and which can serve to differentiate between ceramics, whereas other elements give redundant information and do not contribute to sample differentiation. Seven chemical elements were considered principal and provide the necessary information. Two types of element were identified: 1- a group formed by common elements, such as: Ca, Fe, Mg, Mn and 2- another formed by optional elements: K or Na and Ba or Sr and Al or Ti.     

Theoretical study of the calcium dication hydrates

Ab initio SCF calculations have been carried out for calcium dication surrounded by 1–4 or 6 water molecules using several basis sets. The structure of the hydrated ions is examined and the hydration energy estimated. The changes in the structure of water within the complex are qualitatively explained in terms of charge transfer and coulombic interactions among the atoms. Finally, the influence of electron correlations is discussed.