The use of internal standards in ICP-MS

Careful study of the matrix effect in ICP-MS showed that, in all cases studied, the magnitude of the signal suppression or enhancement depends in a regular way on the mass number. Hence, accurate correction for non-spectral interferences is only possible using an internal standard with mass number close to that of the analyte element(s). It is also shown that using an internal standard with mass number close to that of the analyte improves the precision. For both cases, the ionization energy of the internal standard seems to be of no or only secondary importance. To obtain optimal precision and accuracy, the internal standard should be selected as close in mass number as possible to that of the analyte element(s). When a number of elements over a considerable mass range are to be determined, several internal standards have to be used.     

Self-assembled aggregates of the carotenoid zeaxanthin: Time-resolved study of excited states

In this study, we present a way of controlling the formation of the two types of zeaxanthin aggregates in hydrated ethanol: J-zeaxanthin (head-to-tail aggregate, characteristic absorption band at 530 nm) and H-zeaxanthin (card-pack aggregate, characteristic absorption band at 400 nm). To control whether J- or H- zeaxanthin is formed, three parameters are important: (1) pH, that is, the ability to form a hydrogen bond; (2) the initial concentration of zeaxanthin, that is, the distance between zeaxanthin molecules; and (3) the ratio of ethanol/water. To create H-aggregates, the ability to form hydrogen bonds is crucial, while J-aggregates are preferentially formed when hydrogen-bond formation is prevented. Further, the formation of J-aggregates requires a high initial zeaxanthin concentration and a high ethanol/water ratio, while H-aggregates are formed under the opposite conditions. Time-resolved experiments revealed that excitation of the 530-nm band of J-zeaxanthin produces a different relaxation pattern than excitation at 485 and 400 nm, showing that the 530-nm band is not a vibrational band of the S2 state but a separate excited state formed by J-type aggregation. The excited-state dynamics of zeaxanthin aggregates are affected by annihilation that occurs in both J- and H-aggregates. In H-aggregates, the dominant annihilation component is on the subpicosecond time scale, while the main annihilation component for the J-aggregate is 5 ps. The S(1) lifetimes of aggregates are longer than in solution, yielding 20 and 30 ps for H- and J-zeaxanthin, respectively. In addition, H-type aggregation promotes a new relaxation channel that forms the zeaxanthin triplet state.     

Mass spectrometric structure determination of metabolites of 3-[2-(N,N-dimethylaminomethyl)phenylthio]phenol

Metabolites of 3-[2-(N,N-dimethylaminoethyl)phenylthio]phenol (I) were isolated from the urine of rats, mice, rabbit and dog and from the faeces of rats by extraction and thin-layer chromatography. From the mass spectrum of I and using characteristics mass shifts in the spectra of metabolites, the structures of four metabolites were determined, whereas for two other isomeric metabolites the structures were resolved by means of IR spectra. Conclusions regarding the structures derived from the spectra were confirmed by comparing them with synthetic standards. Generally, metabolic changes of I can be characterized by demethylation, hydroxylation combined with methylation, S-oxidation and by a combination of these metabolic reactions.     

Reactions under autogenic pressure at elevated temperature (RAPET) of various alkoxides: formation of metals/metal oxides-carbon core-shell structures

The RAPET reaction at 700 degrees C, of different alkoxides (Ti, V, and Si) led to three different nanocomposites. Carbon is the element common to all three structures. Carbon was found as the core in the decomposition of TEOS, as the shell in the decomposition of VO(OEt)3 and embedded in, or mixed completely with, TiO2 in the decomposition of Ti(OiPr)4. This novel method using only metallic alkoxide precursor, in the absence of catalyst, leads in a one-step process to core-shell structures.     

Kinetics and Mechanism of Oxidation of <Emphasis Type="Italic">L</Emphasis>-Leucine by Alkaline Diperiodatonickelate(IV) – A Free Radical Intervention,Deamination, and Decarboxylation

Summary. The kinetics of oxidation of L-leucine by alkaline diperiodatonickelate(IV) was studied spectrophotometrically using a rapid kinetic accessory. The reaction is first order with respect to [DPN] and apparently less than unit order, each in [L-leucine] and [alkali] under the experimental conditions. However, the order in [L-leucine] and [alkali] changes from first order to zero order as the concentrations change from lower to higher values. Addition of periodate has no effect on the reaction rate. A mechanism involving the deprotonated diperiodatonickelate(IV) (DPN) as the reactive oxidant is proposed. The reaction constants involved in the mechanism were evaluated. There is a good agreement between the observed and calculated rate constants under varying experimental conditions. The isokinetic temperature was determined and the activation parameters with respect to the slow step of the reaction scheme were evaluated and discussed.Received September 19, 2002; accepted (revised) January 1, 2003 Published online september 11,2003     

Carotenoid and pheophytin on semiconductor surface: self-assembly and photoinduced electron transfer

Self-assembling of a carotenoid and pheophytin a into a supramolecular system was observed on the surface of nanocrystalline TiO2, and the photoinduced electron-transfer reactions within the system were studied by means of femtosecond transient absorption and laser flash photolysis techniques. Excitation of the pheophytin moiety results in ultrafast electron transfer from carotenoid to the excited pheophytin, creating a long-lived charge-separated state. Two decay pathways of the formed pheophytin a anion radical are proposed. The first is a direct back electron recombination forming a carotenoid triplet state on the nanosecond time scale, while the other is suggested to occur via electron injection to the TiO2 nanoparticle. These results demonstrate that a self-assembled carotenoid-pheophytin system leads to an efficient reductive quenching of the pheophytin moiety, suggesting that a similar mechanism can operate also in natural photosynthetic systems. Moreover, the formation of a long-lived charge-separated state indicates that such self-assembling strategy may be also considered for novel dye-sensitized solar cell constructions and other artificial systems aiming to mimic the electron-transfer chain in natural photosynthesis.     

Determination of procaine, benzocaine and tetracaine by sequential injection analysis with permanganate-induced chemiluminescence detection

Local anaesthetics procaine hydrochloride (I), benzocaine (II), and tetracaine hydrochloride (III) were determined by the technique of sequential injection analysis (SIA) with chemiluminescence (CL) detection. The CL was emitted during the oxidation of the analytes by permanganate in aqueous sulphuric acid in the presence of various CL enhancers (4-hydroxybiphenyl, Rhodamine B, glycolaldehyde, glutaraldehyde and formic acid). The optimum enhancer or reagent concentrations, order and volumes of the injected zones were: 0.37 M formic acid (40, 23 or 28 mul for I-III, respectively), sample (40 mul), 2.3 M H(2)SO(4) (20, 16 or 18 mul for I-III, respectively), and 0.5 mM KMnO(4) (19, 13 or 15 mul for I-III, respectively). After a double (or single for III) reversal of the flow the mixed zone was pushed into the detector at a flow rate of 100 mul s(-1). The transient CL signal was recorded at >/=390 nm. The calibration graphs relating the intensity of the emission (peak heights) to the concentration of the analytes were curvilinear (a second order polynomial showed the best fit) and they were suitable for determining I-III in the ranges 0.5-50, 0.5-25 and 0.2-25 mug ml(-1), respectively. The limits of detection (3sigma) were 0.3 mug ml(-1) for I and II and 0.1 mug ml(-1) for III. The sample throughput was 120 h(-1). The relative standard deviations were 相似文献