Abstract: The use of a Pt electrode coated with a layer of Nafion has been described in previous works as an attractive way to perform the potentiometric detection of hydrogen peroxide. Despite of the attractive features of this approach, the nature of the non-Nernstian response of this system was not properly addressed. In this work, using a mixed potential model, the open circuit potential of the Pt electrode is shown to be under kinetic control of the oxygen reduction reaction (ORR). It is proposed that hydrogen peroxide acts as an oxygenated species that blocks free sites on the Pt surface, interfering with the ORR. Therefore, the effect of the polyelectrolyte coating can be understood in terms of the modulation of the factors that affects the kinetics of the ORR, such as an increase of the H+ concentration, minimization of the effect of the spectator species, etc. Because of the complexity and the lack of models that accurately describe systems with practical applications, this work is not intended to provide a mechanistic but rather a phenomenological view on problem. A general framework to understand the factors that affect the potentiometric response is provided. Experimental evidence showing that the use of polyelectrolyte coatings are a powerful way to control the mixed potential open new ways for the development of robust and simple potentiometric sensors.

Abstract: The precision of the 14-MeV neutron activation determination of oxygen in steel has been examined as a function of the oxygen content for a large number of steel and cast iron samples. The experimental and the statistically expected standard deviations have been compared. In the conditions used, 2.5 counts from 16N were registered for each p.p.m. of oxygen in the samples. The neutron flux was monitored by a simultaneously irradiated oxygen standard.

Abstract: The high toxicity, endocrine-disrupting effects and low (bio)degradability commonly attributed to phenolic compounds have promoted their recognition as priority toxic pollutants. For this reason, the monitoring of these compounds in industrial, domestic and agricultural streams is crucial to prevent and decrease their toxicity in our daily life. To confront this relevant environmental issue, we propose the use of a combi-electrosensor which combines singlet oxygen (1O2)-based photoelectrochemistry (PEC) with square wave voltammetry (SWV). The high sensitivity of the PEC sensor (being a faster alternative for traditional COD measurements) ensures the detection of nmol L−1 levels of phenolic compounds while the SWV measurements (being faster than the color test kits) allow the differentiation between phenolic compounds. Herein, we report on the development of such a combi-electrosensor for the sensitive and selective detection of phenol (PHOH) in the presence of related phenolic compounds such as hydroquinone (HQ), bisphenol A (BPA), resorcinol (RC) and catechol (CC). The PEC sensor was able to determine the concentration of PHOH in spiked river samples containing only PHOH with a recovery between 96% and 111%. The SWV measurements elucidated the presence of PHOH, HQ and CC in the spiked samples containing multiple phenol compounds. Finally, the practicality of the combi-electrosensor set-up with a dual SPE containing two working electrodes and shared reference and counter electrodes was demonstrated. As a result, the combination of the two techniques is a powerful and valuable tool in the analysis of phenolic samples, since each technique improves the general performance by overcoming the inherent drawbacks that they display independently.

Abstract: Non-destructive 14-MeV neutron activation analysis for silicon in steel has been applied with 56Mn as internal standard.56Mn is formed from the iron matrix via the 56Fe(n,p)56Mn reaction. Several methods of internal standardisation via56Mn are discussed. The 0.84-MeV photopeak of 56Mn is recommended if steel samples of about the same composition are to be analysed. Chemically analysed steel samples are used as silicon standards. A precision of 0.7% was obtained for an analysis plus standardisation time of 13 min. Special attention was paid to interferences produced by concentration changes of impurity elements. Several possible sources of errors were investigated.

Abstract: The determination of silicon via the 28Si(n,p)28 Al reaction by means of 14-MeV neutrons is applied to the analysis of pollution and natural aerosols. A Whatman 41 filter (40 cm2) on which airborne particulate material has been collected is compressed into a 3 × 12.7 mm pellet. Standards are prepared in the same way from clean filters spiked with a silicate solution. After a 50-s irradiation and a 75-s decay time, the sample is counted for 2 min with 5 × 5 NaI(Tl) well detector. The 1.779-MeV photopeak of 28Al is measured with a single channel sealer chain or with a multichannel analyser. The reproducibility, sensitivity and liability to interference from other elements were investigated for both counting systems. The homogeneity of the pellets and the filters was checked. The overall precision of one single-channel determination was estimated to be 3.5% after a 24-h high-volume sampling time. Samples collected in urban, industrial and remote areas with concentrations ranging from 0.05 to 15 μg Si m-3 air were analysed and the results are discussed.

Abstract: A general standardization method is described for the determination of oxygen in solid samples via the 16O(n,p)16N reaction. Two systems of flux monitoring are considered: the sample versus standard comparator method and BF3 monitoring. The average flux in sample and standard, fast neutron shielding, fast neutron scattering, absorption of fast neutrons, absorption of 16N γ-rays and counting efficiency of sample and standard are considered. The influence of the target diameter on the obtained correction factors has also been studied. Total achievable accuracy is believed to be about 1%.

Abstract: Several sample preparation methods for waters for energy-dispersive x.r.f. were examined, as well as the influence of sample size on the analytical characteristics. The most satisfactory simple, rapid method proved to be spotting of 1.5 ml of water sample on a Whatman-41 cellulose filter paper provided with a wax ring of 29-mm diameter and evaporating the water with an unheated air stream from underneath. Sensitivities are below 100 p.p.b. for most elements and often below 50 p.p.b. when the optimal secondary fluorescer is used. Accuracy and precision are usually in the 1520 % range. The method is applicable to many dilute aqueous solutions as is illustrated by analysis of industrial water samples and ashed biological material.

Abstract: A computer routine was developed for qualitative and quantitative analysis of photographically recorded spark-source mass spectra. Particular attention is given to the case of a graphite matrix. The program starts from the line intensities (expressed as Seidel values) and isotope masses calculated from the densitometer readings by a commercially available routine. From the intensities in the different exposures (typically 15 stages), it computes the parameters for the linear parts of the density curves for each ion. Taking into account mutual interferences of multivalent ions, isotope or C-clusters, oxide, carbide and dicarbide ions, the program automatically identifies and then quantifies the elements present. The precision of the results is around 5%. Reading and complete processing of one photoplate is achieved within 23 h.

Abstract: The relative merits of eight procedures for preconcentrations of trace metal ions from natural water samples and synthetic solutions are evaluated. Spikes (100 μg l−1 ) of Mn, Co, Zn, Eu, Cs and Ba and the corresponding radioactive tracers were added to batches of drinking water, estuarine water, sea water, ground water, twice-distilled water and ahumic material solution. After equilibration for 25 months, the following techniques were applied: passage through columns of Dowex Al chelating resin and ofsilylated silica gel, filtration through laminate membrane filters and chelating diethylenetriamine cellulose filters, precipitation with sodium diethyldithiocarbamate and l-(2-pyridylazo)-2-naphthol, extraction with ammonium pyrrolidinedithiocarbamate, and chelation by 8-quinolinol (oxine) followed by adsorption on activated carbon. The quantitative characteristics of these techniques and the influence of the water matrix effects are discussed, as well as the applicability for x-ray fluorescence analysis.

Abstract: Cellulose filters with immobilized 2,2'-diaminodiethylamine (DEN) functional groups are studied for trace anion preconcentration from aqueous solution, with subsequent x-ray fluorescence measurements. For most oxoanions with a central metal atom, nearly quantitative collection can be achieved by 10-cm2 DEN filters under the following optimized conditions: pH 36, filtration rate up to 0.5 ml cm-2 min-1, and sample volume up to 100 ml cm-2. The collection yield is independent of the trace oxoanion concentration up to at least 1.5 μmol cm-2. Although the DEN filter exhibits some selectivity towards oxoanions with a central metal atom, ionic strength affects the results; the collection efficiency is strongly depressed with salt (e.g. NaCl) concentrations above 0.01 M. The applicability of the DEN filter in anion collection is therefore limited to dilute solutions.

Abstract: Conventional energy-dispersive x-ray fluorescence is applied in the analysis of blood serum to give the concentrations of 710 elements simultaneously with minimal manipulation of the samples. Simple spotting onto a Mylar carrier of 250 μl of serum, doped with two internal standards, was chosen as the sample preparation step. Some 200 serum samples, analyzed in replicate (n = 26), were used to evaluate this procedure. The detection limits are 4 μg ml-1 for K and Ca, 0.50.2 μg ml-1 for Fe, Cu, Pb and Zn, and less than 0.1 μg ml-1 for Se, Rb and Sr. Well above these limits, the standard deviation is around 10%. Comparison with the results of other measurements on the same samples indicates an accuracy of that order. The simplicity and high throughput, and the possibility of automating the x.r.f. measurements, make the proposed procedure suitable for screening large numbers of sera.

Abstract: Selenium is determined in the ng g−1 to μg g−1 range in biological and environmental samples. A wet digestion procedure was optimized with respect to volatility losses and recovery yields, by using 75Se metabolically incorporated into rat organs. Selenium is preconcentrated from the digestion liquid by a two-step reduction with 4 M HCl and ascorbic acid. The colloidal selenium formed is adsorbed on activated carbon and filtered on a Nucleopore membrane for measurement by energy-dispersive x-ray fluorescence. Almost complete recovery was obtained, and the detection limit was 20 ng, corresponding to 10 ng g−1 for a 2-g sample. Biological reference materials were analyzed with satisfactory results, and the accuracy of the method was good.