Thermochemolysis of seven nucleobases—adenine, thymine, uracil, cytosine, guanine, xanthine, and hypoxanthine—in tetramethylammonium hydroxide (TMAH) was studied individually by pyrolysis gas ...chromatography mass spectrometry in the frame of the Mars surface exploration. The analyses were performed under conditions relevant to the Sample Analysis at Mars (SAM) instrument of the Mars Curiosity Rover and the Mars Organic Molecule Analyzer (MOMA) instrument of the ExoMars Rover. The thermochemolysis products of each nucleobase were identified and the reaction mechanisms studied. The thermochemolysis temperature was optimized and the limit of detection and quantification of each nucleobase were also investigated. Results indicate that 600°C is the optimal thermochemolysis temperature for all seven nucleobases. The methylated products trimethyl-adenine, 1, 3-dimethyl-thymine, 1, 3-dimethyl-uracil, trimethyl-cytosine, 1, 3, 7-trimethyl-xanthine (caffeine), and dimethyl-hypoxanthine, respectively, are the most stable forms of adenine, thymine, uracil, cytosine, guanine, and xanthine, and hypoxanthine in TMAH solutions. The limits of detection for adenine, thymine, and uracil were 0.075 nmol; the limits of detection for guanine, cytosine, and hypoxanthine were higher, at 0.40, 0.55, and 0.75 nmol, respectively. These experiments allowed to well constrain the analytical capabilities of the thermochemolysis experiments that will be performed on Mars to detect nucleobases.
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•Seven nucleobases were detected by GC-MS under conditions relevant to in situ operating instruments on Mars: SAM and MOMA.•The TMAH thermochemolysis products coming from seven nucleobases were determined.•The LOD and LOQ values of seven nucleobases in TMAH were determined after GC-MS analysis.•Results of the nucleobases thermochemolysis will help to identify those compounds on Mars with SAM and MOMA.
Lignins from different non-woody plants such as hemp (
Cannabis sativa), flax (
Linum usitatissimum), jute (
Corchorus capsularis), sisal (
Agave sisalana) and abaca (
Musa textilis), commonly used ...for manufacturing specialty papers, were analyzed by pyrolysis-gas chromatography–mass spectrometry (Py-GC/MS) in the absence and in the presence of tetramethylammonium hydroxide (TMAH) and by Fourier-transform infrared (FTIR) spectroscopy, after alkaline isolation. Hemp and flax lignins showed a predominance of guaiacyl (1-hydroxy-2-methoxyphenyl) units, while jute, sisal and abaca lignins contained predominantly syringyl (1-hydroxy-2,6-dimethoxyphenyl) units.
p-Hydroxycinnamic acids, namely
p-coumaric and ferulic acids, were also found in the isolated lignins, linked by alkali-resistant ether bonds, especially in abaca and sisal lignins. The presence of the latter compounds in the isolated lignins, as well as in their respective whole fibers, was shown by pyrolysis in the presence of tetramethylammonium hydroxide (Py/TMAH),
p-coumaric acid being especially abundant in abaca.
A rapid, cheap and feasible new approach was used to synthesize the Mg0.375Fe0.375Al0.25-LDH in the presence of tetramethylammonium hydroxide (TMAH), as a nontraditional hydrolysis agent, applying ...both mechano-chemical (MC) and co-precipitation methods (CP). For comparison, these catalysts were also synthesized using traditional inorganic alkalis. The mechano-chemical method brings several advantages since the number of steps and the energy involved are smaller than in the co-precipitation method, while the use of organic alkalis eliminates the possibility of contaminating the final solid with alkaline cations. The memory effect was also investigated. XRD studies showed Fe3O4 as stable phase in all solids. Regardless of the alkalis and synthesis methods used, the basicity of catalysts followed the trend: mixed oxides > parent LDH > hydrated LDH. The catalytic activity of the catalysts in the Claisen−Schmidt condensation between benzaldehyde and cyclohexanone showed a linear dependence to the basicity values. After 2 h, the calcined sample cLDH-CO32−/OH−-CP provided a conversion value of 93% with a total selectivity toward 2,6-dibenzylidenecyclohexanone. The presence of these catalysts in the reaction media inhibited the oxidation of benzaldehyde to benzoic acid. Meanwhile, for the self-condensation of cyclohexanone, the conversions to mono- and di-condensed compounds did not exceed 3.8%.
Synthesis of two-dimensional (2D) materials that is readily scalable, cost-effective, and eco-friendly is important from both scientific and industrial viewpoints. Currently, these 2D materials are ...synthesized either by selective etching of relatively expensive layered solids, viz. using a top-down approach, or by autoclaving metal salts/organic compounds. Herein, we describe a near-ambient, one-pot, inexpensive, scalable pathway to convert—through a bottom-up approach—5 different water-insoluble Mn-bearing precursors, viz. Mn3O4, Mn2O3, MnB, Mn5SiB2, and Mn2AlB2, into birnessite-based 2D flakes that, in some cases, are remarkably crystalline. The precursor powders are immersed in 25 wt % tetramethylammonium hydroxide aqueous solutions at 50°C to 80°C for 2 to 4 days. The structures, compositions, oxidation states, and morphologies of the synthesized flakes are determined using a battery of characterization techniques. The synthesized 2D sheets demonstrate reversible O2 electrocatalysis with activities comparable with those of a commercial Pt/C catalyst.
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•Scalable, inexpensive, one-pot, facile synthesis of 2D birnessite flakes•Near-ambient conversion of water-insoluble, non-layered Mn-precursors•Resulting birnessite flakes are crystalline, with thicknesses around ≈2 ± 0.4 nm•Reversible O2 electrocatalysis with activities comparable with those of a Pt/C catalyst
Typically, materials that are atomically thick but extend over large areas are labeled two-dimensional (2D) materials, which are produced by exfoliating or etching layered bulk materials. The process can be slow and hazardous and can render the production of 2D materials in large quantities challenging and expensive. We developed a one-pot recipe of producing 2D sheets in bulk scale almost at room temperature, starting with inexpensive green precursors. The quality of sheets produced is as high as those prepared using high temperatures and pressures and/or hazardous chemicals. Our one-pot method is thus the simpler, scalable, cheaper, and safer approach.
Electrodes made with our materials could lead to better batteries for cell phones and transportation. The sheets are also biocompatible, which renders them good candidates for use in the biomedical field. This discovery is a true breakthrough in scaling up the production of nanomaterials and will have a direct and positive impact on society.
Two-dimensional materials are usually prepared using special chemicals and higher temperature processes and/or using hazardous chemicals such as hydrofluoric acid. Herein, we present a near-ambient condition (50° to 80°C), non-toxic, one-pot, cheap, scalable pathway to convert five different water-insoluble, non-layered Mn-containing precursors, viz. Mn3O4, Mn2O3, MnB, Mn5SiB2, and Mn2AlB2, into 2D crystalline birnessite flakes. The resulting flakes demonstrated electrocatalytic activities for reversible O2 reactions that are comparable with Pt/C electrodes.
Over the years studies on sporopollenin have reported a wide variety of structures. However, the methods and techniques used to elucidate sporopollenin structures are highly diverse so that much is ...still unclear with respect to the nature and structural diversity of sporopollenins. In order to investigate the structural diversity in sporopollenin between different taxa, extant sporomorphs of ten different species ranging from a mushroom to a cycad were examined using a relatively simple and fast analytical procedure. Sporomorphs, before and after saponification, were analysed for sporopollenin composition by Thermally assisted Hydrolysis and Methylation (THM) using 13Ctetramethylammonium hydroxide (13CTMAH). The sporomorp chemical composition differed markedly between the groups of organisms analysed. Moreover, we not only identified the nature and relative quantities of the well-known sporopollenin constituents p-coumaric acid and ferulic acid but also many other phenolic moieties, such as caffeic acid, which appeared to be the most abundant phenolic constituent in spores of Equisetum palustre, Salvinia molesta, Cyrtomium falcatum and Anemia phyllitidis. Within the two Equisetum species analysed as well as in the closely related Azolla and Salvinia species the same suite of phenolic constituents were observed, but their relative distribution varied largely. We thus demonstrate the existence of a high structural diversity, both qualitatively and quantitatively in sporopollenins enabling future studies related to the evolution, phylogeny and (palaeo)environment of sporopollenin-producing organisms. Furthermore, a better knowledge of sporopollenin and its structural variety is of relevance to the rapidly growing application of spores and pollen as a drug delivery agent in medicine.
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•Spores of 10 species were analysed for their sporopollenin composition.•Spores before and after extraction and saponification were examined.•Thermally assisted Hydrolysis and Methylation using 13C-labeled tetramethylammonium hydroxide was applied.•Caffeic acid appeared to be a major component of sporopollenin.
In this study, heteroepitaxial ZnO(0001) thin films were grown directly on α-Al2O3(0001) substrates at room temperature by laser-molecular beam epitaxy without buffer layers. The epitaxial films were ...grown on substrates pretreated with strongly alkaline tetramethylammonium hydroxide solution, while the substrates cleaned by hydrochloric acid resulted in uniaxially oriented growth with poor in-plane structural anisotropy. The surfaces of the thin films were ultraflat and composed of ultrafine nanocrystallites based on low-growth temperatures, which well reflected the atomically stepped morphology of the substrates. In addition, the ZnO thin films exhibited high transparency with a transmittance of >80 % in the visible region and optical bandgaps of ∼3.4 eV. The epitaxial and uniaxially oriented ZnO(0001) films were both n-type degenerate semiconductors and demonstrated relatively low resistivities of ∼3.1 × 10−3 and ∼2.1 × 10−3 Ω cm, respectively, at room temperature. The epitaxial thin film indicated a relatively higher carrier concentration of ∼3.4 × 1020 cm−3 and lower mobility of ∼6.1 cm2/V s as compared with the uniaxially oriented film. This suggested a greater number of crystal defects were introduced, including oxygen vacancies based on relaxed epitaxial strain.
•Online pyrolytic methylation-GC/MS was developed for the determination of GHB in human hair.•Low consumption of sample was achieved by the method without laborious pretreatment.•It required only one ...hair (0.3 mg) in every treatment, which can reduce the difficulty of sampling.•The developed method has profound significance for monitoring GHB abuse.
A novel method based on on-line pyrolytic methylation-gas chromatography/mass spectrometry was established and applied into rapid determination of γ-hydroxybutyric acid (GHB) in human hair. With the pyrolytic methylation by tetramethylammonium hydroxide (TMAH), GHB was converted into its corresponding methyl ester in the pyrolyzer at 500 °C. Thus, GHB in hair samples could be well determined by GC/MS on a capillary column in SIM mode. To obtain optimum methylation conditions, important factors were investigated and then applied to the following experiments. The results were obtained as following: 500 °C as reaction temperature, 2:1 as the proportion of TMAH to GHB. The achieved calibration curve exhibited good linearity with the correlation coefficient of 0.9997 at the concentraion range of 2−200 ng/mg, and relative standard deviation (n = 3) from 1.3 %–7.7 %. The limit of detection and the limit of quantification were down to 0.3 ng/mg and 1.0 ng/mg, respectively. The precision ranged from 2.0 %–7.9 % and the recovery ranged from 98.5 %–107.4 %. The results of on-line pyrolytic methylation-GC/MS were in agreement with those observed by the referenced off-line silylation-GC/MS method. The method was applied to a real case of a woman addicted to GHB, the drug concentration in hair segments of 0−5 cm and 5−10 cm from the root strand well correlated with the use of GHB in different periods of her life. The proposed method demonstrated significant potential in the application of rapid determination for GHB in human hair.
A low-damaged wet process utilizing electrochemical (EC) etching and subsequent chemical etching has been developed for the fabrication of GaN porous structures. Superior controllability in depth and ...diameter could be obtained by achieving anisotropic nature of the vertical direction to the substrate by EC etching and horizontal direction by tetramethylammonium hydroxide (TMAH) etching, respectively. The optical and photoelectrochemical properties of GaN porous structures were very sensitive to the structural properties. Photoreflectance measurement revealed that porous sample had an effective refractive index that could be controlled by TMAH etching time. In photoelectrochemical measurement, the incident-photon-to-current conversion efficiency (IPCE) was dramatically enhanced to as high as 91% by the formation of porous structures. A series of experimental results were consistently explained by the change of thickness of pore wall and width of space charge region.
This article studies the sub-linearity of the output characteristics measured in Schottky-barrier metal-oxide-semiconductor field-effect transistors with simulations and experiments. It is shown that ...the sub-linearity is not due to the forward-biased Schottky diode at the drain contact interface but due to the drain bias impact on the source-side Schottky-barrier, resulting in an increased carrier injection with increasing drain-source voltage. The simulation results are confirmed with the measurements of fabricated dual-gate Schottky-barrier transistors.
Sample preparation in alkaline media Nóbrega, Joaquim A.; Santos, Mirian C.; de Sousa, Rafael A. ...
Spectrochimica acta. Part B: Atomic spectroscopy,
05/2006, Letnik:
61, Številka:
5
Journal Article
Recenzirano
The use of tetramethylammonium hydroxide, tertiary amines and strongly alkaline reagents for sample treatment involving extraction and digestion procedures is discussed in this review. The ...preparation of slurries is also discussed. Based on literature data, alkaline media offer a good alternative for sample preparation involving an appreciable group of analytes in different types of samples. These reagents are also successfully employed in tailored speciation procedures wherein there is a critical dependence on maintenance of chemical forms. The effects of these reagents on measurements performed using spectroanalytical techniques are discussed. Several undesirable effects on transport and atomization processes necessitate use of the method of standard additions to obtain accurate results. It is also evident that alkaline media can improve the performance of techniques such as inductively coupled plasma mass spectrometry and accessories, such as autosamplers coupled to graphite furnace atomic absorption spectrometers.
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