Measurement of δ13C values of soil amino acids by GC–C–IRMS using trimethylsilylation: a critical assessment

In this study, we evaluated trimethylsilyl (TMS) derivatives as derivatization reagents for the compound-specific stable carbon isotope analysis of soil amino acids by gas chromatography–combustion–isotope ratio mass spectrometry (GC–C–IRMS). We used non-proteinogenic amino acids to show that the extraction–derivatization–analysis procedure provides a reliable method to measure δ¹³C values of amino acids extracted from soil. However, we found a number of drawbacks that significantly increase the final total uncertainty. These include the following:

• production of multiple peaks for each amino acid, identified as di-, tri- and tetra-TMS derivatives;

• a number of TMS-carbon (TMS-C) atoms added lower than the stoichiometric one, possibly due to incomplete combustion;

• different TMS-C δ¹³C for di-, tri- and tetra-TMS derivatives.

For soil samples, only four amino acids (leucine, valine, threonine and serine) provide reliable δ¹³C values with a total average uncertainty of 1.3?‰. We conclude that trimethylsilyl derivatives are only suitable for determining the ¹³C incorporation in amino acids within experiments using ¹³C-labelled tracers but cannot be applied for amino acids with natural carbon isotope abundance until the drawbacks described here are overcome and the measured total uncertainty significantly decreased.     

Mechanisms of the oxidation and combustion of normal paraffin hydrocarbons: Transition from C1–C10 to C11–C16

Recently, detailed kinetic mechanisms of the oxidation and combustion of higher hydrocarbons, composed of hundreds of components and thousands of elementary reactions, have been proposed. Despite the undoubtful advantages of such detailed mechanisms, their application to simulations of turbulent combustion and gas dynamic phenomena is difficult because of their complexity. At the same time, to some extent limited, they cannot be considered exhaustive. This work applies previously proposed algorithm for constructing an optimal mechanism of the high- and low-temperature oxidation and combustion of normal paraffin hydrocarbons, which takes into account the main processes determining the reaction rate and the formation of key intermediates and final products. The mechanism has the status of a nonempirical detailed mechanism, since all the constituent elementary reactions have a kinetic substantiation. The mechanism has two specific features: (1) it does not include reactions of so-called double oxygen addition (first to the peroxide radical, and then to its isomeric form), i.e., the first addition turns out to be sufficient; (2) it does not include isomeric compounds and their derivatives as intermediates, since this oxidation pathway is slower than the oxidation of molecules and radicals with normal structure. Application of the algorithm makes it possible to compile a compact mechanism, which is important for modeling chemical processes involving paraffin hydrocarbons C _n with large n. Previously, based on this algorithm, compact mechanisms of the oxidation and combustion of propane, n-butane, n-pentane, n-hexane, n-heptane, n-octane, n-nonane, and n-decane have been constructed. In this work, we constructed a nonempirical detailed mechanism of the oxidation and combustion of hydrocarbons from n-undecane to n-hexadecane. The most important feature of the new mechanism is its staged nature, which manifests itself through the emergence of cool and blue flames during low-temperature autoignition. The calculation results are compared with experimental data.     

Fundamental investigation into characteristics of particulate matter produced from rapid pyrolysis of biochar in a drop-tube furnace at 1300 °C

This paper reports the emission characteristics of leaf and wood biochar (LC500 and WC500) pyrolysis in a drop tube furnace at 1300 °C in argon atmosphere. The char yields at 1300 °C are ～ 65% and ～ 73% respectively for LC500 and WC500. Over 60% Mg, Ca, S, Al, Fe and Si are retained in char after pyrolysis at 1300 °C. The retentions of Na and K in the char from LC500 pyrolysis are lower than those in the char from WC500 pyrolysis due to release via enhanced chlorination as a result of much higher Cl content in LC500. Particulate matter (PM) with aerodynamic diameter of < 10 µm (i.e. PM₁₀) from LC500 and WC500 pyrolysis exhibits a bimodal distribution with a fine mode diameter of 0.011 µm and a coarse mode diameter of 4.087 µm. The PM₁₀ yield for LC500 pyrolysis is ～ 8.2 mg/g, higher than that of WC500 pyrolysis (～2.1 mg/g). Samples in PM_1-10 (i.e. PM with aerodynamic diameter 1 µm – 10 µm) are char fragments that have irregular shapes and similar molar ratio of (Na+K + 2Mg+2Ca)/(Cl+2S+3P) as the char collected in the cyclone. In PM₁ (i.e. PM with aerodynamic diameter < 1 µm), the main components in sample are inorganic species, and carbon only contributes to ～5% and ～8% the PM₁ produced from rapid pyrolysis of LC500 and WC500, respectively. Na, K and Cl are main inorganic species in PM₁, contributing ～ 98.8% and ～ 97.5% to all inorganic species. Na, K and Cl from rapid pyrolysis of biochar have a unimodal distribution with a mode diameter of 0.011 µm. In PM_1–10, Ca is the main inorganic specie, contributing to ～71.2% and ～65.3% to all inorganic species in PM_1–10 from pyrolysis of LC500 and WC500, respectively.     

Effect of exposure to air on sheet resistance and phase composition of Co silicides annealed at a low temperature of 470 °C

Attention has been attracted to Co silicides due to their superior properties in deep-submicron integrated circuit technology. In this paper, the effect of exposure to air on the properties of Co silicides has been studied. Co films of 20-nm thickness were deposited onto polysilicon layers using Ar sputtering. After deposition, the samples were exposed to air at room temperature for different times, ranging from 0 to 48 h, before a rapid thermal annealing (RTA) at 470 °C. It is found that exposure to air significantly changes the sheet resistance (R_s) and the phase composition of the silicides. The sample exposed to air for 48 h has R_sof∼71 Ω/sq, which is about 10% lower than that for the sample annealed immediately. This is due to the fact that more Co₂Si phase and less CoSi phase are formed in the former sample. The mechanism can be attributed to the gases in air (e.g. O₂), which contaminate the Co/Si interface and act as a kinetic barrier during the subsequent RTA. It has been demonstrated that gaseous contamination from air strongly influences the CoSi_x phase transformation. Received: 3 June 2002 / Accepted: 29 June 2002 / Published online: 28 October 2002 RID="*" ID="*"Corresponding author. E-mail: qiang.huang@philips.com     

Temperature–rate dependences of the flow stress and the resistance to fracture of a VT6 titanium alloy under shock loading at a temperature of 20 and 600°C

The evolution of elastic-plastic shock compression waves in a VT6 titanium alloy is measured at a distance of 0.16–17 mm at room temperature and 600°C. The results of measuring the decay of an elastic precursors and the compression rate in a plastic shock wave are used to determine the temperature–rate dependences of the flow stress in the strain-rate range 10³–10^7s–1. New data for the spall strength of the alloy at normal and elevated temperatures are obtained.     

NMR relaxation study of disorder in condensed matter – low temperature studies in the mixed system − BPI(1−x)BP x – transition from classical to quantum rotation

¹H NMR spin-lattice relaxation time (T₁ ) studies have been carried out in the mixed system, betaine phosphite–betaine phosphate (BPI_(1?x)BP _x ), to study the effects of disorder. The T₁ data in the temperature range 100 K to 4 K, at two Larmor frequencies 11.4 and 23.3 MHz, has been analysed following Lourens’ model, which shows a gradual transition from classical reorientations to quantum tunnelling motions. At lower temperatures, (when thermal motions become too slow) differences (due to disorder) in the local environments of the reorienting groups result in a distribution in the activation energy (E_a ) and the energy gap of the ground to the first excited torsional level (E₀₁ ). Below 50 K, the system moves into the quantum tunneling regime and the magnetisation recovery shows biexponential behaviour which is another signature of disorder. These results are compared with those of the parent compounds BP and BPI.     

Dielectric spectroscopy of PbZr1 ? xTixO3 solid solutions (0.495 ≤ x ≤ 0.51) in a temperature range of 100–300 K at frequencies from 1 × 10?2 to 2 × 107 Hz

The behavior of the relative permittivity ɛ/ɛ₀ of PbZr_{1 − x} Ti _x O₃ (PZT) solid solutions (0.495 ≤ x ≤ 0.51) in the temperature range of 100–300 K at frequencies from 1 × 10⁻² to 2 × 10⁷ Hz was investigated. Diffuse, strongly relaxing maxima at T = 230−260 K (x = 0.495−0.505) and 150–160 K (x = 0.510) were observed in the PZT studied. The relaxation processes are well described by the Vogel-Fulcher law, and the dielectric spectra are approximated by the Cole-Cole formula.