Micropowder collecting technique for stable isotope analysis of carbonates

Micromilling is a conventional technique used in the analysis of the isotopic composition of geological materials, which improves the spatial resolution of sample collection for analysis. However, a problem still remains concerning the recovery ratio of the milled sample. We constructed a simple apparatus consisting of a vacuum pump, a sintered metal filter, electrically conductive rubber stopper and a stainless steel tube for transferring the milled powder into a reaction vial. In our preliminary experiments on carbonate powder, we achieved a rapid recovery of 5 to 100 µg of carbonate with a high recovery ratio (>90%). This technique shortens the sample preparation time, improves the recovery ratio, and homogenizes the sample quantity, which, in turn, improves the analytical reproducibility. Copyright © 2011 John Wiley & Sons, Ltd.     

Site-specific carbon isotope analysis of aromatic carboxylic acids by elemental analysis/pyrolysis/isotope ratio mass spectrometry

Site-specific carbon isotope composition of organic compounds can provide useful information on their origin and history in natural environments. Site-specific isotope analyses of small amounts of organic compounds (sub-nanomolar level), such as short-chain carboxylic acids and amino acid analogues, have been performed using gas chromatography/pyrolysis/isotope ratio mass spectrometry (GC/pyrolysis/IRMS). These analyses were previously limited to volatile compounds. In this study, site-specific carbon isotope analysis has been developed for non-volatile aromatic carboxylic acids at sub-micromolar level by decarboxylation using a continuous flow elemental analysis (EA)/pyrolysis/IRMS technique. Benzoic acid, 2-naphthylacetic acid and 1-pyrenecarboxylic acid were pyrolyzed at 500-1000 degrees C by EA/pyrolysis/IRMS to produce CO2 for delta13C measurement of the carboxyl group. These three aromatic acids were most efficiently pyrolyzed at 750 degrees C. Conventional sealed-tube pyrolysis was also conducted for comparison. The delta13C values of CO2 generated by the continuous flow technique were within 1.0 per thousand of those performed by the conventional technique, indicating that the new continuous flow technique can accurately analyze the carbon isotopic composition of the carboxyl group in aromatic carboxylic acids. The new continuous flow technique is simple, rapid and uses small sample sizes, so this technique will be useful for characterizing the isotopic signature of carboxyl groups in organic compounds.     

Carbon isotope analyses of cellulose using two different on-line techniques (elemental analysis and high-temperature pyrolysis)--a comparison

Even though the recent development in on-line methods for the stable isotope determination in cellulose has led to a significant increase in sample throughput and decrease in sample preparation expenditure, there still is a large potential for optimizing the analytical procedures by simultaneously measuring the isotope ratios of two or even more elements. Therefore, the main objective of this study was to answer the question whether high-temperature pyrolysis (HTP) is a suitable and reliable technique for the determination of the carbon isotopic composition of cellulose simultaneously during the well-known conventional oxygen isotope analysis. This study shows that HTP of cellulose is a technique that can produce reasonable delta(13)C values, matching the requirements of most research problems related to paleoclimatology. The reproducibility of the delta values for (13)C/(12)C is better than 0.2 per thousand. Some deficiencies of the method are related to the incomplete conversion of the organic carbon in the sample to carbon monoxide. A clear isotope effect seems to be related to the non-statistical conversion of the carbon in the cellulose to CO. The extent of this effect appears to be controlled by the relative proportion of crystallized and amorphous matter in the cellulose structure. Those deficiencies can be eliminated by using an appropriate normalization and by applying the principles of identical treatment for reference materials and samples. In general, a very good agreement is achieved for carbon isotope values determined by HTP and elemental analysis (EA).     

An innovative molybdenum column liner for oxygen and hydrogen stable isotope analysis by pyrolysis

The most widely used method for pyrolysing samples for hydrogen or oxygen isotopic analysis involves heating them to greater than 1300 degrees C in a helium stream passed through a glassy carbon tube in an alumina casing. There are a number of difficulties with this. Glassy carbon tubes are expensive and interaction between the carbon tube and the outer casing produces unwanted carbon monoxide by reduction of the alumina at high temperatures. The latter effect is overwhelming if temperatures of 1400 degrees C or greater are used for pyrolysis. We experimented with lining alumina casings with pure molybdenum sheet. It is relatively cheap, conforms well to the interior of the reactor tube (to avoid carrier and sample bypassing of the carbon pack), resists high temperatures and neither oxidises excessively nor absorbs the gases. The main disadvantages are that silver sample cups must be used and that the molybdenum degrades over time by formation of the carbide. We can maintain sharp peaks, high precision and good accuracy over more than 700 solid samples for both hydrogen and oxygen. The reactors last longer for water injections. The molybdenum in the columns does not contribute greatly to memory effects. The precision of analysis is dependent on other factors as well as the pyrolysis column, but for oxygen we typically achieve approximately <0.2 per thousand (sucrose), <0.25 per thousand (water) and <0.25 per thousand (leaf), sometimes using only a linear correction of drift, after dividing the run into 1 to 3 segments.     

Continuous high-temperature fluidized bed pyrolysis of coal in complex atmospheres: Product distribution and pyrolysis gas

This study is devoted to investigating the continuous coal pyrolysis in a laboratory fluidized bed reactor that fed coal and discharged char continuously at temperatures of 750–980 °C and in N₂-base atmospheres containing O₂, H₂, CO, CH₄ and CO₂ at varied contents. The results showed that the designed continuous pyrolysis test provided a clear understanding of the coal pyrolysis behavior in various complex atmospheres free of and with O₂. The effect of adding H₂, CO, CH₄ or CO₂ into the atmosphere on the tar yield was related to the O₂ content in the atmosphere. Without O₂ in the atmosphere, adding H₂ and CO₂ decreased the pyrolysis tar yield, but the tar yield was conversely higher with raising the CO and CH₄ contents in the atmosphere. In O₂-containing atmospheres, the influence from varying the atmospheric gas composition on the product distribution and pyrolysis gas composition was closely related to the oxidation or gasification reactions occurring to char, tar and the tested gas.     

Flash flow pyrolysis: mimicking flash vacuum pyrolysis in a high-temperature/high-pressure liquid-phase microreactor environment

Flash vacuum pyrolysis (FVP) is a gas-phase continuous-flow technique where a substrate is sublimed through a hot quartz tube under high vacuum at temperatures of 400-1100 °C. Thermal activation occurs mainly by molecule-wall collisions with contact times in the region of milliseconds. As a preparative method, FVP is used mainly to induce intramolecular high-temperature transformations leading to products that cannot easily be obtained by other methods. It is demonstrated herein that liquid-phase high-temperature/high-pressure (high-T/p) microreactor conditions (160-350 °C, 90-180 bar) employing near- or supercritical fluids as reaction media can mimic the results obtained using preparative gas-phase FVP protocols. The high-T/p liquid-phase "flash flow pyrolysis" (FFP) technique was applied to the thermolysis of Meldrum's acid derivatives, pyrrole-2,3-diones, and pyrrole-2-carboxylic esters, producing the expected target heterocycles in high yields with residence times between 10 s and 10 min. The exact control over flow rate (and thus residence time) using the liquid-phase FFP method allows a tuning of reaction selectivities not easily achievable using FVP. Since the solution-phase FFP method does not require the substrate to be volatile any more--a major limitation in classical FVP--the transformations become readily scalable, allowing higher productivities and space-time yields compared with gas-phase protocols. Differential scanning calorimetry measurements and extensive DFT calculations provided essential information on pyrolysis energy barriers and the involved reaction mechanisms. A correlation between computed activation energies and experimental gas-phase FVP (molecule-wall collisions) and liquid-phase FFP (molecule-molecule collisions) pyrolysis temperatures was derived.     

A reactor for high-temperature pyrolysis and oxygen isotopic analysis of cellulose via induction heating

A reactor for converting cellulose into carbon monoxide for subsequent oxygen isotopic analysis via continuous flow isotope ratio mass spectrometry is described. The system employs an induction heater to produce temperatures >or=1500 degrees C within a molybdenum foil crucible positioned by boron nitride (BN) spacers within a quartz outer sleeve. For samples of a homogeneous working standard cellulose between 300 and 400 microg in size, the blank/signal ratio is <5%, and the long-term precision is 0.30 per thousand (N = 232). For samples of 30 to 100 microg in size, a gas pressure sintered silicon nitride (Si(3)N(4)) outer sleeve replaces the quartz sleeve, the BN spacers are not used, and 6.0-grade carrier He must be used to minimize the blank signal. With these modifications a blank/sample ratio of <5% and long-term precision of 0.30 per thousand (N = 144) are obtained. These results are similar to those achieved using standard high-temperature furnaces, but the reactor is simpler to pack, the system is more economical to run, and samples as small as 30 microg cellulose may be measured. For both reactors memory is significant in the subsequent sample and is believed to be due to exchange with reactor oxygen at temperatures above 1000 degrees C. Further applications might include online preparation of other materials requiring temperatures of 1500-2600 degrees C.     

Lack of kinetic hydrogen isotope effect in acetylene pyrolysis

Second order rate constants for C₂H₂ or C₂D₂ polymerizations into vinylacetylene and higher C_nH_n products have been measured in a static reactor by dynamic mass spectrometry between 770–980 K. They are nearly identical within experimental error (±50%). It is shown that these results are consistent with the participation of thermally equilibrated vinylidene H₂C ? C: as a reactive intermediate: (1) since this assumption only introduces a modest reverse equilibrium isotope effect (K_iH/K_iD ca. 0.48 in this range) into overall rate constants. At the same time they seem to discriminate in general against alternative mechanisms in which the required H-atom transfers take place in rate determining steps. Present evidence, in conjunction with an updated analysis of relevant issues such as experimental and theoretical vs. termochemical estimates of the heat of formation of H₂C?C:, the nature of the transition states of singlet vinylidene addition reactions and the likelihood of discrete biradical intermediates in C₂H₂ dimerization, seems to lend further support to the notion that acetylene behaves as a singlet carbene at high temperatures.     

Determination of boron in marine carbonates by prompt gamma-ray analysis

Boron in carbonate reference samples was measured by neutron-induced prompt gamma-ray analysis (PGA) using cold and thermal neutron guide beams of the JRR-3M reactor. In order to determine B contents in marine carbonates, the Doppler-broadened -ray peak of 478 keV was used together with the correction of interference from Na-peak of 472 keV. We determined B in coral samples within 3% of analytical precision. The data obtained by the present method are mostly consistent with reported values. Here, we report PGA of B in marine carbonates.     

Oxidation of S and SO by O2 in high-temperature pyrolysis and photolysis reaction systems

The reaction of S atoms with O₂ was studied behind reflected shock waves applying atomic resonance absorption spectroscopy (ARAS) for concentration measurements of S and O atoms. S atoms were generated either by laser-flash photolysis (LFP) of CS₂ or by the high-temperature pyrolysis of COS, respectively. The concentrations of O₂ in the mixtures ranged between 50 ppm and 400 ppm, and those of the S precursors, CS₂ and COS, between 5 and 25 ppm. The rate coefficient of the reaction was determined from the observed decay of the S absorption signals for temperatures 1220 K ? T ? 3460 K. The measured O-atom concentration profiles in COS/O₂/Ar reaction systems were evaluated, using simplified kinetic mechanism, to verify the given rate coefficient k₅. In experiments with the highest value of the [O₂]/[COS] ratio the measured O-atom concentrations were found to be sensitive to the reaction: The fitting of the calculated O-atom profiles to the measured ones results in mean value of: which is to be valid for the temperature range 2570 K ? T ? 2980 K. A first-order analysis of the observed S absorption decay in LFP shock wave experiments on CS₂/Ar gas mixtures resulted in a rate coefficient of the background reaction (R4): for temperatures 1260 K ? T ? 1820 K. © 1995 John Wiley & Sons, Inc.     

Oxygen determination in organic fluorine compounds by means of a glassy-carbon pyrolysis tube

A conventional apparatus for determination of oxygen in organic compounds has been improved for application to organic fluorine compounds. A feature of the apparatus is the use of a pyrolysis tube made of glassy carbon instead of quartz, which eliminates effects due to hydrogen fluoride produced in pyrolysis of the sample. Ten analyses of dexamethasone with the apparatus gave a mean value of 20.44% for oxygen (theory, 20.38%), with a standard deviation of 0.16%. Oxygen in 9 organic fluorine compounds was accurately determined by using the apparatus, with an average error of +0.1%. One analysis by a gravimetric or a coulometric method took about 40 or 25 min, respectively.     

Oxygen isotope exchange in oxygen-bearing compounds of uranium

Isotope exchange is reported for gaseous oxygen in contact with the following uranium compounds: -Na₂UO₄, -Na₂UO₄, Na₂U₂O₇, UO₃(A), -UO₃, -UO_2.94 and U₃O₈.; qualitative tests have also been done with UO₂F₂ and Cs₂UO₂Cl₄. The times of half-exchange have been determined as functions of temperature for U₃O₈, -UO_2.94, Na₂U₂O₇ and -Na₂UO₄; diffusion coefficients for oxygen have been calculated for UO₃(A), -UO₃, Na₂U₂O₇, -Na₂UO₄ and -Na₂UO₄. Activation energies have been deduced for diffusion and surface exchange. All the oxygen atoms in these compounds are equivalent as regards isotope exchange; the above activation energies increase with the UO ratio in some cases. Diffusion-limited exchange tends to show periodic oscillations in rate not ascribable to errors of measurement; a mechanism is proposed for this.     

Calibration of isotope amount ratios by analysis of isotope mixtures

The question as to what constitutes a fully calibrated isotope amount ratio measurement still remains a topic of active research. For years, the definitive calibration approach has been by means of synthetic mixtures of highly enriched isotopes with known chemical purity to give gravimetrically defined ratios. This article outlines the core concepts and assumptions of this method and illustrates the recent developments in the practical metrology of isotope amount ratio measurements.     

Routine hydrogen isotope measurement of cellulose nitrate by high-temperature pyrolysis--reference materials and precision

The determination of isotope ratios of non-exchangeable hydrogen in tree-ring cellulose is commonly based on the nitration of wood cellulose followed by online high-temperature pyrolysis and isotope ratio mass spectrometry measurement of cellulose nitrate samples. The application of this method requires a proper calibration using appropriate reference materials whose delta(2)H values have been reliably normalized to the V-SMOW/SLAP scale. In our study, we achieve this normalization by a direct alternating measurement of reference waters (V-SMOW and SLAP) and three cellulose nitrates chosen as reference materials. For that purpose, both water and solid organic samples are introduced into the pyrolysis reactor by silver capsule injection. The analytical precision of the water measurement using the capsule method is +/-1.5 per thousand. The hydrogen isotopic composition of three cellulose nitrate standards measured ranges from -106.7 to -53.9 per thousand. The standard deviation of the calculated means from five measurement periods of those standards is better than 1 per thousand. Twenty-four different measurements of the hydrogen isotope composition of cellulose nitrate were evaluated in order to assess the precision of the described method. We obtained an analytical precision of +/-3.0 per thousand as representative for the 95% confidence interval applicable for routine measurements of cellulose nitrate samples. Evidence was found for significant differences in the behavior of cellulose nitrate and PE foil during the pyrolitic conversion that emphasizes the need for a proper calibration of the routine measurements. This calibration can only be successful if the reference materials used have a very similar chemical composition and undergo the same preparation procedure as the samples.