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).     

Stable carbon, nitrogen, and oxygen isotope analysis as a potential tool for verifying geographical origin of beef

Stable isotope analysis of organic elements such as carbon and nitrogen has been employed as a powerful tool for provenance determination of food materials, because isotopic compositions of the materials reflect many factors in natural environment. In this study, we examined carbon, nitrogen, and oxygen isotope signatures of beef from Australia, Japan, and USA, in order to confirm the method as a potential tool for verifying geographical origin of beef commercially distributed in Japan. Defatted dry matter of beef from USA was characterized by higher carbon isotopic composition (-13.6 per thousand to -11.1 per thousand) than that from Japan (-19.6 per thousand to -17.0 per thousand) and Australia (-23.6 per thousand to -18.7 per thousand). That from Australia was characterized by higher oxygen isotopic composition (+15.0 per thousand to +19.4 per thousand) than that from Japan (+7.3 per thousand to +13.6 per thousand) and USA (+9.5 per thousand to +11.7 per thousand). The oxygen isotopic composition in Japanese beef showed a positive correlation with the isotopic composition of cattle drinking water, the difference in which is clearly latitude dependent. These results suggest that a comparison of carbon, nitrogen, and oxygen isotopic compositions is applicable as a potential tool to discriminate the provenance of beef not only between different countries (i.e. Australia, Japan, and USA) but also among different regions within Japan.     

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.     

Determination of light elements in metals and semiconductors by charged particle activation analysis

The determination of boron, carbon, nitrogen and oxygen in metals and semiconductors by charged particle activation analysis (CPAA) is reviewed. It is shown that CPAA is a sensitive and accurate method suitable for the analysis of reference materials.     

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.     

Open tube combustion method of organic samples for stable carbon isotope analysis

A simple and effective method for the conversion of organic carbon into carbon dioxide for analysis of stable carbon isotopes (delta(13)C) in samples of various organic substances, soils, sedimentary rocks, oils and volatile organic liquids is presented. The conversion of organic carbon of the samples is carried out in a quartz reactor connected to a vacuum line for CO(2) freezing and purification. A solid organic sample mixed with CuO is placed at the reactor bottom and the reactor is subsequently filled with granular CuO. One end of the CuO column is preheated to 850 degrees C while the other end of the column in contact with the sample is kept at ambient temperature. Heating of the sample (850 degrees C) and the remainder of the column is then performed. The preheated part of the column provides efficient conversion of carbon into CO(2). The reactor for the conversion of volatile liquid organic compounds is filled with granular CuO. The column of CuO is heated to 850 degrees C. Samples of volatile liquids are introduced into the reactor through a septum using a microsyringe. Complete conversion takes 10 min for solid samples and 3 min for volatile liquids. The precision of the delta(13)C analysis for solid and volatile liquid organic substances is +/-0.1 per thousand and +/-0.04 per thousand, respectively.     

delta34S measurements on organic materials by continuous flow isotope ratio mass spectrometry

Sulfur (S) isotope ratios of thoroughly dried organic samples were measured by direct thermal decomposition in an elemental analyzer coupled to an isotope ratio mass spectrometer in continuous flow mode (EA-CF-IRMS). For organic samples of up to 13 mg weight and with total S contents of more than 10 microg, the reproducibility of the delta34S(organic) values was +/-0.4 per thousand or better. However, the delta34S values of organic samples measured directly by online EA-CF-IRMS analysis were between 0.3 and 2.9 per thousand higher than those determined on BaSO4 precipitates produced by Parr Bomb oxidation from the same sample material. Our results suggest that structural oxygen in organic samples influences the oxygen isotope ratios of the SO2 produced from organic samples. Consequently, SO2 generated from organic samples appears to have different 18O/16O ratios than SO2 generated from BaSO4 precipitates and inorganic reference materials, resulting in a deviation from the true delta34S values because of 32S16O18O contributions to mass 66. It was shown that both the amount of structural oxygen in the organic sample, and the difference of the oxygen isotope ratios between organic samples and tank O2, influenced the magnitude of the observed deviation from the true delta34S value after direct EA-CF-IRMS analysis of organic samples. Suggestions are made to correct the difference between measured delta34S(organic) and true delta34S values in order to obtain not only reproducible, but also accurate S isotope ratios for organic materials by EA-CF-IRMS.     

Automatic microanalyzers: II. Automatic analyzer for rapid microdetermination of oxygen

A commercially available analyzer for the microanalytic determination of oxygen in organic and some inorganic compounds is described. The technique rests upon the application of the Schütze-Unterzaucher principle; it involves flash pyrolysis of the sample in nitrogen atmosphere and reduction of pyrolitic gases with carbon black at 1120 °C in order to produce carbon monoxide which is oxidized into carbon dioxide; the latter is finally titrated by protonometric coulometry.The use of a vertical reactor makes possible a complete automation of the technique and besides it preserves the initial properties of the aging carbon black filling which is a very important feature for correct oxygen determination. The flash pyrolysis of samples in the vertical tube makes also possible a very important shortening of analysis time: for routine operation, a 7 min duration is generally chosen by the operator but it can be reduced to only 5 min at will.This technique covers a very broad analytical field and the apparatus can be provided with an automatic sample dispenser now commercially available, so that series determinations are carried out without operator save for the previous weighing out of all samples.     

Automated quantitative and isotopic (13C) analysis of dissolved inorganic carbon and dissolved organic carbon in continuous-flow using a total organic carbon analyser

A method for the automated (13)C analysis of dissolved inorganic and organic carbon species has been developed to operate on a continuous-flow isotope ratio mass spectrometer (CF-IRMS). For natural and anthropogenic carbon species, the (13)C stable isotope has proven to be an excellent environmental tracer. Analytical performance tests were carried out on various organic compounds from easily oxidisable (sugar) to difficult (humic acid). A set of natural samples was also analysed to confirm the flexibility of the system. Analytical precision (2sigma) is typically <0.20 per thousand with sample reproducibility from 0.10-0.35 per thousand depending on reactivity of material. We believe this to be the first successful use of a total organic carbon (TOC) analyser for both dissolved inorganic and, specifically, dissolved organic species for (13)C stable isotope analysis in an automated CF-IRMS system. Routine analysis is achieved fairly quickly, is relatively simple with little or no sample manipulation, and will allow new and exciting studies for stable isotope research in both natural abundance and organic tracer studies not easily achieved before.     

Arsenic speciation in solid biological specimens by temperature-controlled pyrolysis and NAA

A pyrolysis-neutron activation analysis (NAA) procedure has been developed and applied to the speciation of arsenic in solid biological samples. The method involves the retention of the inorganic arsenic in the pyrolysis boat by the addition of NaOH, the volatilization and trapping of the organic arsenic on a cation exchange resin and the subsequent NAA of the resin for the determination of the trapped arsenic. The method, developed with the aid of radiochemically labelled arsenic compounds, has been applied to the determination of the ratio of inorganic to organic arsenic species in commercical shrimps as well as in NBS standard reference materials such as oysters and orchard leaves. The results show different relative amounts of inorganic arsenic content in the samples analysed. In the shrings the fraction of inorganic arsenic was of the order of 20%, in the oysters the inorganic arsenic consfituted 60% of the total arsenic concentration while in the samples of vegetable origin more than 98% of the arsenic was of inorganic nature.     

The International Measurement Evaluation Programme,IMEP-8: carbon and oxygen isotope ratios in CO<Subscript>2</Subscript>

In IMEP-8, two samples of high purity CO(2)(g), with different carbon and oxygen isotope ratios were distributed to 27 participants, originating from 14 countries and from various isotopic measurement domains (geochemistry, atmospheric and food chemistry), but particularly set up for food laboratories. In total 19 laboratories reported results. The outcome of this comparison exercise shows that the laboratories reported carbon and oxygen isotope results in good agreement with the reference values across the domains. The reported results for delta(13)C(VPDB) (carbon) for both materials are within 1 per thousand. However, for the reported results of delta(18)O(VPDB) (oxygen) for both materials the overall spread of the reported results is about 11 per thousand. Within this spread two distinct groups of participants can be identified, where the results within each group vary about 2 per thousand. The latter seems to be caused by calculation errors by participants of the reporting delta(18)O(VPDB) values. As requested, participants also reported the isotope amount ratio for carbon and oxygen in the CO(2) samples. For carbon, all reported results for both materials agree with the isotope ratio value, which can be traced back to the value reported by Craig. For oxygen, all results are in good agreement and deviate by a maximum of 0.5% from the reference values measured at IRMM. Work carried out indicates the carbon isotope ratio, for both samples IMEP-8A and IMEP-8B, differ from those reported by Craig by as much as 1.2%. In the case of oxygen, this deviation is far smaller. Both data sets, i.e. the one realised by Craig and the one realised at IRMM, demonstrate traceability to SI. It is clear that both values significantly disagree.     

High-precision determination of 18O/16O ratios of silver phosphate by EA-pyrolysis-IRMS continuous flow technique

A high-precision, and rapid on-line method for oxygen isotope analysis of silver phosphate is presented. The technique uses high-temperature elemental analyzer (EA)-pyrolysis interfaced in continuous flow (CF) mode to an isotopic ratio mass spectrometer (IRMS). Calibration curves were generated by synthesizing silver phosphate with a 13 per thousand spread in delta(18)O values. Calibration materials were obtained by reacting dissolved potassium dihydrogen phosphate (KH(2)PO(4)) with water samples of various oxygen isotope compositions at 373 K. Validity of the method was tested by comparing the on-line results with those obtained by classical off-line sample preparation and dual inlet isotope measurement. In addition, silver phosphate precipitates were prepared from a collection of biogenic apatites with known delta(18)O values ranging from 12.8 to 29.9 per thousand (V-SMOW). Reproducibility of +/- 0.2 per thousand was obtained by the EA-Py-CF-IRMS method for sample sizes in the range 400-500 microg. Both natural and synthetic samples are remarkably well correlated with conventional (18)O/(16)O determinations. Silver phosphate is a very stable material and easy to degas and, thus, could be considered as a good candidate to become a reference material for the determination of (18)O/(16)O ratios of phosphate by high-temperature pyrolysis.     

Pyrolysis behaviour of titanium dioxide-poly(vinyl pyrrolidone) composite materials

Inorganic-organic hybrid materials are studied due to the unique properties they exhibit. As these materials become more widely applied, particularly as precursor materials for forming inorganic materials, it is essential that the pyrolysis behaviour is understood. Transparent yellow hybrid materials consisting of titanium dioxide and poly(vinyl pyrrolidone) were prepared using sol-gel processing techniques. The hybrids maintained their transparency up to the highest achieved inorganic loading of 57 wt.%. These materials were characterised using thermogravimetric analysis in which the organic component was pyrolysed. The resultant chars were then investigated using optical microscopy, x-ray diffraction, scanning electron microscopy, and atomic force microscopy. The inorganic loading had an effect on char formation, with higher loadings leading to the formation of pyrolysis intermediates which were less apparent in samples of lower inorganic content. The pyrolysis intermediates were found to be carbon-rich.     

Deuterium/hydrogen isotope ratio measurement of water and organic samples by continuous-flow isotope ratio mass spectrometry using chromium as the reducing agent in an elemental analyser

A rapid continuous-flow technique for quantitative determination of hydrogen isotope ratios in water and organic materials at natural abundance levels is described. Water and organic samples were reduced in a helium stream at temperatures in excess of 1000 degrees C over chromium metal. delta(2)H per thousand values of water and organic samples were determined by calibration against International Atomic Energy Agency reference materials V-SMOW and SLAP water. The accuracy of the method was demonstrated through the analysis of the intermediate water standard GISP and IAEA water intercomparison materials OH-1, OH-2 and OH-3. Values obtained using this technique compared well with reference values (maximum difference 2.2 per thousand). The precision of water analyses was less than 2.3 per thousand (1 sigma or 1 standard deviation) in all cases. No apparent memory effect was observed when measuring samples at the natural abundance level. The application of the technique to organic molecules and the salts of organic acids was successfully demonstrated by measuring the delta(2)H per thousand values of an n-hexadecane laboratory reference and anhydrous calcium formate versus water calibration materials. Copyright Crown copyright 2001. Reproduced with permission of the Controller of Her Majesty's Stationery Office. Published by John Wiley & Sons, Ltd.     

Spectral-isotopic method of determination of gas-forming elements in organic and inorganic substances

A review is given of the latest papers on the application of the spectral-isotopic method of determination of gas-forming impurities in organic and inorganic materials. Main attention is paid to a new kind of investigation, i.e. the development of spectroscopic methods for isotopic analyses of nitrogen, carbon, oxygen, and hydrogen in biological objects for use in analytical monitoring in biological experiments with “tracers”. A universal method is described for the preparation of natural samples, for the separation of the gases to be analyzed from the sample, and for their purification from the impurities. The main characteristics are given of the methods for individual determination of the above elements and for joint analyses of some elements (N¹⁵-C¹³-H²) in one analytical cycle (spectrum excitation conditions, isotope concentration calculations, etc.). The characteristics of the methods have been compared with the results of other researches and the scope of our method for applications in biological investigations with tracers has been defined. The main results of spectral-isotopic method development in the traditional direction, i.e. for quantitative analysis of impurities in solid inorganic materials and for gas analysis, are produced.     

A strategy for selection of reference materials in stable oxygen isotope analyses of solid materials

The propagation of uncertainties associated with the stable oxygen isotope reference materials through a multi-point normalisation procedure was evaluated in this study using Monte Carlo (MC) simulation. We quantified the normalisation error for a particular selection of reference materials and their number of replicates, when the choice of standards is restricted to either nitrates, sulphates or organic reference materials alone, and in comparison with when this restriction was relaxed. A lower uncertainty in stable oxygen isotope analyses of solid materials performed using High-Temperature Pyrolysis (HTP) can be readily achieved through an optimal selection of reference materials. Among the currently available certified reference materials the best performing pairs minimising the normalisation errors are USGS35 and USGS34 for nitrates; IAEA-SO-6 and IAEA-SO-5 for sulphates; and IAEA-601 and IAEA-602 for organic materials. The normalisation error can be reduced further--by approximately half--if each of these two analysed reference materials is replicated four times. The overall optimal selection among all nine considered reference materials is the IAEA-602 and IAEA-SO-6 pair. If each of these two reference materials is replicated four times the maximum predicted normalisation error will equal 0.22‰, the minimum normalisation error 0.12‰, and the mean normalisation error 0.15‰ over the natural range of δ(18)O variability. We argue that the proposed approach provides useful insights into reference material selection and in assessing the propagation of analytical error through normalisation procedures in stable oxygen isotope studies.     

Analysis of gases containing inorganic and organic compounds using a combination of a thick-film capillary column and a packed adsorption column

A gas chromatographic system consisting of one multiport valve, two (hot-wire and flame-ionization) detectors and two analytical columns (one thick-film capillary and one packed adsorption column) is used for the analysis of gas samples containing a number of inorganic compounds (hydrogen, argon, oxygen, nitrogen, carbon monoxide, and carbon dioxide) and organic compounds. Examples include samples containing hydrocarbons up to n-nonane and benzene and toluene. The system also permits the analysis of more complicated samples containing, for example, alcohols, in addition to hydrocarbons.     

Inter‐laboratory comparison of elemental analysis and gas chromatography/combustion/isotope ratio mass spectrometry. II. δ15N measurements of selected compounds for the development of an isotopic Grob test

An inter‐laboratory exercise was carried out by a consortium of five European laboratories to establish a set of compounds, suitable for calibrating gas chromatography/combustion/isotope ratio mass spectrometry (GC‐C‐IRMS) devices, to be used as isotopic reference materials for hydrogen, carbon, nitrogen and oxygen stable isotope measurements. The set of compounds was chosen with the aim of developing a mixture of reference materials to be used in analytical protocols to check for food and beverage authentication. The exercise was organized in several steps to achieve the certification level: the first step consisted of the a priori selection of chemical compounds on the basis of the scientific literature and successive GC tests to set the analytical conditions for each single compound and the mixture. After elimination of the compounds that turned out to be unsuitable in a multi‐compound mixture, some additional oxygen‐ and nitrogen‐containing substances were added to complete the range of calibration isotopes. The results of δ¹³C determinations for the entire set of reference compounds have previously been published, while the δD and δ¹⁸O determinations were unsuccessful and after statistical analysis of the data the results did not reach the level required for certification. In the present paper we present the results of an inter‐laboratory exercise to identify and test the set of nitrogen‐containing compounds present in the mixture developed for use as reference materials for the validation of GC‐C‐IRMS analyses in individual laboratories. Copyright © 2009 John Wiley & Sons, Ltd.     

High-temperature pyrolysis/gas chromatography/isotope ratio mass spectrometry: simultaneous measurement of the stable isotopes of oxygen and carbon in cellulose

Stable isotope analysis of cellulose is an increasingly important aspect of ecological and palaeoenvironmental research. Since these techniques are very costly, any methodological development which can provide simultaneous measurement of stable carbon and oxygen isotope ratios in cellulose deserves further exploration. A large number (3074) of tree-ring α-cellulose samples are used to compare the stable carbon isotope ratios (δ(13)C) produced by high-temperature (1400°C) pyrolysis/gas chromatography (GC)/isotope ratio mass spectrometry (IRMS) with those produced by combustion GC/IRMS. Although the two data sets are very strongly correlated, the pyrolysis results display reduced variance and are strongly biased towards the mean. The low carbon isotope ratios of tree-ring cellulose during the last century, reflecting anthropogenic disturbance of atmospheric carbon dioxide, are thus overestimated. The likely explanation is that a proportion of the oxygen atoms are bonding with residual carbon in the reaction chamber to form carbon monoxide. The 'pyrolysis adjustment', proposed here, is based on combusting a stratified sub-sample of the pyrolysis results, across the full range of carbon isotope ratios, and using the paired results to define a regression equation that can be used to adjust all the pyrolysis measurements. In this study, subsamples of 30 combustion measurements produced adjusted chronologies statistically indistinguishable from those produced by combusting every sample. This methodology allows simultaneous measurement of the stable isotopes of carbon and oxygen using high-temperature pyrolysis, reducing the amount of sample required and the analytical costs of measuring them separately.     

卡尔菲休滴定仪测定液体石油产品中水含量

对运用卡尔菲休滴定仪库仑法测定液体石油产品中微量水的简单原理、分析方法的建立与分析流程进行了论述.实验结果表明,该方法测定结果的相对标准偏差为0.66%～1.30%(n=8),用该方法测定3种标准物质,其相对误差不大于2%.该方法分析快速、简便,适用于含有醇类、无机含氧弱酸特殊样品的测定.