Oxygen isotope studies of phosphite oxidation: purification and analysis of reactants and products by high-temperature conversion elemental analyzer/isotope ratio mass spectrometry

RATIONALE

Increased attention has been recently focused on the origin and reactions of reduced-P oxyanions such as phosphite [PO3 (III)] in terrestrial and biological systems. We present new methods for studying O-isotopic reactions between PO3 (III) and other oxygen sources during oxidation of PO3 (III) to PO4 (V).

METHODS

Na2 (HPO3 )·5H2 O, used as a PO3 (III) source, contains structural water due to its hygroscopic nature; thus, we developed a method for determining the δ(18) O value of PO3 (III) after the removal of structural water. Next, we tested two techniques for purifying PO4 (V) from aqueous PO3 (III)/PO4 (V) mixtures: (1) precipitation of PO4 (V) as ammonium phosphomolybdate (APM); and (2) precipitation of PO4 (V) as magnesium ammonium phosphate (MAP). The O-isotope compositions, (18) O:(16) O (δ(18) O values), of Na2 (HPO3 ) and Ag3 PO4 were analyzed by TC/EA/IRMS.

RESULTS

Structural water was removed from Na2 (HPO3 )·5H2 O after drying at 100 °C under vacuum and the δ(18) O value of PO3 (III) was obtained. The δ(18) O values of PO4 (V), which was extracted from (18) O-labeled PO3 (III)/PO4 (V) mixtures by APM and MAP precipitations, were not altered by the precipitation process. This result confirms that PO3 (III) is not converted into PO4 (V) by oxidation or hydrolysis under either strong acidic/oxidizing (APM) or alkaline (MAP) conditions for up to a 24-h period.

CONCLUSIONS

We conclude that both APM and MAP precipitation are reliable and effective methods for the separation and purification of PO4 (V) from aqueous PO3 (III)/PO4 (V) mixtures. The methods described here will permit the study of the isotopic evolution of various pathways of geochemical as well as biological PO3 (III) oxidation.

Microwave extraction-isotope ratio infrared spectroscopy (ME-IRIS): a novel technique for rapid extraction and in-line analysis of δ18O and δ2H values of water in plants, soils and insects

RATIONALE

Traditionally, stable isotope analysis of plant and soil water has been a technically challenging, labour-intensive and time-consuming process. Here we describe a rapid single-step technique which combines Microwave Extraction with Isotope Ratio Infrared Spectroscopy (ME-IRIS).

METHODS

Plant, soil and insect water is extracted into a dry air stream by microwave irradiation within a sealed vessel. The water vapor thus produced is carried to a cooled condensation chamber, which controls the water vapor concentration and flow rate to the spectrometer. Integration of the isotope signals over the whole analytical cycle provides quantitative δ(18)O and δ(2) H values for the initial liquid water contained in the sample. Calibration is carried out by the analysis of water standards using the same apparatus. Analysis of leaf and soil water by cryogenic vacuum distillation and IRMS was used to validate the ME-IRIS data.

RESULTS

Comparison with data obtained by cryogenic distillation and IRMS shows that the new technique provides accurate water isotope data for leaves from a range of field-grown tropical plant species. However, two exotic nursery plants were found to suffer from spectral interferences from co-extracted organic compounds. The precision for extracted leaf, stem, soil and insect water was typically better than ±0.3 ‰ for δ(18)O and ±2 ‰ for δ(2) H values, and better than ±0.1 ‰ for δ(18)O and ±1 ‰ for δ(2) H values when analyzing water standards. The effects of sample size, microwave power and duration and sample-to-sample memory on isotope values were assessed.

CONCLUSIONS

ME-IRIS provides rapid and low-cost extraction and analysis of δ(18)O and δ(2) H values in plant, soil and insect water (≈10-15 min for samples yielding ≈ 0.3 mL of water). The technique can accommodate whole leaves of many plant species.

δ18O in the tropical conifer Agathis robusta records ENSO-related precipitation variations

Long-lived trees from tropical Australasia are a potential source of information about internal variability of the El Niño-Southern Oscillation (ENSO), because they occur in a region where precipitation variability is closely associated with ENSO activity. We measured tree-ring width and oxygen isotopic composition (δ18O) of α-cellulose from Agathis robusta (Queensland Kauri) samples collected in the Atherton Tablelands, Queensland, Australia. Standard ring-width chronologies yielded low internal consistency due to the frequent presence of false ring-like anatomical features. However, in a detailed examination of the most recent 15 years of growth (1995-2010), we found significant correlation between δ18O and local precipitation, the latter associated with ENSO activity. The results are consistent with process-based forward modeling of the oxygen isotopic composition of α-cellulose. The δ18O record also enabled us to confirm the presence of a false growth ring in one of the three samples in the composite record, and to determine that it occurred as a consequence of anomalously low rainfall in the middle of the 2004/5 rainy season. The combination of incremental growth and isotopic measures may be a powerful approach to development of long-term (150+ year) ENSO reconstructions from the terrestrial tropics of Australasia.

18O enrichment in phosphorus pools extracted from soybean leaves

The objective of this study was to investigate the isotopic composition of oxygen bound to phosphate (δ(18)O-PO(4)) in different phosphorus (P) pools in plant leaves. As a model plant we used soybean (Glycine max cv Toliman) grown in the presence of ample P in hydroponic cultures. The leaf blades were extracted with 0.3 M trichloroacetic acid (TCA) and with 10 M nitric acid. These extractions allowed measurement of the TCA-soluble reactive P (TCA P) that is rapidly cycled within the cell and the total leaf P. The difference between total leaf P and TCA P yielded the structural P which includes organic P compounds not extractable by TCA. P uptake and its translocation and transformation within the soybean plants lead to an (18)O enrichment of TCA P (δ(18)O-PO(4) between 16.9 and 27.5‰) and structural P (δ(18)O-PO(4) between 42.6 and 68.0 ‰) compared with 12.4‰ in the phosphate in the nutrient solution. δ(18)O values of phosphate extracted from soybean leaves grown under optimal conditions are greater than the δ(18)O-PO(4) values of the provided P source. Furthermore, the δ(18)O-PO(4) of TCA P seems to be controlled by the δ(18)O of leaf water and the activity of inorganic pyrophosphatase or other pyrophosphatases.

A high-performance, safer and semi-automated approach for the delta18O analysis of diatom silica and new methods for removing exchangeable oxygen

The determination of the oxygen isotope composition of diatom silica in sediment cores is important for paleoclimate reconstruction, especially in non-carbonate sediments, where no other bioindicators such as ostracods and foraminifera are available. Since most currently available analytical techniques are time-consuming and labour-intensive, we have developed a new, safer, faster and semi-automated online approach for measuring oxygen isotopes in biogenic silica. Improvements include software that controls the measurement procedures and a video camera that remotely records the reaction of the samples under BrF(5) with a CO(2) laser. Maximum safety is guaranteed as the laser-fluorination unit is arranged under a fume hood in a separate room from the operator. A new routine has been developed for removing the exchangeable hydrous components within biogenic silica using ramp degassing. The sample plate is heated up to 1100 degrees C and cooled down to 400 degrees C in approximately 7 h under a flow of He gas (the inert Gas Flow Dehydration method--iGFD) before isotope analysis. Two quartz and two biogenic silica samples (approximately 1.5 mg) of known isotope composition were tested. The isotopic compositions were reproducible within an acceptable error; quartz samples gave a mean standard deviation of <0.15 per thousand (1sigma) and for biogenic silica <0.25 per thousand (1sigma) for samples down to approximately 0.3 mg. The semi-automated fluorination line is the fastest method available at present and enables a throughput of 74 samples/week.

Stable oxygen isotopic fractionation during photolytic O(2) consumption in stream waters

Oxygen (O(2)) is required for life in higher organisms, however, processes such as respiration, the oxidation of reduced inorganic species, and the photolytic breakdown of dissolved organic matter (DOM) decrease the O(2) concentrations in aquatic systems. Filtered, inoculated, and sterile samples of stream waters from Ontario, Canada, were incubated in natural sunlight to examine the effects of photolysis of DOM, respiration, and abiotic reactions on O(2) consumption and delta(18)O of dissolved oxygen (delta(18)O-O(2)). Oxygen consumption rates in the light were up to an order of magnitude greater than in the dark, suggesting light-mediated processes controlled O(2) consumption. Rates of O(2) loss were the same for each treatment (i.e. filtered, inoculated, and sterile) indicating that photolysis was the dominant O(2) consuming process over respiration in these incubations. O(2) consumption rates were different between streams, even when normalized to the change in dissolved organic carbon (DOC), signifying that DOM photolability varied among streams. During DOM breakdown to CO(2), the lighter (16)O isotopomer was preferentially consumed. Fractionation factors observed for photolysis, respiration, and abiotic reactions ranged between 0.988 and 0.995, and were similar in both the light and in the dark incubations in all streams. These fractionation factors are not a function of O(2) consumption rates, and are outside the range published for respiration (0.975-0.982). In current models of O(2) and delta(18)O-O(2), photolysis and respiration are not considered separately and the isotopic fractionation during respiration that is measured in the dark is used in the light. In these incubations, DOM degradation and abiotic reactions are important O(2) consuming and delta(18)O-O(2) fractionating processes. Current models of O(2) and delta(18)O-O(2) incorporate photolysis of DOM and other abiotic processes into the respiratory component of O(2) consumption, thereby overestimating respiration and underestimating photosynthesis to respiration ratios. Consequently, photolysis and abiotic reactions should be considered separately, particularly in shallow aquatic systems with high DOC.

Routine quality control of recycled target [18O]water by capillary electrophoresis and gas chromatography

Recycling of [(18)O]water for [(18)F]fluoride production can be accomplished with reliable results. We have developed sensitive, robust, and rapid analyses of impurities in [(18)O]water. Anions were quantitated by capillary electrophoresis and organic residuals were quantitated by gas chromatography using methods with excellent reproducibility and linearity. Kryptofix 222 (K-222) was quantitated by a sensitive LC-MS-MS technique. Isotopic composition was determined by GC-MS with satisfactory accuracy and precision. These methods were employed to evaluate recovered [(18)O]water purified by a novel electrolysis method. 2-[(18)F]FDG yields using purified [(18)O]water with very low levels of impurities are indistinguishable from newly purchased [(18)O]water. High (> 300 ppm) carbonate concentration reduces the fluoride trapping efficiency of QMA. The analyses of anions, organics, and isotopic enrichment were applied routinely for quality control of [(18)O]water to predict a satisfactory outcome of 2-[(18)F]FDG production.

Mass-independent fractionation of oxygen isotopes during thermal decomposition of carbonates

Nearly all chemical processes fractionate 17O and 18O in a mass-dependent way relative to 16O, a major exception being the formation of ozone from diatomic oxygen in the presence of UV radiation or electrical discharge. Investigation of oxygen three-isotope behavior during thermal decomposition of naturally occurring carbonates of calcium and magnesium in vacuo has revealed that, surprisingly, anomalous isotopic compositions are also generated during this process. High-precision measurements of the attendant three-isotope fractionation line, and consequently the magnitude of the isotopic anomaly (delta17O), demonstrate that the slope of the line is independent of the nature of the carbonate but is controlled by empirical factors relating to the decomposition procedure. For a slope identical to that describing terrestrial silicates and waters (0.5247 +/- 0.0007 at the 95% confidence level), solid oxides formed during carbonate pyrolysis fit a parallel line offset by -0.241 +/- 0.042 per thousand. The corresponding CO2 is characterized by a positive offset of half this magnitude, confirming the mass-independent nature of the fractionation. Slow, protracted thermolysis produces a fractionation line of shallower slope (0.5198 +/- 0.0007). These findings of a 17O anomaly being generated from a solid, and solely by thermal means, provide a further challenge to current understanding of the nature of mass-independent isotopic fractionation.