Adapting an elemental analyser to perform high-temperature catalytic oxidation for dissolved organic carbon measurements in water.

RATIONALE: Many laboratories employ elemental analyzers (EAs) coupled to isotope ratio mass spectrometers (IRMSs) to measure carbon stable isotope ratios (δ13 C) in solid samples. Dissolved organic carbon (DOC) in most natural water samples cannot be analyzed using this approach unless time-consuming preconcentration is employed. METHODS: An EA-IRMS can be used to measure DOC δ13 C in natural waters without the need for sample preconcentration by employing high-temperature catalytic oxidation. An autosampler injects water into the EA reactor at 680°C filled with platinum catalyst beads, where all carbon is converted to CO2 . Remaining water and halides are removed, while CO2 is trapped in a cryotrap and later released to the IRMS. RESULTS: Measurements were accurate (deviation <0.3‰ compared to solid sample measurements) and precise (error of 0.3‰ for concentrations ≥46 µM). Blanks were present and accounted for. Salinity up to seawater level did not affect accuracy or precision, but limited the number of samples that could be run before cleaning of the reactor was needed. DOC δ13 C in a river/estuary varied between -25.7 and -23.2‰, with higher values for waters with higher salinity, as expected. Deep-sea water reference material had a value of -22.9 ± 0.5‰, very similar to those found in recent reports employing similar techniques. CONCLUSION: Adapting an EA is a feasible approach for the measurement of DOC δ13 C in natural waters. The low cost and simplicity of the system allow its use in any laboratory already equipped with EA-IRMS.

Interlaboratory test for stable carbon isotope analysis of dissolved inorganic carbon in geothermal fluids.

RATIONALE: Stable carbon isotope ratios have many applications in natural sciences. In the first worldwide interlaboratory proficiency test, the discrepancies in measured δ13 CDIC values of natural waters were up to σ = ±3‰. Therefore, we continued the investigation on the analytical data quality assurance of individual laboratories and internal consistency among laboratories worldwide. METHODS: We designed and performed an interlaboratory comparison exercise for δ13 C analyses of ten water and two solid samples (Na2 CO3 , CaCO3 ), including two synthetic samples prepared by dissolving the carbonates individually. Three laboratories analyzed an additional sample set to assess solution stability, at least one month after the first set analysis period. The δ13 C values were measured using dual inlet isotope ratio mass spectrometry (DI-IRMS) or continuous flow (CF)-IRMS. RESULTS: The δ13 C values of solid Na2 CO3 and its aqueous solution were -5.06 ± 0.21‰ and 5.32 ± 0.24‰, respectively, while the δ13 C value of solid CaCO3 was -4.49 ± 0.93‰. Similarly, the lake water has a consistent value (2.45 ± 0.19‰). The δ13 C values of geothermal water have a wide dispersion among individual laboratory measurements and among those of different laboratories; however, a trend exists in the δ13 C values measured at the three sampling points of each well. CONCLUSIONS: The δ13 C values of solid Na2 CO3 and its solution, and lake water (i.e. DIC concentration samples >100 mg/L carbon) are consistent among all the participating laboratories. The dispersion in the δ13 C values of solid CaCO3 is associated with its lower chemical affinity than that of Na2 CO3 . The poor reproducibility in the δ13 C values of geothermal fluids, collected at three points of a geothermal well, despite overall consistent trends regarding their collection points suggests inadequate sample handling (atmospheric CO2 exchange) and/or inappropriate analytical approaches (incomplete H3 PO4 acid reaction).

Light respiration by subtropical seaweeds.

Here, we report the first-ever measurements of light CO2 respiration rate (CRR) by seaweeds. We measured the influence of temperature (15-25°C) and light (irradiance from 60 to 670 µmol · m-2  · s-1 ) on the light CCR of two subtropical seaweed species, and measured the CRR of seven different seaweed species under the same light (150 µmol · m-2  · s-1 ) and temperature (25°C). There was little effect of irradiance on light CRR, but there was an effect of temperature. Across the seven species light CRR was similar to OCR (oxygen consumption rate in the dark), with the exception of a single species. The outlier species was a coralline alga, and the higher light CRR was probably driven by calcification. CRR could be estimated from OCR, as well as carbon photosynthetic rates from oxygen photosynthetic rates, which suggests that previous studies have probably provided good estimations of gross photosynthesis for seaweeds.

Bulk hydrogen stable isotope composition of seaweeds: Clear separation between Ulvophyceae and other classes.

Little is known about the bulk hydrogen stable isotope composition (δ2 H) of seaweeds. This study investigated the bulk δ2 H in several different seaweed species collected from three different beaches in Brazil, Australia, and Argentina. Here, we show that Ulvophyceae (a group of green algae) had lower δ2 H values (between -94‰ and -130‰) than red algae (Florideophyceae), brown algae (Phaeophyceae), and species from the class Bryopsidophyceae (another group of green algae). Overall the latter three groups of seaweeds had δ2 H values between -50‰ and -90‰. These findings were similar at the three different geographic locations. Observed differences in δ2 H values were probably related to differences in hydrogen (H) metabolism among algal groups, also observed in the δ2 H values of their lipids. The marked difference between the δ2 H values of Ulvophyecae and those of the other groups could be useful to trace the food source of food webs in coastal rocky shores, to assess the impacts of green tides on coastal ecosystems, and to help clarify aspects of their phylogeny. However, reference materials for seaweed δ2 H are required before the full potential of using the δ2 H of seaweeds for ecological studies can be exploited.

Improving the measurement of nitrogen stable isotopes in organic materials containing high C:N ratios using a 5A molecular sieve column.

The nitrogen stable isotope composition (δ15N) of plant materials has numerous applications. Plant materials like bark can have a very high C:N ratio. Incomplete C combustion in such samples interferes with the δ15N measurement due to CO production. We modified the standard setup for δ15N measurement using an elemental analyzer (EA) coupled to an isotope ratio mass spectrometer (IRMS) by incorporating a 5A molecular sieve column, which better separates N2 from CO. We compared this new modified setup and the standard one for the measurement of bark samples. Precision and accuracy for δ15N in standards with low C:N ratio were equivalent for the two methods. However, for bark the results obtained with the new method had better precision and accuracy than the standard method. Replicates are nevertheless recommended with the new method to ensure confidence in the results.•During elemental analysis, incomplete combustion of material with high C:N ratio can lead to CO formation, which interferes with δ15N IRMS measurements.•Here we use a 5A molsieve column to remove the CO interference in δ15N measurements Precision and accuracy on δ15N measurements of samples with high C content are significantly improved.

Stable carbon isotope analysis of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) in natural waters--results from a worldwide proficiency test.

RATIONALE: Stable carbon isotope ratios of dissolved inorganic (DIC) and organic carbon (DOC) are of particular interest in aquatic geochemistry. The precision for this type of analysis is typically reported in the range of 0.1‰ to 0.5‰. However, there is no published attempt that compares δ(13)C measurements of DIC and DOC among different laboratories for natural water samples. METHODS: Five natural water samples (lake water, seawater, two geothermal waters, and petroleum well water) were analyzed for δ(13)CDIC and δ(13)CDOC values by five laboratories with isotope ratio mass spectrometry (IRMS) in an international proficiency test. RESULTS: The reported δ(13)CDIC values for lake water and seawater showed fairly good agreement within a range of about 1‰, whereas geothermal and petroleum waters were characterized by much larger differences (up to 6.6‰ between laboratories). δ(13)CDOC values were only comparable for seawater and showed differences of 10 to 21‰ for other samples. CONCLUSIONS: This study indicates that scatter in δ(13)CDIC isotope data can be in the range of several per mil for samples from extreme environments (geothermal waters) and may not yield reliable information with respect to dissolved carbon (petroleum wells). The analyses of lake water and seawater also revealed a larger than expected difference and researchers from various disciplines should be aware of this. Evaluation of analytical procedures of the participating laboratories indicated that the differences cannot be explained by analytical errors or different data normalization procedures and must be related to specific sample characteristics or secondary effects during sample storage and handling. Our results reveal the need for further research on sources of error and on method standardization.

Novel use of cavity ring-down spectroscopy to investigate aquatic carbon cycling from microbial to ecosystem scales.

Development of cavity ring-down spectroscopy (CRDS) has enabled real-time monitoring of carbon stable isotope ratios of carbon dioxide and methane in air. Here we demonstrate that CRDS can be adapted to assess aquatic carbon cycling processes from microbial to ecosystem scales. We first measured in situ isotopologue concentrations of dissolved CO2 ((12)CO2 and (13)CO2) and CH4 ((12)CH4 and (13)CH4) with CRDS via a closed loop gas equilibration device during a survey along an estuary and during a 40 h time series in a mangrove creek (ecosystem scale). A similar system was also connected to an in situ benthic chamber in a seagrass bed (community scale). Finally, a pulse-chase isotope enrichment experiment was conducted by measuring real-time release of (13)CO2 after addition of (13)C enriched phytoplankton to exposed intertidal sediments (microbial scale). Miller-Tans plots revealed complex transformation pathways and distinct isotopic source values of CO2 and CH4. Calculations of δ(13)C-DIC based on CRDS measured δ(13)C-CO2 and published fractionation factors were in excellent agreement with measured δ(13)C-DIC using isotope ratio mass spectroscopy (IRMS). The portable CRDS instrumentation used here can obtain real-time, high precision, continuous greenhouse gas data in lakes, rivers, estuaries and marine waters with less effort than conventional laboratory-based techniques.

Automated weighing in the stable isotope lab: When less is more.

Automated powder weighing is an elusive goal in scientific laboratories. The main problem is that powders are much more heterogeneous than liquids, making difficult the development of a unified automation solution for their handling. A compromise has been presented with miau, a low-cost, open-source autosampler for microbalance. Miau was demonstrably useful to perform the automated weighing of some powders, as long as the same powder is weighed repeatedly, which is useful for preparing standards to be measured along samples. However, in stable-isotope laboratories it is also necessary to weigh samples, which are often very heterogeneous, and thus not amenable for miau. Here it is demonstrated how miau can be adapted to handle not only standards, but also samples, using the "less is more" philosophy:•Miau is simplified to perform only the manipulation of weighing capsules, becoming "miau redux"•Miau redux can be used not only for standards, but for a variety of samples as well•Miau redux saves 64% of operator time when using a microbalance.

Miau, a microbalance autosampler.

Powder weighing is an essential but tedious activity in many branches of science. Here I describe a MIcrobalance AUtosampler (miau) that transfers solids in the sub-mg range to a microbalance. Miau is a pick-and-place machine which moves a gripper with dual function: 1) move tin capsules; 2) deliver powder from a container to tin capsules. In our laboratory we routinely use miau to prepare working standards for quality control of elemental and isotopic analyses. In a test, miau produced standards between 0.3 and 1.1 mg, which is a useful range in our laboratory. Failure to produce a weighed standard happened in 5% of the cases. A comparison with manual measurements demonstrated that obtained amounts for automated samples were as accurate and precise as manually prepared ones. Setup for daily use is simple, and the microbalance can be easily used alternately with or without miau. Miau is a low-cost device that can work with microbalances from many manufacturers, and can be readily adopted by many laboratories.

Portable open-source autosampler for shallow waters.

Automated water sampling can be very useful, but open-source choices are limited. Here I present an autosampler which consists of a gantry robot that delivers water from a syringe pump to 24 capped 40 ml vials. The autosampler is controlled using an Android tablet automatized using Macrodroid. Three rinsing cycles ensure negligible carryover between consecutive samples. Hourly sampling from a creek under rainy conditions suggested that total organic carbon in water was diluted by the rain. Some important limitations: 1) the autosampler must be on a steady, flat, horizontal surface; 2) unattended sampling can only last as long as the batteries powering the tablet and the motors; 3) distance from the syringe pump to water cannot exceed ~2 m in height and ~4 m in length for 3 mm tubing; 4) sampling frequency does not exceed one sample every eleven minutes. However, because of its open design, the autosampler can be modified and improved to not only overcome these limitations, but also potentially expand its scope to more demanding sampling if necessary.

Open-source autosampler for elemental and isotopic analyses of solids.

Elemental and isotopic analyses are performed using elemental analyzers, and are widely employed for diverse scientific fields. An elemental analyzer is typically equipped with an autosampler. Here we present an open-source autosampler for elemental and isotopic analysis of solid samples. The autosampler consists of 1) a sampling table, on which a carousel pushes samples inside an orifice, and 2) a purging pipe, placed directly beneath the orifice, where the sample is purged off surrounding air, and then delivered to the reaction tube. The action of the purging pipe ensured that air contamination, an issue for the analysis of some elements like nitrogen and oxygen, was negligible, and results for elemental and isotopic composition of nitrogen and carbon were inside specs. Compared to commercial alternatives, the autosampler presented here has the advantages of lower cost to build and maintain, universal compatibility with instruments from different manufacturers, capacity do deal with bulky samples, capacity for a larger number of samples in a single run, and no necessity for time-consuming purging in-between sample loading. The autosampler could potentially also be employed for the analyses of other elements (e.g. oxygen, hydrogen and sulfur) because they are performed using similar equipment.

Integration of analytical instruments with computer scripting.

Automation of laboratory routines aided by computer software enables high productivity and is the norm nowadays. However, the integration of different instruments made by different suppliers is still difficult, because to accomplish it, the user must have knowledge of electronics and/or low-level programming. An alternative approach is to control different instruments without an electronic connection between them, relying only on their software interface on a computer. This can be achieved through scripting, which is the emulation of user operations (mouse clicks and keyboard inputs) on the computer. The main advantages of this approach are its simplicity, which enables people with minimal knowledge of computer programming to employ it, and its universality, which enables the integration of instruments made by different suppliers, meaning that the user is totally free to choose the devices to be integrated. Therefore, scripting can be a useful, accessible, and economic solution for laboratory automation.