Uncertainty of Blank Correction in Isotope Ratio Measurement

New method for simultaneous determination of dissolved organic carbon and its stable carbon isotope ratio in liquid samples: environmental applications

This study reports the development of an all-in-one elemental analyser isotope ratio mass spectrometry (EA-IRMS) system modified for simultaneous analysis of dissolved organic carbon (DOC) concentration and its stable carbon isotope footprint (δ¹³C_DOC) in aqueous samples. The method involves a quantitative oxidation of DOC in a 200 µL liquid sample to CO₂, after sample acidification and stripping by nitrogen. The detection limit of the method for DOC quantification was 0.2 mg C/L with an analytical precision of 12 %. Uncertainty of stable isotope determinations was 2 % at 0.2 mg DOC/L, while decreasing to 0.3 % at 20 mg DOC/L. Quantitative oxidation of DOC in aqueous samples was validated by using ring test water samples and Deep Sea reference seawater. The method performances of isotope analysis were evaluated by analysing different isotopic standard solutions. The applicability of the method was tested through the analysis of different environmental types of water, showing that δ¹³C_DOC ranged from - 23.30 to -31.85 ‰, allowing to characterize samples of different environmental origin. The developed method offers several advantages including rapidity, use of small sample volumes and minimal sample pre-treatment, making it a valuable tool for routine DOC concentration measurements paired with isotopic characterization.

Seasonal changes in stable carbon isotopic composition in the bulk aerosol and gas phases at a suburban site in Prague

Isotope fractionation between the gas and aerosol phases is an important phenomenon for studying atmospheric processes. Here, for the first time, seasonally resolved stable carbon isotope ratio (δ¹³C) values are systematically used to study phase interactions in bulk aerosol and gaseous carbonaceous samples. Seasonal variations in the δ¹³C of total carbon (TC; δ¹³C_TC) and water-soluble organic carbon (WSOC; δ¹³C_WSOC) in fine aerosol particles (PM_2.5) as well as in the total carbon of part of the gas phase (TCgas; δ¹³C_TCgas) were studied at a suburban site in Prague, Czech Republic, Central Europe. Year-round samples were collected for the main and backup filters from 14 April 2016 to 1 May 2017 every 6 days with a 48 h sampling period (n = 66). During all seasons, the highest ¹³C enrichment was found in WSOC, followed by particulate TC, whereas the highest ¹³C depletion was found in gaseous TC. We observed a clear seasonal pattern for all δ¹³C, with the highest values in winter (avg. δ¹³C_TC = -25.5 ± 0.8‰, δ¹³C_WSOC = -25.0 ± 0.7‰, δ¹³C_TCgas = -27.7 ± 0.5‰) and the lowest values in summer (avg. δ¹³C_TC = -27.2 ± 0.5‰, δ¹³C_WSOC = -26.4 ± 0.3‰, δ¹³C_TCgas = -28.9 ± 0.3‰). This study supports the existence of different aerosol sources at the site during the year. Despite the different seasonal compositions of carbonaceous aerosols, the isotope difference (Δδ¹³C) between δ¹³C_TC (aerosol) and δ¹³C_TCgas (gas phase) was similar during the seasons (year avg. 1.97 ± 0.50‰). Moreover, Δδ¹³C between WSOC and TC in PM_2.5 showed a difference between spring and winter, but in general, these values were also similar year-round (year avg. 0.71 ± 0.37‰). During the entire period, TCgas and WSOC were the most ¹³C-depleted and most ¹³C-enriched fractions, respectively, and although the resulting difference Δ(δ¹³C_WSOC - δ¹³C_TCgas) was significant, it was almost invariant throughout the year (2.67 ± 0.44‰). The present study suggests that the stable carbon isotopic fractionation between the bulk aerosol and gas phases is probably not entirely dependent on the chemical composition of individual carbonaceous compounds from different sources.

Successive and automated stable isotope analysis of CO2 , CH4 and N2 O paving the way for unmanned aerial vehicle-based sampling

RATIONALE

Measurement of greenhouse gas (GHG) concentrations and isotopic compositions in the atmosphere is a valuable tool for predicting their sources and sinks, and ultimately how they affect Earth's climate. Easy access to unmanned aerial vehicles (UAVs) has opened up new opportunities for remote gas sampling and provides logistical and economic opportunities to improve GHG measurements.

METHODS

This study presents synchronized gas chromatography/isotope ratio mass spectrometry (GC/IRMS) methods for the analysis of atmospheric gas samples (20-mL  glass vessels) to determine the stable isotope ratios and concentrations of CO₂ , CH₄ and N₂ O. To our knowledge there is no comprehensive GC/IRMS setup for successive measurement of CO₂ , CH₄ and N₂ O analysis meshed with a UAV-based sampling system. The systems were built using off-the-shelf instruments augmented with minor modifications.

RESULTS

The precision of working gas standards achieved for δ¹³ C and δ¹⁸ O values of CO₂ was 0.2‰ and 0.3‰, respectively. The mid-term precision for δ¹³ C and δ¹⁵ N values of CH₄ and N₂ O working gas standards was 0.4‰ and 0.3‰, respectively. Injection quantities of working gas standards indicated a relative standard deviation of 1%, 5% and 5% for CO₂ , CH₄ and N₂ O, respectively. Measurements of atmospheric air samples demonstrated a standard deviation of 0.3‰ and 0.4‰ for the δ¹³ C and δ¹⁸ O values, respectively, of CO₂ , 0.5‰ for the δ¹³ C value of CH₄ and 0.3‰ for the δ¹⁵ N value of N₂ O.

CONCLUSIONS

Results from internal calibration and field sample analysis, as well as comparisons with similar measurement techniques, suggest that the method is applicable for the stable isotope analysis of these three important GHGs. In contrast to previously reported findings, the presented method enables successive analysis of all three GHGs from a single ambient atmospheric gas sample.

Extending the Limit of Measurement for Dual H and O Isotope Ratios Using Thermolysis

Hydrogen and oxygen isotope ratios are of use to determine the origin of matter. Thermolysis is used to convert matter to H₂ and CO gases, which are the respective substrates for measurement of these two isotope ratios, using isotope ratio mass spectrometry (IRMS). This work was done in response to the need for analysis of small invasive insects, requiring a decrease in the limit of measurement for isotope ratiometry of hydrogen and oxygen, while determining both isotope ratios on the same sample. Miniaturization of a thermolysis reactor using commercially available components is presented that results in improvement in the limit of measurement for both hydrogen and oxygen isotope ratios. δ²H was determined on 0.4 μg of H and δ¹⁸O determined on 5 μg of O with precisions of 3 mUr and 0.7 mUr, respectively. To extend the usable sample size range or increase the resolution of sampling gives obvious advantages in forensic and environmental sciences. The technique has been applied to determining the natural origin of Tephritidae fruit flies for which only the wing is suitable for analysis and provides just 60 μg of material for analysis.

Spatial Microanalysis of Natural 13C/12C Abundance in Environmental Samples Using Laser Ablation-Isotope Ratio Mass Spectrometry

The stable ¹³C/¹²C isotope composition usually varies among different organic materials due to isotope fractionation during biochemical synthesis and degradation processes. Here, we introduce a novel laser ablation-isotope ratio mass spectrometry (LA-IRMS) methodology that allows highly resolved spatial analysis of carbon isotope signatures in solid samples down to a spatial resolution of 10 μm. The presented instrumental setup includes in-house-designed exchangeable ablation cells (3.8 and 0.4 mL, respectively) and an improved sample gas transfer, which allow accurate δ¹³C measurements of an acryl plate standard down to 0.6 and 0.4 ng of ablated carbon, respectively (standard deviation 0.25‰). Initial testing on plant and soil samples confirmed that microheterogeneity of their natural ¹³C/¹²C abundance can now be mapped at a spatial resolution down to 10 μm. The respective δ¹³C values in soils with C3/C4 crop sequence history varied by up to 14‰ across a distance of less than 100 μm in soil aggregates, while being partly sorted along rhizosphere gradients of <300 μm from Miscanthus plant roots into the surrounding soil. These very first demonstrations point to the appearance of very small metabolic hotspots originating from different natural isotope discrimination processes, now traceable via LA-IRMS.

A closer look into the nitrogen blank in elemental analyser/isotope ratio mass spectrometry measurements

RATIONALE

One important limitation for the precise measurement of minute amounts of nitrogen (N) in solid samples by elemental analyser/isotope ratio mass spectrometry (EA/IRMS) is the accurate determination of the analyser blank value. This study was performed to identify different sources, amounts and isotopic composition of N blanks in EA/IRMS in order to identify measures for minimising the effect of the N blank on N isotopic data quality.

METHODS

Different types of autosamplers, with and without zero-blank functionality, were tested by analysing different amounts of substances of varying isotopic composition by EA/IRMS.

RESULTS

Using zero-blank autosamplers reduces the atmospheric N₂ blank from 60 nmol to between 4 and 5 nmol depending on the autosampler type. This blank is derived from atmospheric N₂ leaking into the elemental analyser, trapped in the sample tin capsules or contained in the oxygen added for combustion. Another source of blank is the reaction tube. As the sources of the blank differ, the isotopic composition of the blank is very variable. In addition, cross-contamination from previous samples may contribute up to 3.3 nmol N.

CONCLUSIONS

For precise measurements of minute amounts of N in solid samples, reduction of the N blank is the most promising strategy. Correcting for the remaining N blank is only meaningful if the sample isotopic composition is very different from that of the N blank, because the precise determination of the isotopic composition of the N blank is not possible.

High-throughput method for simultaneous quantification of N, C and S stable isotopes and contents in organics and soils

RATIONALE

Information about the sulfur stable isotope composition (δ(34) S value) of organic materials and sediments, in addition to their nitrogen (δ(15) N value) and carbon (δ(13) C value) stable isotope compositions, can provide insights into mechanisms and processes in different areas of biological and geological research. The quantification of δ(34) S values has traditionally required an additional and often more difficult analytical procedure than NC dual analysis. Here, we report on the development of a high-throughput method that simultaneously measures the elemental and isotopic compositions of N, C and S in a single sample, and over a wide range of sample sizes and C/N and C/S ratios.

METHODS

We tested a commercially available CHNOS elemental analyzer in line with an isotope ratio mass spectrometer for the simultaneous quantification of N, C and S stable isotope ratios and contents, and modified the elemental analyzer in order to overcome the interference of (18) O in δ(34) S values, to minimize any water condensation that could also influence S memory, and to achieve the complete reduction of nitrogen oxides to N2 gas for accurate measurement of δ(15) N values. A selection of organic materials and soils was analyzed with a ratio of 1:1.4 standards to unknowns per run.

RESULTS

The modifications allowed high quality measurements for N, C and S isotope ratios simultaneously (1 SD of ±0.13‰ for δ(15) N value, ±0.12‰ for δ(13) C value, and ±0.4‰ for δ(34) S value), with high throughput (>75 unknowns per run) and over a wide range of element amount per capsule (25 to 500 μg N, 200-4000 μg C, and 8-120 μg S).

CONCLUSIONS

This method is suitable for widespread use and can significantly enhance the application of δ(34) S measurements in a broad range of soils and organic samples in ecological and biogeochemical research. Copyright © 2016 John Wiley & Sons, Ltd.

Evaluation strategies and uncertainty calculation of isotope amount ratios measured by MC ICP-MS on the example of Sr

This paper critically reviews the state-of-the-art of isotope amount ratio measurements by solution-based multi-collector inductively coupled plasma mass spectrometry (MC ICP-MS) and presents guidelines for corresponding data reduction strategies and uncertainty assessments based on the example of n((87)Sr)/n((86)Sr) isotope ratios. This ratio shows variation attributable to natural radiogenic processes and mass-dependent fractionation. The applied calibration strategies can display these differences. In addition, a proper statement of uncertainty of measurement, including all relevant influence quantities, is a metrological prerequisite. A detailed instructive procedure for the calculation of combined uncertainties is presented for Sr isotope amount ratios using three different strategies of correction for instrumental isotopic fractionation (IIF): traditional internal correction, standard-sample bracketing, and a combination of both, using Zr as internal standard. Uncertainties are quantified by means of a Kragten spreadsheet approach, including the consideration of correlations between individual input parameters to the model equation. The resulting uncertainties are compared with uncertainties obtained from the partial derivatives approach and Monte Carlo propagation of distributions. We obtain relative expanded uncertainties (U rel; k = 2) of n((87)Sr)/n((86)Sr) of < 0.03 %, when normalization values are not propagated. A comprehensive propagation, including certified values and the internal normalization ratio in nature, increases relative expanded uncertainties by about factor two and the correction for IIF becomes the major contributor.

Combined ¹³C and ¹⁵N isotope analysis on small samples using a near-conventional elemental analyzer/isotope ratio mass spectrometer setup

RATIONALE

A high sensitivity elemental analyzer/isotope ratio mass spectrometer setup was developed to allow analysis of (13)C and (15)N isotopic composition on microgram amounts of C and N, respectively.

METHODS

Increased sensitivity of a conventional elemental analyzer equipped with a low blank autosampler was obtained by decreased carrier gas flow of 35 mL/min. The diameters of the oxidation and reduction reactors and water trap were reduced to 7.8, 7.8 and 4 mm i.d., respectively, to obtain sharp sample peaks in the mass spectrometer. To increase the lifetime of the reduction reactor, a 1:1 He/O2 mixture was used as oxidizing agent in the elemental analyzer.

RESULTS

Sample amounts of 0.6 µg N and 1 µg C were sufficient for accurate isotopic analysis with <1 ‰ standard error after blank correction. One major advantage of the setup is the easy switching between conventional EA and μEA as only consumable parts need to be exchanged.

CONCLUSIONS

The proposed setup proved to be suitable to analyze minute amounts of C and N in one analytical run simultaneously.