Ignoring IUPAC guidelines for measurement and reporting of stable isotope abundance values affects us all

Perspective: Hidden biases in isotope delta results and the need for comprehensive reporting

Measurements of stable-isotope composition on an isotope-delta scale can be subject to bias between laboratories or over time within a single laboratory. This bias can arise not just from differences in method protocol but also from changes in reporting guidelines, or even to the isotope-delta scales themselves. Without a clear description of method protocols, including all sample preparation steps, instrumental parameters and settings, data processing including calibration of results and estimation of measurement uncertainty, the traceability and comparability of isotope-delta values cannot be assured as bias(es) may remain hidden. To address this need, there are now clear guidelines published by IUPAC for reporting isotope-delta values for the "light" elements hydrogen, carbon, nitrogen, oxygen and sulfur.1 We recommend that authors and reviewers adhere to those guidelines when preparing and reviewing future publications.

Differential response of two acidophytic lichens to increased reactive nitrogen availability

Lichens are one of the most responsive components of the ecosystem to reactive forms of nitrogen. In this work, we selected the lichen genera Cladonia and Usnea, composed of terricolous and epiphytic lichens respectively, and described as sensitive to nitrogen, to test the effects of different doses of nitrogen on lichen physiological parameters (photobiont and mycobiont vitality, chitin quantification, nitrogen content and stable isotopes analysis). The main objectives were to check if the activation of protective mechanisms could be stimulated in case of chronic stress (low nitrogen increase for prolonged time), and, if so, if a toxicity threshold could be identified above which these mechanisms fail. The two lichen genera were generally affected by prolonged exposure to increased nitrogen availability. However, Cladonia rangiformis was able to maintain physiological functioning at the lowest nitrogen doses used, whereas thalli of Usnea become overwhelmed. Moreover, the mycobiont appeared to be more sensitive than the photobiont responding to lower nitrogen doses. Although only studies of longer duration and testing more nitrogen doses will be able to determine an accurate toxicity threshold, these results give important clues on the use of lichens as biomonitors for the establishment of environmental policies.

From a Lose-Lose to a Win-Win Situation: User-Friendly Biomass Models for Acacia longifolia to Aid Research, Management and Valorisation

Woody invasive species pose a big threat to ecosystems worldwide. Among them, Acacia longifolia is especially aggressive, fundamentally changing ecosystem structure through massive biomass input. This biomass is rarely harvested for usage; thus, these plants constitute a nuisance for stakeholders who invest time and money for control without monetary return. Simultaneously, there is an increased effort to valorise its biomass, e.g., for compost, growth substrate or as biofuel. However, to incentivise A. longifolia harvest and usage, stakeholders need to be able to estimate what can be obtained from management actions. Thus, the total biomass and its quality (C/N ratio) need to be predicted to perform cost-benefit analyses for usage and determine the level of invasion that has already occurred. Here, we report allometric biomass models for major biomass pools, as well as give an overview of biomass quality. Subsequently, we derive a simplified volume-based model (BM ~ 6.297 + 0.982 × Vol; BM = total dry biomass and Vol = plant volume), which can be applied to remote sensing data or with in situ manual measurements. This toolkit will help local stakeholders, forest managers or municipalities to predict the impact and valorisation potential of this invasive species and could ultimately encourage its management.

Guidance for characterization of in-house reference materials for light element stable isotope analysis

RATIONALE

Preparation of in-house reference materials (RMs) is an important aspect of light element stable isotope analysis. While some relevant information is available, there is as yet no clear set of guidelines available covering all aspects of in-house production and characterization of RMs.

METHODS

To address this need, the experience of production of certified reference materials under accreditation to ISO 17034:2016 and ISO/IEC 17025:2017 has been distilled into guidance for production of in-house RMs that are fit-for-purpose.

RESULTS

The guidance provided covers five areas: (i) planning; (ii) material considerations including preparation, packaging, and storage; (iii) measurements and assessments; (iv) value and uncertainty assignment; and (v) monitoring and use.

CONCLUSIONS

In-house RMs prepared by following this guidance can be used to provide traceability to measurement results when used for normalization or for quality control and/or assurance purposes.

Elemental profile of native lichens displaying the impact by agricultural and artificial land uses in the Atlantic island of São Miguel (Azores)

Smaller oceanic islands, often hosting endangered native habitats, are particularly vulnerable to the impact of human activities. Using lichens as bioindicators, this study aimed to test if agricultural (AGR) and artificial (ART) land uses are noticeably more impacted than forest (FOR) land use on an oceanic island (São Miguel, Azores). Livestock and farming practices in AGR areas involve the intensive application of synthetical agrochemicals as well as organic fertilizers and manure. ART areas accommodate vehicular traffic besides industries dedicated to waste management, energy production or exploration and transformation of raw materials. Naturally occurring Parmotrema lichens were collected in 28 sampling sites distributed between each land use. The concentrations of 58 elements as well as the percentage (%N) and the isotopic composition of nitrogen (δ¹⁵N) were determined on lichen samples. An overall pattern of significant elemental enrichment was observed in lichens from AGR and ART sites compared with FOR lichens, including several rare-earth elements. FOR lichens were noticeably cleaner, thus providing background concentrations for the calculation of bioaccumulation ratios. Bioaccumulation levels were generally low to moderate in AGR lichens and moderate to high in ART lichens, including toxic heavy metals. %N was highest in AGR lichens and its isotopic signature was distinguishable from ART lichens by significantly lower δ¹⁵N values. This study provides a comprehensive baseline of bioaccumulation data across major land uses for comparison with other insular regions, highlighting the greater vulnerability of island ecosystems to anthropogenic impacts even if by relatively small-scale human activities.

Spatially modelling the risk areas of chronic exposure to hydrothermal volcanic emissions using lichens

Human populations living in volcanically active areas are chronically exposed to volcanogenic air pollution, potentially contributing to long-term adverse health effects. However, mapping chronic exposure is difficult due to low spatial resolution of monitoring data on air pollutants and the need for time integration. To overcome these problems, lichens were tested as ecological indicators of hydrothermal volcanic air pollution, considering their bioaccumulation capacity over time, by transplanting them from a reference area to several sites (n = 39) in a volcanic area. The test was developed at Furnas volcano (Azores, Portugal). A stratified sampling design was followed using previous measurements of soil CO₂ flux at ground level and the distance to the main fumarolic fields. After 6 months of exposure, lichen transplants were analyzed for S isotopic ratio (δ³⁴S), which strongly related with the distance to fumarolic fields on a logarithmic regression, serving as an appropriate hydrothermal exposure biomarker. Considering kriging interpolated δ³⁴S values as tracer of airborne hydrothermal emissions and habitational areas as proxy of ongoing human presence, a map was built relating both information per area unit to spatially model risk areas. It was estimated that 26% of habitational areas in the study area stand at high or very high risk of outdoors chronic exposure to airborne hydrothermal emissions. This methodologic approach to produce chronic exposure risk maps is applicable to other volcanically active and inhabited areas of the world, with time-integration and high spatial resolution, contributing in this way for spatially focusing future human health assessments.

From stable isotope ecology to forensic isotope ecology - Isotopes' tales

Stable isotope ecology and forensic isotope ecology are not only linked by name. More often than not, knowledge and insights gained through the former serve as a springboard for application focused work of the latter. This review aims to offer a glimpse into the fascinating world of both though with more emphasis on forensic isotope ecology. To this end a selection of past and recent published work is presented and discussed to highlight both potential and limitations of isotopic analytical approaches to the detection of illegal trade in plants and animals.

δ15N of lichens reflects the isotopic signature of ammonia source

Although it is generally accepted that δ¹⁵N in lichen reflects predominating N isotope sources in the environment, confirmation of the direct correlation between lichen δ¹⁵N and atmospheric δ¹⁵N is still missing, especially under field conditions with most confounding factors controlled. To fill this gap and investigate the response of lichens with different tolerance to atmospheric N deposition, thalli of the sensitive Evernia prunastri and the tolerant Xanthoria parietina were exposed for ten weeks to different forms and doses of N in a field manipulation experiment where confounding factors were minimized. During this period, several parameters, namely total N, δ¹⁵N and chlorophyll a fluorescence, were measured. Under the experimental conditions, δ¹⁵N in lichens quantitatively responded to the δ¹⁵N of released gaseous ammonia (NH₃). Although a high correlation between the isotopic signatures in lichen tissue and supplied N was found both in tolerant and sensitive species, chlorophyll a fluorescence indicated that the sensitive species very soon lost its photosynthetic functionality with increasing N availability. The most damaging response to the different N chemical forms was observed with dry deposition of NH₃, although wet deposition of ammonium ions had a significant observable physiological impact. Conversely, there was no significant effect of nitrate ions on chlorophyll a fluorescence, implying differential sensitivity to dry deposition versus wet deposition and to ammonium versus nitrate in wet deposition. Evernia prunastri was most sensitive to NH₃, then NH₄⁺, with lowest sensitivity to NO₃^-. Moreover, these results confirm that lichen δ¹⁵N can be used to indicate the δ¹⁵N of atmospheric ammonia, providing a suitable tool for the interpretation of the spatial distribution of NH₃ sources in relation to their δ¹⁵N signal.

Recent applications of isotope analysis to forensic anthropology

Isotope analysis has become an increasingly valuable tool in forensic anthropology casework over the past decade. Modern-day isotopic investigations on human remains have integrated the use of multi-isotope profiles (e.g. C, N, O, H, S, Sr, and Pb) as well as isotopic landscapes (“isoscapes”) from multiple body tissues (e.g. teeth, bone, hair, and nails) to predict possible region-of-origin of unidentified human remains. Together, data from various isotope analyses provide additional lines of evidence for human identification, including a decedent’s possible region-of-birth, long-term adult residence, recent travel history, and dietary choices. Here, we present the basic principles of isotope analysis and provide a brief overview of instrumentation, analytical standards, sample selection, and sample quality measures. Finally, we present case studies that reflect the diverse applications of isotope analysis to the medicolegal system before describing some future research directions. As shown herein, isotope analysis is a flexible and powerful geolocation tool that can provide new investigative leads for unidentified human remains cases.

Calling all archaeologists: guidelines for terminology, methodology, data handling, and reporting when undertaking and reviewing stable isotope applications in archaeology

Stable isotope analysis has been utilized in archaeology since the 1970s, yet standardized protocols for terminology, sampling, pretreatment evaluation, calibration, quality assurance and control, data presentation, and graphical or statistical treatment still remain lacking in archaeological applications. Here, we present recommendations and requirements for each of these in the archaeological context of: bulk stable carbon and nitrogen isotope analysis of organics; bulk stable carbon and oxygen isotope analysis of carbonates; single compound stable carbon and nitrogen isotope analysis on amino acids in collagen and keratin; and single compound stable carbon and hydrogen isotope analysis on fatty acids. The protocols are based on recommendations from the Commission on Isotopic Abundances and Atomic Weights of the International Union of Pure and Applied Chemistry (IUPAC) as well as an expanding geochemical and archaeological science experimental literature. We hope that this will provide a useful future reference for authors and reviewers engaging with the growing number of stable isotope applications and datasets in archaeology.

Using nitrogen concentration and isotopic composition in lichens to spatially assess the relative contribution of atmospheric nitrogen sources in complex landscapes

Reactive nitrogen (Nr) is an important driver of global change, causing alterations in ecosystem biodiversity and functionality. Environmental assessments require monitoring the emission and deposition of both the amount and types of Nr. This is especially important in heterogeneous landscapes, as different land-cover types emit particular forms of Nr to the atmosphere, which can impact ecosystems distinctively. Such assessments require high spatial resolution maps that also integrate temporal variations, and can only be feasibly achieved by using ecological indicators. Our aim was to rank land-cover types according to the amount and form of emitted atmospheric Nr in a complex landscape with multiple sources of N. To do so, we measured and mapped nitrogen concentration and isotopic composition in lichen thalli, which we then related to land-cover data. Results suggested that, at the landscape scale, intensive agriculture and urban areas were the most important sources of Nr to the atmosphere. Additionally, the ocean greatly influences Nr in land, by providing air with low Nr concentration and a unique isotopic composition. These results have important consequences for managing air pollution at the regional level, as they provide critical information for modeling Nr emission and deposition across regional as well as continental scales.

Characterization of Substrate, Cosubstrate, and Product Isotope Effects Associated With Enzymatic Oxygenations of Organic Compounds Based on Compound-Specific Isotope Analysis

Enzymatic oxygenations are among the most important biodegradation and detoxification reactions of organic pollutants. In the environment, however, such natural attenuation processes are extremely difficult to monitor. Changes of stable isotope ratios of aromatic pollutants at natural isotopic abundances serve as proxies for isotope effects associated with oxygenation reactions. Such isotope fractionations offer new avenues for revealing the pathway and extent of pollutant transformation and provide new insights into the mechanisms of catalysis by Rieske non-heme ferrous iron oxygenases. Based on compound-specific C, H, N, and O isotope analysis, we present a comprehensive methodology with which isotope effects can be derived from the isotope fractionation measured in substrates, the cosubstrate O₂, and organic oxygenation products. We use dioxygenation of nitrobenzene and 2-nitrotoluene by nitrobenzene dioxygenase as illustrative examples to introduce different mathematical procedures for deriving apparent substrate and product isotope effects. We present two experimental approaches to control reactant and product turnover for isotope fractionation analysis in experimental systems containing purified enzymes, E. coli clones, and pure strains of environmental microorganisms. Finally, we present instrumental procedures and sample treatment instructions for analysis of C, H, and N isotope analysis in organic compounds and O isotope analysis in aqueous O₂ by gas and liquid chromatography coupled to isotope ratio mass spectrometry.

Nitrogen stable isotope variability in tissues of juvenile tilapia Oreochromis aureus: empirical and modelling results

RATIONALE

Studies on diet or trophic interactions of organisms based on stable isotopes require accurate estimates of how quickly stable isotope ratios change in the investigated tissues. However, rates of isotope turnover in fish tissues, especially in omnivorous species, are poorly understood.

METHODS

We conducted a diet-shift study using juvenile tilapia to (i) empirically estimate the isotopic turnover rates of nitrogen in the dorsal muscle, liver, fin and backbone; (ii) model the relative contributions of metabolism and growth to the total isotopic turnover in each tissue; and (iii) develop a non-lethal approach for estimating body nitrogen stable isotope ratios for threatened or endangered species. Isotopic analyses were performed using a Flash EA CN elemental analyser coupled to a ThermoFinnigan Delta Plus mass spectrometer.

RESULTS

Nitrogen isotopic turnover rates were consistently ranked in the order backbone > liver > muscle > fin due to the relatively lower metabolic rates of muscle and fin tissue. Backbone tissue turned over significantly faster than other tissues, suggesting the potential for a multiple-tissue stable isotope approach to the study of movement and trophic position over different time scales for omnivorous fish. However, fin tissue had the longest half-life, at 57.81 days, indicating that this tissue is more useful than muscle as a long-term dietary indicator.

CONCLUSIONS

The change in nitrogen isotope ratios in dorsal muscle was mainly regulated by somatic growth, but metabolic activity markedly stimulated the turnover rate of backbone. This study is one of a few to demonstrate significant variation in the δ(15) N turnover rates among multiple tissues of a single organism, especially for omnivorous fish. Our results, to some extent, also indirectly contribute to the conservation of threatened or endangered species. Copyright © 2016 John Wiley & Sons, Ltd.

Improved accuracy and precision in δ15 NAIR measurements of explosives, urea, and inorganic nitrates by elemental analyzer/isotope ratio mass spectrometry using thermal decomposition

RATIONALE

Elemental analyzer systems generate N(2) and CO(2) for elemental composition and isotope ratio measurements. As quantitative conversion of nitrogen in some materials (i.e., nitrate salts and nitro-organic compounds) is difficult, this study tests a recently published method - thermal decomposition without the addition of O(2) - for the analysis of these materials.

METHODS

Elemental analyzer/isotope ratio mass spectrometry (EA/IRMS) was used to compare the traditional combustion method (CM) and the thermal decomposition method (TDM), where additional O(2) is eliminated from the reaction. The comparisons used organic and inorganic materials with oxidized and/or reduced nitrogen and included ureas, nitrate salts, ammonium sulfate, nitro esters, and nitramines. Previous TDM applications were limited to nitrate salts and ammonium sulfate. The measurement precision and accuracy were compared to determine the effectiveness of converting materials containing different fractions of oxidized nitrogen into N(2).

RESULTS

The δ(13) C(VPDB) values were not meaningfully different when measured via CM or TDM, allowing for the analysis of multiple elements in one sample. For materials containing oxidized nitrogen, (15) N measurements made using thermal decomposition were more precise than those made using combustion. The precision was similar between the methods for materials containing reduced nitrogen. The %N values were closer to theoretical when measured by TDM than by CM. The δ(15) N(AIR) values of purchased nitrate salts and ureas were nearer to the known values when analyzed using thermal decomposition than using combustion.

CONCLUSIONS

The thermal decomposition method addresses insufficient recovery of nitrogen during elemental analysis in a variety of organic and inorganic materials. Its implementation requires relatively few changes to the elemental analyzer. Using TDM, it is possible to directly calibrate certain organic materials to international nitrate isotope reference materials without off-line preparation.

Forensic Mass Spectrometry

Developments in forensic mass spectrometry tend to follow, rather than lead, the developments in other disciplines. Examples of techniques having forensic potential born independently of forensic applications include ambient ionization, imaging mass spectrometry, isotope ratio mass spectrometry, portable mass spectrometers, and hyphenated chromatography-mass spectrometry instruments, to name a few. Forensic science has the potential to benefit enormously from developments that are funded by other means, if only the infrastructure and personnel existed to adopt, validate, and implement the new technologies into casework. Perhaps one unique area in which forensic science is at the cutting edge is in the area of chemometrics and the determination of likelihood ratios for the evaluation of the weight of evidence. Such statistical techniques have been developed most extensively for ignitable-liquid residue analyses and isotope ratio analysis. This review attempts to capture the trends, motivating forces, and likely impact of developing areas of forensic mass spectrometry, with the caveat that none of this research is likely to have any real impact in the forensic community unless: (a) The instruments developed are turned into robust black boxes with red and green lights for positives and negatives, respectively, or (b) there are PhD graduates in the workforce who can help adopt these sophisticated techniques.