A three-phase liquid chromatographic method for δ13C analysis of amino acids from biological protein hydrolysates using liquid chromatography–isotope ratio mass spectrometry

Application of a porous graphitic carbon column to carbon and nitrogen isotope analysis of underivatized individual amino acids using high-performance liquid chromatography coupled with elemental analyzer/isotope ratio mass spectrometry

RATIONALE

Isolation of underivatized amino acids (AAs) using high-performance liquid chromatography (HPLC) is becoming a popular method for carbon (δ13 C) and nitrogen isotope (δ15 N) analyses of AAs because of the high analytical precision and for performing dual-isotope analysis. However, some AAs in natural samples, especially small, hydrophilic AAs, are not suitably separated using reversed-phase columns (e.g., C18) and ion-exchange columns (e.g., Primesep A).

METHODS

We developed a new method for HPLC using a porous graphitic carbon column for the separation of nine hydrophilic AAs. After purification, δ13 C and δ15 N values of AAs were determined using elemental analyzer/isotope ratio mass spectrometry (EA/IRMS). We demonstrated the application of this method by determining δ13 C and δ15 N values of individual hydrophilic AAs in a biological sample, the muscle of blue mackerel (Scomber australasicus).

RESULTS

Chromatographically, the baseline separation of hydrophilic AAs was achieved in both the standard mixture and the biological sample. We confirmed that δ13 C and δ15 N values of AA standards remained unchanged during the whole experimental procedure. The δ13 C values of AAs in mackerel muscle are also in good agreement with the values obtained using another verified method for δ13 C analysis.

CONCLUSIONS

The good separation performance of hydrophilic AAs and the reliability of δ13 C and δ15 N analyses of individual AAs using the porous graphite column offer a significant advantage over conventional settings. We suggest that, in the future, the HPLC × EA/IRMS method can be used for reliable δ13 C and δ15 N analyses of AAs in natural samples.

Fingerprinting eukaryotic metabolism across the animal kingdom using position-specific isotope analysis (PSIA) 13C/12C measurements

Despite differences in their overall metabolism, eukaryotes share a common mitochondrial biochemistry. We investigated how this fundamental biochemistry supports overall metabolism using a high-resolution carbon isotope approach, position-specific isotope analysis. We measured carbon isotope ¹³C/¹²C cycling in animals, focusing on amino acids that are formed in mitochondrial reactions and are most metabolically active. Carboxyl isotope determinations for amino acids showed strong signals related to common biochemical pathways. Contrasting isotope patterns were measured for metabolism associated with major life history patterns, including growth and reproduction. Turnover of proteins and lipids as well as gluoconeogensis dynamics could be estimated for these metabolic life histories. The high-resolution isotomics measurements fingerprinted metabolism and metabolic strategies across the eukaryotic animal kingdom, yielding results for humans, ungulates, whales, and diverse fish and invertebrates in a nearshore marine food web.

Bioarchaeological evidence of one of the earliest Islamic burials in the Levant

The Middle East plays a central role in human history harbouring a vast diversity of ethnic, cultural and religious groups. However, much remains to be understood about past and present genomic diversity in this region. Here we present a multidisciplinary bioarchaeological analysis of two individuals dated to the late 7th and early 8th centuries, the Umayyad Era, from Tell Qarassa, an open-air site in modern-day Syria. Radiocarbon dates and burial type are consistent with one of the earliest Islamic Arab burials in the Levant. Interestingly, we found genomic similarity to a genotyped group of modern-day Bedouins and Saudi rather than to most neighbouring Levantine groups. This study represents the genomic analysis of a secondary use site with characteristics consistent with an early Islamic burial in the Levant. We discuss our findings and possible historic scenarios in the light of forces such as genetic drift and their possible interaction with religious and cultural processes (including diet and subsistence practices).

Ancient genomic and archaeological data combine to identify a surprisingly early Islamic burial in modern day Syria.

OUP accepted manuscript

Stable isotope analysis of teeth and bones is regularly applied by archeologists and paleoanthropologists seeking to reconstruct diets, ecologies, and environments of past hominin populations. Moving beyond the now prevalent study of stable isotope ratios from bulk materials, researchers are increasingly turning to stable isotope ratios of individual amino acids to obtain more detailed and robust insights into trophic level and resource use. In the present article, we provide a guide on how to best use amino acid stable isotope ratios to determine hominin dietary behaviors and ecologies, past and present. We highlight existing uncertainties of interpretation and the methodological developments required to ensure good practice. In doing so, we hope to make this promising approach more broadly accessible to researchers at a variety of career stages and from a variety of methodological and academic backgrounds who seek to delve into new depths in the study of dietary composition.

Rainfall as a trigger of ecological cascade effects in an Australian groundwater ecosystem

Groundwaters host vital resources playing a key role in the near future. Subterranean fauna and microbes are crucial in regulating organic cycles in environments characterized by low energy and scarce carbon availability. However, our knowledge about the functioning of groundwater ecosystems is limited, despite being increasingly exposed to anthropic impacts and climate change-related processes. In this work we apply novel biochemical and genetic techniques to investigate the ecological dynamics of an Australian calcrete under two contrasting rainfall periods (LR—low rainfall and HR—high rainfall). Our results indicate that the microbial gut community of copepods and amphipods experienced a shift in taxonomic diversity and predicted organic functional metabolic pathways during HR. The HR regime triggered a cascade effect driven by microbes (OM processors) and exploited by copepods and amphipods (primary and secondary consumers), which was finally transferred to the aquatic beetles (top predators). Our findings highlight that rainfall triggers ecological shifts towards more deterministic dynamics, revealing a complex web of interactions in seemingly simple environmental settings. Here we show how a combined isotopic-molecular approach can untangle the mechanisms shaping a calcrete community. This design will help manage and preserve one of the most vital but underrated ecosystems worldwide.

A method for stable carbon isotope measurement of underivatized individual amino acids by multi-dimensional high-performance liquid chromatography and elemental analyzer/isotope ratio mass spectrometry

RATIONALE

To achieve better precision and accuracy for δ¹³ C analysis of individual amino acids (AAs), we have developed a new analytical method based on multi-dimensional high-performance liquid chromatography (HPLC) and elemental analyzer/isotope ratio mass spectrometry (EA/IRMS). Unlike conventional methods using gas chromatography, this approach omits pre-column chemical derivatization, thus reducing systematic errors associated with the isotopic measurement.

METHODS

The separation and isolation of individual AAs in a standard mixture containing 15 AAs and a biological sample, spear squid (Heterololigo bleekeri) were performed. AAs were isolated using an HPLC system equipped with a reversed-phase column and a mixed-mode column and collected using a fraction collector. After the chromatographic separation and further post-HPLC purification, the δ¹³ C values of AAs were measured by EA/IRMS.

RESULTS

The complete isolation of all 15 AAs in the standard mixture was achieved. The δ¹³ C values of these AAs before and after the experiment were in good agreement. Also, 15 AAs in the biological sample, H. bleekeri, were successfully measured. The δ¹³ C values of AAs in H. bleekeri varied by as much as 30‰ with glycine being most enriched in¹³ C.

CONCLUSIONS

The consistency between the δ¹³ C values of reference and processed AAs demonstrates that the experimental procedure generates accurate δ¹³ C values unaffected by fractionation effects and contamination. This method is therefore suitable for δ¹³ C analysis of biological samples with higher precision than conventional approaches. We propose this new method as a tool to measure δ¹³ C values of AAs in biological, ecological and biogeochemical studies.

Heavy reliance on plants for Romanian cave bears evidenced by amino acid nitrogen isotope analysis

Heavy reliance on plants is rare in Carnivora and mostly limited to relatively small species in subtropical settings. The feeding behaviors of extinct cave bears living during Pleistocene cold periods at middle latitudes have been intensely studied using various approaches including isotopic analyses of fossil collagen. In contrast to cave bears from all other regions in Europe, some individuals from Romania show exceptionally high δ¹⁵N values that might be indicative of meat consumption. Herbivory on plants with high δ¹⁵N values cannot be ruled out based on this method, however. Here we apply an approach using the δ¹⁵N values of individual amino acids from collagen that offsets the baseline δ¹⁵N variation among environments. The analysis yielded strong signals of reliance on plants for Romanian cave bears based on the δ¹⁵N values of glutamate and phenylalanine. These results could suggest that the high variability in bulk collagen δ¹⁵N values observed among cave bears in Romania reflects niche partitioning but in a general trophic context of herbivory.

Preparative HPLC Separation of Underivatized Amino Acids for Isotopic Analysis

Single-compound analysis of stable or radioactive isotopes has found application in a number of fields ranging from archaeology to forensics. Often, the most difficult part of these analyses is the development of a method for isolating the compound(s) of interest, which can derive from a wide range of sample types including the hair, nails, and bone.Here we describe three complementary preparative HPLC techniques suitable for separating and isolating amino acids from bone collagen and hair keratin. Using preparative reversed-phase, ion-pair, or mixed-mode chromatography in aqueous carbon-free mobile phases, or those from which carbon can easily be removed, underivatized single amino acids can be isolated and further analyzed using mass spectrometric techniques.

Stable carbon isotope diagnostics of mammalian metabolism, a high-resolution isotomics approach using amino acid carboxyl groups

The carbon isotopic compositions of amino acids are increasingly measured to characterize diets and metabolic response to diets. We report a new high-resolution system to measure the stable carbon isotopic composition of carboxyl atoms within amino acids. The automated system used HPLC to separate amino acids followed by addition of ninhydrin for decarboxylation and transfer of the evolved CO2 to a stable isotope ratio mass spectrometer for δ13CCARBOXYL measurement. The ninhydrin reaction was conducted at acidic pH (1.5) and elevated temperature (160 oC) giving yields close to 100% for most common amino acids. Eight mammalian keratin samples from herbivores (kudu and caribou), omnivores (humans) and carnivores (bowhead and humpback zooplanktivorous whales) were analysed with this new system. The data provide an initial calibration of reference materials to be used in studies of this type and is the first report of carboxyl carbon isotope distributions in mammals. Results showed widespread 13C enrichments in both essential and non-essential amino acid carboxyl groups, likely linked to decarboxylation of amino acids during normal metabolism. Analyses of non-essential amino acid isotope profiles showed (1) consistent and general taxon-level metabolic differences between the herbivore, human and whale samples, (2) marked differences among individual humans, ruminants and whales (3) evidence for gluconeogenesis in the wildlife samples, and (4) extensive 13C enrichment likely associated with fasting in the humpback whale sample. Future mammalian research related to the metabolism of growth, reproduction, aging and disease may benefit from using this technique. Values obtained for internationally available samples USGS42 and USGS43 (Tibetan and Indian human hair) provide a first characterization of reference materials for δ13CCARBOXYL profiles.

Elucidating stygofaunal trophic web interactions via isotopic ecology

Subterranean ecosystems host highly adapted aquatic invertebrate biota which play a key role in sustaining groundwater ecological functioning and hydrological dynamics. However, functional biodiversity studies in groundwater environments, the main source of unfrozen freshwater on Earth, are scarce, probably due to the cryptic nature of the systems. To address this, we investigate groundwater trophic ecology via stable isotope analysis, employing δ¹³C and δ¹⁵N in bulk tissues, and amino acids. Specimens were collected from a shallow calcrete aquifer in the arid Yilgarn region of Western Australia: a well-known hot-spot for stygofaunal biodiversity. Sampling campaigns were carried out during dry (low rainfall: LR) and the wet (high rainfall: HR) periods. δ¹³C values indicate that most of the stygofauna shifted towards more ¹³C-depleted carbon sources under HR, suggesting a preference for fresher organic matter. Conversion of δ¹⁵N values in glutamic acid and phenylalanine to a trophic index showed broadly stable trophic levels with organisms clustering as low-level secondary consumers. However, mixing models indicate that HR conditions trigger changes in dietary preferences, with increasing predation of amphipods by beetle larvae. Overall, stygofauna showed a tendency towards opportunistic and omnivorous habits—typical of an ecologically tolerant community—shaped by bottom-up controls linked with changes in carbon flows. This study provides baseline biochemical and ecological data for stygofaunal trophic interactions in calcretes. Further studies on the carbon inputs and taxa-specific physiology will help refine the interpretation of the energy flows shaping biodiversity in groundwaters. This will aid understanding of groundwater ecosystem functioning and allow modelling of the impact of future climate change factors such as aridification.

Optimal Conditions for the Direct RP-HPLC Determination of Underivatized Amino Acids with Online Multiple Detection

The combined use of a dual-UV detector as well as a fluorimetric and a multielectrode electrochemical detector (equipped with a dual electrode, consisting of a conventional size 3 mm diameter glassy carbon electrode (GCE) and of a pair of 30 μm thick carbon microfibers) is proposed for the detection of the following 15 underivatized amino acids: L-histidine (His), L-cysteine (Cys), creatine (Crn), S-methyl-L-cysteine (Me-Cys), DL-homocysteine (Hcy), L-methionine (Met), beta-(3,4-dihydroxyphenyl)-L-alanine (DOPA), L-tyrosine (Tyr), DL-m-tyrosine (m-Tyr), L-a-methyl-DOPA (Me-DOPA), L-phenylalanine (Phe), DL-alpha-methyltyrosine (Me-Tyr), 5-hydroxy-tryptophan (5-HTP), 3-nitro-L-tyrosine (NO₂Tyr), and L-tryptophan (Trp), as well as of 2 dipeptides L-cystathionine (Cysta) and L-carnosine (Car), and of creatinine (Cre). A multilinear solvent (acetonitrile) gradient elution program, determined by a simple optimization algorithm, is required for the efficient reversed-phase separation of the above mixture of 18 solutes within 27 min at a flow rate of 1.0 mL/min and at 25 °C.

New Concepts for the Determination of Oxidation Efficiencies in Liquid Chromatography-Isotope Ratio Mass Spectrometry

In liquid chromatography coupled to isotope ratio mass spectrometry (LC-IRMS), analytes are separated on an LC system and consecutively oxidized to CO₂, which is required for the determination of compound-specific carbon isotope ratios. Oxidation is performed in an online reactor by sulfate radicals. Reaction conditions in the interface depend on the flow conditions determined by the LC method and the flow rates and concentrations of oxidation agent and phosphoric acid added in the interface. To determine accurate isotope ratios, a quantitative conversion of the carbon contained in the analyte to the CO₂ measurement gas is a prerequisite. Oxidation efficiencies are not commonly evaluated during method development, although certain analytes are known to be difficult to be oxidized by sulfate radicals. For the assessment of the oxidation efficiency of the LC-IRMS system, three different approaches were evaluated. (1) Residual organic carbon in the eluent stream of the interface was determined to calculate oxidation yields depending on the initial analyte concentration. (2) The IRMS response was calibrated to an inorganic carbon reference material to determine oxidation efficiencies with the help of the IRMS as a detector. (3) The oxidation temperature was deliberately reduced while monitoring the δ¹³C and signal intensity. The common assumption that a linear relation of IRMS signal to analyte concentration is an indicator for complete oxidation in LC-IRMS could be disproved. All three approaches can be applied for future method development in LC-IRMS, monitoring of existing flow injection applications, as well as for verification of complete oxidation in established LC-IRMS methods.

13C- and 15N-Isotope Analysis of Desphenylchloridazon by Liquid Chromatography-Isotope-Ratio Mass Spectrometry and Derivatization Gas Chromatography-Isotope-Ratio Mass Spectrometry

The widespread application of herbicides impacts surface water and groundwater. Metabolites (e.g., desphenylchloridazon from chloridazon) may be persistent and even more polar than the parent herbicide, which increases the risk of groundwater contamination. When parent herbicides are still applied, metabolites are constantly formed and may also be degraded. Evaluating their degradation on the basis of concentration measurements is, therefore, difficult. This study presents compound-specific stable-isotope analysis (CSIA) of nitrogen- and carbon-isotope ratios at natural abundances as an alternative analytical approach to track the origin, formation, and degradation of desphenylchloridazon (DPC), the major degradation product of the herbicide chloridazon. Methods were developed and validated for carbon- and nitrogen-isotope analysis (δ¹³C and δ¹⁵N) of DPC by liquid chromatography-isotope-ratio mass spectrometry (LC-IRMS) and derivatization gas chromatography-IRMS (GC-IRMS), respectively. Injecting standards directly onto an Atlantis LC-column resulted in reproducible δ¹³C-isotope analysis (standard deviation <0.5‰) by LC-IRMS with a limit of precise analysis of 996 ng of DPC on-column. Accurate and reproducible δ¹⁵N analysis with a standard deviation of <0.4‰ was achieved by GC-IRMS after derivatization of >100 ng of DPC with 160-fold excess of (trimethylsilyl)diazomethane. Application of the method to environmental-seepage water indicated that newly formed DPC could be distinguished from "old" DPC by the different isotopic signatures of the two DPC sources.

Analysis of the 13C isotope ratios of amino acids in the larvae, pupae and adult stages of Calliphora vicina blow flies and their carrion food sources

Adult blow flies are one of the first necrophagous insects to colonize fresh carcasses. The eggs they lay hatch into larvae, which then feed on the decomposing body. Like all organisms, blow flies "are what they eat," meaning that the isotopic composition of their body tissues reflects their diet. This manuscript combines ecology with a forensic application by using isotope ratio mass spectrometry (IRMS) to understand the relationship between the δ¹³C of amino acids in different carrion sources and the blow fly that feed on them. We also measure the amino acid-level fractionation that occurs at each major life stage of the blow flies. Adult blow flies from a commercial strain of Calliphora vicina (Robineau-Desvoidy) (Diptera: Calliphoridae) oviposited on raw pork muscle, beef muscle, or chicken liver. Larvae, pupae, and adult blow flies from each carrion were selected for amino acid compound-specific isotope analysis. Canonical discriminant analysis showed that flies were correctly classified to specific carrion types in 100% (original rules) and 96.8% (leave-one-out cross-validation [LOOCV]) of cases. Regarding life stages, we obtained 100% and 71% of correct classification in original rules and LOOCV, respectively. The isotope ratios of most of the essential amino acids did not significantly change between life stages (at 95% CI). However, some non-essential amino acids (Ala, Ser, and Glu) and some conditionally essential amino acids (Gly and Pro) were isotopically depleted in the adult stage. Except for the essential amino acids, the amino acids in larvae and pupae were enriched in ¹³C, and adult blow flies were depleted in ¹³C relative to the carrion on which they fed. These results make it possible to exclude potential sources of carrion as larval food. Amino acid-specific IRMS could help inform entomologists whether a fly has just arrived from another location to feed on a corpse or has emerged from a pupa whose feedstock was the corpse. Such insight could enhance the significance of blow flies for post-mortem interval determinations. The analytical ability to link organisms from one trophic level to another through the use of compound-specific isotope analysis of amino acids could have wide-reaching consequences in a variety of disciplines. Graphical abstract ᅟ.

Online wet oxidation/isotope ratio mass spectrometry method for determination of stable carbon isotope ratios of water-soluble organic carbon in particulate matter

RATIONALE

Water-soluble organic carbon (WSOC) is formed by oxidation of organic compounds in particulate matter (PM) and accounts for 25-80% of the organic carbon in PM. Stable carbon isotope ratio (δ¹³ C) analysis is widely used to identify the sources of PM, but determining the δ¹³ C values of WSOC is complicated and requires a time-consuming pretreatment process.

METHODS

We have developed an online wet oxidation/isotope ratio mass spectrometry method with a reduced pretreatment time. We have measured the δ¹³ C values of WSOC by using this method.

RESULTS

The method showed high accuracy (0.1‰) and precision (0.1‰) for levoglucosan, and the limit of detection was sufficiently low for WSOC analysis. Using this method, we determined δ¹³ C values of WSOC in PM_2.5 samples collected in Japan during the period from July to November 2017 and found that the values ranged from -26.5‰ to -25.0‰ (average, -25.8‰).

CONCLUSIONS

Our simple, low-blank method could be used for rapid quantitative analysis of the δ¹³ C values of WSOC in PM_2.5 . We propose that this online method be used as a standard method for δ¹³ C analysis of WSOC.

Determination of carbon isotope ratios for honey samples by means of a liquid chromatography/isotope ratio mass spectrometry system coupled with a post-column pump

RATIONALE

Liquid chromatography/isotope ratio mass spectrometry (LC/IRMS) has been used to authenticate and trace products such as honey, wine, and lemon juice, and compounds such as caffeine and pesticides. However, LC/IRMS has several disadvantages, including the high cost of the CO₂ membrane and blocking by solidified sodium persulfate. Here, we developed an improved system for determining carbon isotope ratios using LC/IRMS.

METHODS

The main improvement was the use of a post-column pump. Using the improved system, we determined δ¹³ C values for glucose with high accuracy and precision (0.1‰ and 0.1‰, respectively; n = 3). The glucose, fructose, disaccharide, trisaccharide, and organic acid constituents of honey samples were analyzed using LC/IRMS.

RESULTS

The δ¹³ C values for glucose, fructose, disaccharides, trisaccharides, and organic acids ranged from -27.0 to -24.2‰, -26.8 to -24.0‰, -28.8 to -24.0‰, -27.8 to -22.8‰, and - 30.6 to -27.4‰, respectively. The analysis time was a third to a half of that required for analysis by previously reported methods.

CONCLUSIONS

The column flow rate could be arbitrarily adjusted with the post-column pump. We applied the improved method to 26 commercial honey samples. Our results can be expected to be useful for other researchers who use LC/IRMS.

Position-specific 13 C/12 C analysis of amino acid carboxyl groups - automated flow-injection analysis based on reaction with ninhydrin

RATIONALE

The fundamental level of stable isotopic knowledge lies at specific atomic positions within molecules but existing methods of analysis require lengthy off-line preparation to reveal this information. An automated position-specific isotope analysis (PSIA) method is presented to determine the stable carbon isotopic compositions of the carboxyl groups of amino acids (δ¹³ C_CARBOXYL values). This automation makes PSIA measurements easier and routine.

METHODS

An existing high-performance liquid chromatography (HPLC) gas handling interface/stable isotope ratio mass spectrometry system was modified by the addition of a post-column derivatisation unit between the HPLC system and the interface. The post-column reaction was optimised to yield CO₂ from the carboxyl groups of amino acids by reaction with ninhydrin.

RESULTS

The methodology described produced δ¹³ C_CARBOXYL values with typical standard deviations below ±0.1 ‰ and consistent differences (Δ¹³ C_CARBOXYL values) between amino acids over a 1-year period. First estimates are presented for the δ¹³ C_CARBOXYL values of a number of internationally available amino acid reference materials.

CONCLUSIONS

The PSIA methodology described provides a further dimension to the stable isotopic characterisation of amino acids at a more detailed level than the bulk or averaged whole-molecule level. When combined with on-line chromatographic separation or off-line fraction collection of protein hydrolysates the technique will offer an automated and routine way to study position-specific carboxyl carbon isotope information for amino acids, enabling more refined isotopic studies of carbon uptake and metabolism.

Liquid chromatography/isotope ratio mass spectrometry analysis of halogenated benzoates for characterization of the underlying degradation reaction in Thauera chlorobenzoica CB-1T

RATIONALE

Halogenated benzoic acids occur in the environment due to their widespread agricultural and pharmaceutical use. Compound-specific stable isotope analysis (CSIA) has developed over the last decades for investigation of in situ transformation and reaction mechanisms of environmental pollutants amenable by gas chromatography (GC). As polar compounds are unsuitable for GC analysis we developed a method to perform liquid chromatography (LC)/CSIA for halogenated benzoates.

METHODS

LC/isotope ratio mass spectrometry (IRMS) utilizing a LC-Surveyor pump coupled to a MAT 253 isotope ratio mass spectrometer via a LC-Isolink interface was applied. For chromatographic separation a YMC-Triart C18 column and a potassium hydrogen phosphate buffer (150 mM, pH 7.0, 40°C, 200 μL mL^-1 ) were used, followed by wet oxidation deploying 1.5 mol L^-1 ortho-phosphoric acid and 200 g L^-1 sodium peroxodisulfate at 75 μL mL^-1 .

RESULTS

Separation of benzoate and halogenated benzoates could be achieved in less than 40 min over a concentration range of 2 orders of magnitude. Under these conditions the dehalogenation reaction of Thauera chlorobenzoica 3CB-1^T using 3-chloro-, 3-bromo- and 4-chlorobenzoic acid was investigated resulting in inverse carbon isotope fractionation for meta-substituted benzoic acids and minor normal fractionation for para-substituted benzoic acids. Together with the respective growth rates this led to the assumption that dehalogenation of para-halobenzoic acids follows a different mechanism from that of meta-halobenzoic acids.

CONCLUSIONS

A new LC/IRMS method for the quantitative determination of halogenated benzoates was developed and used to investigate the in vivo transformation pathways of these compounds, providing some insights into degradation and removal of these widespread compounds by T. chlorobenzoica 3CB-1^T .

Four millennia of Iberian biomolecular prehistory illustrate the impact of prehistoric migrations at the far end of Eurasia

Population genomic studies of ancient human remains have shown how modern-day European population structure has been shaped by a number of prehistoric migrations. The Neolithization of Europe has been associated with large-scale migrations from Anatolia, which was followed by migrations of herders from the Pontic steppe at the onset of the Bronze Age. Southwestern Europe was one of the last parts of the continent reached by these migrations, and modern-day populations from this region show intriguing similarities to the initial Neolithic migrants. Partly due to climatic conditions that are unfavorable for DNA preservation, regional studies on the Mediterranean remain challenging. Here, we present genome-wide sequence data from 13 individuals combined with stable isotope analysis from the north and south of Iberia covering a four-millennial temporal transect (7,500-3,500 BP). Early Iberian farmers and Early Central European farmers exhibit significant genetic differences, suggesting two independent fronts of the Neolithic expansion. The first Neolithic migrants that arrived in Iberia had low levels of genetic diversity, potentially reflecting a small number of individuals; this diversity gradually increased over time from mixing with local hunter-gatherers and potential population expansion. The impact of post-Neolithic migrations on Iberia was much smaller than for the rest of the continent, showing little external influence from the Neolithic to the Bronze Age. Paleodietary reconstruction shows that these populations have a remarkable degree of dietary homogeneity across space and time, suggesting a strong reliance on terrestrial food resources despite changing culture and genetic make-up.

Labelled microbial culture as a calibration medium for (13) C-isotope measurement of derivatized compounds: application to tert-butyldimethylsilyl amino acids

RATIONALE

Compound-specific stable carbon isotope analysis by gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS) is widely used in studies of environmental or biological functioning. In the case of derivatized molecules, a calibration might be required due to added non-analyte carbon and in some cases non-stoichiometric recovery by the mass spectrometer.

METHODS

Two biological materials of known isotopic composition were produced by microbial cell cultures on either (13) C-labelled glucose or non-labelled glucose as sole source of carbon. Subsequent hydrolyzed amino acids were derivatized as tert-butyldimethylsilyl (tBDMSi) derivatives and analyzed by GC/C/IRMS. The (13) C-enrichment measurements were used as a direct calibration to calculate the original (13) C/(12) C ratios of individual amino acids. We tested this calibration on both known and unknown samples.

RESULTS

For the main proteinogenic amino acids we could determine the number of non-analyte added carbon atoms and assess the non-stoichiometrical recovery of tBDMSi carbon atoms, due to their incomplete oxidation in the combustion step of GC/C/IRMS. The calibration enabled the determination of the natural abundances (δ(13) C values) of amino acids with an average accuracy of ±1.1 ‰. We illustrate the application of the calibration to determine the (13) C/(12) C ratios of amino acids, and the associated uncertainty, in biological and plant materials.

CONCLUSIONS

The analysis of a labelled microbial cell culture offers a straightforward, rapid and reliable estimate of non-analyte carbon contribution to stable isotope composition. We recommend this method as a calibration or a control in artificial or natural (13) C-tracing experiments. Copyright © 2016 John Wiley & Sons, Ltd.

Variability in the carbon isotope composition of individual amino acids in plant proteins from different sources: 1 Leaves

The natural carbon isotope composition of individual amino acids from plant leaf proteins has been measured to establish potential sources of variability. The plant leaves studied, taken from a range of plant groups (forbs, trees, grasses, and freshwater aquatic plants), showed no significant influence of either season or environment (water and light availability) on their Δδ(13)C values. Plant groups did, however, differ in carbon isotope composition, although no consistent differences were identified at the species level. A discriminant analysis model was constructed which allowed leaves from (1) nettles, (2) Pooideae, (3) other Poales, (4) trees and (5) freshwater higher plants to be distinguished from each other on the basis of their natural abundance (13)C/(12)C ratios of individual amino acids. Differences in carbon isotope composition are known to be retained, to some extent, in the tissues of their consumers, and hence an understanding of compound-specific variation in (13)C/(12)C fractional abundance in plants has the potential to provide dietary insights of value in archaeological and ecological studies.

Performance of the wet oxidation unit of the HPLC isotope ratio mass spectrometry system for halogenated compounds

The performance of liquid chromatography-isotope ratio mass spectrometry (LC-IRMS) for polar halogenated compounds was evaluated. Oxidation capacity of the system was tested with halogenated acetic acids and halogenated aromatic compounds. Acetic acid (AA) was selected as a reference compound for complete oxidation and compared on the molar basis to the oxidation of other analytes. The isotope values were proofed with calibrated δ(13)C values obtained with an elemental analyzer (EA). Correct isotope values were obtained for mono- and dichlorinated, fluorinated, and tribrominated acetic acids and also for aniline, phenol, benzene, bromobenzene, chlorobenzene, 1,2-dichlorobenzene, 2,4,6-trichlorophenol, pentafluorophenol, and nitrobenzene. Incomplete oxidation of trichloroacetic acid (TCA) and trifluoroacetic acid (TFA) resulted in lower recovery compared to AA (37% and 24%, respectively) and in isotopic shift compared to values obtained with EA (TCA Δδ(13)C(EA/LC-IRMS) = 8.8‰, TFA Δδ(13)C(EA/LC-IRMS) = 6.0‰). Improvement of oxidation by longer reaction time in the reactor and increase in the concentration of sulfate radicals did not lead to complete combustion of TCA and TFA needed for δ(13)C analysis. To the best of our knowledge, this is the first time such highly chlorinated compounds were studied with the LC-IRMS system. This work provides information for method development of LC-IRMS methods for halogenated contaminants that are known as potential threats to public health and the environment.

A versatile method for simultaneous stable carbon isotope analysis of DNA and RNA nucleotides by liquid chromatography/isotope ratio mass spectrometry

RATIONALE

Liquid chromatography/isotope ratio mass spectrometry (LC/IRMS) is currently the most accurate and precise technique for the measurement of compound-specific stable carbon isotope ratios ((13)C/(12)C) in biological metabolites, at their natural abundance. However, until now this technique could not be applied for the analysis of nucleic acids, the building blocks of the carriers of genetic information in living cells and viruses, DNA and RNA.

METHODS

Mixed-mode chromatography (MMC) was applied to obtain the complete separation of nine nucleotides (eight originating from DNA/RNA and one nucleotide (inosine monophosphate) that may serve as an internal standard) in a single run using LC/IRMS. We also developed and validated a method for DNA and RNA extraction and an enzymatic hydrolysis protocol for natural samples, which is compatible with LC/IRMS analysis as it minimizes the carbon blank. The method was used to measure the concentration and stable carbon isotope ratio of DNA and RNA nucleotides in marine sediment and in the common marine macro alga (Ulva sp.) at natural abundance levels as well as for (13)C-enriched samples.

RESULTS

The detection limit of the LC/IRMS method varied between 1.0 nmol for most nucleotides and 2.0 nmol for late-eluting compounds. The intraday and interday reproducibility of nucleotide concentration measurements was better than, respectively, 4.1% and 8.9% and for δ(13)C measurements better than, respectively, 0.3‰ and 0.5‰. The obtained nucleic acid concentrations and nucleic acid synthesis rates were in good agreement with values reported in the literature.

CONCLUSIONS

This new method gives reproducible results for the concentration and δ(13)C values of nine nucleotides. This solvent-free chromatographic method may also be used for other purposes, such as for instance to determine nucleotide concentrations using spectrophotometric detection. This sensitive method offers a new avenue for the study of DNA and RNA biosynthesis that can be applied in various fields of research.

High-resolution food webs based on nitrogen isotopic composition of amino acids

Food webs are known to have myriad trophic links between resource and consumer species. While herbivores have well-understood trophic tendencies, the difficulties associated with characterizing the trophic positions of higher-order consumers have remained a major problem in food web ecology. To better understand trophic linkages in food webs, analysis of the stable nitrogen isotopic composition of amino acids has been introduced as a potential means of providing accurate trophic position estimates. In the present study, we employ this method to estimate the trophic positions of 200 free-roaming organisms, representing 39 species in coastal marine (a stony shore) and 38 species in terrestrial (a fruit farm) environments. Based on the trophic positions from the isotopic composition of amino acids, we are able to resolve the trophic structure of these complex food webs. Our approach reveals a high degree of trophic omnivory (i.e., noninteger trophic positions) among carnivorous species such as marine fish and terrestrial hornets.This information not only clarifies the trophic tendencies of species within their respective communities, but also suggests that trophic omnivory may be common in these webs.

Long-term resilience of late holocene coastal subsistence system in Southeastern South america

Isotopic and molecular analysis on human, fauna and pottery remains can provide valuable new insights into the diets and subsistence practices of prehistoric populations. These are crucial to elucidate the resilience of social-ecological systems to cultural and environmental change. Bulk collagen carbon and nitrogen isotopic analysis of 82 human individuals from mid to late Holocene Brazilian archaeological sites (∼6,700 to ∼1,000 cal BP) reveal an adequate protein incorporation and, on the coast, the continuation in subsistence strategies based on the exploitation of aquatic resources despite the introduction of pottery and domesticated plant foods. These results are supported by carbon isotope analysis of single amino acid extracted from bone collagen. Chemical and isotopic analysis also shows that pottery technology was used to process marine foods and therefore assimilated into the existing subsistence strategy. Our multidisciplinary results demonstrate the resilient character of the coastal economy to cultural change during the late Holocene in southern Brazil.

High-precision mass spectrometric analysis using stable isotopes in studies of children

The use of stable isotopes combined with mass spectrometry (MS) provides insight into metabolic processes within the body. Herein, an overview on the relevance of stable isotope methodology in pediatric research is presented. Applications for the use of stable isotopes with MS cover carbohydrate, fat, and amino acid metabolism as well as body composition, energy expenditure, and the synthesis of specific peptides and proteins, such as glutathione and albumin. The main focus of these studies is on the interactions between nutrients and the endogenous metabolism within the body and how these factors affect the health of a growing infant. Considering that the early imprinting of metabolic processes hugely impacts metabolism (and thus functional outcome) later in life, research in this area is important and is advancing rapidly. The major fluxes on a metabolic level are the synthesis and breakdown rates. They can be quantified using kinetic tracer analysis and mathematical modeling. Organic MS and isotope ratio mass spectrometry (IRMS) are the two most mature techniques for the isotopic analysis of compounds. Introduction of the samples is usually done by coupling gas chromatography (GC) to either IRMS or MS because it is the most robust technique for specific isotopic analysis of volatile compounds. In addition, liquid chromatography (LC) is now being used more often as a tool for sample introduction of both volatile and non-volatile compounds into IRMS or MS for (13)C isotopic analyses at natural abundances and for (13)C-labeled enriched compounds. The availability of samples is often limited in pediatric patients. Therefore, sample size restriction is important when developing new methods. Also, the availability of stable isotope-labeled substrates is necessary for measurements of the kinetics and concentrations in metabolic studies, which can be a limiting factor. During the last decade, the availability of these substrates has increased. Furthermore, improvements in the accuracy, precision, and sensitivity of existing techniques (such as GC/IRMS) and the development of new techniques (such as LC/IRMS) have opened up new avenues for tackling these limitations.

The glycemic response does not reflect the in vivo starch digestibility of fiber-rich wheat products in healthy men

Starchy food products differ in the rate of starch digestion, which can affect their metabolic impact. In this study, we examined how the in vivo starch digestibility is reflected by the glycemic response, because this response is often used to predict starch digestibility. Ten healthy male volunteers [age 21 ± 0.5 y, BMI 23 ± 0.6 kg/m² (mean ± SEM)] participated in a cross-over study, receiving three different meals: pasta with normal wheat bran (PA) and bread with normal (CB) or purple wheat bran (PBB). Purple wheat bran was added in an attempt to decrease the rate of starch digestion. The meals were enriched in ¹³C and the dual isotope technique was applied to calculate the rate of appearance of exogenous glucose (RaE). The ¹³C-isotopic enrichment of glucose in plasma was measured with GC/combustion/isotope ratio MS (IRMS) and liquid chromatography/IRMS. Both IRMS techniques gave similar results. Plasma glucose concentrations [2-h incremental AUC (iAUC)] did not differ between the test meals. The RaE was similar after consumption of CB and PBB, showing that purple wheat bran in bread does not affect in vivo starch digestibility. However, the iAUC of RaE after men consumed PA was less than after they consumed CB (P < 0.0001) despite the similar glucose response. To conclude, the glycemic response does not always reflect the in vivo starch digestibility. This could have implications for intervention studies in which the glycemic response is used to characterize test products.

Preparative HPLC separation of underivatized amino acids for isotopic analysis

Single-compound analysis of stable or radio-isotopes has found application in a number of fields ranging from archaeology to forensics. Often, the most difficult part of these analyses is the development of a method for isolating the compounds of interest.Here, we describe three complementary preparative HPLC procedures suitable for separating and isolating single amino acids from bone collagen or hair keratin with minimal isotopic contamination. Using preparative reversed-phase, ion-pair, or mixed-mode chromatography of underivatized amino acids in aqueous mobile phases, single amino acids can be isolated and further analyzed using mass spectrometric techniques.

Optimal conditions for the direct RP-HPLC determination of underivatized amino acids with online multiple detection

The combined use of a dual-UV detector as well as a fluorimetric and a multielectrode electrochemical detector (equipped with a dual electrode, consisting of a conventional size 3 mm diameter glassy carbon electrode (GCE) and of a pair of 30 μm thick carbon microfibers) is proposed for the detection of the following 15 underivatized amino acids: L: -histidine (his), L: -cysteine (cys), creatine (crn), S-methyl-L: -cysteine (me-cys), DL: -homocysteine (hcy), L: -methionine (met), beta-(3,4-dihydroxyphenyl)-L: -alanine (dopa), L: -tyrosine (tyr), DL: -m-tyrosine (m-tyr), L: -a-methyl-dopa (me-dopa), L: -phenylanine (phe), DL: -alpha-methyltyrosine (me-tyr), 5-hydroxy-tryptophan (5htp), 3-nitro-L: -tyrosine (NO(2)Tyr) and L: -tryptophan (trp), as well as of two dipeptides: L: -cystathionine (cysta), L: -carnosine (car), and of creatinine (cre). A multilinear solvent (acetonitrile) gradient elution program, determined by a simple optimization algorithm, is required for the efficient reversed phase separation of the above mixture of 18 solutes within 27 min at a flow rate of 1.0 mL/min and at 25°C.

Liquid chromatography/isotope ratio mass spectrometry measurement of δ13C of amino acids in plant proteins

In archaeological studies, the isotopic enrichment values of carbon and nitrogen in bone collagen give a degree of information on dietary composition. The isotopic enrichments of individual amino acids from bone collagen and dietary protein have the potential to provide more precise information about the components of diet. A limited amount of work has been done on this, although the reliability of these studies is potentially limited by fractionation arising through hydrolysis of whole plant tissue (where reaction between amino acids and carbohydrates may occur) and, for certain amino acids, the use of derivatives (particularly trifluoroacetyl derivatives) for gas chromatography/isotope ratio mass spectrometry (GC/IRMS) analysis. The present study takes the approach of extracting the protein components of plant tissues before hydrolysis and using liquid chromatography/isotope ratio mass spectrometry (LC/IRMS), which does not require derivatisation, for measurement of the isotopic enrichment of the amino acids. The protocol developed offers a methodology for consistent measurement of the δ(13)C values of amino acids, allowing isotopic differences between the individual amino acids from different plant tissues to be identified. In particular, there are highly significant differences between leaf and seed protein amino acids (leaf minus grain) in the cases of threonine (-4.1‰), aspartic acid (+3.5‰) and serine (-3.2‰). In addition to its intended application in archaeology, the technique will be of value in the fields of plant sciences, nutrition and environmental food-web studies.

Comparison of liquid chromatography-isotope ratio mass spectrometry (LC/IRMS) and gas chromatography-combustion-isotope ratio mass spectrometry (GC/C/IRMS) for the determination of collagen amino acid δ13C values for palaeodietary and palaeoecological reconstruction

Results are presented of a comparison of the amino acid (AA) δ(13)C values obtained by gas chromatography-combustion-isotope ratio mass spectrometry (GC/C/IRMS) and liquid chromatography-isotope ratio mass spectrometry (LC/IRMS). Although the primary focus was the compound-specific stable carbon isotope analysis of bone collagen AAs, because of its growing application for palaeodietary and palaeoecological reconstruction, the results are relevant to any field where AA δ(13)C values are required. We compare LC/IRMS with the most up-to-date GC/C/IRMS method using N-acetyl methyl ester (NACME) AA derivatives. This comparison involves the analysis of standard AAs and hydrolysates of archaeological human bone collagen, which have been previously investigated as N-trifluoroacetyl isopropyl esters (TFA/IP). It was observed that, although GC/C/IRMS analyses required less sample, LC/IRMS permitted the analysis of a wider range of AAs, particularly those not amenable to GC analysis (e.g. arginine). Accordingly, reconstructed bulk δ(13)C values based on LC/IRMS-derived δ(13)C values were closer to the EA/IRMS-derived δ(13)C values than those based on GC/C/IRMS values. The analytical errors for LC/IRMS AA δ(13)C values were lower than GC/C/IRMS determinations. Inconsistencies in the δ(13)C values of the TFA/IP derivatives compared with the NACME- and LC/IRMS-derived δ(13)C values suggest inherent problems with the use of TFA/IP derivatives, resulting from: (i) inefficient sample combustion, and/or (ii) differences in the intra-molecular distribution of δ(13)C values between AAs, which are manifested by incomplete combustion. Close similarities between the NACME AA δ(13)C values and the LC/IRMS-derived δ(13)C values suggest that the TFA/IP derivatives should be abandoned for the natural abundance determinations of AA δ(13)C values.

Review: Current applications and challenges for liquid chromatography coupled to isotope ratio mass spectrometry (LC/IRMS)

High-precision isotope analysis is recognized as an essential research tool in many fields of study. Until recently, continuous flow isotope ratio mass spectrometry (CF-IRMS) was available via an elemental analyzer or a gas chromatography inlet system for compound-specific analysis of light stable isotopes. In 2004, however, an interface that coupled liquid chromatography with IRMS (LC/IRMS) became commercially available for the first time. This brought the capability for new areas of application, in particular enabling compound-specific δ(13)C analysis of non-volatile, aqueous soluble, compounds from complex mixtures. The interface design brought with it several analytical constraints, however, in particular a lack of compatibility with certain types of chromatography as well as limited flow rates and mobile phase compositions. Routine LC/IRMS methods have, however, been established for measuring the δ(13)C isotopic ratios of underivatized individual compounds for application in archeology, nutrition and physiology, geochemistry, hydrology, soil science and food authenticity. Seven years after its introduction, we review the technical advances and constraints, methodological developments and new applications of liquid chromatography coupled to isotope ratio mass spectrometry.

Quantitation of plasma 13C-galactose and 13C-glucose during exercise by liquid chromatography/isotope ratio mass spectrometry

The utilisation of carbohydrate sources under exercise conditions is of considerable importance in performance sports. Incorporation of optimal profiles of macronutrients can improve endurance performance in athletes. However, gaining an understanding of the metabolic partitioning under sustained exercise can be problematical and isotope labelling approaches can help quantify substrate utilisation. The utilisation of oral galactose was investigated using (13)C-galactose and measurement of plasma galactose and glucose enrichment by liquid chromatography/isotope ratio mass spectrometry (LC/IRMS). As little as 100 μL plasma could readily be analysed with only minimal sample processing. Fucose was used as a chemical and isotopic internal standard for the quantitation of plasma galactose and glucose concentrations, and isotopic enrichment. The close elution of galactose and glucose required a correction routine to be implemented to allow the measurement, and correction, of plasma glucose δ(13)C, even in the presence of very highly enriched galactose. A Bland-Altman plot of glucose concentration measured by LC/IRMS against glucose measured by an enzymatic method showed good agreement between the methods. Data from seven trained cyclists, undergoing galactose supplementation before exercise, demonstrate that galactose is converted into glucose and is available for subsequent energy metabolism.

Advances in natural stable isotope ratio analysis of human hair to determine nutritional and metabolic status

PURPOSE OF REVIEW

We review the literature on the use of stable isotope ratios at natural abundance to reveal information about dietary habits and specific nutrient intakes in human hair protein (keratin) and amino acids. In particular, we examine whether hair isotopic compositions can be used as unbiased biomarkers to provide information about nutritional status, metabolism, and diseases.

RECENT FINDINGS

Although the majority of research on the stable isotope ratio analysis of hair has focused on bulk protein, methods have been recently employed to examine amino acid-specific isotope ratios using gas chromatography or liquid chromatography coupled to an isotope ratio mass spectrometer. The isotopic measurement of amino acids has the potential to answer research questions on amino acid nutrition, metabolism, and disease processes and can contribute to a better understanding of the variations in bulk protein isotope ratio values. First results suggest that stable isotope ratios are promising as unbiased nutritional biomarkers in epidemiological research. However, variations in stable isotope ratios of human hair are also influenced by nutrition-dependent nitrogen balance, and more controlled clinical research is needed to examine these effects in human hair.

SUMMARY

Stable isotope ratio analysis at natural abundance in human hair protein offers a noninvasive method to reveal information about long-term nutritional exposure to specific nutrients, nutritional habits, and in the diagnostics of diseases leading to nutritional stress and impaired nitrogen balance.

Strong anion-exchange liquid chromatography coupled with isotope ratio mass spectrometry using a Liquiface interface

The introduction of liquid chromatography coupled with isotope ratio mass spectrometry (LC/IRMS) as an analytical tool for the measurement of isotope ratios in non-volatile analytes has somewhat simplified the analytical cycle from sample collection to analysis mainly due to the avoidance of the extensive sample processing and derivatisation that were necessary for gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS). Here we test the performance of coupling strong anion exchange to IRMS using only the second commercially available interface; the Liquiface. The system was modified from installation specification to improve peak resolution in the interface and maintain peak separation from the column to the mass spectrometer. The system performance was assessed by the determination of sensitivity, accuracy and precision attained from carbohydrate separations. The system performed satisfactorily after modifications, resulting in maintenance of peak resolution from column to mass spectrometer. The sensitivity achieved suggested that approximately 150 ng carbon could be analysed with acceptable precision (<0.3 per thousand). Accuracy was maintained in the interface as determined by correlation with offline techniques, resulting in regression coefficient of r(2) = 0.98 and a slope of 0.99. The average precision achieved for the separation of seven monosaccharides was 0.36 per thousand. The integration of a carbonate removal device limited the effect of background carbon perturbations in the mass spectrometer associated with eluent gradients, and the coupling of strong anion-exchange chromatography with IRMS was successfully achieved using the Liquiface.

Amino acid delta13C analysis of hair proteins and bone collagen using liquid chromatography/isotope ratio mass spectrometry: paleodietary implications from intra-individual comparisons

We report a novel method for the chromatographic separation and measurement of stable carbon isotope ratios (delta(13)C) of individual amino acids in hair proteins and bone collagen using the LC-IsoLink system, which interfaces liquid chromatography (LC) with isotope ratio mass spectrometry (IRMS). This paper provides baseline separation of 15 and 13 of the 18 amino acids in bone collagen and hair proteins, respectively. We also describe an approach to analysing small hair samples for compound-specific analysis of segmental hair sections. The LC/IRMS method is applied in a historical context by the delta(13)C analysis of hair proteins and bone collagen recovered from six individuals from Uummannaq in Greenland. The analysis of hair and bone amino acids from the same individual, compared for the first time in this study, is of importance in palaeodietary reconstruction. If hair proteins can be used as a proxy for bone collagen at the amino acid level, this validates compound-specific isotope studies using hair as a model for palaeodietary reconstruction. Our results suggest that a small offset observed in the bulk delta(13)C values of the hair and bone samples may be attributed to two factors: (i) amino acid compositional differences between hair and bone proteins, and (ii) differential turnover rates of the tissues and the amino acid pools contributing to their synthesis. This application proposes that hair may be a useful complementary or alternative source of compound-specific paleodietary information.

Analysis of [U-13C6]glucose in human plasma using liquid chromatography/isotope ratio mass spectrometry compared with two other mass spectrometry techniques

The use of stable isotope labelled glucose provides insight into glucose metabolism. The 13C-isotopic enrichment of glucose is usually measured by gas chromatography/mass spectrometry (GC/MS) or gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS). However, in both techniques the samples must be derivatized prior to analysis, which makes sample preparation more labour-intensive and increases the uncertainty of the measured isotopic composition. A novel method for the determination of isotopic enrichment of glucose in human plasma using liquid chromatography/isotope ratio mass spectrometry (LC/IRMS) has been developed. Using this technique, for which hardly any sample preparation is needed, we showed that both the enrichment and the concentration could be measured with very high precision using only 20 microL of plasma. In addition, a comparison with GC/MS and GC/IRMS showed that the best performance was achieved with the LC/IRMS method making it the method of choice for the measurement of 13C-isotopic enrichment in plasma samples.