High-precision measurement of phenylalanine δ15N values for environmental samples: a new approach coupling high-pressure liquid chromatography purification and elemental analyzer isotope ratio mass spectrometry

Alkaline persulfate oxidation as an intermediate step for the development of a wet chemical oxidation interface for compound-specific δ15N analysis by LC-IRMS

For the measurement of compound-specific isotope ratios by liquid chromatography isotope ratio mass spectrometry (LC-IRMS), complete mineralization of organic compounds to a single species of measurement gas is required so that isotopic fractionation can be minimized and corrected by identical treatment with standards. The established use of peroxydisulfate in an acidic environment has its limitations, especially when it comes to the complete oxidation of nitrogen-containing compounds with aromatic ring systems. Under acidic oxidation conditions, ammonium and nitrate were identified as the main nitrogen containing mineralization products of the oxidation of different model compounds. In contrast to the oxidation in an acidic environment, alkaline peroxydisulfate oxidation leads to nitrate as a final mineralization product. The concept of alkaline oxidation was transferred from large-scale batch experiments to a commercially available oxidation reactor used in LC-IRMS systems. The obtained nitrate recoveries indicate that alkaline oxidation could be a promising step towards the measurement of compound-specific nitrogen isotope ratios by LC-IMRS. In our work, we show that alkaline peroxydisulfate oxidation allows faster and more complete mineralization of nitrogen-containing compounds. For several model compounds, 63 to 100% of the initially present nitrogen was converted to nitrate within a reaction time of 43 s.

Special delivery of proteinaceous matter to deep-sea microbes

Earth's deep ocean holds a vast reservoir of dissolved organic carbon, traditionally considered old and resistant to microbial degradation. Radiocarbon analyses indicate the hidden occurrence of younger dissolved organic carbon components, assumed to be accessible to deep-sea microorganisms but not yet demonstrated. Using compound-class radiocarbon analysis, molecular characterization, and bioassay experiments, we provide direct evidence for rapid microbial utilization of young, labile, high-molecular weight proteinaceous material in bathypelagic waters. The abundance of labile proteinaceous material diminishes from epipelagic to mesopelagic waters but notably increases in bathypelagic waters, where it exhibits a short turnover time (days) and resembles surface plankton in molecular composition. This observation coincides with peak zooplankton biomass recorded over the year. The nonmonotonic depth trend suggests a deep-sea replenishment of organic particles from mesopelagic migrating zooplankton. Our results indicate the presence of labile organic molecules at bathypelagic depths and reveal a nonlinear supply of plankton-derived substrates that support microbial metabolism and carbon sequestration in the deep ocean.

Anchovy boom and bust linked to trophic shifts in larval diet

Although massive biomass fluctuations of coastal-pelagic fishes are an iconic example of the impacts of climate variability on marine ecosystems, the mechanisms governing these dynamics are often elusive. We construct a 45-year record of nitrogen stable isotopes measured in larvae of Northern Anchovy (Engraulis mordax) in the California Current Ecosystem to assess patterns in food chain length. Larval trophic efficiency associated with a shortened food chain increased larval survival and produced boom periods of high adult biomass. In contrast, when larval food chain length increased, and energy transfer efficiency decreased, the population crashed. We propose the Trophic Efficiency in Early Life (TEEL) hypothesis, which states that larval fishes must consume prey that confer sufficient energy for survival, to help explain natural boom-bust dynamics of coastal pelagic fishes. Our findings illustrate a potential for trophic indicators to generally inform larval survival and adult population dynamics of coastal-pelagic fishes.

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.

Application of amino acids nitrogen stable isotopic analysis in bioaccumulation studies of pollutants: A review

Accurately quantifying trophic positions (TP) to describe food web structure is an important element in studying pollutant bioaccumulation. In recent years, compound-specific nitrogen isotopic analysis of amino acids (AAs-N-CSIA) has been progressively applied as a potentially reliable tool for quantifying TP, facilitating a better understanding of pollutant food web transfer. Therefore, this review provides an overview of the analytical procedures, applications, and limitations of AAs-N-CSIA in pollutant (halogenated organic pollutants (HOPs) and heavy metals) bioaccumulation studies. We first summarize studies on the analytical techniques of AAs-N-CSIA, including derivatization, instrumental analysis, and data processing methods. The N-pivaloyl-i-propyl-amino acid ester method is a more suitable AAs derivatization method for quantifying TP. The AAs-N-CSIA application in pollutant bioaccumulation studies (e.g., Hg, MeHg, and HOPs) is discussed, and its application in conjunction with various techniques (e.g., spatial analysis, food source analysis, and compound tracking techniques, etc.) to research the influence of pollutant levels on organisms is summarized. Finally, the limitations of AAs-N-CSIA in pollutant bioaccumulation studies are discussed, including the use of single empirical values of β_glu/phe and TDF_glu/phe that result in large errors in TP quantification. The weighted β_glu/phe and the multi-TDF_glu/phe models are still challenging to solve for accurate TP quantification of omnivores; however, factors affecting the variation of β_glu/phe and TDF_glu/phe are unclear, especially the effect of pollutant bioaccumulation in organisms on internal AA metabolic processes.

Stable Isotope Analysis of Residual Pesticides via High Performance Liquid Chromatography and Elemental Analyzer-Isotope Ratio Mass Spectrometry

To broaden the range of measurable pesticides for stable isotope analysis (SIA), we tested whether SIA of the anthranilic diamides cyantraniliprole (CYN) and chlorantraniliprole (CHL) can be achieved under elemental analyzer/isotope ratio mass spectrometry with compound purification in high-performance liquid chromatography (HPLC). Using this method, carbon isotope compositions were measured in pesticide residues extracted from plants (lettuce) grown indoors in potting soil that were treated with 500 mg/kg CHL and 250 mg/kg CYN and were followed up for 45 days. Our results show that the CYN and CHL standard materials did not have significant isotope differences before and after clean-up processing in HPLC. Further, when applied to the CYN product and CHL product in soil, stable isotope differences between the soil and plant were observed at <1.0‱ throughout the incubation period. There was a slight increase in the variability of pesticide isotope ratio detected with longer-term incubation (CHL, on average 1.5‱). Overall, we measured the carbon isotope ratio of target pesticides from HPLC fraction as the purification and pre-concentration step for environmental and biological samples. Such negligible isotopic differences in pesticide residues in soils and plants 45 days after application confirmed the potential of CSIA to quantify pesticide behavior in environments.

Rapid, sensitive analysis method for determining the nitrogen stable isotope ratio of total free amino acids in soil

RATIONALE

The amino acid-nitrogen (AA-N) isotope analysis of naturally abundant or isotope-labeled samples is indispensable for tracing nitrogen transfer in soil nitrogen biogeochemical cycling processes. Despite the usefulness of AA-N isotope analysis, the preparation methods are complex and time-consuming, and necessitate the use of toxic reagents.

METHODS

We present an improved, rapid method for AA-N isotope analysis with high precision. At a high pH, AA-N was released and oxidized to N₂ O using ClO^- under vacuum. Additionally, purge-and-trap isotope ratio mass spectrometry was used to analyze N₂ O. Moreover, we investigated the effect of various factors on the N₂ O conversion process with glycine and applied the results to seven representative single-N AAs (alanine, serine, cysteine, aspartic acid, glutamic acid, leucine, and phenylalanine) and five poly-N AAs (lysine, arginine, histidine, tryptophan, and asparagine), as well as side-chain analogs, blank reagent, and other N forms.

RESULTS

The concentration of ClO^- and the pH were determined to be crucial factors for achieving desirable AA-N to N₂ O conversion efficiencies. Glycine-N had the highest N₂ O yield of 70%, with isotopic results consistent with those of the reference values at a high precision (within 0.5‰ for natural abundance and 0.01 atom% for ¹⁵ N-enrichment) at the nanomolar N level. Additionally, the α-NH₂ AAs were labile, and the single-N AAs were more easily converted to N₂ O than poly-N AAs. With the exception of γ-aminobutyric acid, the N₂ O conversion efficiencies of the side-chain N analogs were very low (below 5%). This method was also applicable to the ¹⁵ N analysis of the total free AAs in complex soil samples without interference from analytical blanks and other forms of N.

CONCLUSIONS

Our method is highly selective for the α-NH₂ groups of an amino acid, and the oxidation of the side chain is difficult. In addition, the method is sensitive, rapid, and convenient, and does not require toxic reagents.

High-precision measurement of phenylalanine and glutamic acid δ15 N by coupling ion-exchange chromatography and purge-and-trap continuous-flow isotope ratio mass spectrometry

RATIONALE

Nitrogen isotopic compositions (δ¹⁵ N) of source and trophic amino acids (AAs) are crucial tracers of N sources and trophic enrichments in diverse fields, including archeology, astrobiochemistry, ecology, oceanography, and paleo-sciences. The current analytical technique using gas chromatography-combustion-isotope ratio mass spectrometry (GC/C/IRMS) requires derivatization, which is not compatible with some key AAs. Another approach using high-performance liquid chromatography-elemental analyzer-IRMS (HPLC/EA/IRMS) may experience coelution issues with other compounds in certain types of samples, and the highly sensitive nano-EA/IRMS instrumentations are not widely available.

METHODS

We present a method for high-precision δ¹⁵ N measurements of AAs (δ¹⁵ N-AA) optimized for canonical source AA-phenylalanine (Phe) and trophic AA-glutamic acid (Glu). This offline approach entails purification and separation via high-pressure ion-exchange chromatography (IC) with automated fraction collection, the sequential chemical conversion of AA to nitrite and then to nitrous oxide (N₂ O), and the final determination of δ¹⁵ N of the produced N₂ O via purge-and-trap continuous-flow isotope ratio mass spectrometry (PT/CF/IRMS).

RESULTS

The cross-plots of δ¹⁵ N of Glu and Phe standards (four different natural-abundance levels) generated by this method and their accepted values have a linear regression slope of 1 and small intercepts demonstrating high accuracy. The precisions were 0.36‰-0.67‰ for Phe standards and 0.27‰-0.35‰ for Glu standards. Our method and the GC/C/IRMS approach produced equivalent δ¹⁵ N values for two lab standards (McCarthy Lab AA mixture and cyanobacteria) within error. We further tested our method on a wide range of natural sample matrices and obtained reasonable results.

CONCLUSIONS

Our method provides a reliable alternative to the current methods for δ¹⁵ N-AA measurement as IC or HPLC-based techniques that can collect underivatized AAs are widely available. Our chemical approach that converts AA to N₂ O can be easily implemented in laboratories currently analyzing δ¹⁵ N of N₂ O using PT/CF/IRMS. This method will help promote the use of δ¹⁵ N-AA in important studies of N cycling and trophic ecology in a wide range of research areas.

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.

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.

Correction to: Measurement of the 15N/14N ratio of phenylalanine in fermentation matrix by isotope ratio mass spectrometry

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