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1. High‐precision measurement of phenylalanine and glutamic acid δ 15 N by coupling ion‐exchange chromatography and purge‐and‐trap continuous‐flow isotope ratio mass spectrometry

   https://doi.org/10.1002/rcm.9085  

   Zhang, Lin ; Lee, Wing‐man ; Kreider‐Mueller, Ava ; Kuhnel, Evelyn ; Baca, Jesus ; Ji, Chongxiao ; Altabet, Mark ( May 2021 , Rapid Communications in Mass Spectrometry)

   Nitrogen isotopic compositions (δ¹⁵N) 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.

   We present a method for high‐precision δ¹⁵N measurements of AAs (δ¹⁵N‐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 (N₂O), and the final determination of δ¹⁵N of the produced N₂O via purge‐and‐trap continuous‐flow isotope ratio mass spectrometry (PT/CF/IRMS).

   The cross‐plots of δ¹⁵N 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 δ¹⁵N 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.

   Our method provides a reliable alternative to the current methods for δ¹⁵N‐AA measurement as IC or HPLC‐based techniques that can collect underivatized AAs are widely available. Our chemical approach that converts AA to N₂O can be easily implemented in laboratories currently analyzing δ¹⁵N of N₂O using PT/CF/IRMS. This method will help promote the use of δ¹⁵N‐AA in important studies of N cycling and trophic ecology in a wide range of research areas.

    

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2. Effects of chemical preservation on bulk and amino acid isotope ratios of zooplankton, fish, and squid tissues

   https://doi.org/10.1002/rcm.8408  

   Hetherington, Elizabeth D. ; Kurle, Carolyn M. ; Ohman, Mark D. ; Popp, Brian N. ( April 2019 , Rapid Communications in Mass Spectrometry)

   It is imperative to understand how chemical preservation alters tissue isotopic compositions before using historical samples in ecological studies. Specifically, although compound‐specific isotope analysis of amino acids (CSIA‐AA) is becoming a widely used tool, there is little information on how preservation techniques affect amino acidδ¹⁵N values.

   We evaluated the effects of chemical preservatives on bulk tissueδ¹³C andδ¹⁵N and amino acidδ¹⁵N values, measured by gas chromatography/isotope ratio mass spectrometry (GC/IRMS), of (a) tuna (Thunnus albacares) and squid (Dosidicus gigas) muscle tissues that were fixed in formaldehyde and stored in ethanol for 2 years and (b) two copepod species,Calanus pacificusandEucalanus californicus, which were preserved in formaldehyde for 24–25 years.

   Tissues in formaldehyde‐ethanol had higher bulkδ¹⁵N values (+1.4,D. gigas; +1.6‰,T. albacares), higherδ¹³C values forD. gigas(+0.5‰), and lowerδ¹³C values forT. albacares(−0.8‰) than frozen samples. The bulkδ¹⁵N values from copepods were not different those from frozen samples, although theδ¹³C values from both species were lower (−1.0‰ forE. californicusand −2.2‰ forC. pacificus) than those from frozen samples. The mean amino acidδ¹⁵N values from chemically preserved tissues were largely within 1‰ of those of frozen tissues, but the phenylalanineδ¹⁵N values were altered to a larger extent (range: 0.5–4.5‰).

   The effects of preservation on bulkδ¹³C values were variable, where the direction and magnitude of change varied among taxa. The changes in bulkδ¹⁵N values associated with chemical preservation were mostly minimal, suggesting that storage in formaldehyde or ethanol will not affect the interpretation ofδ¹⁵N values used in ecological studies. The preservation effects on amino acidδ¹⁵N values were also mostly minimal, mirroring bulkδ¹⁵N trends, which is promising for future CSIA‐AA studies of archived specimens. However, there were substantial differences in phenylalanine and valineδ¹⁵N values, which we speculate resulted from interference in the chromatographic resolution of unknown compounds rather than alteration of tissue isotopic composition due to chemical preservation.

    

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3. Dietary protein content and digestibility influences discrimination of amino acid nitrogen isotope values in a terrestrial omnivorous mammal

   https://doi.org/10.1002/rcm.9073  

   Whiteman, John P. ; Rodriguez Curras, Mauriel ; Feeser, Kelli L. ; Newsome, Seth D. ( April 2021 , Rapid Communications in Mass Spectrometry)

   Ecologists increasingly determine the δ¹⁵N values of amino acids (AA) in animal tissue; “source” AA typically exhibit minor variation between diet and consumer, while “trophic” AA have increased δ¹⁵N values in consumers. Thus, trophic‐source δ¹⁵N offsets (i.e., Δ¹⁵N_T‐S) reflect trophic position in a food web. However, even minor variations in δ¹⁵N_{source AA}values may influence the magnitude of offset that represents a trophic step, known as the trophic discrimination factor (i.e., TDF_T‐S). Diet digestibility and protein content can influence the δ¹⁵N values of bulk animal tissue, but the effects of these factors on AA Δ¹⁵N_T‐Sand TDF_T‐Sin mammals are unknown.

   We fed captive mice (Mus musculus) either (A) a low‐fat, high‐fiber diet with low, intermediate, or high protein; or (B) a high‐fat, low‐fiber diet with low or intermediate protein. Mouse muscle and dietary protein were analyzed for bulk tissue δ¹⁵N using elemental analyzer‐isotope ratio mass spectrometry (EA‐IRMS), and were also hydrolyzed into free AA that were analyzed for δ¹⁵N using gas chromatography‐combustion‐IRMS.

   As dietary protein increased, Δ¹⁵N_{Consumer‐Diet}slightly declined for bulk muscle tissue in both experiments; increased for AA in the low‐fat, high‐fiber diet (A); and remained the same or decreased for AA in the high‐fat, low‐fiber diet (B). The effects of dietary protein on Δ¹⁵N_T‐Sand on TDF_T‐Svaried by AA but were consistent between variables.

   Diets were less digestible and included more protein in Experiment A than in Experiment B. As a result, the mice in Experiment A probably oxidized more AA, resulting in greater Δ¹⁵N_{Consumer‐Diet}values. However, the similar responses of Δ¹⁵N_T‐Sand of TDF_T‐Sto diet variation suggest that if diet samples are available, Δ¹⁵N_T‐Saccurately tracks trophic position. If diet samples are not available, the patterns presented here provide a basis to interpret Δ¹⁵N_T‐Svalues. The trophic‐source offset of Pro‐Lys did not vary across diets, and therefore may be more reliable for omnivores than other offsets (e.g., Glu‐Phe).

    

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4. A sealed‐tube method for offline δ 13 C analysis of CO 2 via a Gas Bench II continuous‐flow isotope ratio mass spectrometer

   https://doi.org/10.1002/rcm.9040  

   Walker, Brett D. ; Beaupré, Steven R. ; Griffin, Sheila ; Walker, Jennifer ; Druffel, Ellen ; Xu, Xiaomei ( February 2021 , Rapid Communications in Mass Spectrometry)

   The isotopic measurement of environmental sample CO₂via isotope ratio mass spectrometry (IRMS) can present many analytical challenges. In many offline applications, exceedingly few samples can be prepared per day. In such applications, long‐term storage (months) of sample CO₂is desirable, in order to accumulate enough samples to warrant a day of isotopic measurements. Conversely, traditional sample tube cracker systems for dual‐inlet IRMS offer a capacity for only 6–8 tubes and thus limit throughput. Here we present a simple method to alleviate these concerns using a Gas Bench II gas handling device coupled with continuous‐flow IRMS.

   Sample preparation entails the cryogenic purification and quantification of CO₂on a vacuum line. Sample CO₂splits are expanded from a known volume to several sample ports and allowed to isotopically equilibrate (homogenize). Equilibrated CO₂splits are frozen into 3 mm outer diameter Pyrex break‐seals and sealed under vacuum with a torch to a length of 5.5 cm. Sample break‐seals are scored, placed into 12 mL Labco Exetainer^®vials, purged with ultrahigh‐purity helium, cracked inside the capped helium‐flushed vials and subsequently measured via a Gas Bench equipped IRMS instrument using a CTC Analytics PAL autosampler.

   Our δ¹³C results from NIST and internal isotopic standards, measured over a time period of several years, indicate that the sealed‐tube method produces accurate δ¹³C values to a precision of ±0.1‰ for samples containing 10–35 μgC. The tube cracking technique within Exetainer vials has been optimized over a period of 10 years, resulting in decreased sample failure rates from 5–10% to <1%.

   This technique offers an alternative method for δ¹³C analyses of CO₂where offline isolation and long‐term storage are desired. The method features a much higher sample throughput than traditional dual‐inlet IRMS cracker setups at similar precision (±0.1‰).

    

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5. A simple CO 2 equilibration method for measuring blood oxygen isotope compositions

   https://doi.org/10.1002/rcm.9256  

   Green, Daniel R. ; Olack, Gerard ; Tütken, Thomas ; Leichliter, Jennifer ; Winkler, Daniela E. ; Clauss, Marcus ; Vonhof, Hubert ; Colman, Albert S. ( February 2022 , Rapid Communications in Mass Spectrometry)

   Blood water oxygen isotope compositions can provide valuable insights into physiological processes and ecological patterns. While blood samples are commonly drawn for medical or scientific purposes, blood fractions are infrequently measured for oxygen isotopic compositions (δ¹⁸O) because such measurements are time consuming and expensive.

   We sampled blood from sheep, goats, and iguanas raised in field and animal laboratories into serum, EDTA, heparin, and uncoated plastic vials commonly used in medical and scientific research, then separated red blood cell (RBC) and plasma or serum blood fractions. These were injected into helium‐flushed Exetainer tubes where they naturally outgassed endogenous CO₂(goat blood), or into He‐ and CO₂‐flushed tubes (iguana blood). The CO₂gas was sampled on a GasBench II system, and δ¹⁸O was measured by an isotope ratio mass spectrometer (IRMS).

   Repeated δ¹⁸O measurements were stable over multiple days. The addition of desiccated blood solids to water standards had little impact on their δ¹⁸O measurements, suggesting that organic molecular constituents within blood serum and plasma do not interfere with blood water δ¹⁸O values. We observed slight but statistically significant δ¹⁸O offsets between plasma, serum and RBC fractions. Mass‐dependent body water turnover times for iguanas were derived from the data.

   We demonstrate that a simple blood‐CO₂equilibration method using the GasBench can quickly, reliably and accurately characterize water δ¹⁸O in the plasma, RBC, and whole blood fractions of mammalian and reptilian blood samples (precision ≤ 0.1‰). This method will expand the application of blood stable isotope analysis in physiological and medical research.

    

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