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  1. Rationale

    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.

    Methods

    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.

    Results

    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‰).

    Conclusions

    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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