Preservation of blood and tissue samples for stable-carbon and stable-nitrogen isotope analysis

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Abstract
Researchers engaged in collecting animal material for stable-carbon and stable-nitrogen isotope analysis are frequently faced with the need to preserve tissues prior to transportation to the laboratory. In many cases, freezing is not possible in the field, so we investigated the potential of several techniques for preserving tissues for this purpose. We also included preservation techniques used for DNA analyses in order to evaluate how they might alter δ13C and δ15N values in tissues and, ultimately, whether archived DNA samples could be used for stable-isotope assay. Tissues included blood and pectoral muscle from quail (Coturnix coturnix japonica) and blood from sheep (Ovis aries). Preservation techniques for blood included freeze-drying (control), drying on precombusted glass-fibre filter paper, and storing in 70% ethanol, 10% buffered formalin, ABI lysis buffer, and Queen's lysis buffer. After 8 weeks, the use of both lysis buffers and formalin resulted in significant depletion of 13C and 15N in blood. Values for samples dried on glass-fibre filter paper or stored in 70% ethanol did not differ significantly from those for the control. Muscle tissue was freeze-dried (control) or stored in 70% ethanol, 10% buffered formalin, or DMSO solution. Both the DMSO and formalin treatments resulted in significant depletion of 13C and 15N compared with the control. Only the 70% ethanol treatment did not result in changes to either isotope ratio in muscle. Where freezing is not possible, we recommend that blood samples be dried or stored in 70% ethanol. Our study provides an estimate of isotopic correction factors that may be applied to tissues archived for DNA analysis or stored in formalin.