Sample preparation effects on stable C and N isotope values: a comparison of methods in Arctic marine food web studies

摘要

We compared effects of different sample preparation techniques on the stable carbon and nitrogen isotope values of fish muscle, whole crustaceans and particulate organic matter (POM). Comparisons were also made to untreated (i.e. dried and homogenised) samples. Relatively carbonate-rich samples treated with weak acid (0.1 N HCl), either by quickly wetting or acid dampening, were on average 1.3per thousand more enriched in ~(13)C than duplicates soaked in 2 N HCl for 5 min, indicating incomplete carbonate removal with the weaker acid. In comparison, no differences in δ~(15)N values were found between acid treatments, and a following water rinse had no effect on the δ~(13)C or δ~(15)N values. Chloroform-methanol (2:1 by volume) extraction overnight removed less lipids than Soxhlet extraction with 7% methanol in dichloromethane for 2 h, resulting in ～1.2per thousand difference in δ~(13)C values between treatments of lipid-rich duplicates. Different lipid-extraction methods did not lead to consistent differences in δ~(15)N values, however. Depending on the lipid and carbonate content, untreated samples were depleted in ~(13)C by 0.8 to 4.4per thousand and in ~(15)N by 0.6 to 1.4per thousand compared to treated duplicates. We conclude that δ~(13)C and δ~(15)N values of samples with low lipid and carbonate content are highly comparable among studies regardless of pre-treatment methods, whereas the δ~(13)C values of relatively lipid- and/or carbonate-rich samples must be carefully considered based on the pre-treatment applied to samples. In comparison, δ~(15)N values are relatively robust to differences in carbonate and lipid-removal methods, and δ~(15)N values of untreated vs. carbonate- and lipid-treated samples are comparable within ±1.0per thousand.