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

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 1.3 parts per thousand. more enriched in C-13 than duplicates soaked in 2 N HCl for 5 min, indicating incomplete carbonate removal with the weaker acid. In comparison, no differences in 615 N values were found between acid treatments, and a following water rinse had no effect on the 8 13C or 6 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 similar to 1.2 parts per thousand difference in delta C-13 values between treatments of lipid-rich duplicates. Different lipid-extraction methods did not lead to consistent differences in delta N-15 values, however. Depending on the lipid and carbonate content, untreated samples were depleted in C-13 by 0.8 to 4.4 parts per thousand and in N-15 by 0.6 to 1.4 parts per thousand compared to treated duplicates. We conclude that delta C-13 and delta N-15 values of samples with low lipid and carbonate content are highly comparable among studies regardless of pre-treatment methods, whereas the VC values of relatively lipid- and/or carbonate-rich samples must be carefully considered based on the pretreatment applied to samples. In comparison, delta N-15 values are relatively robust to differences in carbonate and lipid-removal methods, and delta N-15 values of untreated vs. carbonate- and lipid-treated samples are comparable within 1.0 parts per thousand.