Stable hydrogen and carbon isotopic compositions of long-chain (C21–C33) n-alkanes and n-alkenes in insects

We report the molecular and stable isotopic (δD and δ13C) compositions of long-chain n-alkanes in common insects including the cabbage butterfly, swallowtail, wasp, hornet, grasshopper, and ladybug. Insect n-alkanes are potential candidates of the contamination of soil and sedimentary n-alkanes that are believed to be derived from vascular plant waxes. Long-chain n-alkanes (range C21–33; maximum C23–C29) are found to be abundant in the insects (31–781 μg/dry g), with a carbon preference index (CPI) of 5.1–31.5 and an average chain length (ACL) of 24.9–29.3. The isotopic compositions (mean ± 1σ, n = 33) of the n-alkanes are −195 ± 16‰ for hydrogen and −30.6 ± 2.4‰ for carbon. The insect n-alkanes are depleted in D by approximately 30–40‰ compared with wax n-alkanes from C3 (−155 ± 25‰) and C4 vascular plants (−167 ± 13‰), whereas their δ13C values fall between those of C3 (−36.2 ± 2.4‰) and C4 plants (−20.3 ± 2.4‰). Thus, the contribution of insect-derived n-alkanes to soil and sediment could potentially shift δD records of n-alkanes toward more negative values and potentially muddle the assumed original C3/C4 balance in the δ13C records of the soil and sedimentary n-alkanes. n-Alkenes are also found in three insects (swallowtail, wasp and hornet). They are more depleted in D relative to the same carbon numbered n-alkanes (δDn-alkene − δDn-alkane = −17 ± 16‰), but the δ13C values are almost identical to those of the n-alkanes (δ13Cn-alkene − δ13Cn-alkane = 0.1 ± 0.2‰). These results suggest that these n-alkenes are desaturated products of the same carbon numbered n-alkanes.