Proceed following the protocol for sampling of grains or leaves for CID in Chapter 6 in Pask et al., 2012. Mass spectrometers generate differential values of stable carbon isotope composition (δ13C), expressed negative values in parts per thousand (‰) (Farquhar et al., 1989): δ13C (‰) = [(Rsample/Rstandard)–1] × 1000. Where: the ratio of heavy to light isotope (R = 13C/12C) for the sample is in comparison to Peedee Belemnite (PDB) carbonate (limestone) standard. Following Farquhar et al. (1989) – rather than using carbon isotope composition values (δ13C), positive values for CID (Δ13C) can be calculated for easier statistical analysis as: Δ13C = [(δa-δp)]/[1 + (δp/1000)]
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