Figure 4. EIC analysis of the labeled coelenteramines.

(A) Extracted ion count chromatograms (EICs, extraction window ±10 ppm) for natural ¹²C-coelenteramine (M + H) under ¹³C-labeling and non-labeling conditions. (B/C) EICs for the ¹³C₉-labeled (B) and the ¹³C₈-labeled (C) coelenteramine ions centered at m/z 287.1590 and m/z 286.1556 are shown. Data support that the carbons depicted in red are isotopically labeled at low abundance. The peak at 14.4 min has the same retention time as the coelenteramine standard. Observed for ¹³C₈-coelenteramine: 286.1561, 1.7 ppm error. Observed for ¹³C₉-coelenteramine: 287.1598, 2.8 ppm error.