GC–MS protocols and analysis were similar to those described before53, except a modified temperature gradient was used for GC: Initial temperature was 130 °C, held for 4 min, rising at 6 °C min−1 to 243 °C, rising at 60 °C min−1 to 280 °C, held for 2 min. Data were processed as before with quantification against varied amounts of standard mixtures run in parallel performed using MetaQuant. Quantities were corrected for recovery using the L-norvaline internal standard.
