Fig 2. Schematic overview of the PSIA analytical instrument.

Amino acids are separated by HPLC and a fraction of the eluent (150 from 750 μ l/min) is mixed 1:1 with acidic ninhydrin reagent prior to entering the 1.5 ml stainless steel coil at 160 ^oC. Amino acids are decarboxylated as part of the ninhydrin reaction and the carboxyl CO₂ is separated from the aqueous stream into a helium flow, dried and passed to an isotope ratio mass spectrometer to measure the carbon isotopic composition. B = back-pressure regulator, C = non-metallic check-valve. NB when the system is flushed with acetone (weekly) the connection between the CO₂ separator and H₂O separator must be removed.