Determination of phenylthiohydantoin-amino acids by two-step laser desorption/multiphoton ionization.

The 20 primary phenyithlohydantoln (PTH)-amino acids can be detected and quantltated by time-of-flight (TOF) mass spectrometry using a two-step laser methodology. First a CO, laser pulse desorbs the PTH-amlno acid or a mlxture thereof prepared as a thin film on the Inside wall of a rotating glass cup. The latter Is part of the first electrode of the TOF apparatus. The desorptlon process Is demonstrated to be essentlaity complete In the laser spot area. After a dah the delay, a second UV laser pulse (266 nm) causes 1 + 1 resonance-enhanced multiphoton ionization (REMPI) of the neutral cloud of desorbed molecules. The mass spectra obtained are domlnated by the parent Ion peak In almost all cases. Knowledge of the velocity dlstrlbutlon permlts flux measurement. The ion slgnai is linear in PTH-amino acid concentration in the range of picomoles io nanomoles. This Is the fhst demonstration of quantltative analysls of molecules by laser desorptlon/multlphoton Ionization.

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