Ammonia chemical ionization mass spectrometry of substituted phenylpropanoids and phenylalkyl phenyl ethers

The ammonia chemical ionization (NH₃/CI) spectra of phenylpropanoids and substituted phenylalkyl phenyl ethers containing a-hydroxy or a-carbonyl functional groups are discussed. Fragmentation reactions which occur in the temperature range 180-450 °C are examined to model pyrolysis mass spectrometric experiments of lignin. Hydroxycinnamyl alcohols and a-hydroxy phenylalkyl phenyl ethers show abundant [M + H — H₂O]⁺ elimination ions and to a lesser extent [M + NH₄ — H₂O]⁺ substitution ions while hydroxycinnamic acids and a-carbonyl phenylalkyl phenyl ethers show predominantly [M + H]⁺ and [M + NH₄]⁺ ions. The interpretation of functional group information is facilitated by using trideuterioammonia (ND₃) and [¹⁵N]ammonia (¹⁵NH₃) as reagent gases. Labile hydrogens undergo essentially complete exchange upon ND₃/CI and the number of hydroxyl functional groups is readily determined by comparison of the [M + NH₄]⁺ and [M—nH+nD+ND₄]⁺ adduct ions. Incomplete H-D exchange is observed for enolizable and aromatic protons.