A Hewlett-Packard (Palo Alto, CA, USA) HP 1100 Series LC–MS system equipped with a binary solvent pump, an autosampler, and a mass-selective detector coupled with an analytical work station was employed. The mass-selective detector consisted of a standard atmospheric pressure ionization (API) source configured as APCI. Typical operating conditions of the APCI interface in the negative mode were: vaporizer temperature, 450 8C; nebulizer gas