Mass spectrometric analyses were performed in the negative ion mode. The nebulising gas was N2 (20 psi), the drying gas was air (300 C, 25 psi), the needle voltage was 4500 V, the shield voltage was 600 V, the detector voltage was 1650 V, the capillary voltage was 60 V and the collision energy voltage was 16 V. The deprotonated molecule (m/z = 295) was fragmented to its product ion (m/z = 265) and used for quantification purposes. The deprotonated molecule (m/z = 295) was fragmented to its product ion (m/z = 138) and used for identification purposes