Desorption electrospray ionization mass spectrometry for fast detection of Sudan dyes in foods without sample pretreatment

Desorption electrospray ionization mass spectrometry (DESI-MS) was employed for fast direct detection of Sudan dyes such as Sudan I, Sudan II, Sudan III and Sudan IV presented in various foods, including chili powder, tomato sauce, sausage and fried egg. The operation parameters for both DESI and LTQ were optimized so that reliable signal of analyte was generated using a mixture of methanol/water/acetic acid (49:49:2) as electrospray liquid. All the analytes gave predominant signal at m/z 249, 277, 353, 381, which was corresponding to the protonated molecules of Sudan I, Sudan II, Sudan III and Sudan IV, respectively. The structures of the protonated molecules were confirmed with high confidence by tandem DESI-MS/MS, which yielded characteristic fragments (e.g. m/z 156) for the four dyes, leading into elimination of false positive alert, which is usually common in results of direct fast detection of trace analyte in such a complicated matrix as tomato sauce, fried egg, etc. Detection limits of Sudan dyes in chili powder, sauce tomato, sausage, fried egg were found to be less than 0.01 pg/mm(2), 0.02 pg/mm(2), 0.02 pg/mm(2), 1.0 pg/mm(2), respectively. The mean analysis time for a single sample was about 1 minute. It was also proposed to couple DESI source for miniature mass spectrometer for fast in-situ analysis of complicated samples without pretreatment.