Study On Direct Analysis Of Food Contaminants By Using Desorption Electrospray Ionization Mass Spectrometry

Ambient mass spectrometry is now playing an essential role in scientific research and public security. Desorption electrospray ionization mass spectrometry (DESI-MS), the first introduced ambient ionization method, has the most applications in various areas. Herein, we have developed a DESI-MS method for direct analysis of food contaminant including (i) high throughput screening of clenbuterol in urine samples and (ii) in situ arsenic speciation on solid surfaces without standards. The detailed research contents are listed as following:1. Rapid screening of clenbuterol (CLB) in urine was performed by DESI-MS and DESI-MS/MS. Optimization experiments were carried out including the selection of substrates, spray solutions, nebulizing gas pressures, high-voltage power supplies and flow rates of spray solution. The limit of detection (LOD), defined as the lowest quantity that can be detected, was 5.0 pg for the pure compound. Using DESI coupled with solid-phase extraction (SPE), the linear response range was from 10 to 400 ng/mL (R~2 = 0.993) and the concentration LOD for urine sample was 2.0 ng/mL. The analysis for one spiked urine sample was achieved within 4min. In addition to the fast analysis speed, MS/MS provided structural information for the confirmation of clenbuterol. Urine samples from different people were investigated and the recoveries were within 100±20 %. The developed method can potentially be used for screening of clenbuterol in doping control.2. A simple and fast (<5 s) method for in situ arsenic speciation on solid surfaces was developed. Arsenic-polluted environmental samples such as animal feed and plant tissues could be directly monitored by DESI-MS. Each arsenic species in this study, including monomethylarsonic acid (MMA), dimethylarsinic acid (DMA), arsenobetaine (AsB), arsenocholine (AsC), 4-arsanilic acid (p-ASA), 4-hydroxyphenylarsonic acid (4-OH), Nitarsone, Roxarsone and two inorganic arsenic species, arsenate As(V) and arsenite As(III), could be detected by their typical m/z and collision induced dissociation (CID) behavior respectively. By the characteristic information, mixtures of different arsenic species could be detected without any sample preparation and separation process. This method could give absolute detection limits of the arsenic species at ng/mm~2 to pg/mm~2 level (3σ) with a best RSD of 5.3 % (n = 5). The method could be potentially applied to in situ environmental monitoring of arsenic pollution, especially that caused by arsenic pesticides, animal feed additives, herbicides and wood treatment.