Rapid Analysis Of Pollutants In Seawater Using Ambient Ionization Mass Spectrometry

In recent years,the amount of organic pollutants in seawater is increasing.Although the concentrations of antibiotics and hormones are low,they can easily be enriched in the marine animals,which will threaten the health of mankind.Therefore,it is very important to monitor marine organic pollutants for both marine environmental protection and the human health.Mass spectrometry（MS）has already been used to analyze the organic pollutants in seawater.However,the traditional mass spectrometer cannot analyze high-salt samples directly.The sample should be pretreated including filtration,enrichment and purification prior to MS analysis.The pretreatment operations are time-consuming,which might increase the errors of the analysis.It should be put on agenda to develop new technologies for rapid and convenient analysis of marine organic pollutants.This study developed a spontaneous desalting-droplet spray ionization-mass spectrometry（SD-DSI-MS）method to achieve the rapid and convenient analysis of analytes in seawater samples.Prior to sample desorption/ionization,the sample solution was loaded on the corner of a cover slip adjacent to the MS inlet and dried naturally.After desalting,a spray solvent was pipetted onto the corner to dissolve the separated target molecules,which were then motived by high electric field to spray out from the corner and detected by MS.The parameters for improving desalting efficiency and MS sensitivity were systematically optimized.The most suitable spray solvent is acetonitrile,and the selected volume of acetonitrile is 4μL.The optimal redisslution time is 4 s.In addition,other salts（MgCl₂,KCl,Na₂SO₄,etc.）were investigated in either positive or negative ion modes,and the results illustrated that SD-DSI-MS is suitable for other high-salt samples as well.Both antibiotis and hormones in simulated seawater samples were analyzed by SD-DSI-MS.The limits of detection（LODs）were determined to be 0.10μg/L,0.26μg/L and 29.4μg/L for the analysis of trimethoprim,clarithromycin and diethylstilbestrol,respectively.The relative standard deviation（RSD）was illustrated to be 6.8%（n=6）for analysis of trimethoprim.These results improved that the sensitivity and stability of SD-DSI-MS are excellent.Futhermore,the analysis of real seawater samples was achieved,and the results obtained showed that this method is well in anti-interference.The recoveries（80.1-107.8%）and RSDs（≤8.3%）were obtained according to the analysis of trimethoprim,clarithromycin and diethylstilbestrol in real seawater samples,and illustrated a satisfactory accuracy and precision of SD-DSI-MS.Finally,the mechanism of SD-DSI-MS was explored at the microscopic level.By comparing the changes of the surfaces of the sample crystals,it was found that the analytes were deposited on the surfaces of the cover slip and the salt crystals instead of doping into the salt crystals.After that,a spray solvent（acetonitrile）was used to quickly dissolve the separated analytes instead of salt crytals.The mechanism further demonstrated the feasibility for analysis of high-salt samples by SD-DSI-MS.The SD-DSI-MS achieved the rapid and direct analysis of antibiotics and hormones in seawater samples.This method is simple,time-saving（⁵ min）,easy to set up,and need no sample pretreatment.In addition,as the two processes of desalting and detection are separated,this method is easy to achieve high-throughput analysis of the sample.We believe that SD-DSI will play an important role in the field of marine organic pollutants monitoring.