Direct Ionization Analysis Of Food Samples Using Tip-Assisted Ambient Arc Ionization Mass Spectrometry

Mass spectrometry,a traditional analytical technique,has been widely used in various fields for many years because of its excellent detection performance.Among them,ambient mass spectrometry(AMS),an emerging mass spectrometry technique,which relies on the construction of ambient ion source(AIS),has been gradually developed due to its simple operation steps and fast analysis process.AIS based on arc plasma ionization is a new type of AIS developed in the past two years.It usually uses commercial arc as the plasma generator,and has the advantages of high ionization efficiency and wide applicability.The detection of samples with complex matrices such as food has always been a difficult point in mass spectrometric analysis.Generally,complex extraction and separation pretreatments are required before analysis,which not only leads to large consumption of reagents,but also is not conducive to high-throughput and rapid analysis of samples.Although arc plasma ionization mass spectrometry has advantages in the in situ analysis of samples,it still has strong matrix interference due to the sugars and salts contained in complex food samples.Therefore,the method is in urgent need of improvement and optimization.In this study,we improved the ambient electric arc ionization(AEAI)source established by our group,based on which we developed a tip-assisted ambient arc ionization-mass spectrometry(TAAEAI-MS)method.Thus we successfully applied it to the analysis of trace pesticide residues in fruits and free unsaturated fatty acids(UFA)in edible oils.The main work can be divided into the following two parts:1.Rapid detection of trace pesticide residues in fruits using TAAEAI-MS:Based on AEAI source,we introduced a micro-syringe tip to established a new TAAEAI-MS method,and applied it to the detection and analysis of trace pesticide residues in fruits.The strong electric field distribution at the tip of the syringe,combining with the thermal desorption effect of the arc,can reduce matrix interference and improve the sample ionization efficiency.Using commercially available fruit juice to simulate complex matrices,high-intensity protonated pesticides were detected by TAAEAI-MS in positive ion mode.Quantitative curves were prepared for nine pesticides with good linearity and low limits of detection,which were applied to the quantitative analysis of pesticide residues in various fruits.Without extraction and separation,fruit samples only need to be squeezed into juice to achieve rapid ionization analysis of trace pesticides by this method.2.Rapid detection of free unsaturated fatty acids in edible oil using TAAEAIMS: The quantitative detection and Carbon-Carbon Double Bond(C=C)localization of free UFA in edible oil were realized using TAAEAI-MS.The arc plasma can both efficiently ionize the UFA and epoxidize C=C without any derivatization reagents or auxiliary gases.Fragmentation was performed on the epoxidized UFA,and the double bond position information was located according to the fragment ions.In this study,TAAEAI-MS was used to detect six kinds of UFA(including a pair of C=C position isomers),and the corresponding quantitative curves were obtained based on primary and tandem mass spectrometric data,respectively.They were successfully applied to the quantitative analysis of free UFA in edible oil.Without extraction and separation,edible oil samples only need to be diluted with methanol before being detected using TAAEAI-MS.