Single Grain Electrospray Ionization Mass Spectrometry And Its Applications To Direct Analysis Of Rice

Rice(Oryza sativa L.)is one of the most important crops in the world,and nearly half of the world's population are using rice or rice products as their staple food.Therefore,quality monitoring and safety control of rice are important matters of concern.The traditional detection methods of rice are chromatography,chromatography combined with mass spectrometry,Fourier transformation infrared spectroscopy(FTIR),nuclear magnetic resonance spectroscopy(NMR),and etc.Most of these methods require multi-step sample pretreatments including grinding,extraction,derivatization,separation and enrichment,following by complicated operations and large amounts of toxic/hazardous reagents,which may not meet the actual needs of high-throughput analysis of a large number of samples.In recent years,ambient mass spectrometry has been gradually developed,due to its simplified operations,high analysis-speed,high throughput and high sensitivity.Complex matrix samples can be analyzed continuously,in real-time and on-line by mass spectrometry without sample pretreatment.This thesis was devoted to the working principle,the implementation and technical characteristics of single grain electrospray ionization mass spectrometry(SG-ESI-MS),and applying this technique to different aspects in terms of quality analysis,fatty acids metabolism and rapid screening of pesticide residues in rice.Based on the brief overview of the traditional detection techniques of rice,the principles and classification of ambient ionization techniques and its application in food,the main contents of this thesis are as follows:1.The principle,device structure,technical characteristics,analytical speed and quantitative capability of SG-ESI technique were studied.2.The application of SG-ESI-MS in rice quality analysis was conducted.In this study,single rice grain was placed in a specially designed tiny chamber,and then charged solvents(e.g.,methanol/water/acetic acid,40:40:20,v/v/v)were guided through a quartz capillary to extract rice grain for liberation of analytes(free fatty acids),which became an ionic plume at the chamber outlet to generate the analyte ions for mass spectrometric analysis.The characteristic fingerprint of every single rice grain was rapidly recorded by the single grain mass spectrometry without tedious sample pretreatments(such as off-line extraction,centrifugation and/or separation).The target ions were selected for the collision-induced dissociation(CID)test,and the structures of the target analytes were further identified.Semi-quantitative determination of heptanoic acid in the single rice grain samples was successfully achieved.The rice grains of different cultivation areas and/or storage time were differentiated by the full scan mass spectral data processed using multivariate statistical analysis(PCA and PLS-DA).The experimental results demonstrate that the established single grain mass spectrometry has differentiated the rice grains of different cultivation areas and/or storage time at the molecular level,which is a useful tool for direct analysis of rice grains with minimal sample preparation,low sample consumption,high sensitivity and high throughput.3.The SG-ESI-MS was applied to the study of fatty acids metabolism in rice.Herein we attempted to differentiate 4 rice cultivars by analyzing rice-specific metabolites(free fatty acids)of each individual grain using the single grain electrospray ionization mass spectrometry(SG-ESI-MS)combined with multivariate statistical analysis(e.g.PCA and ANOVA)and correlation analysis.The mass spectral features indicated that the content of fatty acids varied along the 4 rice cultivars planted under the non-differentiable natural environment.Both PCA and ANOVA revealed that the 4 rice samples were notably differential.The correlation analysis provided correlations between 4 metabolites that participated in commonly related fatty acid metabolic pathways.The proposed method with features of minimal sample consumption,high analysis speed(< 2 min)and no sample pretreatment,revealed the differential metabolites in 4 cultivars at molecular levels,which provided a new mass spectrometric tool for direct analysis of single granular grain samples and plant metabolomics studies.4.The SG-ESI-MS was applied to rapid screening of pesticide residues in rice.Under the positive ion detection mode,the SG-ESI-MS was employed to directly analyze organophosphorus pesticides(OPPs)residues in rice.Omethoate and dichlorvos were identified via collision-induced dissociation(CID)experiments.Semi-quantitative determination of omethoate and dichlorvos in the single rice grain samples were successfully achieved,providing the limits of detection(LODs)of 0.38~1.30 ng/g,the recovery rates of 77~91% and the relative standard deviation(RSD)values less than 9.54%.Those OPPs in single rice sample were rapidly detected less than 2 min.The rapid analysis of trace pesticide residues in rice was achieved using SG-ESI-MS without tedious sample pretreatments.The result shows that the SG-ESI-MS can accomplish the rapid detection of trace residues in rice samples without any sample pretreatment,providing a powerful technique for the rapid screening of toxic and hazardous substances at the molecular level in food and food safety supervision.