Research And Application Of Matrix Assisted Laser Desorption Ionization Mass Spectrometry Imaging In Alkaloids

Objective:The visualized distribution of alkaloids in food and traditional Chinese medicine samples was established by matrix assisted laser desorption ionization mass spectrometry imaging.(1)The distribution and change regulation of four toxic glycoalkaloids in three different parts of potato tubers(sprout,periderm and medulla)were studied under different storage time;(2)A simple and fast sample preparation method based on sandpaper was discussed,and it was successfully applied in the alkaloid imaging distribution in Fritillaria samples.Methods:The potato samples were stored for 0,10,15,20,30,40 and 60 days under normal storage conditions.After sampling,the samples were frozen rapidly by liquid nitrogen,and then sectioned in a cryomicrotome at–18 ~oC,and then transferred to the conductive glass slides coated with indium tin oxide(ITO).Then,sample sections were placed on an imaging mass microscope(iMScope)to collect the optical images.We used the 2,5-dihydroxybenzoic acid(DHB)as the matrix and“two-step matrix application”as the matrix coating method to detect the glycoalkaloids in potato tubers.In the positive ionization mode,tissue sections were ionized by primary and secondary mass spectrometry.The Growth/sigmoidal model with Logistic equation was used to fit and predict the growth trend of glycoalkaloids in potato tubers.(2)The traditional Chinese medicine of Fritillaria was sliced in a cryomicrotome at–20 ~oC,and then transferred to ITO slides with the aid of conductive double-sided adhesive,and the optical image was collected on the iMScope.In addition,the Fritillaria sample was placed on sandpaper to obtain the surface content of the sample tissue,and the optical image was collected on the iMScope.α-cyano-4-hydroxycinnamic acid(CHCA)was used as the matrix,and the method of airbrushing was applied as the matrix deposition method to detect the alkaloids in Fritillaria.In the positive ionization mode,the mass spectrum and distribution images of four alkaloids in Fritillaria were obtained.Results:The slice thickness of potato tissue was 20μm.Compared to other organic matrices,DHB used in positive ionization mode showed less background interference and enhanced MS signal intensity.A“two-step matrix application”method,which combined with matrix sublimation and airbrushing,was chosen as the optimal matrix deposition way.The signals of glycoalkaloids were the strongest and the matrix crystallization was more uniform.The results clearly revealed that,the distribution of four glycoalkaloids in the sprout,periderm and medulla of potato tubers was heterogeneous.The growth curves of glycoalkaloids were fitted by a growth model with Logistic equation,and the correlation coefficients(r~2)of this model were in the range of 0.97-0.99.The growth rate and relative content for four glycoalkaloids in sprout and periderm were higher than that in medulla.For dehydrosolanine and?-solanine,rapid growth period of these two glycoalkaloids in sprout(about at the day 23,similar to these in medulla)was later than which period in periderm(about at the day 17),while rapid growth of dehydrochaconine and?-chaconine appeared at almost the same time(about at the day 20);(2)The imaging distribution of four alkaloids in Fritillaria samples can be obtained by frozen section method and sandpaper friction sample cross section method.The results showed that alkaloids were evenly distributed in the Fritillaria tissue and relatively more in the external.Compared with the frozen section method,the sandpaper friction sample cross section method was simpler,more reliable and cheaper.This sample preparation can be realized without the help of microtome,imprinting or other technologies.This method can provide a new idea for molecular imaging of traditional Chinese medicine samples.Conclution:(1)The distribution and variation of glycoalkaloids in potato tubers were studied by matrix assisted laser desorption/ionization mass spectrometry imaging.This method had simple sample pretreatment and high spatial resolution.The established model can better predict the growth trend of glycoalkaloids and provide guidance for potato storage and food safety.(2)The developed sandpaper friction cross section method can be used for the sample preparation of traditional Chinese medicine for mass spectrometry imaging.This method was simple,fast and cheap,which can provide a reference for the preparation of Chinese herbal tissues that are difficult to section.