DESI-MSI and Its Application into Examination of Document/Cultural Relics

Mass spectrometry imaging is a new approach of imaging technology, capable of providing rich in situ spatial distribution information about the analyte. Desorption electrospray ionization (DESI) - mass spectrometry imaging (MSI) has in recent years been focused owing to its merits of ambient operation, simple pre-processing, less damage to the treated sample and high throughput. DESI plays its role through the solvent (usually methanol or isopropyl alcohol) being sprayed onto the surface of sample via capillary atomizing at open natural environment so that the desorption and ionization could be carried out meanwhile. During the process, the sampling ions would pass by droplet through the ion transfer tube to enter the mass spectrometer for detection. However, if the droplet flow is sprayed speedily, smearing phenomena would appear to result in damaging the tested document and analyte (e.g., ink). The another influential factor is the distance between the mass spectrometer’s sampling inlet and the analyte, also able to affect the sampling integrity. DESI-MSI has been widely used with environmental/food testing, criminal forensic detection and other fields due to its simple sample preparation and experimental operation. For example, document/cultural relic inspection is usually related to testing relevant polar compounds and art masterpiece of high price value, DESI-MSI is thus an appropriate choice to undertake the inspection/examination. When DESI-MSI is used to analyze document materials, each sampling micro-area could be extracted of the charge-to-mass-ratio images up to more than 1000 varieties at the same time, therewith bringing forth a large amount of data. Chemometric methods, e.g., principal component analysis and partial least squares-discriminant analysis, would be available to modularly group and analyze the data of mass spectrometry to identify differences and/or conclude results. Accordingly, species discrimination and source tracking could be conducted with the identified differences and conlcuded results. At present, DESI-MSI is still in the relatively preliminary stage for document and writing materials to inspect and examine. An expectation is how to apply this emerging technology to achieve breakthroughs in difficult tasks such as document chronicling, writing time deduction although relevant further researches are required. This article reviewed the related literature about document/cultural relic inspection/examination since DESI-MSI came on stage. The scientific rationality and main problems were also discussed with a purpose to provide reference for the peers of document inspection/examination.