Surface analysis of historic manuscripts in ambient conditions by desorption electrospray ionisation (DESI) imaging mass spectrometry and direct infusion mass spectrometry (DIMS)

Robert Burns, the much-loved national (Scottish poet), has been the target of forgers, the most prolific being 'Antique' Smith, who realised the demand for authentic manuscripts which could bring a high price. Antique Smith's forgeries have continued to appear for auction as the real thing long after Smith's time, and suspicion sometimes falls on authentic Burns' documents because of the awareness of the Smith corpus. A chemical method for distinguishing forged from real Burns could have a significant impact on the value of many manuscripts attributed to the poet. We chose to focus on the chemical composition of the inks used in both authentic manuscripts and forged ones. In this study, desorption electrospray ionisation (DESI) and direct infusion mass spectrometry (DIMS) have been applied for the analysis of black inks made using traditional recipes from the 18th and 19th centuries and for historic manuscripts. DESI is an ambient mass spectrometry method that can be used for spatial analysis and imaging of sample surfaces, with very little sample preparation. DESI has been engineered in house, using an existing nano-spray source. DESI directly desorbs sample analytes from a surface under ambient conditions using electrospray ionisation. Optimisation of a DESI method can be very complex as there is a wide array of dependent parameters that can alter the performance which include; nebulising gas flow, solvent flow, voltage and geometrical parameters. Once fully optimised DESI can be very sensitive and non-destructive. The lab-built DESI, based on the Abbassi-Ghadi source, was coupled to Thermo Scientific ExactiveTM OrbitrapTM mass spectrometer. DIMS, which is similar in principle to liquid extraction surface analysis (LESA), operates by picking up sample analytes using a solvent droplet on the surface. Using a methanol water solvent, it has been shown to be minimally destructive. DIMS is performed by placing a pipette tip just above the surface of a document, and pipetting up and down three times. Direct infusion was then carried out, using the solvent containing the sample analytes with an Advion TriVersa nanomate set up for chip-based infusion coupled to a Thermo Scientific Orbitrap FusionTM TribridTM mass spectrometer. The sampling process of DIMS can be performed out of the laboratory setting, which eliminates the need for transportation of the historic documents. DESI imaging has been successfully performed on rhodamine 6G, and for the analysis iron gall and ivory black inks. The word 'Polyomics' written in rhodamine 6G on a glass slide, was imaged using low spatial resolution of 1.25 mm steps per line as a 'proof of concept' experiment. An array of letters was written onto handmade cotton paper where ivory black ink was successfully imaged in positive ionisation mode with a unique peak at 375.21 m/z. In negative ionisation mode iron gall inks could be spatially identified using a unique peak at 220.15 m/z. It was concluded that further optimisation of the DESI method would be required however, before being suitable for the analysis of historic documents. The DESI source was investigated for the analysis of cow's milk and amino acids. In DESI there are two main groups of analytes, electrospray ionisation (ESI) and atmospheric-pressure chemical ionisation (APCI), which require different DESI parameters for successful ionisation. The analysis of cow's milk produced a complex lipid spectrum. Lipids belong to the ESI group of analytes, in the experiment groups of glycerolipids were identified. This indicates that the lab-built DESI source is capable of ionising this group of analytes. High mass spectral resolution mass spectrometry was used, providing high mass accuracy, which eliminates the requirement for chromatographic separation, although tandem mass spectrometry would be required for further identification of the lipids. Amino acids however, are within the APCI group of analytes, and have been shown to be successfully spatially detected using DESI. Ink components will be within the same group of analytes as amino acids, which demonstrates suitability for this group of analytes. DIMS was used for the first time to analyse the inks and paper used by Burns and Smith. It was shown using DIMS that iron gall inks and ivory black inks can be distinguished successfully, therefore, 9 manuscripts known to be written by A. Smith and 3 manuscripts written by R. Burns were analysed. A unique chemical fingerprint was created for each of these manuscripts. It was found that there were 94 discriminatory peaks (Q < 0.05) distinguishing Burns and Smith manuscripts, of which 42 of these were not detected in any of the solvent peaks. A support vector machine classifier was produced with a 0.76 AUC value which distinguishes these authors works. Eight significant peaks were carefully selected for further analysis with annotations where available. DIMS could have a broad application, aiding collectors of historic literary works and auction houses with the authentication of spurious documents. It would be necessary to create a library using DIMS, sampling a wide range of manuscripts from different time periods from around the world, incorporating different historic figures literary works. A number of chemical classifiers could be produced and used to identify the origin and time period, and therefore authenticity, of spurious manuscripts.