Traditional analytical methods for analysing ink involve destroying the sample through extraction procedures and then analysis via thin-layer chromatography (TLC) or liquid chromatography. More recently, several atmospheric ionisation mass spectrometry techniques have been investigated for the analysis of modern inks. These methods include desorption electrospray ionisation (DESI)1, direct analysis in real time (DART)2, laser desorption ionisation (LDI)3 and matrix-assisted laser desorption ionisation (MALDI)4, but no high-resolution mass spectrometry method has been applied to historical inks. We demonstrate here a method for the analysis of historic writing inks and paper using liquid extraction directly from the paper surface, analysed using direct infusion nanospray mass spectrometry. The advantages over the existing methods are such that no matrix is required and that sampling can be performed rapidly without needing to move the historic artefact, as it can be performed outside of the laboratory.

Historic documents can have great significance in many respects. They can, for example, give insight into the opinions of leading individuals whose influence on society is profound. In some cases they might be of legal importance, underpinning ownership of land or valued property. A global fascination with the works of great writers has made original handwritten documents items of considerable value too.

Scotland’s ‘national’ bard, Robert Burns (1759–96) is a world-renowned writer whose fame as a song-writer and a lover of women, as well as one of the first celebrities of the Romantic age, makes him a figure of enduring fascination. His first collection, Poems, Chiefly in the Scottish Dialect (1786), sells for anything between £40,000 and £80,000, when – as rarely happens – one comes on the market, one selling on 28th October 2010 at Sotheby’s for £73,2505. A single manuscript of a letter or poem by Burns might fetch £6,000 to £90,000 (the Mitchell Library, Glasgow, paid a seven figure sum in the past decade to acquire a manuscript (one of several) of ‘Auld Lang Syne’). The latest estimate of Burns’s value to the Scottish economy derived over a decade ago reached a figure of £157,000,000 per annum6. This figure would undoubtedly now be much higher, and did not take account of the international market for rare copies, manuscripts and memorabilia of Burns. The finer auctioneers of London, New York and elsewhere frequently feature such items in their sales. Burns has had several forgers, the most notorious of whom, Alexander Howland (‘Antique’) Smith served a prison spell for his efforts in the 1890s7. Many examples of his handiwork remain at large and, for instance, the New York Public Library, has a large collection of Smith’s ‘Burns’ manuscripts, now valuable in their own right but originally acquired in the late nineteenth century as genuine. Famous (or any) handwriting can be difficult to authenticate, as the Antique Smith and other cases demonstrate8. A skilled draftsman such as Smith can fool even experts for a time, indeed, the Earl of Rosebery (UK Prime Minister 1894–1895) bought Smith manuscripts sold as authentic Burns.

Deeper investigation, however, can sometimes identify inconsistences (as happened in the Smith case, where he failed to spot that Burns’s hand went through four identifiably different phases through his career). The colour of ink, authentic watermarked and aged paper (and especially lines in the paper) are all telling of an authenticity (along with other things) that require considerable expertise to fake9. However, these methods for assessing authenticity require considerable expertise, and the existence of a chemical model to distinguish real from faked manuscripts could have a considerable effect on the field of authenticity with important economic impacts.

When analysing unique and precious manuscripts it is important to minimise damage to them. X-ray fluorescence spectrometry (XRF), which measures the quantities of certain elements and impurities and gives a chemical fingerprint has been used in the identification of iron gall inks10, pigments and inks on oil paintings11 and coloured pencils12. XRF, however, is confined to analysing metallic elements in the substrate. Another minimally or non-destructive method, Raman microscopy, has been used to study the degradation of iron gall ink on manuscripts13,14, but the chemical information it can give is very restricted although it is more powerful in distinguishing coloured pigments in manuscripts15.

In order to obtain greater chemical information there has been more focus recently in using non-destructive atmospheric ionisation mass spectrometry techniques to interrogate ink and paper surfaces. In forensic science desorption electrospray ionisation (DESI) has been used to characterise a range of modern blue and black ballpoint, gel and rollerball inks16,17. Direct analysis in Real Time (DART) is another minimally-destructive technique which has been established as a document authenticity technique18. One of the major advantages DESI has over DART is the imaging capability for the analysis of documents which may have been altered at different time points19. As of yet however the analysis of historic black writing inks such as iron gall and ivory black has not been documented for analysis by minimally-destructive atmospheric ionisation mass spectrometry techniques.

Here we use direct infusion nanospray mass spectrometry for the first time to investigate the poetry of Robert Burns and distinguish it from manuscripts written by a skilled, contemporary forger. Authentic Robert Burns documents “A letter written to ‘Rev and Venerable Sir’” (MSS11), “A note” (MSS10) and “The Five Carlins” poem (MSS14) were sampled minimally-destructively from both ink and paper and subjected to analysis using ultra-high resolution mass spectrometry. These were compared to seven different forgeries produced by Alexander Smith. Baseline authenticity for both Smith and Burns was obtained using the gold standard methods of provenance and transmission of ownership, as well as expert inspection of the documents. Figure 1 shows a schematic of the technique used for the analysis with two of the most significant features distinguishing Smith and Burns highlighted.

Figure 1 Schematic of the extraction and analytical process with direct Infusion data for authentic manuscripts. All samples for Antique Smith (A–C) have an ions at m/z 327.0782 and 344.1049; (A). The Holy fair in the hand of R. Burns, also signed by J. Hogg (B). Dainty dive poem in the hand of R. Burns and (C). The first psalm in the hand of R. Burns. All samples for Robert Burns have ions at m/z 113.9639 and 272.0655 (D–F); (D). A note written by R. Burns, (E). A letter written by R. Burns and (F). The five carlins poem. Full size image

To assess the ability of untargeted spectral analysis to distinguish between real and forged documents, a classifier was produced. It is able to distinguish between real Burns and the Alexander Smith forgeries with an AUC of 0.77 (see Fig. 2). Peaks were selected as features if they were present in at least three samples and in none of the solvent blanks, resulting in 496 peak features. Classification performance was evaluated using a cross-validation procedure in which samples from a single document were held out as test data (to replicate the use case of training on documents of known provenance and testing on a new document) and the remaining documents were used for training. A hard margin Support Vector Machine classifier was used with an RBF kernel function20. Both paper and ink samples were combined in the classifier.

Figure 2 ROC curve demonstrating the sensitivity, specificity and false positive rate for a classifier based on mass spectrometric discrimination of Burns and Smith. To generate the model, a hard margin Support Vector Machine classifier was used with an rbf kernel function. Signals from both ink and paper were combined in this instance. Full size image

To further explore the discriminatory peaks, 94 significant differences (Q < 0.05) were found distinguishing Burns and Smith manuscripts, with 42 features not detectable in any solvent blank. After manual analysis of each significant peak not found in the solvent blanks, we selected eight from each author (see Fig. 3 and Table 1) that were high intensity, not isotopic peaks derived from other compounds and that were only detected in the respective author’s work. While many of the distinguishing features could not be annotated due to the minimal sample volume, predicted formulae were suggested for eight compounds, with annotations (obtained from ChemSpider) available for four of these. Two compounds are clearly organically-derived, probably from plant extracts (citropen and citrate), while the other compounds are potentially inorganically derived. Significant differences between the inks and paper were also detected in Burns, with a majority of features found in the paper also detected in the ink samples. There was little significant difference between ink and paper samples extracted from the Smith manuscript, likely due to his habit of tea- or tobacco-staining paper to artificially age it, producing strong features from the staining that overwhelmed signatures from the ink.

Figure 3 Heatmap of features found between Burns inks and paper and Smith inks and paper. Full size image

Table 1 List of diagnosing peaks, their predicted formulae, and, where available, a suggested compound name. Full size table

After obtaining contemporary recipes for inks, especially iron gall and ivory black, we were able to match individual documents to ink signatures. In the The Holy Fair manuscript by A. Smith and in the letter written by R. Burns, we detect iron gall ink, and in the Dainty dive poem by A. Smith, we are able to detect the presence of ivory black, as shown in Fig. 4. Interestingly, in the Five Carlins manuscript by R. Burns, we detect both features, demonstrating that Burns, as was common at the time, mixed inks to obtain a desired lustre and consistency in his writing.

Figure 4 (A–D) Comparison of ink spectra to identifying peak for iron gall at m/z 90.9479 (A). A. Smith - The Holy Fair in the hand of R. Burns – also signed by J. Hogg. (B) A letter written by R. Burns. (C) Ink spectrum for simple ivory black. (D) Ink Spectrum for ivory black made with treacle. (E–H) Comparison of ink spectra to identifying peak for ivory black at m/z 130.5259 (E). A. Smith - Dainty dive poem 2 in the hand of R. Burns. (F) The Five Carlins poem written by R. Burns. (G) Ink spectrum for Blots iron gall. (H) Ink Spectrum for home-made iron gall. Full size image

We anticipate applying the analysis in a tiered manner, such that an area of the document may be analysed with no effect on the written material, and if (as was commonly done by Smith) a forgery has been written on a document excised from an older, Burns-contemporary work, the ink may be used in a minimally destructive analysis to further clarify the authenticity.

Burns is a significant part of Scotland’s national heritage, and the provenance and authenticity of his manuscripts is a strength of collections both public and private. Manuscripts of dubious authenticity continue to appear at auctions, and a robust and quantifiable method that distinguishes the most prolific forger from authentic Burns is a significant step in this direction. Extreme caution is exercised over the discovery of any new potentially authentic manuscript: appearance is scrutinised and contents and ownership, if incomplete in record, usually can at least be credibly hypothesised. For approximately 80 per cent of the Burns corpus of manuscripts, it is possible to account for full provenance. However, 20% of manuscripts have less strong authenticity and a chemical classifier such as the one described in this paper will have a significant impact, not only within the field of Burns, but in literary authenticity in general.

To the best of our knowledge this is the first high resolution mass spectrometry analysis of contemporary inks and historical manuscripts. Signatures from the inks produced using historical recipes and original manuscripts from Robert Burns and ‘Antique’ Smith were compared in order to create a platform which questioned manuscripts can be authenticated. Techniques such as XRF, DART and DESI15,16,18 must be performed in a laboratory environment as a mass spectrometer is impracticable to move about and sources such as DESI can take time to fully optimise21. The simplicity however of the sample preparation method described herein means that the sampling can be easily performed at the site where the manuscripts are stored, which in turn could make it an ideal technique for auction houses to confirm authenticity.