DESI-MS: analysing ancient proteins from artefacts

Organic residues sorbed to archaeological artefacts provide a valuable source of information applicable to different aspects of ancient life. For example, analysis of lipids from ceramics dating from the Neolithic to the Iron Age is changing our perception of the origins of dairying. Milk contains proteins, notably alpha-casein, that are readily absorbed onto ceramics and which although less robust, are more specific than lipids in terms of the animal from which they originate. In our previous work we have used destructive sampling of the artefact and immunological detection, which has enabled us to identify the species that was milked. Milk proteins are doubly interesting, because evidence of their persistence over time has relevance to claims for the detection of ancient proteins on both ceramics and stone tools. We are increasingly using analytical science, especially methods based on the instrumental technique of mass spectrometry, as these are able not only to identify proteins, but give insights into this protein preservation. Consequently when a new analytical technique called desorption electrospray ionisation mass spectrometry (DESI-MS) was invented two years ago, we saw obvious application to archaeology. The method makes it possible to analyse compounds directly from the surface of materials as diverse as vegetables or pieces of fruit, bricks, and wood, without any sample preparation. We therefore designed and built a DESI interface for one of our mass spectrometers and showed that it was possible to use it to detect and analyse small pieces of protein on a Perspex surface. We propose to improve the design of our DESI-MS so that it is both easier to use and accepts typical archaeological samples. We will then analyse both experimental samples that were prepared by boiling and storing unpasteurized cow's milk in unglazed ceramic pots and buried in the ground for 1, 2, 4 and 8 years, as well as broken samples of pottery from Gujarat (India) used for many years to prepare milk and then recovered from a refuse heap. If we can detect casein on these increasingly degraded samples using DESI-MS, we will finally test the analyser on archaeological pottery from a range of sites and periods from Neolithic to Iron Age.