Identifying Samples Using PMF

The PMF of yellowish skin on boots.
"Peptide mass fingerprint" of yellowish skin making up strap on the boots. By comparing the sample with the known spectrum of bearded seal, Peabody conservators identified the strap materials as bearded seal.

 

Samples of each material were taken from areas of damage or loss, thus minimizing impact to the object. Samples needed for analysis are no more than the size of a pinhead, however slightly larger samples are usually taken to ensure there is enough material for reanalysis if necessary.

Example of size of sample removed from an object for analysis. 

The samples are subjected to an extraction/digestion protocol in order to prepare them for analysis by Matrix Assisted Laser Desorption Ionization Time-of-Flight Mass Spectrometry (MALDI-ToF-MS). During analysis, a mass spectrum is produced for each sample. Because each protein sequence is unique, each mixture of peptides is unique. This means that each spectrum contains characteristic marker peptides, or a “peptide mass fingerprint,” that can be matched to reference spectra. Bearded seal can be identified to the species level based on its unique peptide markers; other mammals can presently be identified only to the family, or sometimes tribe, level. 

For example, in the chart above, the mass spectrum of the sample is shown above and compared to a reference spectrum from bearded seal. The matching spectra, then, allow conservators to deduce that the yellowish skin on the boots is bearded seal. In addition, the sinew was identified as caribou; the thick yellowish skin as bearded seal; and the orange-brown colored skin as a seal in the Phocini tribe. It is interesting to note that several different species of animal (including bearded seal (Phocidae family), caribou (Cervidae family), and a seal from the Phocini tribe (Phocidae family) were used to make this single pair of boots.

For more on the PMF process, click here for detailed report.