Analysis of protein additives degradation in aged mortars using mass spectrometry and principal component analysis

Abstract Enrichment of mortar mixtures by the addition of natural organic materials has a long history. Already in ancient times it has been empirically proved that proteinaceous additives significantly increase mortar durability, strength or plasticity. The knowledge of the protein occurrence in historical mortars would be very useful for the restoration work. However, some of historical recipes did not survive till these days or it is difficult to interpret them if they have survived, and, therefore,we are not able to prepare all kinds of mortars of an appropriate quality nowadays. The goal of this work was to identify and relatively quantify proteins contained in the model mortars samples enriched with four different protein additives (animal glue, blood, egg, and milk). For this purpose, aged mortar samples were analysed by LC-ESI-Q-TOF MS (Liquid Chromatography and ElectroSpray Ionization Quadrupole Time of Flight Mass Spectrometry) and the relative content of proteins was quantified by label-free technique. Number of semi-specific and non-specific peptides were detected in different ageing times and preferential sites in non-specific peptide were found. Principal component analysis (PCA) applied on MALDI-TOF (Matrix-Assisted Laser Desorption/Ionization – Time of Flight) spectra was also used to differentiate between protein additives in mortars. Peptides that are stable in the prepared model mortars and characteristic for the individual protein additives were found and together with the results of relative quantification they confirmed the possibility of unambiguous identification of protein additives in century-old mortars that brings the opportunity to make an improved hypothesis about the old traditional recipes. By the mass spectrometric methods milk additive was identified in historical mortar from Swedish church dated back to 14th century.