Micro-extraction and determination of transition metals in historical ink

A minimally invasive approach is described for sampling Fe, Cu, Zn and Mn in iron-gall ink. Filter paper (2 x 5 mm) that is moistened with de-ionized water is used to extract metals from an iron-gall ink surface. The extraction requires about 30-120 s for a small quantity of ink to diffuse into the moistened filter paper. The metals in the ink are extracted from the filter paper using 50 A mu L of concentrated nitric acid in a 2 mL polystyrene beaker for approximately 10 min. Graphite furnace atomic absorption spectrometry (GFAAS) was used to quantify Fe, Cu, Zn and Mn in iron-gall ink. In order to test the feasibility of this extraction procedure, the ratio of Fe:Cu was determined from ink lines that were drawn with a commercially available iron-gall ink on modern acid-free paper. The measured ratio of Fe:Cu was 18.2:1 which is comparable to the expected ratio of 20:1. The slightly lower ratio achieved using the extraction procedure and GFAAS was possibly due to the higher solubility of the Cu species in the ink. One of the advantages of using a moistened filter paper is that only the ink surface is sampled which avoids the possible measurement of metals of different inks of other components that may be present in the paper or on the opposite side of the paper.