Ambrogio Giacomelli

GC-MS characterization of proteinaceous and lipid binders in UV aged polychrome artifacts

This paper presents the most significant results obtained from the characterization of protein and lipid binders in a broad range of reference paint materials prepared and stored at Opificio delle Pietre Dure in Florence (Italian Ministry of the Cultural Heritage, Italy). The amino acid distribution for protein binders and the fatty acid distribution for lipid binders were determined by gas chromatography-mass spectrometry before and after being artificially aged through exposure to UV light at 254 nm or 366 nm for different time periods in a climatic chamber (20°C and 80% RH). The results were compared with those relevant to old paintings of different ages, and showed that UV light aging does not significantly affect the amino acid profile of protein binders. Consequently, protein binders in old paintings can be reliably identified by comparing the amino acid composition with that of reference paint materials which have not been aged. However, the composition of lipid binders is substantially affected by UV irradiation, leading to a lowering of oleic acid and the formation of azelaic acid and other dicarboxylic acids including oxalic acid. An oleic to stearic acid ratio of less than 0.5 was observed in all the samples of works of art, and this parameter can be used to evaluate the extent of the artificial aging process. The formation of oxalic acid was also observed starting with pure unsaturated fatty acids, thus supporting the chemical origin of oxalate patina.

Two procedures for suppressing interference from inorganic pigments in the analysis by gas chromatography-mass spectrometry of proteinaceous binders in paintings

Abstract Two methods for suppressing the interference of inorganic pigments in the determination of amino acids in hydrolysates of wall painting samples by gas chromatography–mass spectrometry are described. One is based on the extraction of proteinaceous matter from the sample by a 2.5 M ammonia solution prior to the hydrolysis step, and the other on the elimination of inorganic ions from the hydrolysate by means of a cation-exchange resin. The proteinaceous binders present in the paint layer were identified using principal component analysis on the relative amino acid percentage. Some samples from “Giudizio Universale” in Florence Cathedral and from the Monumental Cemetery in Pisa (Italy), were analysed using both procedures. The presence of milk binder as the main organic component of the tempera was highlighted.

Characterization of proteinaceous binders in wall painting samples by microwave-assisted acid hydrolysis and GC-MS determination of amino acids

AbstractAn analytical procedure for the characterization of proteinaceous binders (milk or casein, egg and animal glue) in artistic wall painting samples is described. The method is based on microwave-assisted acid hydrolysis of the sample, and on derivatization of the amino acids freed with a silylating agent and their quantitative determination by gas chromatography/mass spectrometry. The procedure was tested on wall painting reference samples, and showed a typical protein recovery of about 70% at 0·1mg sample size. Fourteen amino acids were determined and some ratios between their concentrations were selected, so that the proteinaceous binders in the sample could be identified. Results on some samples from frescoes from the Monumental Celnetery in Pisa (Italy) showed that the amino acid composition of casein differed significantly from that expected, as a result of degradation processes due to aging and moisture.

Activity coefficients of electrolytes from the emf of liquid membrane cells. III: LaCl[sub 3], K[sub 3]Fe(CN)[sub 6], and LaFe(CN)[sub 6]

The activity coefficients of LaCl3, K3Fe(CN)6, and LaFe(CN)6 were measured down to about 1×10−4, 3×10−5, and 2×10−5 mol-kg−1 respectively, by means of cells with ion-exchange liquid membranes. In the diluted region, the trend of lanthanum chloride agrees with the Debye-Huckel theory and corroborates earlier findings in the literature relevant to more concentrated solutions, with minor systematic corrections of the γ± values. K3Fe(CN)6 attains (rather than tends to attain) the Debye-Huckel limiting slope at≈1×10−3 mol-kg−1, and lanthanum ferricyanide in the diluted region shows negative deviations from the limiting law, similar to the ones predicted for large-sized, highly-charged ions in the diluted region by Bjerrums, IPBE, and Mayers theories. The behavior of LaCl3 in the concentrated solutions proves that lanthanum ion drags along with it into the membrane many molecules of water which were then found to be twelve. Pitzers theory best-fit coefficients that meet the experimental curves to be reproduced satisfactorily are reported.

Activity and osmotic coefficients from the emf of liquid-membrane cells. VII : Co(ClO4)2, Ni(ClO4)2, K2SO4, CdSO4, CoSO4, and NiSO4

The activity coefficients of CdSO4 CoSO4, and NiSO4 are determined from the emf of liquid-membrane cells, like those described in the papers I–VI of this series. The activity coefficients of the auxiliary salts Co(ClO4)2, Ni(ClO4)2, and K2SO4, needed to eliminate the problem of extrapolation to infinite dilution of the 2–2 salts, are also measured. CdSO4 CoSO4 and NiSO4 in the dilute region deviate downward from the limiting law by a larger extent than believed in the past, thus creating the need for the activity coefficients to be recalculated and systematically lowered by 8–16%. The activity coefficients of Co(ClO4)2 and Ni(ClO4)2, too, need to be corrected by around –3%. For K2SO4, the original values, although scattered, were substantially correct. Pitzers theory best-fit parameter, able to provide the activity and osmotic coefficients of the salts considered, are reported.

A NEW SYNTHETIC ROUTE FOR TRANSITION-METAL COMPLEXES WITH SCHIFF-BASES

Abstract A new preparation method of Schiff base metal complexes is proposed, based on the transimination reaction between ammonium or ammonium-like salts and the imino portion of some N-substituted salicylaldimine-metal chelates. The reaction was carried out through a general acid catalysis induced by the ammonium ions and by the metal centre. It was mainly applied to the preparation of Ni(II), Cu(II) and Co(II) complexes. The reaction took place rapidly on addition of a slight excess of the ammonium salt to a non-aqueous solution of the Schiff base complex, at room temperature. The method can also be applied to the preparation of N -salicylidene amino acid complexes.

Activity coefficients of electrolytes from the E.M.F. of liquid membrane cells. II - multicharged electrolyte solutions

The activity coefficients of Co(en)3Cl3 and K2SO4 were measured by means of a cell with ion-exchange liquid membranes following the method described in paper I. The results prove that this method is even more valuable with multicharged salts than with 1-1 electrolytes. The values obtained are precise and reliable down to dilution limits never before accessible, e.g., 4×10−5 mol-kg−1 in Co(en)3Cl3. High dilution levels are of particular importance when dealing with highly charged electrolytes since the trend at higher concentrations often leads to errors both in extrapolation to infinite dilution and in the absolute activity coefficients. As an application, the activity coefficients of [Co(en)3]2(SO4)3-suspected to be wrongly evaluated in past literature-were measured, and their values at low concentrations were actually lower than those quoted before.

Activity coefficients from the emf of liquid membrane cells V. alkaline earth hexacyanoferrates (III) in aqueous solutions at 25°C

The relative mean activity coefficients of the M3[Fe(CN)6]2 salts, M=Mg, Ca, Sr, Ba, are measured down to about 5×10−6 mol-kg−1 using the liquid membrane cell method. In the dilute region these salts display negative instead of positive deviations from the limiting law, contrary to Debye-Hückels theory predictions. An indirect method based on auxiliary emf measurements in MCl2, K3Fe(CN)6 and KCl, rather than a theory-assisted direct extrapolation to zero of the relative activity coefficients, is used to identify the actual values of the activity coefficients. The data are compared with Mayers theory, ion-pair theory and numerical integration of the Poisson-Boltzmann equation. Best-fit coefficients of Pitzers equation which meet the activity coefficients of the M3[Fe(CN)6]2 salts to be reproduced, are reported.

Substitution reactions at palladium(II) complexes producing dimeric species: stability constants determination for simultaneous equilibria

Substitution equilibria in Pd(dien)-like aquocomplexes are parallel to dimerization equilibria of the starting monomeric species. Accurate potentiometric measurements of the hydrogen ion concentration lead to the simultaneous determination of the stability constants of both kinds of equilibria: Pd(dien)H2O2+ + X− ⇄ Pd(dien)X+ + H2O, and 2 Pd(dien)H2O2+ + X− ⇄ [Pd(dien)]2X3+ + 2H2O. For the monomeric complexes, the following relative stability series was ascertained: X− = Cl− < Br− < I− < SCN− < OH−. The bridging ligand affects the relative stability of the dimeric species in the order: X− = I− ⪡ OH− < SCN−.

Activity coefficients of 3:3 electrolytes in aqueous solutions

Abstract Activity coefficients of 3:3 electrolytes have not been liable to determination until 1994, when one of the authors devised a new type potentiometric cell able to trace La[Fe(CN) 6 ] down to about 5×10 −6 mol kg −1 . La[Co(CN) 6 ], [Co(en) 3 ][Co(CN) 6 ], and [Co(en) 3 ][Fe(CN) 6 ], are now studied by the same method. Like La[Fe(CN) 6 ], these electrolytes display in the dilute regions large negative deviations from the limiting slope instead of the moderate positive deviations predicted by the Debye–Huckel theory. Although the negative deviations can be easily interpreted in terms of only a partial dissociation, one should not conclude that the 3:3 salts are only sparingly dissociated. Theoretical calculations are presented, showing how long-range interactions, in a population of non associated +3 and −3 charged spheres, are able to produce effects that simulate the behaviour of a weak electrolyte. Preliminary information is provided of an unexplored method, here devised, to determine the mean activity coefficients in mixed electrolyte solutions.