A Forensic Application of Solid-State 13C NMR Spectroscopy: the Date of a Photographic Development

Resumo

During a penal trial, it was requested to our laboratory to contribute the public minister prosecutor to study with a solid-state technique the effect of the time on a photographic negative and, eventually, to determine if the date of a certain photographic development can be located in a particular time range. It was important the determination of the time when a picture has been developed. Particularly, it was asked to define an estimate, more precisely possible, of the time elapsed from when the picture was taken assuming that it was close to the time of the processing of the negative of the photograph. The ¹³C cross polarization magic angle spinning nuclear magnetic resonance spectroscopy is known as a technique that is able to characterize the crystallinity of a sample. Among many materials, cellulose matrix is one whose crystallinity is more influenced on time. Assuming that after the wet development of the negative, the transformation of the crystallinity decreases upon time toward the amorphous state, it was possible, with a suitable reference scale, to obtain the approximate date of the development of the picture as required in the trial. This was possible by the preparation of a crystallinity reference scale obtained by photographic negatives developed at precise intervals along time.