Analytical Spectral Semi-quantitative Dating Method of Printed Text Toner Using Raman Spectroscopy and the Fluorescence signal of Paper Sheet Cellulose
Authors: Kochemirovskaya Svetlana , Kozhakhmetov Serik , Borisov Evgeniy , Ischuk Diana , Mokhorov Daniil , Kochemirovsky Vladimir 
Published: September 2024

The chemical processes that occur during the surface degradation of document paper under the influence
of external factors were studied by Raman spectroscopy. The possibility of comparing the same processes occurring
on clean free paper and under printed text toner letters is of interest. It is assumed that the transformation of cellulose
can occur at different rates in the areas of paper exposed to direct atmospheric and light exposure and in the areas
containing an intermediate layer of the toner. The most stable temporal changes occur in the region of C–C–C and C–
C–O vibrations of glycosidic rings in the short-wavelength region of the Raman spectrum.

Background:

The main areas of research in the field of document dating, mainly focused on chromatographic methods.
These methods are based on studying the dynamics of evaporation of volatile ink components. There is also a
variety of chromatography techniques, including gas chromatography, ion chromatography, high performance liquid
chromatography, thin layer chromatography and gas chromatography-mass spectrometry. This indicates the wide
range of approaches and techniques used to determine the age of written materials. However, all these methods are
used mainly during the first two years after writing the document details, which does not allow examining documents
that are more than two years old.

Methods:

Raman spectroscopy was used as the main research method. The object of the study was sheets of
white A4 paper with printed text printed on them, produced on a laser printer. The subject of the study was areas
of paper free of text, as well as areas of paper under toner. The integrated fluorescence of a paper optical brightener
based on stilbene was studied in the wavelength range of 400–500 nm. Also, all objects were studied using
optical microscopy.

Results:

The comparison of peak intensity values averaged over 10 brands over the years made it possible to reveal
the following regularities: – the ratio of peaks in the paper spectra from which the toner was removed is greater than
in the blank paper spectra, and the difference becomes less noticeable as the age of the paper decreases. The ratio of
the intensity of the 330 cm-1 peak to the intensity of the 1603 cm-1 peak: the older the document becomes, the lower
the average ratios of these peaks in both the blank paper spectra and in the spectra of paper from which the toner has
been removed. –the intensity of the peak in the region of 280 cm-1 and 1 086 cm-1 of the Raman shift increases with
decreasing the age of the toner-free paper.– a gradual increase in the intensity of the 380 cm-1 peak is observed when
comparing the paper samples from which the toner was removed for 2016, 2018, 2020, and 2022. Regularity was
also found in the change in the ratio of the average statistical values of the peak at 280 cm-1 in the spectra of the
paper under the toner to a similar peak in the spectra of the paper free from text: the value of this ratio decreases with
decreasing the paper age. At the same time, the paper fluorescence signal is an informative parameter that makes it
possible to establish some facts from the document’s history. With artificial document aging, a significant broadening
of the dispersion region and the appearance of its “splitting” into two or three strata are observed in the distribution of
the fluorescence signal level of a paper sheet relative to the average value. The average fluorescence signal of a naturally
aged sheet decreases with the age of the document.

Conclusion:

As a result of research, it has been established that Raman spectroscopy can be successfully used as
a method for dating documents. When comparing the degree of degradation of the surface of open areas of paper
and areas protected by toner of printed text, different effects of external factors on these areas were established,
leading to different rates of degradation processes. The use of Raman peak intensity ratios for paper components
in exposed areas compared to peaks under printed text further expands the methodology. The emphasis on spectral
methods, in particular fluorescence signal distribution and Raman spectroscopy, to detect surface changes is
well justified. It demonstrates a holistic approach to document analysis, taking into account both the chemical
changes observed using Raman spectroscopy and the fluorescence signal distribution that may indicate signs of
artificial aging through light exposure. This combination of methods appears to be reliable for assessing the age
of documents and can make significant contributions to the field of forensic document examination.