How Does Pen Ink Age? - Chromatography Investigates
Oct 20 2019 Read 361 Times
Knowing how ball point inks age on paper is an important consideration during the forensic analysis of documents. There are many different factors that can affect how the ink dries and changes including ink type, paper type and the storage conditions. And those storage conditions are influenced by the climatic conditions. A recent paper published in the journal Current Chromatography has looked at the ageing profile of ballpoint ink on different papers when stored in Brazilian tropical conditions.
Solvents give the age away
Previous researchers have used different aspects of the ink to try and date it. One option that has been tried was to analyse the dyes used in the inks and see how they had degraded. Could the amount of degradation be used to date an ink sample? Unfortunately, researchers found that the ageing of dyes was too dependant on the physical properties of the ink layer on the surface. This is determined by the ink composition, paper properties and exposure to light and humidity.
So, researchers turned to the volatile solvents used in ballpoint inks. And studies have identified 2-Phenoxyethanol (PE) as the solvent of choice in ballpoint inks - with one study finding PE present in over 83% of 354 blue ballpoint inks that were tested. 2-Phenoxyethanol is a colourless oily liquid that is classed as an aromatic alcohol. It has many uses including in perfumes, insect repellent, lubricants as well as in ballpoint inks.
Too many variables
The analysis of ink ageing on paper is a complex matter due to the many different variables that can affect the ink. The composition of the ink can affect how it ages as ink can be composed of many different components including resins, dyes, solvents and additives. The pressure applied during writing and even the size of the ball in the tip of the pen have all been shown to influence the amount of ink and thus solvent that is deposited on the paper.
In the study carried out in Brazil, 15 blue and 15 black inks were tested. Lines were drawn using the pens on 2 different types of paper for the purpose of the study. Samples were analysed at different periods of time: recent, 4 months, 18 months, 24 months and 36 months. Reference samples were kept in the dark at 20C and 30% relative humidity, whilst other samples were kept in various locations exposed to daylight, varying temperatures and at much higher relative humidity.
Chromatography writes the answer
The samples were then analysed using gas chromatography-mass spectrometry to analyse the 2-Phenoxyethanol content in each sample. GC-MS is a powerful analytical technique as discussed in the article, Workflow solution for antidoping analysis including steroids in urine with GC-QqQ and GC-HRAM. The team report that concentrations of PE in samples at 4 months and 18 months differed from the PE concentration of a recent sample. But there was little difference in PE concentration at 24 and 36 months.
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