Identification of Different Brands Erasable Pens by Infrared Spectroscopy Combined With Chemometrics Methods
ZHAO Yu-xuan1, ZENG Le-yang-zi2, LI Kai-kai1*
1. College of Criminal Investigation, National Experimental Teaching Demonstrating Center, People’s Public Security University of China, Beijing 100038，China
2. Changning Branch of Shanghai Public Security Bureau, Shanghai 200336, China
Abstract：As a new writing tool, an erasable pen has the characteristics of erasable ink. Therefore, in the current public security work, common criminals use an erasable pen to tamper with documents. In order to ensure the integrity of material evidence, it is urgent to establish a non-destructive and rapid ink test of the erasable pen to provide help for finding writing tools and identifying criminal suspects. Common erasable pen inks can be divided into film-forming ink and temperature change ink. In this study, the ink of 30 erasable pens of different brands, types and colors were analyzed with Fourier transform infrared spectroscopy (FTIR). Through the analysis of infrared spectra in the range of 4 000～650 cm-1, the difference in their composition and the spectral difference of erasable pen with different fading mechanism were compared. Through the infrared spectrum information, it is found that the components of the same brand and the same model of erasable pen ink are similar, and the ink color has little influence on the infrared spectra. At the same time, comparing the infrared spectrum information of the erasable pen ink before and after erasing, it is found that there are still some special chemical components in the temperature change ink after erasing, which can be used as the identification standard. However, the residual components of film-forming ink are less after erasing, which is difficult to identify, which may be related to the particle structure of film-forming ink. In addition, the spectra in the range 650～1 500 cm-1 of erasable pen ink were analyzed by principal component analysis (PCA) and Heatmap to classify the types of erasable pen. The relationship between infrared spectrum information and principal component loadings was established. According to the loading plot, the first two principal components summarize almost all the infrared spectrum information, and the cumulative contribution rate is more than 79%. Therefore, the first two principal components are selected to a scatter plot. In order to determine the source of the erasable pen in practical work, 5 samples of the unknown pen were randomly selected from 30 kinds of ink samples, and PCA analysis was carried out with known samples at the same time, and a scatter diagram was made to realize the prediction of the types of unknown erasable pen ink samples and work well. The results indicate that FTIR combined with PCA can be used to classify the erasable pen ink rapidly. It provides a method for examining the erasable pen ink in altered documents with advantages of fast, non-destructive and high sensitivity.
