Evaluation of different types of paper as solid substrates and their effects on LIBS emission signals

Abstract

In the present work, different types of papers were evaluated as a solid substrate for LIBS measurements, aimed at verifying their effects on the emission signals, taking into account their grammage, thickness and metal ions naturally present (minor components/impurities). Cu(II) and Mn(II) ions were used in the present study. The emission signals for Cu I and Mn I significantly varied depending on the paper type, whose intensities decreased after their rinsing with nitric acid solution. No correlation was found between signal intensity and paper grammage or paper thickness, whose effect could be hidden by the metal ions present in the paper, as signals significantly decreased after their removal. In addition, after washing with nitric acid solution, the wettability of the paper (measured as the contact angle with water) increased, allowing a more effective absorption of the metal ion solution by the paper, fact that can also contribute to decrease the signal intensity. The addition of Na(I) and Ca(II) ions in the paper sheet previously rinsed with nitric acid solution caused an increase in the signal emission of Cu I and Mn I, as these ions can contribute with the electron density of the plasma, increasing its temperature. However, the addition of Fe(III) ions and other transition metal ions in the paper also provided an increase in the emission of Cu I and Mn I and signal intensities can be recovered by adding a mix of these metal ions on the paper. The sensitivity of the measurements (taken as the slope of the calibration curve) provided by using office paper as a substrate were around 14 and 2.7 times higher than those obtained with quantitative filter paper for Cu and Mn, respectively. Therefore, depending of the analyte to be determined, an ordinary paper can be used to improve the sensitivity of the desired LIBS method.