Direct determination of trace elements in terephthalic acid by laser induced breakdown spectroscopy

Abstract

The feasibility of laser induced breakdown spectroscopy (LIBS) as an online tool for trace metal analysis in terephthalic acid (TA) was evaluated. Millions of metric tonnes of TA are produced each year as a raw material for various kinds of polyesters used to make many consumer and industrial products. The conventional techniques used for trace metal analysis in TA are electrothermal atomization atomic absorption spectrometry (ETA-AAS) and inductively coupled plasma optical emission spectroscopy (ICP-OES). However, these techniques require digestion of the sample and dissolution and so are very time consuming and are not suitable for online analysis. LIBS, if it meets the sensitivity and accuracy requirements, will be a very effective tool for online analysis of the production of terephthalic acid and other organic particles. In this work, the main elements of interest, including Mn, Co, Fe, and Na, were studied. The strongest emission line of each element was chosen to obtain the best detection limit. The experimental parameters, including delay time, gate time, sample stage moving speed, and laser repetition rate, were optimized to maximize the signal-to-noise and signal-to-background ratios. Two sampling methods were studied: compressed pellet and loose powder on sticky tape. The preliminary results indicated that there was no difference between these two sampling methods in term of sensitivity and precision. Therefore, the loose powder on sticky tape was preferred for making online analysis of loose powders possible. One unknown and two certified samples were used to determine the accuracy of the method, and the results were compared to those obtained by ICP-OES.