A new approach to calibration and determination of selected trace elements in food contact polymers by LA-ICP-MS

Abstract

A calibration strategy using porous nylon disks and reference solutions is proposed for the first time for matrix matching and determination of As, Ba, Cd, Cr, Pb, Sr and Zn in polymers by laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS). Polymer samples commonly used in contact with food were analyzed. The procedure consists on the use of porous nylon disks as support for a dried droplet reference solution. Calibration in the range of 0.50-190μgg^-1 for Ba, Cd, Cr, Pb, Sr and Zn and from 0.30-9.0μgg^-1 for As was used. Laser and ICP-MS instrument conditions were evaluated in order to achieve the best signal-to-background ratio. The radiofrequency power and carrier gas flow rate were fixed at 1300W and 1.25Lmin^-1, respectively. Spot size, repetition rate, scan line speed and laser fluency were set to 100µm, 20Hz, 100µms^-1 and 17.9Jcm^-2, respectively, as the established conditions for analysis of standards and samples. By using these conditions, limits of detection, estimated considering B+3s (where B is the value of the blank and s is the standard deviation of 10 measurements of the blank), ranged from 0.09μgg^-1 (²⁰⁸Pb) to 1.09 (⁵³Cr) and 0.05μgg^-1 (²⁰⁸Pb) to 2.10 (⁵³Cr) for calibration with and without ¹³C as internal standard (IS). In spite to the use of nylon for matrix matching of different polymeric matrices, the normalization with ¹³C as IS was also evaluated. The precision of the method is relatively good (RSD<20%), and the accuracy of the method, evaluated by analysis of certified reference materials (CRM) and by comparison with results obtained from solution analysis by ICP-MS after sample decomposition by microwave induced combustion (MIC) is relatively good. The suitability of the proposed method resulted in direct and reliable analyses of polymer samples with a simplified or unnecessary sample preparation step. In addition, the calibration with dried droplet reference solutions may be considered a promising procedure in view of its advantages to other forms of calibration, as the use of CRM or the preparation of synthetic standards. The use of porous nylon disks spiked with reference solutions for calibration is the main advantage of the present work.

Keywords: Calibration; Inductively coupled plasma-mass spectrometry; Laser ablation; Polymers samples.