Determining Precious Metal Contents in Lettuce

The procedure for determining the elements is described by Pasławski and Migaszewski [51 ] and Kalisz et al. [52 (link)]. Briefly, randomly-selected lettuce leaves were shredded and dried at 70 °C in a dryer. The dried samples were ground using a Pulverisette 14 ball mill (Fritsch GmbH, Idar-Oberstein, Germany; 0.5-mm sieve). After that, 3 g samples were placed in TFM vessels with a volume of 100 cm³ and mineralized in 10 cm³ 65% super pure HNO₃ (Merck no. 100443.2500) in a Mars 5 Xpress (CEM Corporation, Matthews, NC, USA) microwave digestion system. After cooling, the samples were transferred to 25 cm³ flasks with redistilled water. The total contents of the elements Ag, Au, and Pt were analysed by ICP-MS/MS triple quadruple spectrometer iCAP TQ ICP-MS (Thermo Fisher Scientific Inc., Bremen, Germany). Their determination was conducted using the following measurement mode for individual isotopes of elements: S-SQ-KED for ¹⁹⁷Au, ¹⁰⁹Ag, and ¹⁹⁵Pt.