Internal standard evaluation for bioimaging soybean leaves through laser ablation inductively coupled plasma mass spectrometry: a plant nanotechnology approach

Abstract

This work evaluates the use of an internal standard for acquiring the elemental mapping of ¹⁰⁷Ag, ⁵⁵Mn and ⁶³Cu using laser ablation inductively coupled plasma mass spectrometry in the imaging mode from soybean leaves cultivated in the absence or in the presence of silver nanoparticles (40 nm average size) and also using silver nitrate for comparative purposes. Carbon, which is naturally present in plants and currently used as an internal standard for plant analysis is evaluated as ¹²C and ¹³C, as well as ²⁸Si and ³¹P, as viable alternatives. Optimized conditions are established using a certified reference material – CRM 1573a Tomato leaves – considering those parameters involved in the plasma production (RF power – 1000 W and carrier and auxiliary gas flow rates – 1.1 L min⁻¹ and 1.8 L min⁻¹, respectively) as well as in the laser ablation process (laser intensity – 55%, frequency – 20 Hz, spot size diameter – 80 μm, and scan speed – 70 μm s⁻¹). Additionally, the choice of the correct internal standard (¹³C⁺) is made based on the highest precision obtained (RSD of 37, 15, and 4% for ¹⁰⁷Ag⁺, ⁵⁵Mn⁺, and ⁶³Cu⁺, respectively) as well as the highest homogeneity in the sample surface (verified through the images). The analysis of the images reveals not only the basal level of Ag present in the leaves, indicating its poor translocation, but also the homeostasis of Mn and Cu highly affected in plants cultivated in the presence of silver nitrate compared to those cultivated in the presence of silver nanoparticles. The distribution of Mn and Cu in the leaves is similar when the control plants and those cultivated in the presence of nanoparticles are compared.