Investigating the effect of species-specific calibration on the quantitative imaging of iron at mg kg−1 and selenium at μg kg−1 in tissue using laser ablation with ICP-QQQ-MS

There is evidence that iron (Fe) and selenium (Se) are dysregulated in neurodegenerative diseases (ND). Disease-specific impacts on concentration distributions are anticipated for Fe (the most abundant transition metal), and Se (present at trace levels), necessitating imaging strategies that enable their simultaneous quantification in ND pre-clinical models or post-mortem samples. This paper describes methodology for the production of matrix-matched tissue homogenates containing both Fe and Se at pathophysiological levels and investigates their feasibility as calibration standards for quantitative imaging of these elements. For the first time the effect of elemental species-specific calibration on the quality of LA-ICP-MS data is evaluated by investigating whether tissue matched standards spiked with inorganic Fe and Se are suitable alternatives to those spiked with specific species (e.g. ferritin or selenoproteins). To achieve this, the correlation of the calibration graph slopes (inorganic versus metalloprotein-spiked) as well as the homogeneity of the calibration standards were monitored over a laser energy range of 1 to 6 J cm−2. For Fe, such slopes were found to agree well within the associated measurement error. In contrast, the choice of calibrant for Se was observed to have a greater impact; at 1 J cm−2 the regression slope for the selenoprotein calibration was approximately 36% less than that of inorganic Se calibration. Finally, the limit of detection of Se was improved 2-fold by mixing 25% (v/v) methanol with the laser-induced aerosol in a chamber prior to introduction into the ICP, making the simultaneous detection of both elements possible at physiologically relevant concentrations.