Instrumental considerations and applications of elemental bio‐imaging
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Elemental Bio-Imaging (EBI) is an established application of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) that determines spatial distributions of in situ trace element concentrations in thin sections of biological tissues. This project provides an examination of instrumental considerations relating to method development and refinement, and the utilisation of amended methods for the analysis of element concentration distributions in specific biological samples. The first instrumental consideration examined improving the methods used for matrixmatched tissue standard preparation. The main disadvantage to the previously proposed tissue standard preparation was a decrease in the homogeneity of sectioned tissue as the concentrations of spiked elements increased. The decrease in homogeneity was due to the use of relatively low concentration (1000 μg mL-1) certified single element standard solutions. Substituting the low concentration solutions for higher concentration solutions, prepared using soluble metal salts, allowed for smaller quantities of solution to be spiked into the tissue. This increased homogeneity and the ability to spike a greater number of analytes into each tissue standard. The appropriateness of increasing laser scan speed for the reduction of total experimental analysis time was also examined. EBI experiments normally employ scan speeds where the distance traversed in one second is equal to or less than the diameter of the laser beam. Consequently, data for a higher‐resolution (pixel size = 15 μm x 15 μm) image of a 5 mm x 5 mm tissue section can take upwards of 30 hours to acquire. Appropriate laser scan speeds may be calculated by consideration of the relationship between laser scan speed, laser spot diameter and the total scan cycle of the quadrupole mass analyser. A simple method to calculate the laser scan speeds capable of reducing the acquisition time by up to a factor of five whilst maintaining dimensional integrity of the image is presented in this thesis. Two applications were developed utilising increased laser scan speeds. Both applications were related to the study of the neurodegenerative disorder, Alzheimer’s disease, and examined particular variables and their effects on the distribution of metals in the brain. The first application examined the effectiveness of a new drug, PBT-2, on the redistribution of elements in mouse brains with the zinc transporter-3 gene removed. Results indicated PBT‐2 had little effect on the distribution of 66Zn, 63Cu and 56Fe in the midbrain. The second application examined the effect of intermittent hypoxia and a diet of advanced glycation end‐products on element distribution in the brain. The most intriguing results were seen in the images for 59Co. A 100-fold elevation in the concentrations of 59Co was observed between mice exposed to intermittent hypoxia, those in a control environment and wild-type mice not exposed to the experiment’s settings. An examination into the use of H2 as a reaction gas was conducted. The improvement of the analytical performance of imaging experiments was considered for a range of masses with spectral interferences, including the 40Ar16O+ spectral interference on 56Fe+. At low (< 1.0 mL min-1) H2 flow rates, greater spectral interference due to H+ adducts were observed for 57Fe than with the reaction mode off. At higher flow rates, up to 3.0 mL H2 per minute, the spectral interferences were reduced leading to an improvement in the limits of analysis for masses with O- and N-based polyatomic interferences. EBI is typically performed using spatial resolutions of 30 μm x 30 μm and above. Higher resolution imaging is desirable for many biological applications. As a culmination of the project, the combination of the use of improved matrix‐matched tissue standards with the use of the reaction cell and increased image acquisition speeds was employed. The combination of improved parameters resulted in a high quality image of the area of the brain including the substantia nigra being prepared at a resolution of 6 μm x 6 μm for 56Fe.
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