In-vacuum micro-PIXE analysis of biological specimens in frozen-hydrated state

The first in-vacuum measurements of biological specimens in frozen-hydrated state using proton microbeam have been performed at Materials Research Group, iThemba LABS, South Africa. A commercially available cryotransfer system used in electron microscopy has been adapted for this purpose. The analyzed material was frozen in propane cooled by liquid nitrogen, fractured, carbon coated and transferred onto the cold stage (100 K) in the nuclear microprobe chamber. Micro-PIXE and simultaneous proton backscattering was performed using 3 MeV proton beam. Monitoring of water vapour composition during the proton bombardment showed good stability of the analyzed material. Quantitative results were obtained by standardless method, tested using 20% gelatine standards with added PbCl2. The differences between the concentrations obtained for frozen-hydrated and next freeze-dried specimens and the value calculated on the basis of weighted mass of PbCl2 added to gelatine were statistically insignificant. Leaf tissue of Senecio anomalochrous and larvae of Chysolina pardalina were used as examples of plant and animal tissue. Quantitative elemental mapping of frozen-hydrated specimens compared with subsequent analysis of the same areas after freeze-drying revealed the same distribution pattern in both cases.