For comparative purposes the corresponding peak-to-background ratios were: 4.4, 7.1 and 7.9 for 5kV, 10kV and 15kV, respectively. In addition to Nb, other elemental peaks could be detected from the surrounding materials. The Al and Si peaks were generated from the holder (the O peak was visible due to oxidation of the holder itself), the Ni peak originated from the sample grid and the extraction