Scanning Microscopy


In an exploratory study using scanning electron microscope (SEM) based energy dispersive spectroscopy (EDS) of X-rays, differences were detected in the elemental compositions of adult Pectinophora scutigera raised on contrasting diets - cotton, Hibiscus tiliaceus and laboratory medium.

X-ray spectra were collected from the thorax and abdomen of 13 male and 13 female moths. X-ray yields are optimised by ashing the insects for 1 h at 400-600°C, and by operating the SEM at 25 kV.

Spectrum matching, element matching, discriminant analysis and principal components analysis were used to classify the specimens on the basis of diet with 54-100% success. Spectra were considered as a whole or processed using either a digital filter to remove background or a ZAF program to compute semiquantitative elemental concentrations. Background corrected peaks is the favoured data format, having high discriminating power and being simple to obtain and interpret.

Better classification is apparent when subsets of the same sex, body part or both are employed; and when the data for each element is normalised to a mean of 0 and a standard deviation of 1. The chief discriminating elements appear to be F, Na, Mg, P, Cl, K and Ca with Si, S, Mn, Fe and Zn being of little assistance.

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