A key question in determining a method for heavy metal analysis is the technique you have available for the analysis - atomic absorption spectroscopy (AAS) vs. ICP. Another key point is the trace metals that you are expecting to find in your sample and their expected concentration. Basically, you will have to digest the samples to solubilize the trace metals, typically in an acid solution, before introducing the sample for analysis into the spectrometer. You will need to develop a series of standards to get meaningful and reliable concentration data from you samples. Again depending on the elements you expect to find and their concentration, you can adapt a number of published methods for heavy metal analysis in insects or other biological material to use on your samples - there is nothing special here about using Odonata. The following reference from Finland using AAS may help you get started.
Nummelin, M., M. Lodenius, E. Tulisalo, H. Hirvonen & T. Alanko. 2007. Predatory insects as bioindicators of heavy metal pollution. Environmental Pollution 145: 339-347.
Abstract
Heavy metal concentrations of different predatory insects were studied near by a steel factory and from control sites. Waterstriders (Gerridae), dragon fly larvae (Odonata), antlion larvae (Myrmeleontidae) and ants (Formicidae) were analyzed by AAS. In most cases the metal concentrations were higher near the factory, but e.g. waterstriders had higher cadmium concentrations in control area. Discriminant analysis clearly reveals that all these insect groups can be used as heavy metal indicators. However, the commonly used ants were the least effective in indicating the differences between the factory and control sites. Waterstriders are good in detecting differences in iron and manganese, but seem to be poor in accumulating nickel and lead. Antlions are efficient in detecting differences in iron. Antlions and ants are effective in accumulating manganese; as well antlions are efficient in accumulating cadmium. Waterstriders are poor in accumulating lead, but antlions and ants are effective.