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The recent first-ever detections of the light elements comma lithium and beryllium comma in planetary bodies accreted by .polluted. white dwarf stars comma has prompted varied explanations regarding their origin. A remarkable aspect of the observations is that the abundance ratios of Li and Be relative to major elements are significantly overabundant -- for Be the overabundance is two orders of magnitude. On the one hand comma these elements are dosimeters for radiation environments and may be giving us a measure of the degree of spallation production that has occurred while the exoplanetary bodies were forming. On the other hand comma chemical processes such as igneous differentiation (i.e. crust formation) could be responsible. The last member of the set of light elements comma boron comma has never yet been observed in white dwarfs. We propose to conduct observations of the most promising WD that would be expected to display boron comma and we can use the relative abundances of B and Be to discern between the possible formation scenarios. Furthermore comma the proposed observations are expected to easily measure the volatiles comma C and N comma in our target stars comma and provide important constraints on the planetary formation scenarios being tested. Only HST-COS can provide the required UV spectroscopy to observe B comma C comma and N comma which are critical for this science.