It sounds like you're describing a (ribo)nuclease protection assay. I
found the following ref's to be excellent starting points for learning this
Kedzierski and Porter (1990) Mol. Brain Res. 7:45-51
Kedzeirski and Porter (1991) Biotechniques 10:210-214
My experience was consistent with assertations that good results with this
technique *require* empirical optimization.