This may be a really stupid question, and I have seen it done both ways. I am fairly new to mass spectrometry-based quantitation. I am trying to quantitate certain antibiotics (i.e. piperacillin sodium, tobramycin sulfate) utilizing LC-MS/MS. Do you have to calculate the active ingredient (i.e. piperacillin component) prior to analysis to determine the actual concentration? My colleague says no. Some papers do it, some don't. It has left me thoroughly confused. Any guidance would be appreciated.