Activate the evidence scanner and examine thoroughly the area marked by the game. You will find the first trace right next to the drain grating in the floor.
That is not the end yet. Just like in the case of the previous crime scenes analyses, you now need to start using the keys/triggers in order to manipulate the process of replaying of the reconstruction of events. Follow the red line that symbolizes Copperhead's path, and which will take you to the poison sample shown in the screenshot. Scan the sample and listen to the conversation with Alfred that concerns the working out of the antidote.