Yang Ping asked, "What experiment does he need to use toluene for?"

Li Xiong shook his head, "He didn’t say, he just mentioned he needed it, so I helped him buy it. He didn’t allow me to ask about the specifics of the experiment, and I didn’t ask further."

Yang Ping asked again, "After using the two bottles of toluene, where are the bottles?"

Li Xiong thought for a moment and said, "They should be in the drawer of his laboratory bench; he has a habit of keeping empty bottles after use, saying they might be useful later."

Yang Ping walked to the lab bench and opened the drawer. Inside were several empty reagent bottles, all brown glass vessels of varying sizes. Yang Ping took them out one by one and placed them on the surface. Zhaxi leaned over to look; the labels read: Toluene, analytical grade, 500ml, batch number... There were two bottles in total, both toluene.

Yang Ping picked up one of the bottles, unscrewed the cap, and sniffed it. Then he handed the bottle to Zhaxi, "Have a smell."

Zhaxi took it, brought it close to his nose, and a faint organic solvent scent wafted out. He tightened the cap and looked at Yang Ping.

Yang Ping squatted again and opened the second drawer. This drawer contained several old lab notebooks, the covers faded yellow. He took out one and opened the first page. It bore Professor Zhou’s handwriting, neat characters with dates and experiment details. Zhaxi leaned over to look and found the contents were records of organic synthesis experiments, including reaction conditions, material quantities, and product handling, all written in great detail.

Yang Ping flipped through page by page. He stopped at a location roughly half a year back.

On that page was written:

"Experiment objective: Synthesize a new chiral organophosphorus ligand. Reference: Self-designed. Experiment steps: Omitted (see attachment)."

Yang Ping furrowed his brow, flipped to the next page. It was blank. He flipped past a few more pages, still blank.

"Where’s the attachment?" he asked.

Li Xiong leaned over to take a look and shook his head, "I don’t know. Professor Zhou’s lab notebooks are managed by himself. These notebooks were left in the drawer by him; I don’t know what else could be attached."

Yang Ping placed the lab notebook back into the drawer and stood up. He walked over to the bookshelf and glanced at the books. Then he turned around to Dr. Yuan and Officer Yu, saying, "These books, and that bookshelf, all need residue testing. Just like those books at home, organophosphorus, toluene, xylene."

Dr. Yuan nodded, "I’ll arrange for it immediately."

When they walked out of the lab building, it was already noon. The sunlight was intense, glaring white off the floor. Yang Ping stood silently under the sycamore tree in front of the building. Zhaxi stood next to him, not daring to speak.

After quite a while, Yang Ping suddenly said, "Zhaxi, what’s your perspective?"

Zhaxi thought for a moment and said, "The books in the lab are very suspicious. If someone tampered with those books, Professor Zhou would also be exposed to toxins in the lab. Furthermore, Li Xiong mentioned Professor Zhou recently asked him to buy two bottles of toluene; toluene just so happens to be a solvent for organophosphorus."

Yang Ping nodded, "Anything else?"

Zhaxi continued, "The lab notebook mentioned creating a ’new chiral organophosphorus ligand’. If Professor Zhou truly synthesized some new organophosphorus compound, it might not be within the scope of regular toxicology screenings. This would explain why we still haven’t gotten toxicology results."

Yang Ping glanced at him with a hint of approval in his eyes, "Not bad, anything else?"

Zhaxi pondered further and then said, "Chen Wei, he’s Professor Zhou’s student, working on organophosphorus compounds, and he has conflicts with Professor Zhou. He has motive, and the capability."

"Officer Yu, what’s your take?"

Yang Ping asked, leaving the final case-solving to the professionals.

Officer Yu thought briefly, "We have motive and capability, now we need evidence! To be honest, these are the cases we fear most, they are highly technical, but equally, they are the ones we fear the least."

Zhaxi found it difficult to grasp Officer Yu’s contradictory statement.

Officer Yu smiled and explained, "We’re afraid because when a case involves something beyond our forensic expertise, we become passive. However, such cases inherently narrow down the pool of perpetrators; ordinary people can’t employ such high-tech methods."

Dr. Yuan added, "I think the hardest part now isn’t finding suspects; it’s finding evidence that leaves the suspect unable to deny. Right now, all we have are indirect clues. Chen Wei has the motive, his company deals with organophosphorus products. But none of these directly prove he killed Professor Zhou. What we need is that bottle of organophosphorus compound, or the solution applied to the books, or evidence tying him directly to this event."

"Thank you, Professor Yang, seems we also need to strengthen our learning; the knowledge in our brains is truly inadequate now." Officer Yu shook hands with Yang Ping.

Yang Ping smiled and said, "We’re just lending a small hand."