Yang Ping said, "What do you think this letter indicates?"

Zhaxi thought for a moment and said, "The person who wrote the letter knows something about Professor Zhou from thirty years ago, something that if exposed, would be very detrimental to him."

Yang Ping nodded, "Right, what is that matter?"

Zhaxi shook his head.

Yang Ping said, "Officer Yu found out that thirty years ago, Professor Zhou participated in a national research project named ’Development of a New Efficient Organic Phosphorous Pesticide.’ This project was part of the country’s ’Seventh Five-Year Plan’ initiatives, and Professor Zhou was one of the main participants. The project eventually did not succeed and was terminated midway. But according to insiders, the reason for its termination was not technical issues but because the toxicity of one of the compounds was too high, making it unsuitable for use as a pesticide."

He paused and looked at Zhaxi, "Moreover, the toxicity data of that compound was concealed by the project team. The experts responsible for toxicity evaluation deliberately underestimated the compound’s toxicity in their reports, which allowed the project to get approved. Later, when discovered, the project was halted."

Zhaxi’s mouth dropped open, "So, the threatening letter refers to this matter?"

Yang Ping said, "It’s possible! But as for the specific details, Officer Yu is still investigating. What is currently known is that the expert responsible for the toxicity evaluation back then was Professor Zhou himself."

Zhaxi’s mind buzzed, had Professor Zhou concealed the toxicity data? Would a scientist renowned for rigor do such a thing?

Yang Ping seemed to read his mind and said, "Don’t jump to conclusions. Thirty years ago, the context was different from now. The scientific management and ethical norms back then were not as strict as they are today. Moreover, it hasn’t been confirmed if this matter is true, but one thing is certain, someone is using this incident to threaten Professor Zhou."

Zhaxi thought for a moment and then asked a question he had been wanting to ask, "Professor Yang, who do you think the culprit is?"

Yang Ping looked at him and said, "I don’t know either. This matter shouldn’t have distracted you; it’s really a side issue, but it’s just as well. I think involving you fully can help you build curiosity, patience, and a self-driven determination to search for the truth."

"What you need to do now is not to guess who the culprit is but to do your current tasks well. Check the literature, find clues, establish an evidence chain. Take it step by step. Solving a case is like curing a disease, you can’t rush it."

Zhaxi nodded.

Leaving the office, Zhaxi stood in the corridor, took a deep breath.

He took out a notebook and wrote on the top of the latest page: Evidence Chain - Source, Route, Dosage of the Organic Phosphorus, Culprit. 𝘧𝓇𝑒𝑒𝑤ℯ𝑏𝓃𝘰𝑣ℯ𝘭.𝘤ℴ𝘮

Then he drew a big question mark beneath it.

Closing the notebook, he headed back to the dormitory, knowing there was much to do the next day.

On Sunday morning, Zhaxi was awakened by the vibration of his phone. Groggily reaching for it, he saw Professor Yang’s name on the screen; it was only six fifteen. Suddenly awake, he quickly answered, realizing he had gotten up a bit late today.

"Come to the Institute, the small meeting room. Officer Yu and the others are already there." Yang Ping’s voice was calm, but Zhaxi detected an unusual urgency.

This early?

Zhaxi took ten minutes to wash up and get dressed, jogging to the institute. The small meeting room was on the third floor, the door half-open, four people already inside: Yang Ping, Officer Yu, Dr. Yuan, and a middle-aged man Zhaxi hadn’t met before, wearing a dark blue jacket, hair slightly graying, expression serious.

"Zhaxi, take a seat." Yang Ping pointed to a chair beside him, "This is Director Zhang from the Provincial Hall’s Toxicology Laboratory. Last night, preliminary toxicological screening results came out. Director Zhang couldn’t wait to personally interpret them for us."

Zhaxi felt a tightness in his chest, toxicological screening results were out? He quickly sat down and pulled out his notebook.

In front of Director Zhang lay a thick report; his expression was heavy, as if weighing his words. He glanced at Yang Ping, then at Officer Yu, and began speaking in a low and slow voice, "Professor Yang, Captain Yu, in my more than twenty years in the police, this is the first time I’ve encountered a case like this."

He opened the report’s first page, pointing to the data, "We conducted a comprehensive toxicological screening of Professor Zhou’s blood, urine, stomach contents, and liver tissue. All standard toxins — organophosphates, carbamates, opioids, benzodiazepines, barbiturates, alcohol, cyanides, heavy metals — were negative."

Negative for all? Then how to explain the contracted pupils? The pigmentation on the fingertips? The organophosphate residue on the book pages?

Director Zhang continued, "However, we detected an unknown organic compound in the blood and liver tissue. Its chromatographic retention time and mass spectrometry fragmentation pattern are not in any of our databases. In other words, this is not a known common toxin."

Yang Ping asked, "Can its structure be determined?"

Director Zhang shook his head, "Not for now. We conducted high-resolution mass spectrometry, determining its molecular formula to be C₁₈H₂₀NO₄P, with a molecular weight of around 360. However, there are dozens of potential structures corresponding to this formula. To determine the specific one, NMR analysis is required. Moreover..." He paused, "To isolate enough pure sample for NMR, a large amount of sample is needed, and currently, the amount we have is far from sufficient."

Yang Ping’s eyes narrowed slightly, "C₁₈H₂₀NO₄P, is there anything special about this molecular formula?"

Director Zhang said, "We checked the Chemical Abstracts database; this formula has only appeared twice in the literature. Once in a 1987 Japanese patent, describing a new type of organophosphate pesticide intermediate; and another time in a 1992 internal domestic report titled..." He turned a page of the report and read out, "’Synthesis and Toxicological Study of the New Efficient Organophosphorus Pesticide A-8,’ credited to the Nandu University Chemistry Department and the Ministry of Chemical Industry’s Pesticide Research Institute. First author — Zhou Huaixin."