We were excited to be allocated 6 grams of Chang’E 5 lunar soil for the search of organic matter. Our primary concern was terrestrial contamination, because it could lead to a failure to detect any lunar organics. A Class 1000 clean lab was necessary for our analysis, but quickly building such a clean lab was tough during the pandemic. Fortunately, Class 1000 clean booths were commonly used in local electronics manufacturing. So, we asked a local manufacturer to rapidly build such a booth for us, mainly composed of stainless steel and glass to prevent organic contamination. 

        The extremely high contamination-control standard was the most special thing for Li Tengfei (李腾飞), who carried out the analysis with optical microscopy and Raman spectroscopy, and for Liao Yuhong (廖玉宏) and Wang Fu (王甫), who carried out the analysis with pyrolysis-gas chromatography-mass spectrometry. This kind of requirement was not met in their previous work. They tried their best to prevent contamination and found no organic matter, but this also brought them relief because it meant that their samples were free from contamination. Their excellent strategies of contamination control are absolutely significant for future search of organic matter in other space return samples. We were quite worried about contamination of amino acids, since sources of contamination were so common. Linbo Yu (于琳波) set up a very sensitive LC-MSMS method for amino acids measurement and found levels basically below detection limit (~1 ppb or lower). 

       Gan Zhang (张干) proposed a new idea: using the benzene polycarboxylic acid (BPCA) molecular marker method, which had not been used before in lunar sample studies, to probe polycyclic aromatic organic matter. The geochemistry of polycyclic aromatics is recently one of his research interests, with the BPCA method serving as a key tool. The polycyclic aromatics are of environmental significance. Gan Zhang currently leads two NSFC funded projects targeting polycyclic aromatics in terrestrial environment (42192511 and 42030715). He has been interested in polycyclic aromatics in carbonaceous chondrites and in space too. So, he was very excited when the Chang'E 5 lunar soil sample is available for the research community. He hoped that the use of the BPCA method would lead to new findings. 

        Approaching the Chinese New Year, Lele Tian (田乐乐) set up a more sensitive LC-MSMS method to replace our LC-PAD method for BPCA measurement. Soon after the holiday, Yangzhi Mo (莫扬之) worked alongside me during the month-long BPCA analysis marathon, making  comprehensive video logs and double checking the experimental protocol to prevent any mistakes that could lead to a waste of the priceless lunar soil sample. Though not confident to find polycyclic aromatics, I deemed it crucial to conduct the experiment with high quality. Finally, we were excited to find surprisingly high concentrations of polycyclic aromatics in the Chang’E 5 lunar soil. I cooled down over the next few days, since contamination had yet to be ruled out. 

        We started to measure stable carbon isotope composition (δ¹³C) of BPCA to rule out contamination. At the beginning, our team had well-established a method of using preparative liquid chromatography for offline BPCA purification, followed by void-column LC-IRMS detection of BPCA δ¹³C. Later, we realized that the obviously lower BPCA amounts in the lunar soil compared to terrestrial samples posed new challenges. After a lot of trials with Shutao Gao (高淑涛), our LC-IRMS expert, we found that a hybrid of the offline method (higher separation power) and online method (lower contamination) was needed. However, a common problem with online LC-IRMS methods is that the oxidation chamber tends to block. During the process, one of our oxidation chamber was completely blocked and had to be discarded. We were lucky to have a backup and managed to complete all sample analyses before the oxidation chamber was totally blocked. Terrestrial contamination has been ruled out due to the δ¹³C results, which is critical in the search for extraterrestrial organic matter.

         We aimed to identify terrestrial analogs with similar aromatic structures to the polycyclic aromatics in the lunar soil, but the results were contrary to our expectations. Liangliang Wu (吴亮亮), Yunpeng Wang (王云鹏), and Yingjun Chen (陈颖军) meticulously prepared kerogen and coal samples. Xin Yi (怡欣) analyzed BPCAs of all these terrestrial analogs and the BPCA compositions unveiled the unique aromatic structures of lunar-soil polycyclic aromatics.

         Actually, all our team members once worked in the former State Key Laboratory of Organic Geochemistry (SKLOG, 1989-2024), Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. The SKLOG was focused on two fields, including petroleum and natural gas geochemistry, as well as environmental organic geochemistry. Our success stems from the collective expertise of an extraordinary team.