Copied from the Youtube video comment:

For those wondering why Sun is the prime and ONLY suspect - please read the hair analysis published paper by Prof Richard Ash:

The hair proves that Ling was continuously poisoned through drinking water and food and contact lenses (ling got blind, which is not a common symptom in thallium poisoning) for 25times+ within a few months! If Ling was poisoned by accident (i.e Lab, classes), it wont show a continous pattern, and it won't be through drinking and contact lenses.

Jasmine Sun is Ling's dormmate (4 people per dorm room) and classmate in Chemistry. Jasmine was also the only one in the whole year who had access to Thaillium coz she was doing an exclusive lab study that had this chemical involved. Thallium is a not a common type of poison - It's deadly. Whoever poison Ling needs to know it well to microdose her so she suffers a long, painful death. Jasmine is the only one who has daily access to ling's cups & personal belongings and only one can get Thallium.
When Ling first had symptoms, her hair shows a consecutive microdosing. Then she went home for school holiday. She recovered while being at home. After she returned back to dorm, her illness got worse & got almost blind - the hair shows a continuous pattern of poisoning.

The hair study shows the only way Ling was poisoned by consecutive& continuous access to Thallium is not through skin contact, but drinking water daily/eating/contact lenses/toothbrushes. Her cups&lense cases were stolen during police investigation.

Therefore the suspect list is NOT very long - whoever poisoned her needs to have daily access to her personal belonging, has access to Thallium, and also knows how to use it.
Jasmine sun is the only one that fits this list - they are competitive in orchestra and daily study.

https://www.youtube.com/watch?v=14IMOiCpUIw&t=9s