Brain Chemistry

Hi, Could you tell me how I can get an answer to my question?
Thank You, Marie

I don't know if there is such a thing.

are you talking about the detection of chemical doses of toxins in the brain? That is usually done by MAss Spectrometry and specialized Gas Chromatography.

You don't say which chemicals you are interested in!
A Google search using 'brain chemistry' as your search term will yield many web sites which contain a lot of information about normal brain chemicals called neurotransmitters, and how imbalances in them can cause depression or mania or bipolar symptoms.
As far as I have been able to discover there are no techniques to determine amounts of each of the neurotransmitters a person may have at any time.
Rather, it is by the use of some simple questions that a determination of the overall state of a persons mental health is found. Relevant antidepressants or other psychiatrical treatments are then used in an attempt to bring the neurotransmitters back into balance (whatever has caused the imbalance).