Oral 2mg diazepam, half-life 20 h, bioavailability 90%, what is the concentration in saliva after 14 h?
knowing that the saliva drug concentration is 0.1 of the plasma concentration.
The initial distribution phase has a half-life of approximately 1 hour, although it may range up to >3 hours. (...) The initial distribution phase is followed by a prolonged terminal elimination phase (half-life up to 48 hours). The terminal elimination half-life of the active metabolite N-desmethyldiazepam is up to 100 hours. (...) Elimination half-life increases by approximately 1 hour for each year of age beginning with a half-life of 20 hours at 20 years of age.
https://www.accessdata.fda.gov/drugsatfda_docs/label/2016/013263s094lbl.pdf
As you know situation is quite dynamic and complicated ....
Tmax = 1.25h + initial elimination half-life =1h next phase half life= 3h and elimination/terminal =20h (if 20 years old patient).
I think this paper (which covers an oral dose of 5 mg) shows PK profile could be useful.
https://pubmed.ncbi.nlm.nih.gov/6849499/#:~:text=Absolute%20bioavailability%20averaged%2094%25%2C%20indicating,sex%20significantly%20influenced%20oral%20absorption.
From my raw estimation 14 h after a single dose of 2 mg per os in saliva it should be around 1 ng/mL but it's not based on an exact analysis of raw data please double-check....
I hope it helps in your investigation ....
Tomasz