Hi,

HPLC is indeed an expensive technique if you do not already have the machine! Regarding chromatography, you could use TLC for your caffein rich solution. It is quite cheap, you can do it in normal phase (silica) with an adapted solvent combination. You can use several detection to visualize caffein, like iodine reagent, or dragendorff or Mayer reagent... These are not specific reagent for caffein, but for alkaloids and/or aromatic compounds. If you need quantification for you caffein, I would suggest HPTLC, but it is not really cheap since a specific machine is needed. The major problem with these techniques is that you cannot identify your compound for sure, even if you have a commercial standard of caffein, all of it depends of the accuracy you want.

Thank you both Nicolas and M. Farooq. 

M. Farooq, I did't express myself correctly, I apologize about that. I want to measure the exact concentration of caffeine in saliva after it's oral ingestion. Spot tests are only to "detect" caffeine presence, true? 

Nicolas, thanks again. So, no cheap options here. I need to quantify caffeine.