Dear Sir. Concerning your issue about the extraction method for coffee solution . I think the following below links may help you in your analysis:

https://link.springer.com/content/pdf/10.1007%2Fs00217-013-1917-x.pdf

http://www.hkasme.org/News/818/Report_6B_Christiam%2520Alliance%2520SC%2520Chan%2520Memorial%2520Secondary%2520School_Chemical_test_for_caffeine.pdf

Thanks

I suggest a simple method: head-space sampling. You can measure many analytes without extraction. Advantage is, if you have some internal standard. But it ´s not necessary.

extract with methanol using ultrasound