One study found that decaffeinated coffee contained 10 mg of caffeine per cup, compared to approximately 85 mg of caffeine per cup for regular coffee. Detection in body fluids
Caffeine can be quantified in blood, plasma, or serum to monitor therapy in neonates, confirm a diagnosis of poisoning, or facilitate a medicolegal death investigation. Plasma caffeine levels are usually in the range of 2–10 mg/L in coffee drinkers, 12–36 mg/L in neonates receiving treatment for apnea, and 40–400 mg/L in victims of acute overdosage. Urinary caffeine concentration is frequently measured in competitive sports programs, for which a level in excess of 15 mg/L is usually considered to represent abuse. Analogs
Some analog substances have been created which mimic caffeines properties with either function or structure or both. Of the latter group are the xanthines DMPX and 8-chlorotheophylline, which is an ingredient in dramamine. Members of a class of nitrogen substituted xanthines are often proposed as potential alternatives to caffeine. Many other xanthine analogues constituting the adenosine receptor antagonist class have also been elucidated.Some other caffeine analogs:

Dipropylcyclopentylxanthine
8-Cyclopentyl-1,3-dimethylxanthine
8-Phenyltheophylline

Precipitation of tannins
Caffeine, as do other alkaloids such as cinchonine, quinine or strychnine, precipitates polyphenols and tannins. This property can be used in a quantitation method. Natural occurrence
Around thirty plant species are known to contain caffeine. Common sources are the "beans" (seeds) of the two cultivated coffee plants, Coffea arabica and Coffea canephora (the quantity varies, but 1.3% is a typical value); and of the cocoa plant, Theobroma cacao; the leaves of the tea plant; and kola nuts.
During chronic caffeine use, caffeine-induced dopamine release within the nucleus accumbens core is markedly reduced due to drug tolerance. Enzyme targets
Caffeine, like other xanthines, also acts as a phosphodiesterase inhibitor. As a competitive nonselective phosphodiesterase inhibitor, caffeine raises intracellular cyclic AMP, activates protein kinase A, inhibits TNF-alpha and leukotriene synthesis, and reduces inflammation and innate immunity. Caffeine also affects the cholinergic system where it is a moderate inhibitor of the enzyme acetylcholinesterase. Pharmacokinetics
Caffeine from coffee or other beverages is absorbed by the small intestine within 45 minutes of ingestion and distributed throughout all bodily tissues. Peak blood concentration is reached within 1–2 hours. It is eliminated by first-order kinetics. Caffeine can also be absorbed rectally, evidenced by suppositories of ergotamine tartrate and caffeine (for the relief of migraine) and of chlorobutanol and caffeine (for the treatment of hyperemesis). However, rectal absorption is less efficient than oral:  the maximum concentration (Cmax) and total amount absorbed (AUC) are both about 30% (i.e., 1/3.5) of the oral amounts.Caffeines biological half-life – the time required for the body to eliminate one-half of a dose – varies widely among individuals according to factors such as pregnancy, other drugs, liver enzyme function level (needed for caffeine metabolism) and age. In healthy adults, caffeines half-life is between 3 and 7 hours. The half-life is decreased by 30-50% in adult male smokers, approximately doubled in women taking oral contraceptives, and prolonged in the last trimester of pregnancy.