One aspect of the present invention relates to a method of preparing radiofluorinated substituted alkyl, cycloalkyl, aryl, and alkenyl compounds. In a preferred embodiment, potassium fluoride-18 is used. Another aspect of the invention relates to piperazine compounds containing fluorine-18 that are useful as imaging agents. In certain embodiments, the piperazine compounds contain a quaternary amine. Another aspect of the invention relates to arylphosphonium compounds containing fluorine-18 that are useful as imaging agents. In certain embodiments, the phosphonium compound is a tetraaryl phosphonium salt. Another aspect of the present invention relates to a method of obtaining a positron emission image of a mammal, comprising the steps of administering to a mammal a compound of the invention, and acquiring a positron emission spectrum of the mammal.