Alkaloids are organic bases which contain polar groups but whose molecules consist, in large part, of an electrically neutral arrangement of atoms. In aqueous solutions of the free base the molecules are predominantly in a non-ionized state, and in general it is in this state that the compounds are most toxic to animals.
Notwithstanding some contradictions, the published information on the action of alkaloid molecules and ions indicates rather strongly that their characteristic anesthetic and toxic effects on normal animals, tissues and cells are more pronounced when the organisms or tissues are immersed in aqueous solutions of the free bases rather than in solutions of the salts. It appears that the undissociated alkaloid molecules readily pass through intact dermal structures and cell membranes, while under the same conditions the ions penetrate slowly if at all.
The literature reveals few studies concerned with the actual quantities of toxic compound that enter the organism with time, and apparently no such studies have been made with nicotine. A method recently developed for the detection and micro-assay of nicotine in insect tissue (Glover and Richardson, '36) afforded an excellent opportunity to investigate this