The first serious experimental attempts to make biogenic or prebiotic monomers by adding energy to simple gases were the spark discharge experiments that Stanley L. Miller carried out in Harold C. Urey’s laboratory at the University of Chicago. The original experiment consisted of running steam containing a simple gas mixture of hydrogen, ammonia and methane past electrodes supporting a corona spark discharge from an induction coil, then through a steam-condensing loop before emptying back into the boiling flask. The scale of the apparatus was modest; the steam was produced by boiling just 200 ml of water in a 5 l flask and after evacuating air, the apparatus was charged with 10 cm pressure of hydrogen, 20 cm of methane, and 20 cm of ammonia. The boiling and electrical discharge continued for a week;
. . . the water in the flask became noticeably pink after the first day, and by the end of the week the solution was deep red and turbid. Most of the turbidity was due to colloidal silica from the glass. The red color is due to organic compounds adsorbed on the silica. Also present are yellow organic compounds, of which only a small fraction can be extracted with ether . . .
The conditions used by Stanley Miller were based on what were then assumed to be realistic conditions on the ancient Earth (Urey, 1952); specifically, that the early Earth would have had a moist and chemically reducing atmosphere of hydrogen, ammonia and methane with a warm liquid ocean (represented by the water in the boiler) and frequent lightning discharges (represented by the spark discharge). For the time the outcome was amazing, for when Miller analysed the solution by two-dimensional paper chromatography run first in butanol/acetic acid/water followed by water-saturated phenol, a ninhydrin spray (the standard way of detecting amino acids) revealed:
. . . glycine, α-alanine and β-alanine are identified. The identification of the aspartic acid and α-amino-n-butyric acid is less certain because the spots are quite weak. The spots marked A and B are unidentified as yet, but may be β- and γ-amino acids. These are the main amino acids present, and others are undoubtedly present but in smaller amounts. It is estimated that the total yield of amino acids was in the milligram range . . .