Trial and Error Analysis
This page aims to explain the basis for the determination of molecular composition based on just the molecular mass. Althouthe pathgh there is no guarantee that a particular molecular mass will be associated with only one composition, in fact, that is what we found. This doesn't mean that the proposed structure is correct as isomers wouldn't be eliminated. The second set of restrictions is based on the last 50 years of investigation of steroid enzyme biochemistry.
Today, the pharmacologist came into laboratory with a bottle of a clear fluid. He wanted to know if the alcohol was safe to use for hand washing. I turned on the mass spectrometer and measured the molecular mass. If the m/z= 46 Da, it is either ethyl alcohol (CH3-CH2-OH) or diethyl ether (CH3-O-CH3). The table on the left tests all possible molecular combinations that generate a mass of 46 Da. On the right is the table for 60 Da.
Trials and error analysis of molecules with m/z= 46 Da and mz= 60 Da.
Columns 2 and 3 note the number of carbon atoms and the number of oxygen atoms in the molecule on the specific line. The next column (headed C&O) sums the contribution to molecule mass contributed by the carbon and oxygen atoms in column 2 and 3. The next column notes the mass desired. Hreq is the difference between the C&O and proposed mass desired. Hmax is the maximum number of hydrogens. It is 2n+2 where n is the number of carbon atoms. Finally, Delta is the number of rings, and double bonds in the molecule. Each Delta reduces the number of H atoms needed by two hydrogens. In these two examples, no Delta are needed to complete either molecule. The molecular composition is described on line 4 for m/z=46 Da and on line 6 for m/z=60 Da.
For each mass, the composition is known but there are multiple isomers possible. For m/z=46 Da, ethyl alchohol and diethyl ether would be consistent with the mass. For m/z= 60 Da, n-propanol, isopropanol, and methyl ethyl ether would all be consistent with the mass. Other data would be needed to determine which colorless alcohol was actually in the bottle.
Historically, in 1998, Dr. Bradlow and I both knew the molecular masses of C313 (m/z=496 Da) and C341 (m/z=524 Da) but we had not understood that the molecular compositions could have identified just from data.