The Fragment Finder
provides a list of possible formulas for a given
fragment mass. However, many possible formulas are given. The isotopic peak ratios
can then be used to narrow the possibilities for the formula. This "Formula Finder"
application takes a different approach. From the input parent peak and
fragment masses all possible formulas are generated and formulas for
the parent peak that are inconsistent with the fragment formulas
are dropped. For example, a fragment may only have formulas with
one or more nitrogens, therefore the parent formula must have at least
one nitrogen also. For small molecules this procedure decreases the number of
possible formulas by 2030%, so you'll still get a list of 2030 formulas.
To narrow the possibilities further, there are several options that limit the
possible formulas:

Maximum number of double bonds and rings: If the trial formula is
C_{c}H_{h}O_{o}N_{n} then the number of double bonds
and rings is (2ch+n+2)/2. For benzene dbr=4.

Maximum number of nitrogens: Useful if you have information about the functional groups that are present
from other forms of spectroscopy.

List oddelectron molecular ions only: Almost all molecular ions are oddelectron radicals.

Odd electron fragments must correspond to the
loss of a stable neutral molecule
from an odd electron ion: Is useful especially if you are inputting many fragment
masses. Be careful, this option might cause the actual formula to be rejected!
The maximum number of double bonds and rings and the electron parity (odd/even)
are not used in the screening logic. The
maximum and parity are just used as a final filter before the formulas are printed.
Your parent peak mass is also tested for bad and unlikely losses.
If no formulas are printed, then the input parent peak is also
unlikely to be the molecular ion.