Heavy Neighbors Isotope Abundance in Mass Spectra Quiz

Carbon 13 has a natural abundance of approximately 1.1 percent. For every 100 molecules containing Carbon 12, there will be roughly one molecule containing a Carbon 13 atom, resulting in a peak one mass unit higher than the molecular ion.

Bromine has two naturally occurring isotopes, Br-79 and Br-81, which exist in an almost 1 to 1 ratio. This creates a distinctive "doublet" where the M and M plus 2 peaks are nearly the same height.

Chlorine 35 is about three times more abundant than Chlorine 37. Therefore, a molecule containing one chlorine atom will show an M plus 2 peak that is exactly one-third the height of the M peak.

As the number of carbon atoms increases, the probability that at least one of those atoms is a Carbon 13 isotope increases. For a molecule with 10 carbons, the M plus 1 peak will be approximately 11 percent of the height of the M peak.

Chlorine and Bromine have prominent isotopes two mass units apart. Fluorine is monoisotopic (only F-19 exists), and the common isotopes of oxygen are dominated by O-16, with O-18 being very rare (0.2 percent).

The 1 to 1 ratio is the characteristic fingerprint for a single Bromine atom. This allows chemists to instantly identify the presence of this halogen in a mass spectrum without further chemical testing.

When two chlorines are present, the combinations of Cl-35/35, Cl-35/37, and Cl-37/37 result in a triplet of peaks at M, M plus 2, and M plus 4 in a specific 9:6:1 height ratio.

Sulfur-34 has a natural abundance of about 4.4 percent. This produces an M plus 2 peak that is noticeably taller than the standard M plus 1 peak, helping to differentiate sulfur-containing compounds from pure hydrocarbons.

Fluorine is a monoisotopic element, meaning 100 percent of naturally occurring fluorine is Fluorine-19. Unlike Carbon or Nitrogen, it does not have a heavier isotope to produce an M plus 1 signal.

The number of carbons can be estimated by the ratio of M plus 1 to M. The specific height ratios of M plus 2 identify halogens. Hydrogen atoms contribute very little to M plus 1 because the abundance of Deuterium is extremely low (0.015 percent).

By measuring the exact mass of the molecular ion (e.g., 28.0313 for C2H4 vs 28.0061 for N2), scientists can distinguish between molecules that have the same nominal mass but different elemental compositions.

Like Fluorine, Iodine is monoisotopic. It exists only as I-127. While its high mass is easy to see, it does not produce the characteristic isotope clusters associated with Chlorine or Bromine.

Since each carbon atom contributes roughly 1.1 percent to the M plus 1 peak, you divide the relative abundance (4.4) by 1.1, which equals 4 carbon atoms.

The intensity is a direct mathematical result of how common the isotope is in nature and how many chances there are for that isotope to be present in a single molecule. Stability and ionization energy affect the overall peak height but not the relative isotope ratios.

While Oxygen 17 contributes to M plus 1, Oxygen 18 is the isotope located two mass units above the primary Oxygen 16. Its low abundance (0.2 percent) makes the M plus 2 peak very small unless multiple oxygen atoms are present.