HP Data Explorer 4 Series - Manual (Page 391)

Isotopes
Data Explorer ™ Software User’s Guide B-3
B
As the number of carbon atoms in a compound increases, the
possibility of the compound containing a
13 C instead of a 12 C
also increases. A compound with ten carbon atoms includes a
molecular ion M
+ and an isotopic ion (M+1) + one mass unit
greater than the molecular ion, which is approximately
11 percent of the abundance of the molecular ion. The
possibility of including two
13 C atoms in the same molecule
also increases with increasing number of carbon atoms.
Therefore, (M+2)
+ ions become more visible. In a compound
with ten carbon atoms such as decahydro naphthalene,
C
10
H
18
(Figure B-2), relative heights of M + , (M+1) + , and (M+2) +
peaks are 100:11:0.5.
Figure B-2 Mass Spectrum of Decahydro Naphthalene
Isotopic pattern
in mass spectra
All compounds containing carbon include molecular ions and
isotopic ions that are 1 and 2 mass units higher than the
molecular ion. At higher masses, the isotopic pattern of a
mass spectrum is more pronounced as the relative abundance
of isotopes increases. In angiotensin I (Figure B-3) with a
molecular weight of 1,296 Da, (C
62
H
89
O
14
N
17
), a peak
containing one 13 C isotope is approximately 70 percent of the
pure 12 C peak.
M +
(M+1) +
(M+2) +