Previously, we found 4 possible elemental compositions for even-electron ions in our example of a DART-MS spectrum with a peak at m/z 195.0899. If we have a time-of-flight mass spectrometer such as the AccuTOF-DART that can provide accurate isotope measurements, we can use that information to determine the most likely elemental composition.
The elements have characteristic stable isotope patterns. Elements such as carbon, hydrogen, oxygen, and nitrogen have more than one stable isotope. For example, on earth, the isotope carbon-13 (13C) has an average relative abundance of 1.082, normalized to a relative abundance of 100% for 12C. Elements such as fluorine and phosphorus have only one stable isotope. A table of the average relative abundances of the isotopes for these elements on earth is available from the International Union of pure and Applied Chemistry (IUPAC). Using this information, software can calculate the exact masses and relative abundances for any given elemental composition.
Our example DART-MS positive-ion mass spectrum showed the most abundant peak (“base peak”) at m/z 195.0899, but there are also two smaller isotope peaks detected. The isotope measurements are shown below with measured m/z and relative abundances shown below.
|m/z||Rel. abundance (%)|
We can let our elemental composition software calculate the average relative abundances for each of the four possible compositions we determined for our m/z 195.0899 peak and compare the calculated values to the measured values. Now we can rank the elemental compositions based on how well the measured isotope m/z values and relative abundances match the calculated values. Here are the results and the constraints specified for elemental composition determination:
Element Limits: C 0/16 H 0/34 O 0/12 N 0/14
Tolerance: 5 mmu
Even or odd electron ion or both: Even
Electron correction: None
Minimum unsaturation: -1
Maximum unsaturation: 100
The score multiplies the relative abundance match and the accurate-mass (m/z) match and divides by the number of matching isotopes detected. A smaller score denotes a better match, with a perfect score being zero. The NIST column shows the number of entries in the selected NIST-formatted database that have that elemental composition. The correct composition for protonated caffeine C8H11N4O2 has the best matching score, and it is the only one of the four compositions that has matches compounds in the NIST 20 Mass Spectral Database.
Here is a labeled screen shot from the Mass Mountaineer
program of the complete elemental composition
determination including the measured mass spectrum, limits and constraints, results, and the matching isotope measurement
data for the selected (best matching and correct) elemental composition
. The calculated isotope pattern is shown in red in the isotope match tile, and the measured isotope pattern is shown in blue with a slight offset to make it possible to compare the calculated and measured relative abundances of each peak.
Now we have determined the most likely elemental composition
for our “unknown” compound from its positive-ion DART-MS
spectrum. However, we still have a problem! There are 16 isomers in the NIST 20 Mass Spectral Database
. Is there anything we can do to increase our confidence that this is caffeine? Yes, there is! In the next series of articles, I’ll be discussing the role of collision-induced dissociation and fragment ions.
If you have a time-of-flight mass spectrometer
that measures accurate masses
and provides accurate isotope measurements
, you can compare the calculated isotope patterns
for candidate elemental compositions
with the measured isotope patterns
for a chemical compound. You can read more about determining elemental compositions here
. A free Periodic Table app
that lists the m/z
and average relative abundance data for stable isotopes is available for iPhone and Android users. A free MS Calculator app
that can calculate isotope patterns for elemental compositions is available for iPhone users. To learn more about JEOL mass spectrometers
and the AccuTOF-DART
system, please visit us here
1There are slight variations in the isotopic abundances for molecules depending on factors like geographic origin. Measuring these slight variations with high precision and accuracy requires special isotope-ratio mass spectrometers. For the purpose of determining elemental compositions, we can ignore those slight variations and just use the average relative abundances.