This post describes the first piece of information we get from ambient ionization and DART-MS: the molecular weight.
How DART examines molecular weight
In the previous blog, I explained how positive-ion
DART-MS can form protonated molecules [M + H]
+ for compounds like caffeine. Here is the DART-MS caffeine mass spectrum again, with the caffeine structure and elemental composition:
Positive-ion DART mass spectrum of caffeine measured on the JEOL AccuTOF-DART system
Mass spectrometers measure the mass-to-charge ratio (symbol “m/z”). DART-MS only produces single-charge ions, so for the purpose of this discussion we can ignore the charge.
The exact masses in unified atomic mass units (u) of the most abundant stable isotopes of the elements carbon, hydrogen, oxygen, and nitrogen are:
We measured the protonated molecule, so the peak at
195.088 tells us that the
molecular weight of the compound must be
195.088 minus the mass of the proton (1.0078), or 194.080. Caffeine has the elemental composition C8H10N4O2, so the
molecular weight of caffeine is 194.080, which matches what we measured.
If we only have a low-resolution mass spectrometer (such as a quadrupole MS), we will only be able to measure the molecular weight to about one decimal place, but the JEOL AccuTOF-DART measures accurate masses with errors in the 3rd or 4th decimal place. Accurate molecular weight determination is important information for synthetic chemists.
If we want to identify an unknown compound, the molecular weight
is useful information, but not enough to uniquely identify a molecule. Knowing the accurate mass
is considerably more selective than just having an integer mass. If we search the NIST 20 mass spectral database
for compounds with an integer molecular weight
of 194, we find 1277 compounds. Limiting the search to 194.080 with an error tolerance of +/- 0.001 u returns 37 compounds.
Fortunately, an accurate-mass
system like the AccuTOF-DART
provides more information than just the molecular weight
. In the next few articles, we’ll see how to determine the elemental composition from accurate-mass DART-MS
can be used to determine the molecular weight
of molecules. To learn more about JEOL mass spectrometers
and the AccuTOF-DART
system, please visit us here