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:
12C: 12.000000
1H: 1.007825
16O: 15.994915
14N: 14.003074
We measured the protonated molecule, so the peak at
m/z
195.088 tells us that the
molecular weight of the compound must be
m/z
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 with
DART-MS 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 data.
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.