If we have accurate mass data, DART-MS can tell us more than the molecular weight of a molecule. Let’s see how to determine the elemental composition from accurate-mass DART-MS data.
Note: The next few articles focus on the interpretation of accurate-mass
spectra. Although I am using a DART-MS
example, the information here applies to any data from an accurate-mass
mass spectrometer, such as the JEOL AccuTOF GC-Alpha
. We’ll come back to DART-MS
Elemental composition determination from an accurate mass
The first step in determining an elemental composition from an accurate mass is to specify a set of elements that might be present and set some limits on the minimum and maximum number of each element that might be present. We’ll use the caffeine example from the previous posts. Let’s assume that we know that we don’t know that the compound is caffeine, but that the compound only contains carbon, hydrogen, nitrogen and oxygen. Because accurate mass measurements are not always exactly correct, let us also assume that the measured m/z is 195.0899 instead of the exact calculated value of 195.0882. We can set the element limits based on the maximum number of each element that could give a molecular weight less than 195.
Next, we specify an error tolerance of 5 millimass units (5 mmu) for the accurate mass measurement. That’s 0.005 u, a bit wider than the error we expect, but wide enough that we won’t miss any possible compositions. Software then calculates the weight (actually, the m/z) for all possible combinations of the elements within the specified limits and only reports those elemental compositions that have a calculated m/z within 5 mmu. There are nine compositions that fit those constraints.
||C4 H7 N10
||C5 H13 O5 N3
||C6 H9 O1 N7
||C7 H15 O6
||C8 H11 O2 N4
||C10 H13 O3 N1
||C13 H11 N2
||H9 O2 N11
Here’s a summary of the process:
What does DBE mean, and how do we know that the correct
elemental composition for the
195.0899 peak is C8H11N4O2 instead of one of the other 8 compositions? Stay tuned for the next piece in this series!
information obtained with DART-MS
can be used with to determine the elemental composition
of molecules. You can read more about how elemental compositions
are determined from accurate-mass
. To learn more about JEOL mass spectrometers
and the AccuTOF-DART
system, please visit us here