In this post, I explain, step-by-step how ambient ionization with DART-MS forms positive ions by reactions of excited-state helium with atmospheric water.
The first step in DART-MS involves the transfer of energy from highly energetic excited-state atoms to neutral molecules to form a positive ion and an electron. This is called Penning ionization after a short article published by F. W. Penning in 1927. A simplified DART-MS mechanism is represented in the following reaction:
He* refers to a long-lived excited-state (“Metastable”) atom, H
2O denotes a water molecule,
H2O+• denotes a positively charged water ion and
e— denotes an electron with its negative charge. The exact mechanism may be more complex -- for example, nitrogen in the air may play a role in reactions that ionize water molecules. In any case, we can see the result of the next step.
Charged water clusters
The water ions formed in the previous step react with neutral water molecules to produce hydronium ions (H3O+) and protonated water clusters [(H2O)n + H]+•.
Protonated water clusters and trace background ions in positive-ion DART
The sample is ionized
In the last step, the protonated water clusters transfer a proton to the sample:
This will happen if the sample must have a higher proton affinity than water. This is generally true for basic molecules (for example, drugs like caffeine and cocaine) and for many samples with a heteroatom (O, S, N, P) or site of unsaturation (multiple bonds). This reaction won’t happen for very nonpolar molecules (for example, saturated hydrocarbons) and it isn’t the most favorable reaction for molecules like carboxylic acids and phenols. The figure below shows an AccuTOF-DART
positive-ion mass spectrum from the NIST DART Forensic Database
Positive-ion DART mass spectrum of caffeine measured on the JEOL AccuTOF-DART
The simplest mechanism for forming positive ions with DART involves reactions of charged water clusters with the sample to produce a protonated molecule [S + H]+. In future posts, I’ll describe other reactions that can happen, but the next series of articles will discuss the information we can obtain from the accurate-mass data and isotope information in the DART-MS positive-ion mass spectrum. To learn more about JEOL mass spectrometers and the AccuTOF-DART system, please visit us here.