Collision Cross Sections of Phosphoric Acid Cluster Anions in Helium Measured by Drift Tube Ion Mobility Mass Spectrometry.

In the last years, ion mobility mass spectrometry (IMS-MS) has improved structural analysis and compound identification by giving access to the collision cross section (CCS). An increasingly wide and accurate database of CCS values is now available but often without assessment of the influence of different instrumental settings on CCS values. Here, we present 75 CCS values in helium (DTCCSHe) for phosphoric acid cluster anions [(H3PO4)n - zH]z- with charge state (z) up to 4-. The CCS values, noted DTCCSHe, were obtained with a commercial drift tube ion mobility mass spectrometer, in helium, by applying a classic multifield approach. Phosphoric acid clusters are fragile structures that allow to evaluate the effect of different experimental conditions on the retention of weak bonds and their effect on CCS values. We probed harsh and soft voltage gradients in the electrospray (ESI) source before the IMS and two different voltage gradients in the post-IMS region. The variations in the ion mobility and mass spectra consisted in a change in the distribution of the cluster anions aggregation numbers (n) and charge states (z), with a higher amount of multiply charged species for the soft pre-IMS voltage gradient and a lower proportion of cluster dissociation for soft post-IMS conditions. However, the CCS values did not change with experimental conditions for a given cluster, as long as it stays intact from the IMS to the mass analyzer. The DTCCSHe were found in good agreement among 3 to 10 replicated values, with a relative standard deviation between 0.1 and 1.7%.