Collision cross sections of negative cluster ions of phosphoric acid in N2 determined by drift tube ion mobility and their use in travelling wave ion mobility

Abstract

Travelling wave ion mobility usually uses a calibration procedure to determine collision cross sections. Here, we investigate phosphoric acid cluster anions as possible calibrants to increase the pool of reference ions for singly and multiply charged anions with nitrogen as drift gas.Phosphoric acid solutions (5–50 mM) in water and acetonitrile were analyzed using electrospray ionization in the negative ion mode. Collision cross sections were obtained using an IMS-TOF drift tube ion mobility mass spectrometer (Tofwerk, Thun, Switzerland) (DTIMS-MS) on which we varied the electric field. Twenty-six collision cross sections from DTIMS-MS in nitrogen gas, termed DTCCS(N2), were obtained for singly, doubly and triply charged phosphoric acid cluster anions which range from 146 to 530 Å2. Reduced mobilities K0 ranged from 0.767 to 1.435 cm2 V−1 s−1 for singly charged cluster anions, 1.177 to 1.006 cm2 V−1 s−1 and 1.250 to 1.0143 cm2 V−1 s−1 for doubly and triply charged cluster anions, respectively. The DTCCS(N2) were found fitted to a power function 80n0.51 where n is the aggregation number.Ion mobility mass spectrometry measurements were also carried out on a travelling wave ion mobility mass spectrometry (TWIMS-MS) device (SYNAPT G2 HDMS, Waters Corp., Manchester, UK). The DTCCS(N2) were well correlated to the drift times measured on the TWIM instrument (R2 above 0.9999). The correlation function was found to depend on the charge state of the ions. Accuracy tests evidenced a systematic error of 9–11 Å2 (4–6%) on singly charged ions of polyalanine, dextran and hexakis(fluoroalkoxy)phosphazine compared to literature values. Although absolute values were shifted, CCS differences were preserved.