A calibration framework for the determination of accurate collision cross sections of polyanions using...

Polyoxometalates (POMs) are remarkable oxo-clusters forming compact highly charged anions. We measured their collision cross sections (CCS) in N₂ with drift tube ion mobility spectrometry (DTIMS). These values were then used to calibrate a traveling wave ion mobility spectrometry (TWIMS) device and the accuracy of the calibration was tested.

Six POM standards were analyzed by DTIM-MS (Tofwerk, Thun, Switzerland) at different voltages to determine absolute ^DT CCS (N₂ ) values. Five POM compounds (Lindqvist TBA₂ Mo₆ O_19; decatungstate TBA₄ W₁₀ O_32; Keggin TBA₃ PMo₁₂ O₄₀ ; TBA₃ PW₁₂ O₄₀ and Dawson TBA₆ P₂ W₁₈ O₆₂ ) were used for the calibration of the TWIM-MS instrument (Synapt G2 HDMS, Waters, Manchester, UK) and a sixth Dawson POM, TBA₉ P₂ Nb₃ W₁₅ O₆₂ , was used to compare the accuracy of the calibrations with POM or with polyalanine and dextran reference ions.

We determined 45 ^DT CCS (N₂ ) values at 30°C or 60°C. Fourteen ^DT CCS (N₂ ) values at 30°C were used to perform calibration of the TWIMS instrument. Better correlations were observed than when ^DT CCS values in helium from the literature were used. The accuracy tests on six ions of Dawson POM TBA₉ P₂ Nb₃ W₁₅ O₆₂ led to relative errors below 3.1% while relative errors of 3.6% to 10.1% were observed when calibration was performed with polyalanine and dextran reference ions.

Our novel calibration strategy for determination of CCS values of multiply negatively charged ions on TWIM-MS devices based on ^DT CCS (N₂ ) of standard POM structures covered a wider range of CCS and improved the accuracy to 2.1% relative error on average compared with 6.9% using polyalanine and dextran calibration.