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

RationalePolyoxometalates (POMs) are remarkable oxo-clusters forming compact highly charged anions. We measured their collision cross sections (CCS) in N-2 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. MethodsSix POM standards were analyzed by DTIM-MS (Tofwerk, Thun, Switzerland) at different voltages to determine absolute (CCS)-C-DT (N-2) values. Five POM compounds (Lindqvist TBA(2)Mo(6)O(19;) decatungstate TBA(4)W(10)O(32;) Keggin TBA(3)PMo(12)O(40); TBA(3)PW(12)O(40) and Dawson TBA(6)P(2)W(18)O(62)) were used for the calibration of the TWIM-MS instrument (Synapt G2 HDMS, Waters, Manchester, UK) and a sixth Dawson POM, TBA(9)P(2)Nb(3)W(15)O(62), was used to compare the accuracy of the calibrations with POM or with polyalanine and dextran reference ions. ResultsWe determined 45 (CCS)-C-DT (N-2) values at 30 degrees C or 60 degrees C. Fourteen (CCS)-C-DT (N-2) values at 30 degrees C were used to perform calibration of the TWIMS instrument. Better correlations were observed than when (CCS)-C-DT values in helium from the literature were used. The accuracy tests on six ions of Dawson POM TBA(9)P(2)Nb(3)W(15)O(62) 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. ConclusionsOur novel calibration strategy for determination of CCS values of multiply negatively charged ions on TWIM-MS devices based on (CCS)-C-DT (N-2) 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.