The concept of the diffusion calibration for accurate drift time measurement by traveling wave ion mobilty

Abstract

Ion mobility spectrometry (IMS) is a gas phase separation technique which relies on differences in the collisional cross section (CCS) of ions. Ionic clouds of unresolved conformers overlapped if the CCS difference is below the instrumental resolution expressed as Ω/ΔΩ. The experimental arrival time distribution (ATD) peak is then a superimposition of the various contributions weighted by their relative intensities. We have developed a strategy for accurate drift time determination using traveling wave ion mobility spectrometry (TWIMS) of poorly and unresolved conformers. This method implements through a calibration procedure the link between the peak full width at half maximum (FWHM) and the drift time of model compounds for wide range of settings for wave heights and velocities. We modified a Gaussian equation which achieves the deconvolution of ATD peaks where the FWHM is fixed according to our calibration procedure. The new fitting Gaussian equation only depends on two parameters: The apex of the peak (A) and the mean drift time value (µ). The standard deviation parameter (correlated to FWHM) becomes a function of the drift time. This correlation function between µ and FWHM is obtained using the TWIMS calibration procedure which determines the maximum instrumental ion beam diffusion using ionic compounds which are detected as single conformers in the gas phase. This deconvolution process has been used to highlight the presence of poorly resolved conformers for couples of crown ethers and peptides leading to CCS determination in better agreement with quantum chemistry predictions