Space Charge Effects

  • the observed m/z difference of these ions in the same spectrum is 13 mTh,* 33% less than the theoretical value of 18 mTh
  • in separate spectra, the observed m/z difference is 20 mTh, a more accurate value (that is, after the ejection of either ion species, the space charge effects are eliminated)
  • this is experimentally significant compared to uncertainty in the measured m/z ratio: the standard deviation in the observed frequency for either peak was ~0.6 Hz, roughly 1 part per million. This corresponds to a 2 part per million standard deviation in the mass, or only 1 mTh. 
  • such mass shifts and coalescence  phenomena are consistent with space charge effects and have been observed in 3-D Paul traps and FT-ICR.
  • both ion species are doubly charged, increasing the strength of the Coulomb interaction fourfold over the interaction of two singly charged species