While the first term in this equation gives rise to the periodicity of the ionic signal (by generating equidistant peaks separated by 44.0 Da), the two other terms are responsible for the offsets of these ladders with respect to one another, as well as for appearance of multiple ladders within the same m/z region (a feature that is particularly pronounced in the mass spectrum shown in Figure ). Since Na + and K + (23.0 and 39.0 Da, respectively) are usually the only two ubiquitous metal ions that act as cationizing agents for PEG, the third term in equation ( ) alone cannot explain the observed variation in masses of PEG ions, and one needs to consider the second term (end group mass).