Estimation of Activation Energy from the Survival Yields: Fragmentation Study of Leucine Enkephalin and Polyethers by Tandem Mass Spectrometry

Abstract

A simple collision model for multiple collisions occurring in quadrupole type mass spectrometers was derived and tested with leucine enkaphalin a common mass spectrometric standard with well-characterized properties. Implementation of the collision model and Rice-Ramsperger-Kassel-Marcus (RRKM) algorithm into a spreadsheet software allowed a good fitting of the calculated data to the experimental survival yield (SY) versus collision energy curve. In addition, fitting also ensured to estimate the efficiencies of the kinetic to internal energy conversion for Leucine enkephalin in quadrupole-time-of-flight and triple quadrupole instruments. It was observed that the experimental SY versus collision energy curves for the leucine enkephalin can be described by the Rice-Ramsperger-Kassel (RRK) formalism by reducing the total degrees of freedom (DOF) to about one-fifth. Furthermore, this collision model with the RRK formalism was used to estimate the critical energy (E(o)) of lithiated polyethers, including polyethylene glycol (PEG), polypropylene glycol (PPG), and polytetrahydrofurane (PTHF) with degrees of freedom similar to that of leucine enkephalin. Applying polyethers with similar DOF provided the elimination of the effect of DOF on the unimolecular reaction rate constant. The estimated value of E(o) for PEG showed a relatively good agreement with the value calculated by high-level quantum chemical calculations reported in the literature. Interestingly, it was also found that the E(o) values for the studied polyethers were similar.

An approach to estimate the activation energies of fragmentation occurring in quadrupole collision cell of the mass spectrometer

“…In this simplified approach, the step‐by‐step calculation of precursor ion internal energy and reaction time was simplified by using a single E and τ value in the RRK model and in rate equation, respectively. As an estimation, τ = 100 μs was used which is approximately equals to the flight time in the collision cell . The uncertainty of τ, eg, the dependence on the mass of the ions, will appear in the estimated value of the “frequency factor” (ν) in the RRK equation (Equation ), because the product of τ and ν is used in the model.…”

“…As it was already mentioned, the RRK theory overestimates the number of vibrational states, so the number of oscillators is often empirically reduced by a factor 2‐5. In our previous experiments, the observed SY plots could be well described by the RRK model by reducing the total degrees of freedom to approximately one fifth . Therefore, the number of effective oscillators ( S eff in Equations and ) is also approximated as 0.2 × s (where s is the total DOF defined as 3 × N − 6, where N is the number of the atoms in the precursor ion).…”

“…In this study, our aim was only to give a rough estimation on the E o values, using ν only as a fitting parameter not a goal value, because the product τ×ν was used in the fitting procedure so the uncertainties in the τ values hinder the estimation of ν. In addition, using the RRK model, the variation of S eff with the internal energy was also neglected resulting inaccurate ν values . The relative error of the calculated E o values originated from the approximations and from the uncertainties of the model parameters (initial internal energy, number of effective oscillators, kinetic to internal energy conversion ratio) was evaluated to be 30%.…”

“…In a quadrupole‐type mass spectrometer, the kinetic energy is transferred to the precursor ion through its multiple collisions with neutral gas molecules or atoms . In our previous study, a simple collision model was developed for the calculation of the precursor ion internal energy increase due to multiple collisions and for the computation of the time intervals between collisions. The internal energy‐dependent rate coefficient k ( E ) was calculated using both the RRK and the RRKM theory, and the survival yield (SY) was determined after each collisions.…”

Can Nonpolar Polyisobutylenes be Measured by Electrospray Ionization Mass Spectrometry? Anion-Attachment Proved to be an Appropriate Method

“…In order to rule out that the shifts are not due to the effects such mass dependent scattering of the ions in the collision cell or any other mass discrimination issues, we selected polypropylene glycol (PPG) with similar molecular weight to that of the PIBs cationized by Li + ions. Li + ions form adduct with PPG with relative high activation energy for fragmentation [26], thus [PPG + Li] + adduct ions do not fragment significantly either by losing cation or backbone cleavages in the collision energy range of 8-30 eV. In this experiment, we used the same parameter set as in the negative ion mode by reversing the polarity.…”