An empirical approach to estimation of critical energies by using a quadrupole ion trap

A simple energy-resolved mass spectrometric technique is described for the estimation of critical energies for dissociation of ions via threshold collisional activation measurements in a quadrupole ion trap. The method is calibrated by using compounds with well-defined dissociation energies, and separate calibration curves must be constructed for radical ions that are bound by covalent bonds versus hydrogen-bonded complexes. For these sets of experiments the threshold point is defined as the activation voltage required for the fragment ion intensity to be 10% of the total ion intensity. A plot of threshold activation voltage of the calibrant versus literature critical energies shows a near-linear function, and accuracies are estimated as better than +/- 6 kcal/mol. The q(z) value during activation seems to have little effect on the threshold voltages as long as very low q(z) values that cause ion ejection are avoided. Activation periods that are substantially longer than 10-ms result in nonlinear behavior in the calibration curves for ions that have critical energies above 30 kcal/mol. This energy-resolved method was also useful for the estimation of critical energies of complexes bound by electrostatic forces, such as hydrogen-bonding interactions. A quantitative evaluation of proton-bound polyether-amine complexes showed that the number of available hydrogen-binding sites, the gas-phase basicities of the polyether and amine components, and the ability of the complex to attain the most favorable near-linear hydrogen bonds correlate with the threshold values. (C) 1996 American Society for Mass Spectrometry