Collision-induced dissociation breakdown surfaces for n-alkylbenzene molecular ions in a quadrupole ion trap mass spectrometer

Complete collision-induced dissociation (CID) breakdown surfaces showing th e 91(+)/92(+) ratios arising from the molecular ions (M+) of n-butylbenzene , n-pentylbenzene, n-hexylbenzene, and n-octylbenzene have been obtained on a quadrupole ion trap mass spectrometer (QITMS). The 91(+)/92(+) ratios ar e plotted as a function of both resonant excitation time and voltage at a q (z)(M+) = 0.300 and at a constant He buffer gas pressure of 1.0 x 10(-4) To rr. (uncorrected ion gauge reading). Because the 91(+)/92(+) ratios reflect the average internal energy of the M+ ions, the surfaces provide a complet e map of the change in ion energy over a wide range of resonant excitation conditions. Comparisons of the 91(+)/92(+) ratios of the n-alkylbenzenes wi th those obtained using charge-exchange mass spectrometry (CEMS) indicate t hat the amount of internal energy deposited upon resonant excitation varies from similar to 1.5 eV for the M+ ion of n-butylbenzene using 100 mV excit ation, to similar to 7.5 eV for the M+ ion of n-octylbenzene using 500 mV e xcitation. Moreover, calculations of the amount of internal energy deposite d, as well as the percent energy transferred (%E), are found to increase as the size of the n-alkyl goup of the M+ ion is increased, paralleling the v end seen for the same MT ions following low-energy CID in a triple quadrupo le mass spectrometer (TQMS) under single-collision conditions. This trend i n the %E as a function of the size of the n-alkyl group parallels that foun d for the low-energy CID studies performed on a triple quadrupole mass spec trometer and thus indicates that the mechanism of CID in the QITMS is simil ar to that thought to occur in rf-only quadrupole collision cells. (Int J M ass Spectrom 194 (2000) 121-132) (C) 2000 Elsevier Science B.V.