A phenomenological description of a black hole for collisionally induced decomposition products in ion‐trap mass spectrometry

Abstract

Through extensive studies of collisionally induced decompositions of different precursors by ion-trap mass spectrometry, the presence of a shadow region was demonstrated. If fragment ions fall into the stability diagram at a qz value of -0.78, their absolute intensities drop almost to zero. In order to test this phenomenon, different precursors and different collisionally produced fragment ions have been examined and in every case the fragment at qz= 0.78 disappeared, suggesting that such qr values can be considered as black holes in the microcosmos of the ion-trap mass spectrometer.Ion-trap mass spectrometry (ITMS) is, in our opinion, one of the most interesting mass spectrometric techniques to have been developed in recent years.' The potentialities of the instruments are currently under investigation: to date, their valid capabilities in mass analysis, ion isolation and multiple tandem mass spectrometry (MSIMS) experiments have been well p r~v e d .~.~ Mass range extension up to 25 000 Da, linked with collision yields very close to unity, make this instrument unique. We ourselves have tested the capabilities of ITMS in one of the most important Author to whom correspondence should be addressed. current mass spectrometric fields i.e., ion structure studies and isomer characterization based on collisional spectroscopy. In many cases exciting results were obtained.As has been described previously,' the equations of motion within the ion trap are described by second order differential equations (Mathieau equations), of the generalized form d2uwhere where U and V are the amplitudes of the DC and RF potentials applied at the ring electrode, ro the radius of the ring electrode, Q the angular frequency of the RF drive, m the mass of the ion. The a, and qz parameters are essential for describing the stability (trapping) or the instability (ejection) of an ion in the ion trap. Ions generated under EI conditions or from collisional experiments are usually detected at a,=O by massselective ejection of ions, occurring at q, = 0.908.'In MSIMS experiments if a third coordinate of the stability diagram is considered, so that the intensity of a daughter ion is plotted with respect to (az, qz) values, an unusual and unexpected behaviour has been observed by us: when the daughter ion falls in the range of ql values close to 0.78, its intensity drops dramatically, nearly to zero (Fig. 1). We will discuss here the results obtained by using different precursors and different daughter ions, and by variation of the main instrumental parameters, so as to obtain a phenomenological description of this strange behaviour.In Figure 2(a) the fragmentation pattern of chalcone, already studied by US,^ is reported together with its electron ionization mass spectrum (Fig. 2(b)). In Fig. 2(c) and (d) the daughter-ion spectrum of ionic species at rnlz 130 for tickle voltage of 220 mV and a tickle time of 15 ms is reported: this ion mainly decomposes by CO loss, giving rise to a daughter ion at mlz 102. Moving the parent ion position along the qz...

Escape velocity and resonant ion dynamics in Paul trap mass spectrometers

“…It was seen that nonlinear resonance occurred when there was a simple rational relationship between the axial, radial and rf drive frequency. For instance, the two major regions of instability [12,13] were seen to have β values of 1/2 and 2/3 implying that the secular frequency was 1/4 and 1/3 of the rf drive frequency, respectively.…”

“…Eqs. (9) and (12), which have the form of nonlinear Mathieu equations, governs the motion of ions in practical Paul traps in the radial and axial directions, respectively. The nonlinear and coupling terms arise as a result of the hexapole and octopole superpositions in the expression for the potential function.…”

“…Eqs. (9) and (12) are numerically integrated for a given initial velocity magnitude, at a given initial angle in the r-z plane, and a given choice of t 0 (or, phase of the forcing term). Then the initial velocity magnitude is iteratively adjusted so that the ion just reaches the trap boundary within a prespecified "large" time.…”

“…These are regions where ions do not experience as strong a trapping field as in other regions and here ion motion has a tendency to become unstable. Such observations have resulted in the characterization of regions and points where the trapping strength is low as "black canyons" and "black holes," respectively [12][13][14][15]. It has been suggested by Wang et al [16] that such a phenomenon occurs on account of nonlinear resonance when axial and radial ion secular frequencies and the rf drive frequency have a simple rational relationship with each other.…”

Nonlinear Ion Harmonics in the Paul Trap with Added Octopole Field: Theoretical Characterization and New Insight into Nonlinear Resonance Effect

“…This effect can give rise to possible ion losses and sensitivity degradation in the ion trap instruments. For instance, in the MS/MS experiments, the loss of daughter ions because of the nonlinear resonance has been knows as the black holes or black canyons [29]. But in the dipolar resonance experiments, the effect can accelerate the ion ejection and improve the mass resolution [30].…”