Collision energetics in a tandem time-of-flight (TOF/TOF) mass spectrometer with a curved-field reflectron

Abstract

Collisions of fullerene ions (C₆₀⁺) with helium and neon were carried out over a range of laboratory energies (3–20 keV) on a unique tandem time-of-flight (TOF/TOF) mass spectrometer equipped with a curved-field reflectron (CFR). The CFR enables focusing of product ions over a wide kinetic energy range. Thus, ions extracted from a laser desorption/ionization (LDI) source are not decelerated prior to collision, and collision energies in the laboratory frame are determined by the source extraction voltages. Comparison of product ion mass spectra obtained following collisions with inert gases show a time (and apparent mass) shift for product ions relative to those observed in spectra obtained by metastable dissociation (unimolecular decay), consistent with impulse collision models, in which interactions of helium with fullerene in the high energy range are primarily with a single carbon atom. In addition, within a narrow range of kinetic energies an additional peak corresponding to the capture of helium is observed for fragment ions C₅₀⁺, C₅₂⁺, C₅₄⁺, C₅₆⁺ and C₅₈⁺.

Introduction

Collision-induced dissociation (CID) is one of the methods used in many different types of mass spectrometers to produce structurally informative fragmentation. The collision process and the transfer of kinetic energy to internal energy of the projectile ion have undergone detailed investigation and are better understood for small molecules [1], [2]. The dissociation of large molecules is complicated by the increase in the number of degrees of freedom, which restrict the rate of dissociation and the fragment ion yield [3]. To increase the relative collision energy (in the center-of-mass frame) and the internal excitation energy, it is common to use higher acceleration voltages and heavier target gases; however, it has been noted that the lower ionization potential and the lower energy between excited and ground electronic states of heavier noble gases results in their excitation as well, thus decreasing the energy available for transfer to the analyte ion [4], [5], [6], [7]. Another consequence of this additional inelasticity is that heavier target atoms result in a somewhat smaller shift in the velocity of the parent ion (and fragment ions) than would be expected solely from consideration of their relative collision energy and excitation of the projectile molecular ion [8]. In the time-of-flight (TOF) mass spectrometer, collisions with a target gas will result in fragment ions with longer arrival times than their counterparts resulting from unimolecular (or post-source) decay. These shifts in arrival times (expressed as shifts in apparent mass) are used here to probe the mechanisms involved in very high energy collisions by comparing the experimental results with those predicted from impulse collision models [9], [10].

In the case of the spherical fullerene C₆₀ molecule it is also possible to capture different targets [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]. For noble gas atoms this interaction does not involve the formation of a chemical bond. The atom penetrates the fullerene sphere, losing sufficient energy in the process to prevent its escape. The threshold for penetration of He through a six-membered ring of C₆₀ has been evaluated as 9.35 eV, and through a five-membered ring as 13.1 eV [13], while the decomposition energy of C₆₀⁺ with C₂ loss has been evaluated as 7.0–7.6 eV [14], [16]. A molecular dynamics simulation predicts the maximum yield at a laboratory collision energy of 8 keV [13], though the maximum yield in our experiments (see below) was observed at 6 keV (33 eV in the center-of-mass frame) [12]. Most of the experiments involving high-energy collisions of fullerenes have been carried out using sector instruments or time-of-flight mass spectrometers combined with a retarding field energy analyzer [10], [11], [12], [13], [16], [17], [18], [20], [21]. In this investigation we detect helium-trapped fragments using a unique time-of-flight mass analyzer with a curved field reflectron. In this case, it is not necessary to decelerate the ions to very low kinetic energies prior to collision in order to accommodate the focusing bandwidth of the reflectron [25]; therefore, it is possible to investigate a wide range of collision energies by simply reducing the initial acceleration voltage. Within each single spectrum we detected metastable, CID and (for lower collision energies) trapped helium fragments, from which we determined the time (apparent mass) shifts that enabled us to assess the different types of collisions based on impulse collision theory (ICT) [9].