Collision energetics in a tandem time-of-flight (TOF/TOF) mass spectrometer with a curved-field reflectron
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].
Section snippets
Experimental
Tandem mass spectra were obtained on a Kratos (Manchester, England) AXIMA CFR time-of-flight mass spectrometer modified, as described previously [26], [27], with a collision cell mounted at the top of the ion source and ion focusing optics in the region ahead of the mass selection gate (Fig. 1). The collision chamber is a stainless steel cylinder (1.13 in. long, 0.2 in. i.d.) and the collision gas is injected into the chamber through a long (2 m) 0.07 mm i.d. glass capillary tube at a flow rate of
Impact collision theory
Collision induced dissociation (CID) is essentially a two step process that involves excitation of a precursor ion upon impact with a target atom or molecule (usually an inert gas atom), followed by cleavage of chemical bonds in the ion to form separated ionic and neutral fragments [1], [2], [3], [9]. After an elastic collision the precursor ion and target atom fly apart with velocities and kinetic energies that are different from their initial ones, but which conserve the center-of-mass
Collision-induced dissociation versus post-source decay
Tandem mass spectra were obtained for the product ions formed from fullerene C60 with no collision gas, with helium and with neon, over a range of ion accelerating energies from 3 to 20 keV. Without a collision gas only the C58+ fragment is visible [25], [36]. An MS/MS spectrum of fullerene with helium, at high attenuation and at 20 keV, is shown in Fig. 5a. The major ions in the upper mass region are the even-carbon species from C34+ to C58+, with the lower mass ions arising from multiple
Conclusions
The tandem (TOF/TOF) time-of-flight mass spectrometer provides a unique opportunity to evaluate collision energetics and mechanisms based on the small differences in arrival times of the product ions that reflect the partitioning of the initial collision energy into the kinetic and internal energy of the target and products. The curved-field reflectron is helpful for this approach in that it enables us to access very high collision energies, because it does not require reacceleration of the
Acknowledgements
This work was supported by a grant to RJC (GM64402) from the National Institutes of Health and a contract (N01 HV28180) to J. Van Eyk from the National Heart Lung and Blood Institute.
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Current address: Center for Proteomics, Case Western Reserve University, Cleveland, OH 44106, United States.