No abstract

In many cases, an unknown to an investigator is actually known in the chemical literature. We refer to these types of compounds as "known unknowns." Chemical Abstracts Service (CAS) Registry is a particularly good source of these substances as it contains over 54 million entries. Accurate mass measurements can be used to query the CAS Registry by either molecular formulae or average molecular weights. Searching the database by the web-based version of SciFinder is the preferred approach when molecular formulae are available. However, if a definitive molecular formula cannot be ascertained, searching the database with STN Express by average molecular weights is a viable alternative. The results from either approach are refined by employing the number of associated references or minimal sample history as orthogonal filters. These approaches were shown to be successful in identifying "known unknowns" noted in LC-MS and even GC-MS analyses in our laboratory. In addition, they were demonstrated in the identification of a variety of compounds of interest to others.

Photodissociation of mass-selected populations of trapped ions is used as a tool to determine the spatial distributions of the ion clouds under a variety of trapping conditions. These ion tomography experiments are performed in both the axial and radial dimensions, and the results show that the ion cloud expands significantly in the radial dimension as the number of trapped ions is increased. This expansion correlates with an increasing error in mass assignment due to delayed ion ejection. Furthermore, both effects appear to be related to the occurrence of compound-dependent (rather than mass/charge ratio-dependent) effects on ion ejection. The molecular ions of nitrobenzene and n-butylbenzene, and the benzoyl cation, examined under fixed conditions using the same number of ions, each displays different mass shifts which correlate with differences in the magnitudes of their radial distributions. These results demonstrate that the spatial distribution of a collection of ions depends on their physicochemical properties. Furthermore, alterations in the geometry of the trap are shown to be a means of controlling the compound-dependent positional distributions as well as the corresponding mass shifts. Ion tomography measurements of the size of the ion clouds are made for all three types of ions as a function of the number of trapped ions. They show that the compound-dependent mass shifts can be eliminated by symmetrically increasing the spacing of the end cap electrodes, a procedure which deliberately increases the positive octapolar field component. The implications of these results for exact mass measurements using ion traps are considered.

Trapped ions of different masses can be separated in space within a quadrupole ion trap, making it possible to perform efficient mass-selective photodissociation on a mixture of ions. This method employs an axial quadrupolar dc pulse (1 µs) to force all ions into coherent radial motion; ions of the same mass-to-charge ratio are either in phase or phase-shifted by exactly 180°. After this activating pulse, the ions continue to oscillate at their secular frequency in a coherent fashion so that ions of the same mass-to-charge ratio simultaneously cross the z-axis (r ) 0) twice per secular cycle. At this specific moment, a single-pulse (15 ns) laser beam, aligned collinear with the z-axis, can be used to improve the photodissociation efficiency by irradiating these ions that are radially focused along the z-axis and within the confines of the laser beam. The length of the delay time between the dc pulse and the laser pulse is critical, as it controls the phase, and hence the spatial position, of the ions when the laser is fired. Under the given operating conditions, this method improves the photodissociation efficiency from 9% to 35%. The photodissociation efficiency steadily decreases with longer delay times as the oscillating ions undergo increasing numbers of collisions with the helium buffer gas and lose coherence. Since the secular frequencies of trapped ions are mass-dependent at a fixed rf amplitude, ions of different mass-to-charge ratios will cross the z-axis at different times. Massselective photodissociation is illustrated for a mixture of benzoyl-h 5 and -d 5 cations by appropriately adjusting the delay time between the dc pulse and the laser pulse. Simulations using the program ITSIM were used to design this experiment, and the data which describe the ion motion are provided.