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.
A new technique has been developed which allows the direct measurement of frequencies of ions trapped in a quadrupole ion trap mass spectrometer. This pump/probe method employs a fast direct current (DC) pulse (pump) to displace a kinetically cooled ion population from the center of the trap, and a laser (probe) which recognizes when ions reappear at the center of the trap by the formation of photodissociation fragments. The translationally excited ions undergo periodic motion within the confines of the ion trap, and this periodic motion can be followed by recording the intensity of the photodissociation fragment as a function of the delay time between the DC pump and the laser probe. The DC pulse has a rise time of 15 ns; data are taken 1 ms after its application to allow stable ion motion to be sampled. Sampling of the ion cloud is done at 50 ns intervals, and fast Fourier transformation of the time-based data yields the ion frequencies and their relative magnitudes. Data are reported for ions derived from acetophenone (m/z 105) and 1,4-cyclohexadiene (m/z 80) under various trapping conditions corresponding to different Mathieu qz values. The measured fundamental secular frequencies, fz and fr, are found to agree well with those predicted. The presence of higher order multipole contributions to the trapping field is evident from such ion frequencies as the drive frequency, fRF,. The ability to measure ion frequencies under operating conditions provides a new tool for comparing simulated and experimental data. Simulation data from the program ITSIM, modified to account for the effects of collisions, are shown to predict the major frequency components observed in the experimental data.
An early problem in the development of the commercial ion trap mass spectrometer was the phenomena of {open_quotes}mass-shifts{close_quotes}, where two ions, having the same nominal m/z, appear at different points on the mass scale. Although the problem has been circumvented by the use of ion traps elongated in the z-direction (stretched geometry), the phenomena itself remains to be understood and is an area of active interest in ion trap mass spectrometry. One possible explanation is that ions having different chemical properties, e.g., polarizability, have different radial distributions in the ion trap. If these ions experience an inhomogeneous field, say from field faults introduced by holes in the endcaps, the ions will experience different average values of q for a given RF voltage and will be ejected at different times (apparent masses).
New measurements are presented for charged hadron azimuthal correlations at midrapidity in Au+Au collisions at (square root)s(NN) = 62.4 and 200 GeV. They are compared to earlier measurements obtained at (square root)s(NN) = 130 GeV and in Pb + Pb collisions at (square root)s(NN) = 17.2 GeV. Sizeable anisotropies are observed with centrality and transverse momentum (pT) dependence characteristic of elliptic flow (upsilon2). For a broad range of centralities, the observed magnitudes and trends of the differential anisotropy, upsilon2(pT), change very little over the collision energy range (square root)s(NN) = 62-200 GeV, indicating saturation of the excitation function for upsilon2 at these energies. Such a saturation may be indicative of the dominance of a very soft equation of state for (square root)s(NN) approximately 60-200 GeV.
Abstract Mass spectra recorded using a Paul ion trap, operated in the mass‐selective instability scan mode, give peak positions which are shown to depend on ion abundance. The isolated molecular ions of benzene, anisole, cresol, and n‐butylbenzene are ejected during the mass selective instability scan at times which display a linear dependence on ion abundance for low populations of trapped ions. Remarkably, the degree to which the peaks are shifted is also compound dependent. Mass shifts are also influenced by the presence of other ions in the trap and this effect is amplified as the mass difference between the analyte and matrix ions decreases. Mass shifts are also affected by the helium pressure which controls the ion density. Extrapolation to zero ion abundance and to zero helium pressure is suggested as a method of eliminating these effects and hence of improving the mass measurement accuracy attainable using the ion trap.
The motion of an ensemble of ions during their gated injection into the three-dimensional radio-frequency quadrupole ion trap was simulated considering helium buffer gas collisions, injection at certain RF phase angles and using DC impulsive fields to optimize the trapping efficiency. Simulations using a simple model of ion–neutral collisions show that buffer gas alone, even at 1–10 mTorr pressure, is not able to remove sufficient kinetic energy from the injected ions through ion–neutral collisions to prevent their loss and does not solve the problem of low efficiency of ion retention in the trap, even under qz-optimization. Accumulation of ions over long periods is not very effective, in such cases trapping efficiencies are less than 5%. It is shown that under RF phase angles between 110° and 220° the injected ions can be trapped, although only temporarily, with very high efficiencies. The simulations lead to the conclusion that all of the desired injected ions covering a wide mass/charge range can be trapped for indefinite times when short injection pulses are used together with dipolar DC pulses applied during injection. The additional damping achieved through these dipolar DC impulsive fields and the resulting cooling of the ion cloud is shown to give efficient trapping.
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