Ion cyclotron resonance frequency, f, is conventionally converted to ion mass-to-charge ratio, m/z (mass "calibration") by fitting experimental data spanning the entire detected m/z range to the relation, m/z = A/f + B/f(2), to yield rms mass error as low as ~200 ppb for ~10,000 resolved components of a petroleum crude oil. Analysis of residual error versus m/z and peak abundance reveals that systematic errors limit mass accuracy and thus the confidence in elemental composition assignments. Here, we present a calibration procedure in which the spectrum is divided into dozens of adjoining segments, and a separate calibration is applied to each, thereby eliminating systematic error with respect to m/z. Further, incorporation of a third term in the calibration equation that is proportional to the magnitude of each detected peak minimizes systematic error with respect to ion abundance. Finally, absorption-mode data analysis increases mass measurement accuracy only after minimization of systematic errors. We are able to increase the number of assigned peaks by as much as 25%, while reducing the rms mass error by as much as 3-fold, for significantly improved confidence in elemental composition assignment.
Traditional tools for routine environmental analysis and forensic chemistry of petroleum have relied almost exclusively on gas chromatography-mass spectrometry (GC-MS), although many compounds in crude oil (and its transformation products) are not chromatographically separated or amenable to GC-MS due to volatility. To enhance current and future studies on the fate, transport, and fingerprinting of the Macondo well oil released from the 2010 Deepwater Horizon disaster, we created an extensive molecular library of the unadulterated petroleum to compare to a tar ball collected on the beach of Louisiana. We apply ultrahigh resolution Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry to identify compositional changes at the molecular level between native and weathered crude oil samples and reveal enrichment in polar compounds inaccessible by GC-based characterization. The outlined approach provides unprecedented detail with the potential to enhance insight into the environmental fate of spilled oil, improved toxicology, molecular modeling of biotic/abiotic weathering, and comprehensive molecular characterization for petroleum-derived releases. Here, we characterize more than 30,000 acidic, basic, and nonpolar unique neutral elemental compositions for the Macondo well crude oil, to provide an archive for future chemical analyses of the environmental consequences of the oil spill.
The pursuit of ever more precise measures of time and frequency is likely to lead to the eventual redefinition of the second in terms of an optical atomic transition. To ensure continuity with the current definition, based on a microwave transition between hyperfine levels in ground-state 133 Cs, it is necessary to measure the absolute frequency of candidate standards, which is done by comparing against a primary cesium reference. A key verification of this process can be achieved by performing a loop closure-comparing frequency ratios derived from absolute frequency measurements against ratios determined from direct optical comparisons. We measure the 1 S 0 → 3 P 0 transition of 171 Yb by comparing the clock frequency to an international frequency standard with the aid of a maser ensemble serving as a flywheel oscillator. Our measurements consist of 79 separate runs spanning eight months, and we determine the absolute frequency to be 518 295 836 590 863.71(11) Hz, the uncertainty of which is equivalent to a fractional frequency of 2.1 × 10 −16 .This absolute frequency measurement, the most accurate reported for any transition, allows us to close the Cs-Yb-Sr-Cs frequency measurement loop at an uncertainty of <3×10 −16 , limited by the current realization of the SI second. We use these measurements to tighten the constraints on variation of the electron-to-proton mass ratio, µ = m e /m p . Incorporating our measurements with the entire record of Yb and Sr absolute frequency measurements, we infer a coupling coefficient to gravitational potential of k µ = (−1.9 ± 9.4) × 10 −7 and a drift with respect to time oḟ µ µ = (5.3 ± 6.5) × 10 −17 /yr. arXiv:1811.05885v1 [physics.atom-ph]
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