An interleaved ToF-SIMS method extended the reported range for measuring isotope fractions by about 14-fold. At a target 10% precision for each depth layer, the low-fraction limit moved from about 1% with short-pulse acquisition to about 0.07% with interleaving. The stated span widened from about 1.6 to 2.8 decades, or powers of ten. In the tungsten-oxide tests, isotope ratios covered roughly 1:500 to 1:1.
The study asked whether multi-pulse-width ToF-SIMS could quantify isotope ratios spanning orders of magnitude in a single depth profile by assigning each isotope to a phase where its response remained linear. In ordinary terms, an isotope ratio compares signals from two forms of the same element. Each cycle interleaved short and long pulse phases, linked them with a dose factor measured within the crater, and used each isotope only in the phase where it was linear. The reported setup required no hardware additions, and the measurements were made without an extended-dynamic-range option.
Two pulses, one calibrated profile
The bridge between phases was a crater-specific factor called k. It was calculated from the sum of the long-phase oxygen-18 signal divided by the sum of the short-phase oxygen-18 signal. The oxygen-18 fraction in each layer then used the long-phase oxygen-18 count, the short-phase oxygen-16 count and k in the reconstruction. The long phase was set to 35 ns, the longest width whose oxygen-18 peak, measured at 1% of its maximum, stayed inside the water-peak spacing; the short phase was 6 ns and had negligible overlap on dry oxide.
On thermal silicon oxide, the tuned pair reached 2.6% per-layer precision with 13-fold fewer frames than short-pulse acquisition. That corresponded to 12.2-fold less analysis time and 6.6-fold less total time per layer. Single-width runs at higher pulse widths were biased: the 9 ns ratio was 21% to 25% high, while the 35 ns ratio was about 12 times too high. An untuned 6 ns and 9 ns pair reached 3.2% precision but took 1.3 times the tuned-pair time.
The test held across natural and enriched materials
The test materials included thermal silicon oxide, a natural-abundance strontium titanate single crystal and four tungsten-oxide films with nominal oxygen-18 marker fractions of 50%, 25%, 16.7% and 12.5%. For natural-abundance oxygen, oxygen-18/oxygen-16 was 0.9 ± 2.1% above the VSMOW reference in thermal silicon oxide and 3.4 ± 2.3% above it in strontium titanate. Strontium and titanium isotope deviations ranged from -3.5% to +6.8%. Overall, the natural-abundance ratios were reproduced within several percent.
The tungsten-oxide films provided a wider range test. Across nominal enrichments from 12.5% to 50%, the ratio of the interleaved reconstruction to the linear short-phase reference was 0.993 ± 0.013, with no trend. The long-phase oxygen-18 signal reached 0.4 to 1.6 counts per pulse, while the tested isotope ratios spanned about 1:500 to 1:1. The marker values were nominal rather than independently known film compositions.
Calibration still sets the boundaries
The dose ratio drifted by up to 4% over 20 layers. The analysis identifies potential systematic bias if a single crater-level k is applied without calibration layers spanning the region of interest. That caveat limits how broadly the reported range can be carried beyond the tested profile without further calibration checks.
The paper also describes a possible multiphase extension for cases in which one long phase cannot stay valid across the full ratio range. Every neighboring pair would need a same-element bridge isotope that is linear and spectrally clean in both phases. Intermediate phases would be added only when that condition cannot be maintained across the full range.
The work was supported by the U.S. Department of Energy Office of Science, Basic Energy Sciences, Division of Materials Sciences and Engineering, with measurements performed at the CNMS DOE user facility at Oak Ridge National Laboratory.
Paper data and sources
Original title: Quantitative Interleaved-Pulse Acquisition Extends the Range of Isotope-Ratio Measurements in ToF-SIMS
Authors: Anton V. Ievlev
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-09-29
DOI: Not available
Original paper · Full text