Overview
ISO 17862:2022 - "Surface chemical analysis - Secondary ion mass spectrometry - Linearity of intensity scale in single ion counting time-of-flight mass analysers" defines a standardized method to evaluate and correct intensity nonlinearity in single‑ion‑counting TOF‑SIMS instruments. The standard uses isotopic ratios measured from poly(tetrafluoroethylene) (PTFE) spectra to determine a maximum count rate for an acceptable divergence from linearity of the intensity scale. It also provides a practical dead‑time correction applicable to detection chains based on a microchannel plate (MCP) or scintillator + photomultiplier followed by a time‑to‑digital converter (TDC), potentially extending the usable intensity range (95% linearity) by a factor of more than 50 for instruments where the correction is valid.
Key technical topics and requirements
- Scope and intent: Characterize intensity linearity and define a usable maximum count‑rate for TOF‑SIMS single ion counting systems.
- Test material: Measurements are performed on PTFE tape in the “as‑received” condition using isotopic peak ratios.
- Procedure elements:
- Sample mounting and instrument setup (beam, mass analyser, energy acceptance).
- Acquisition of a set of test and analysis spectra (e.g., 16 test spectra to establish settings and 16 data spectra for analysis).
- Calculation and fitting of measured vs corrected counts and isotopic ratios to assess linear range.
- Dead‑time correction: A Poissonian dead‑time correction for MCP/TDC detection chains is provided to compensate for counts lost when multiple secondary ions arrive within the detector dead‑time τ.
- Validation and limits: The method defines validity checks for the correction and sets conservative limits where corrections are reliable. It identifies cases where nonlinearity cannot be simply corrected and a maximum acceptable count rate must be enforced.
- Quality regimen: Recommended to repeat tests after substantive detector/electronics changes (e.g., MCP replacement) or at about one‑year intervals.
Practical applications and users
- Who uses ISO 17862:2022:
- TOF‑SIMS instrument manufacturers and service engineers validating detector performance.
- Surface analysis laboratories and metrology institutes establishing quantitative SIMS capability.
- Quality control and R&D teams that require reliable quantitative depth profiling or surface composition data.
- Practical benefits:
- Establishes a verifiable maximum count rate to avoid quantitative bias from detector saturation.
- Provides an evidence‑based dead‑time correction that can dramatically expand usable dynamic range for eligible instruments.
- Helps diagnose detector faults (e.g., reduced efficiency, non‑single‑ion counting behavior) and optimize measurement settings.
Related standards
- Normative reference: ISO 13084 (Calibration of the mass scale for TOF‑SIMS).
- ISO 17862:2022 complements other surface chemical analysis standards and laboratory best practices for quantitative SIMS.
Keywords: ISO 17862:2022, TOF‑SIMS, secondary ion mass spectrometry, intensity linearity, single ion counting, MCP, TDC, dead‑time correction, PTFE, count rate.