Overview
ISO 21270:2004 - "Surface chemical analysis - X‑ray photoelectron and Auger electron spectrometers - Linearity of intensity scale" defines procedures to evaluate and, where possible, correct the linearity of the intensity scale for X‑ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) instruments. The standard provides two methods to determine the maximum count rate compatible with an acceptable divergence from linear response, and it describes correction relationships (based on detector dead‑time models) that extend usable count‑rate ranges for instruments where those corrections are valid.
Key topics and requirements
- Two measurement methods
- Varying source flux method: for AES and XPS instruments that allow 30 or more approximately evenly spaced source flux settings; uses a sputter‑cleaned polycrystalline copper (Cu) sample and measures count rate vs. beam/X‑ray flux.
- Spectrum ratio method: for XPS instruments with two or more but fewer than 30 predefined X‑ray source settings; compares widescan spectra at high and low emission currents (Cu or stainless‑steel sample).
- Sample preparation
- Preferred sample: polycrystalline Cu (≥ 99.8 %); alternative: stainless steel or a sample holder when no ion gun is available.
- Cleaning: short 1 % nitric acid dip then rinse (if needed); ion sputter cleaning in ultra‑high vacuum to reduce C and O signals below specified limits; adhesives are prohibited for mounting.
- Linearity assessment and correction
- Plot ratios (measured count/flux or spectrum‑ratio vs. count rate) to define linear range.
- Use dead‑time based correction equations (extended/non‑extended dead time) where valid to correct non‑linearities and allow higher maximum count rates.
- When to perform
- Characterization of new instruments, after substantive detection‑circuit modifications, after multiplier replacement, after a substantial increase in multiplier voltage, or at ~12‑month intervals.
- Documentation
- Record spectrometer settings (pass energy, slits, lens settings, etc.) - results apply only to the tested configuration.
Applications and users
- Who uses it: surface analysis labs, XPS/AES instrument manufacturers, calibration and quality control engineers, and R&D groups performing quantitative surface chemical analysis.
- Practical value: ensures accurate quantitative surface composition by identifying count‑rate limits and applying validated corrections; informs acceptable instrument operating ranges to minimize systematic errors from detector non‑linearity.
Related standards
- ISO 18115 (Surface chemical analysis - Vocabulary) is normative for terminology used in ISO 21270.
- Relevant to laboratory quality systems, instrument calibration procedures, and ISO standards on surface analysis practices.
Keywords: ISO 21270:2004, XPS linearity, AES intensity scale, detector dead time, spectrum ratio method, maximum count rate, surface chemical analysis, X‑ray photoelectron spectrometer.
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