Foreword

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This document was prepared by Technical Committee ISO/TC 201, Surface chemical analysis, Subcommittee SC 6, Mass spectrometries.

This third edition cancels and replaces the second edition (ISO 13084:2018), which has been technically revised.

The main changes are as follows:

addition of informative Annex B.

Any feedback or questions on this document should be directed to the user’s national standards body. A complete listing of these bodies can be found at www.iso.org/members.html.

Introduction

Secondary ion mass spectrometry (SIMS) is a powerful technique for the analysis of organic and molecular surfaces. Over the last decade, instrumentation has improved significantly so that modern instruments now have very high repeatability and constancy^[2^]. A growing need is for the identification of the chemical composition of complex molecules from accurate measurements of the mass of the secondary ions. The relative mass accuracy to do this and to distinguish between molecules that contain different chemical constituents, but are of the same nominal mass (rounded to the nearest integer mass), is thus an important parameter. A relative mass accuracy of better than 10 ppm is required to distinguish between C₂H₄ (28,031 30 u) and Si (27,976 92 u) in a parent ion with total mass up to 1 000 u, and between CH₂ (14,015 65 u) and N (14,003 07 u) in parent ions with total mass up to 300 u. However, in a recent interlaboratory study^[3^], the average fractional mass accuracy was found to be 150 ppm, which is significantly worse than the accuracy needed for unambiguous identification of ions. A detailed study^[4^] shows that the key factors degrading the accuracy include the large kinetic energy distribution of secondary ions, non-optimized instrument parameters and extrapolation of the mass scale calibration.

This document describes a simple method, using locally sourced material, to optimize the instrumental parameters, as well as a procedure to ensure that accurate calibration of the mass scale is achieved within a selectable uncertainty.

1 Scope

This document specifies a method to optimize the mass calibration accuracy in time-of-flight secondary ion mass spectrometry (SIMS) instruments used for general analytical purposes. This document is only applicable to time-of-flight instruments but is not restricted to any particular instrument design. This document gives guidance for some of the instrumental parameters that can be optimized using this procedure and the types of generic peaks suitable to calibrate the mass scale for optimum mass accuracy.

2 Normative references

3 Terms and definitions

4 Symbols and abbreviated terms

4.1 Symbols

4.2 Abbreviated terms

5 Outline of method

6 Method for improving mass accuracy

6.1 Obtaining the reference sample for optimization

6.2 Preparation of polycarbonate sample

6.3 Obtaining the SIMS spectral data

6.4 Calculating mass accuracy

6.5 Optimizing instrumental parameters

6.6 Calibration procedure

Bibliography