Isotope Ratio Wide Area
Secondary Ion Mass Spectrometry

Researchers
Peter J. Todd (PI)
Henry S. (Hank) McKown

Introduction

Secondary ion mass spectrometry (SIMS) offers the capability of spatially resolved isotope ratio measurement. This capability is of value to researchers and other workers in fissile material accountability, materials science, biochemistry and geochemistry and archeology. For example, geochemists use the variation of isotope ratios of lighter elements such as oxygen and sulfur to determine the history of various rocks; archeologists can use the distribution of isotopes to identify the age and history of various artifacts; biochemists can use isotope ratio SIMS to map the uptake and deposition of various elements using stable isotope tracers as opposed to radioactive tracers.

Our research is directed toward making isotope ratio measurement by SIMS more precise. Our most determined effort in the past few years has been toward simultaneous detection of isotopic ion beams, e.g. ³⁹K⁺ and ⁴¹K⁺. Simultaneous detection of isotopic ion beams permits improved precision because errors caused by fluctuations in the secondary ion source are cancelled. However, this necessarily means that two or more detectors must be used to measure the ion beams, leading to calibration issues. For examples, the sensitivity of one pulse counting multiplier may be slightly greater than another. While small differences in multiplier gain may be trivial in many applications, the unit of precision in isotope ratio measurement is parts per thousands, and the goal is an error of less than 0.1 part in a thousand. Thus, channel calibration is of profound concern.

Wide Field of View: A/B Referencing

Analytical Instrumentation
The instrument used for this project is based on a JEOL OMS 011 Mattauch-Herzog Mass Spectrometer, but heavily modified.  The wide field of view ion source permits so-called A/B measurement. That is, the primary ion beam – and detection system as well – can be made to switch from an unknown sample to a reference of known or presumed isotope ratio. This feature permits continuous and repeated re-calibration of the sensitivity of the multipliers used to detect the ion beams.

Mattauch-Herzog Secondary Ion Microprobe features a Ga liquid metal ion gun with a spatial resolution of about 1 µm, wide (ca 1 cm) field of view secondary ion source, and the capability for simultaneous detection of secondary ion beams.

Mass spectra obtained simultaneously from a bone sample in the mass region of m/z 35-45.

Future Directions
The key to success for isotope ratio measure by SIMS will be improvement in array detection. At present, we are developing the capability of pulse counting multi-channel plates for this purpose.