Development of Multiplexed ToF-SIMS Instrumentation

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Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is a powerful and widely used method for surface chemical analysis. The technique involves bombarding a sample with a high energy primary ion beam and detecting the chemistry of the molecular secondary ions that are ejected. The research group at the University of Manchester has over 30 years acquired an internationally-leading reputation for the development and analytical application of the ToF-SIMS technique. In recent years the development of novel primary ion beams such as C60 and massive gas clusters (e.g. Ar2000) has extended the range of chemistry that can be detected and allowed in-depth and 3D molecular analysis beyond the surface region. This has greatly accelerated the uptake of the technique in academic and industrial labs, to measure complex molecular systems such as biological cells and advanced materials and devices, and to make advances in healthcare diagnostics and manufacturing. Conventionally, ToF-SIMS measurements rely on signal averaging (SA) over multiple experimental cycles to maximise the signal-to-noise ratio and resulting sensitivity. Each cycle consists of a short (nanosecond) primary ion pulse, followed by the measurement of the flight time (up to 0.2 milliseconds) of secondary ions, ejected from the sample, to a detector to determine their mass-to-charge (m/z) ratio. The m/z ratio in turn provides information about the chemistry of the detected ions and therefore of the sample. In this configuration the system waits for all secondary ions in each cycle to reach the detector before beginning the next cycle - the data is inherently sparse. The resulting poor duty cycle limited by the flight time of the largest m/z ion leads to inefficient (<0.1%) primary ion usage and long experimental measurements. In producing a pixel-by-pixel chemical image of the sample surface very many (~1 million) experimental cycles are used to gain the required sensitivity, often taking several hours of experiment time. Extending the analysis to the sub-surface region (depth-profiling or 3D imaging) requires many times longer or involves a different methodology whereby only a small fraction of the sample is analysed and potentially important information is lost. Here we present a multiplexing methodology in which multiple secondary ion packets are measured simultaneously. This allows much more efficient (up to 50%) usage of the primary beam for signal generation and ensures that the summed mass spectra more rapidly converge to a sensitive and accurate measurement. This represents a completely new paradigm for ToF-SIMS. The development of the necessary hardware (ion optics and electronics), computer control and data processing software is an adventurous task for which we have put together a multidisciplinary academic and industrial team, uniquely positioned to meet this challenge. The result will be greatly improved signal-to-noise and therefore greater sensitivity in shorter experiments. This will increase the throughput and analytical power of the ToF-SIMS technique and extend the range of complex samples that can be analysed. Benefits of improved analytical power will impact on many sectors using this technology including advanced manufacturing and healthcare.