Nanoscale Secondary Ion Mass Spectrometry (NanoSIMS)

What Is Nanoscale Secondary Ion Mass Spectrometry (NanoSIMS)?

NanoSIMS is a powerful analytical technique designed for ultra-high-resolution elemental and isotopic imaging at the nanometer scale. By visualizing how elements and compounds are distributed within materials or biological samples, researchers can diagnose failures, trace processes, and authenticate materials, driving innovation in fields from semiconductors and medicine to green energy and forensic science.

Why Use NanoSIMS?

A key feature of NanoSIMS is its exceptional spatial resolution, capable of imaging features as small as 50 nanometers. This is achieved through a coaxial lens system that positions the ion probe extremely close to the sample surface. The primary ion beam strikes the sample at a perpendicular angle (90°), while the secondary ions are extracted back through the same optical path, minimizing distortions caused by surface topography or shadowing.

NanoSIMS is equipped with up to seven detectors at the mass analyzer’s exit, six of which are movable. This configuration allows for the simultaneous detection of up to seven different ionic species originating from the same sputtered volume. Such parallel detection ensures accurate measurement of isotopic or elemental ratios and enables a direct comparison of their spatial distributions.

The instrument’s mass resolution is also outstanding, with M/ΔM values exceeding 5000. This high mass resolution allows NanoSIMS to distinguish between ions with nearly identical masses, such as ¹³C¹⁴N⁻ (27.000 Da) and ¹²C¹⁵N⁻ (27.006 Da). The use of a double-focusing Mattauch-Herzog-like geometry and optimized transfer optics ensures high transmission efficiency, even at high mass resolution, which is crucial for analyzing small volumes of complex materials.

Unlike time-of-flight SIMS (ToF-SIMS) instruments, NanoSIMS operates in direct current (DC) mode, providing sensitivity that is several orders of magnitude higher and mass resolution that is independent of the spot size. Cs⁺ beams are used for detecting negative ions (e.g., CN⁻) and O⁻ beams for positive ions (e.g., Na⁺). This flexibility allows for the analysis of a wide range of elements, though it does limit which elements can be detected simultaneously.”

How NanoSIMS Works

The process begins with a fine-focused primary ion beam—typically less than 50 nanometers in diameter—composed of ions such as cesium (Cs⁺) or oxygen (O⁻). This beam is rastered, or systematically scanned, across the surface of a sample. As the primary ions strike each pixel of the sample, they sputter secondary ions from the surface. These secondary ions are then collected and analyzed, allowing NanoSIMS to construct detailed, high-resolution maps of elemental and isotopic distributions.

Key Differentiators

• Surface Sensitivity: Extremely high (monolayer sensitivity).
• Mass Range: 1 to >10,000 atomic mass units.
• Molecular Detection: Yes.
• Imaging Capability: Excellent spatial visualization at nanoscale.
• Depth Profiling: Up to a few microns.
• Depth Resolution: ~10 nm.

Sample Requirements

• Metal Samples: A very high-quality metallographic polish is essential to achieve a flat surface and a scratch-free finish, which ensures accurate analysis and minimizes artifacts in the data.
• Biological Samples: Specific protocols must be developed for each experiment to preserve both the structure and the true spatial distribution and abundance of molecules within the sample. Given the operational requirements of the NanoSIMS (ultra-high vacuum), biological samples must be vacuum compatible (volatile free), flat to reduce varying ionization trajectories, and conductive, which can be achieved by sputter coating with Au, Pt, or C. Preparation typically involves fixation using chemical fixation or cryo-fixation, embedding in resin, sectioning into thin slices between 100 nm to 1 μm, and mounting on silicon wafers or slides for analysis.