What is Ion Implantation?

Ion implantation is a process whereby dopant ions are accelerated in intense electrical fields to penetrate the surface of a material, thus changing the material’s properties.

An essential technique in the semiconductor industry, it is used for modifying the conductivity of a semiconductor during the fabrication of integrated circuits. It is also heavily used for making silicon-on-insulator devices and in many other industries, including physics, materials science, and metallurgy.

Virtually all ion implantation applications involve implanting vast numbers of ions over a large area to modify the bulk properties of a material. In contrast, Q-One is designed for implanting single ions with extremely high precision to fabricate quantum devices. However, many of the same concepts apply.

Ion Implanters

Ion implanters consist of a source region that forms the ions, an accelerator region that electrostatically accelerates them to high energy, and a target chamber. Instruments must be pumped to a high vacuum to prevent contamination of the target and breakdown under high voltage.

Ion sources often generate multiple ions depending on design, including different elements, their isotopes, and multiple charge states for each. A magnetic filter, also known as a Wien filter, is often used to select specific ions based on their velocity.

The implanting species, also called dopants, vary considerably with the application. Boron, arsenic and phosphorous are the most common dopants in semiconductor applications, while oxygen and nitrogen are used to process metals. Dopants for quantum applications include phosphorous,1 nitrogen,2–5 silicon,6–8 and germanium,9, and rare earth elements such as erbium.10

Implanters are often categorised by the ion beam current at the target, either low, medium or high. High-current systems for commercial applications operate at up to tens of milliamps and process hundreds of wafers per hour. On the other hand, Q-One operates at extremely low currents when implanting single ions, often tens of femtoamps.11

The ion dose is the integral of the ion current per unit area over time, measured in ions per square centimetre (ions/cm2). The dose determines the concentration of the dopant in the target. Common dose values are in the range 1016 – 1018 ions/cm2.

Beam Energy & Ion Stopping

The energy of the ion beam is a crucial parameter in ion implantation processes as it has several significant effects. The energy is the product of the accelerator voltage and the ion’s charge state, measured in electron volts (eV). For example, a Bi2+ ion accelerated in a 30 kV field has an energy of 60 keV.

Ions hitting a target lose their kinetic energy through collisions with the nuclei and electrons of the material until they stop. The depth to which the ions penetrate depends on their energy and mass, the target mass, and the beam’s angle to the crystal plane in the case of a single crystal. Higher energies penetrate further for a given mass, while lighter elements penetrate further than heavy elements for a given energy.12

The energy range of ion implantation instruments can range from 1 keV to several MeV. Q-One operates in the range of 5 – 30 kV, with an option to extend this to 40 kV. In this range, the ions penetrate approximately 5 – 100 nm beneath the surface – suitable for most quantum applications.

Comparison of SRIM simulations of phosphorous and bismuth implanted into silicon at 25 keV.13 Bismuth penetrates the target less than phosphorous but shows far less straggle due to its higher mass. Courtesy of the University of Surrey.

As the ions stop, they become laterally displaced from their initial trajectory, known as straggle. Straggle is an important consideration when it comes to single-ion implantation. The precision with which ions are positioned is the sum of the beam diameter at the target plus the lateral straggle. In some instances, the straggle is much larger than the beam and, therefore, the limiting factor.

Straggle is proportional to the energy and inversely proportional to the mass. So lowering the implantation energy and selecting heavier elements results in greater precision by reducing the straggle.

Damage

Ion implantation is a violent process. The projectile transfers a large amount of its kinetic energy to the target atoms, displacing them from the lattice sites. The primary collisions result in secondary collisions, and so on, in a process known as collision cascade.

The collision cascade forms a variety of defects in the material, including vacancies, interstitials, amorphous zones, stacking faults, and dislocation loops, among others.12 Thermal annealing post-implantation restores the crystalline order and allows the device to function. High temperatures can also cause diffusion of the implanted atoms, so annealing steps must be designed carefully.

Q-One Single Ion Implantation

Sputter Yield

The beam energy also has an important effect on the sputter yield. Ions impinging on a surface do not just penetrate the surface. They also sputter material, which is an important consideration, particularly at low energy.

The sputter yield (Y) is the mean number of atoms removed from the target surface per incident ion. If Y > 1, the ions remove more target material per implanted ion, resulting in erosion. If Y < 1, the ions sputter fewer target atoms per implanted ion, and material builds up. When Y = 1, there is a one-for-one replacement of target atoms with implanted ions.

Holmes et al. use TRIDYN simulations to model the sputter yield of 28Si impinging natural silicon at various energies.6 They show that the three regimes are energy-dependent and that Y = 1 at two energies, 3 and 45 keV, resulting in a planar surface. However, operating at 45 keV produces a much deeper implant and is thus more suitable for device fabrication.

Want to know more about Q-One and how it can impact your research? Get in touch with our team today, and we’d be happy to help.

References

  1. Morello, A. et al. Single-shot readout of an electron spin in silicon. Nat. 2010 4677316 467, 687–691 (2010) https://doi.org/10.1038/nature09392.
  2. Gruber, A. et al. Scanning confocal optical microscopy and magnetic resonance on single defect centers. Science (80-. ). 276, 2012 (1997) https://doi.org/10.1126/science.276.5321.2012.
  3. Naydenov, B. et al. Increasing the coherence time of single electron spins in diamond by high temperature annealing. Appl. Phys. Lett. 97, 242511 (2010) https://doi.org/10.1063/1.3527975.
  4. Brouri, R., Beveratos, A., Poizat, J.-P. & Grangier, P. Photon antibunching in the fluorescence of individual color centers in diamond. Opt. Lett. 25, 1294 (2000) https://doi.org/10.1364/OL.25.001294.
  5. Mizuochi, N. et al. Electrically driven single photon source at room temperature in diamond. Nat. Photon. 6, 299 (2012) https://doi.org/10.1038/nphoton.2012.75.
  6. Holmes, D. et al. Isotopic enrichment of silicon by high fluence 28Si- ion implantation. Phys. Rev. Mater. 5, 014601 (2021) https://doi.org/10.1103/PhysRevMaterials.5.014601.
  7. Wang, C., Kurtsiefer, C., Weinfurter, H. & Burchard, B. Single photon emission from SiV centres in diamond produced by ion implantation. J. Phys. B At. Mol. Opt. Phys. 39, 37 (2006) https://doi.org/10.1088/0953-4075/39/1/005.
  8. Neu, E. et al. Single photon emission from silicon-vacancy colour centres in chemical vapour deposition nano-diamonds on iridium. New J. Phys. 13, 025012 (2011) https://doi.org/10.1088/1367-2630/13/2/025012.
  9. Iwasaki, T. et al. Germanium-Vacancy Single Color Centers in Diamond. Sci. Reports 2015 51 5, 1–7 (2015) https://doi.org/10.1038/srep12882.
  10. Yin, C. et al. Optical addressing of an individual erbium ion in silicon. Nat. 2013 4977447 497, 91–94 (2013) https://doi.org/10.1038/nature12081.
  11. Cassidy, N. et al. Single Ion Implantation of Bismuth. Phys. status solidi 218, 2000237 (2021) https://doi.org/10.1002/pssa.202000237.
  12. Rimini, E. Ion Implantation: Basics to Device Fabrication. Ion Implant. Basics to Device Fabr. (1995) https://doi.org/10.1007/978-1-4615-2259-1.
  13. Ziegler, J. F., Ziegler, M. D. & Biersack, J. P. SRIM – The stopping and range of ions in matter (2010). Nucl. Instruments Methods Phys. Res. Sect. B Beam Interact. with Mater. Atoms 268, 1818–1823 (2010) https://doi.org/10.1016/j.nimb.2010.02.091.

Ion Beams and their Applications

Ion beams come in many shapes and sizes, with multiple source options and applications. A minefield of options awaits if you are unfamiliar with them. This application note will shed some light on Ionoptika’s range of ion beams to help you choose the right one for your application.

Contents

  1. Sputter vs Analytical Ion Beams
  2. C60 Beams
  3. Gas Cluster Ion Beams
  4. Liquid Metal Ion Beams
  5. Plasma Ion Beams
  6. Conclusions

Sputter vs Analytical Ion Beams

We split our range of ion beams into two groups based on their applications or purpose – sputter beams and analytical beams.

Sputter Beams

While all ion beams will sputter a surface, we make this distinction based on the area and speed with which this occurs. Sputter beams have three characteristic features: high current, large spot size, and wide field of view. They deliver a large dose of ions over a wide area as quickly as possible to optimise etch rates.

Sputter beams remove material before analysis, either for cleaning purposes or for depth profiling through the sample. Techniques employing sputter beams include SIMS, XPS, SEM, TEM, and Auger.

Analytical Beams

Rather than being used to facilitate analysis using a different technique, analytical beams perform the analysis themselves. They also have three characteristic features; wide energy range, small spot size, and variable current control. These features give the user excellent control over the beam characteristics, enabling them to optimise their experiment.

Analytical beams are primarily used for secondary ion mass spectrometry (SIMS) and work well in traditional focused ion beam (FIB) applications such as secondary electron imaging and FIB milling.

C60 Beams

C60 molecule

Carbon-60, or just C60, is a fullerene molecule consisting of sixty carbon atoms formed into a hollow sphere, with a shape very similar to a soccer ball. The first commercial C60 ion beam was produced in 2002 by Ionoptika in collaboration with the University of Manchester, and since then, we have sold more than 150 units worldwide.

Compared to monatomic ion beams, C60 beams result in a much “gentler” sputtering action, reducing molecular fragmentation and damage to sub-surface layers. When employed as an analytical beam, this gentle sputtering action significantly increases sensitivity to intact molecular ions.

As the C60 molecule is larger (~ 7 Å) than the lattice constant for most materials, it also does not channel through the lattice the way monatomic ions do, reducing preferential sputtering. C60 beams exhibit incredibly uniform sputter rates across a wide range of materials, including challenging poly-crystalline materials.

The properties of C60 make it suitable for both sputtering and analysis. Ionoptika offers three C60 ion beam systems: a broad-beam sputtering system, the C60-20S, and two analytical beams, the C60-20 and C60-40.

See our application note all about C60 beams for more information.

Gas Cluster Ion Beams

Illustration of a GCIB sputtering material from a surface

Gas cluster ion beams (GCIB) are high-energy beams of cluster ions, ideal for sputtering and analysing organic matter. GCIBs are an incredibly versatile ion source, as both the ion species and the beam properties can be varied, allowing the user to tune the beam to the needs of their experiment.

The source operates through the adiabatic expansion of gas in a vacuum, causing rapid cooling and cluster formation. The clusters are then ionised through electron bombardment and accelerated towards the target. The size of the cluster is a vital parameter, and users can adjust this over a wide range.

Organic Analysis

GCIBs are the ideal choice for sputtering organic matter. Etch rates of organic matter are orders of magnitude higher than for metals or semiconductors, making cluster beams such as the GCIB 10S an excellent tool for surface cleaning. The cluster distributes the ion’s energy across all constituent atoms/molecules, resulting in a very gentle sputtering effect and almost no damage to layers underneath—GCIBs perform much better than C60 on both fronts.

GCIBs must be operated at high energy to maximise their benefits for SIMS, as the secondary ion yield increases as a function of beam energy. We currently offer a 40kV variant, the GCIB 40, and a 70kV variant, the GCIB SM.

The J105 SIMS utilises the benefits of gas cluster beams for organic analysis. Combining the gentle sputter action of large cluster ions with increased secondary ion yield has extended the usable mass range to > m/z 2500.

Choice of gas

The versatility of GCIBs comes from having a choice of input gas. Argon is the most common as it is an inert gas that forms clusters easily, but Ar/CO2 mixtures and pure CO2 gas are also becoming standard for SIMS applications.

The stronger van der Waals forces between CO2 molecules result in much larger clusters than would be available for Ar – up to 20,000 in some cases. A wider range provides greater control of the all-important E/n value (energy per nucleon). Research has shown that optimising E/n results in an enhancement of the secondary ion signal. The presence of O– ions at the surface may also improve the ionisation probability – further enhancing ion yield.

We have recently developed a GCIB source that runs on water vapour, which is currently available as an optional add-on for the J105 SIMS. Water molecules have even greater binding energy and can form enormous clusters of up to 60,000 molecules. Water clusters provide secondary ion yields up to 500 times greater than argon and are the best choice for state-of-the-art biological SIMS.

See our application note on choosing the best GCIB for your application for more detailed information.

Liquid Metal Ion Beams

Liquid metal ion beams, also known as LMIS, or LMIG, are a well-established source technology. The source operates by a liquid metal reservoir feeding a blunt tungsten tip, from which a strong electric field extracts ions. Due to their elegant and reliable design, FIB systems have been using LMIS for decades. Ionoptika offers a 25 kV LMIG system in two variants; the IOG 25AU gold-cluster system and the IOG 25GA gallium system.

Liquid metal beams produce monatomic or small-cluster ion beams, such as Au+, Ga+, and Au3+. They feature high currents and small spot sizes (< 100 nm), making them ideal for high-resolution analysis applications.

Small, high-energy ions can penetrate far beneath the surface before dissipating their energy. Known as channelling, this causes significant sub-surface damage, making depth profiling unreliable. It also results in considerable fragmentation, making LMIS more suited to analysing hard materials.

Plasma Ion Beams

Plasma ion beam

Plasma ion sources are characterised by incredibly high brightness, making them ideal for high throughput applications. A single plasma source can run on various gases without changing parts, providing flexibility. Gases available for our plasma ion beams include hydrogen, helium, oxygen, nitrogen, argon, and xenon.

Plasma sources are monatomic and do not form clusters, resulting in lower energy distributions and smaller spot sizes. Combined with their high brightness, this leads to a very high current density beam.

Plasma beams are an excellent choice where the primary goal is high-volume etching or milling. For analysis purposes, plasma beams best suit harder materials such as metals, semiconductors, and inorganics.

FLIG – Floating Low Energy Ion Beam

The FLIG 5 is a unique ion beam system based on a floating column design. The design enables ultra-low energy operation to 200 eV while still delivering a high current. Operating at such low impact energies significantly reduces the beam’s penetration depth, improving the depth resolution. Due to its high performance at ultra-low energies, the FLIG 5 has been the industry standard for shallow junction depth profiling for almost two decades.

Conclusion

The table below compares Ionoptika’s ion beam products under several categories discussed in this article (best viewed on desktop).

ION BEAM SPECIES ENERGY RANGE MIN SPOT SIZE BEAM CURRENT APPLICATION BEST FOR
C60 Ion Beams
C60-20S C60+, C60++, C60+++ 5 – 20 kV 100 μm 50 nA SPUTTER Organic, biological, inorganic, metals
C60-20 C60+, C60++, C60+++ 5 –20 kV 2 μm 2 nA ANALYTICAL Organic, biological, inorganic, metals
C60-40 C60+, C60++, C60+++ 10 – 40 kV 300 nm 1 nA ANALYTICAL Organic, biological, inorganic, metals
Gas Cluster Ion Beams
GCIB 10S Arn+, (CO2)n+, or (Ar/CO2)n+ 1 – 10 kV 250 μm 60 nA SPUTTER Organic & biological, polymers
GCIB 40 Arn+, (CO2)n+, (Ar/CO2)n+, or (H2O)n+ 5 – 40 kV 3 μm 200 pA ANALYTICAL Organic & biological, polymers
GCIB 70/SM Arn+, (CO2)n+, (Ar/CO2)n+, or (H2O)n+ 20 – 70 kV 1.5 μm 300 pA ANALYTICAL Inorganic, organic & biological, polymers
Liquid Metal Ion Beams
IOG 25AU Au+, Au++, Au2+, Au3+, Au3++ 5 – 25 kV 100 nm 10 nA ANALYTICAL Inorganics, metals, semiconductors
IOG 25Ga Ga+, 69Ga+ 5 – 25 kV 50 nm 20 nA ANALYTICAL Inorganics, metals, semiconductors
Plasma Ion Beams
IOG 30ECR N2+, O2+, Ar+, & Xe+ 5 – 30 kV 500 nm 500 nA ANALYTICAL Semiconductors, metals, inorganics
IOG 30D H2+, He+, N2+, O2+, & Ar+ 5 – 30 kV 500 nm 500 nA ANALYTICAL Semiconductors, metals, inorganics
FLIG 5 H2+, He+, N2+, O2+, & Ar+ 0.2 – 5 kV 15 μm 500 nA ANALYTICAL Semiconductors, depth profiling

ToF SIMS – Time of Flight Secondary Ion Mass Spectrometry

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What is ToF SIMS? What is it used for, and what sort of information can it provide? Which samples are suitable (and which are not)? In this series, we will answer all these questions and more.

Time-of-Flight Secondary Ion Mass Spectrometry (ToF SIMS) is a surface analysis technique used to study the chemical composition of solid surfaces and thin films in three dimensions.

Illustration describing ToF SIMS

A focused beam of primary ions bombards a target surface, creating a plume of neutral atoms/molecules, secondary ions, and electrons. The secondary ions are collected and analysed using a time-of-flight mass spectrometer. The mass spectrometer measures an ion’s mass-to-charge ratio (m/z) by precisely timing how long it takes to reach the detector – the “time of flight”.

By scanning the primary ion beam across an area of the sample, a chemical map of the surface is formed pixel by pixel. Scientists and technicians use ToF SIMS daily for fundamental research, routine analysis, and quality control in academic and industrial settings.

For many years, the limitations of the primary ion beam confined the analysis to looking at atomic species and small molecules. With advances in instrument and ion beam design, modern instruments such as the J105 SIMS are now routinely imaging large intact molecules. These new capabilities have caused an explosion in new applications, and more papers are published each year in bio and bio-related fields using ToF SIMS.

Anatomy of a ToF SIMS instrument

ToF SIMS instruments are often larger and more expensive than most other analytical instruments found in a lab. High-vacuum conditions (< 1×10-6 mbar) are required to prevent ions from colliding with gas molecules in the air, requiring bigger vacuum pumps, more robust seals, and additional precautions to prevent leaks.

Graphic showing the operation of the J105 SIMS instrument from Ionoptika.
Operation of the J105 SIMS ToF SIMS instrument. 1. The ion beam bombards the sample releasing secondary ions, electrons, and neutrals. 2. The secondary ions are collected. 3. Secondary ions are cooled and focused into the mass spectrometer. 4. The mass spectrometer records the flight time of the ions and converts this to a mass spectrum.
Primary componentsSecondary components
Sample analysis chamber (SAC)Sample introduction System
Primary ion beamCryogenic cooling for low-temperature analysis
Secondary ion extraction opticsCharge compensation, e.g., electron beam
Mass spectrometerSecondary electron imaging

Key Benefits of ToF SIMS

  • Spatial resolution. ToF SIMS achieves significantly higher spatial resolutions than other imaging methods, thanks to beam sizes as small as a few hundred nanometres.
  • Speed. The time-of-flight mass spectrometer operates at much higher rates than other MS techniques. ToF SIMS instruments can run at speeds up to 1000 pixels per second.
  • 3D imaging. The primary ion beam removes a small amount of material each time it scans across the surface. By making multiple passes over the same area, a 3D map of the material builds up layer by layer.
  • Sensitivity. Small spot sizes and shallow impact craters result in tiny analysis volumes, which require great care to prevent signal loss. As a result, SIMS is generally more sensitive than other forms of mass spectrometry.
  • Dynamic range. The ions in a ToF SIMS spectrum can range from a single hydrogen ion to intact protein molecules several thousand daltons in size.
  • Applications. The breadth of applications for ToF SIMS is enormous, ranging from metallurgy to fundamental biology and most things in-between.

Applications of ToF SIMS

ToF SIMS provides a detailed three-dimensional chemical map of a sample. Information about the atoms and molecules that make up the sample, their distribution, and any contamination present are all revealed. This type of information is beneficial for many applications.

Academic research labs, industrial quality control, and research organisations use ToF SIMS daily. Disciplines as diverse as materials science, analytical chemistry, biology, geology, pharmaceutical science, and many others benefit from the detailed chemical information ToF SIMS provides.

2D Imaging

2D images are the most common mode of operation for ToF SIMS applications, whereby the ion beam scans the surface, acquiring a mass spectrum at each pixel. The image resolution can vary from a few hundred pixels to over four million.

Images of individual mass channels show the precise distribution of ions across the field of view. Overlaying multiple mass channels can show the distribution of different ions and how they relate to each other.

The image below shows three individual ion images and an overlay image representing different components of a biological tissue sample.

ToF SIMS image of a rat cerebellum

Spectrometry

Analysis of a ToF SIMS spectrum provides information on the atomic or molecular makeup of the sample and can inform about the general abundance of various compounds. It is also possible to determine atomic ratios in some cases, but this requires well-controlled samples and careful use of reference materials.

Tandem mass spectrometry is a feature on most major ToF SIMS instruments and is extremely useful for confidently identifying ions. Tandem MS, also known as MS/MS, or MS2, involves isolating a secondary ion of interest, fragmenting it, and collecting the resulting fragments in a mass spectrum. By analysing the daughter peaks, it is possible to determine the parent ion with a high level of precision.

MS/MS spectrum of the phospholipid PC34:1+K acquired on the J105 SIMS instrument.
A Tandem MS spectrum of a phospholipid species in a tissue sample acquired on the J105 SIMS instrument. Analysing the fragment pattern confirms the identity of the parent ion as PC34:1+K.

Depth profiling

A powerful analysis mode, depth profiling involves etching vertically through the sample and acquiring a mass spectrum at every layer. The result is a profile of all atoms/molecules through the sampled volume. Large cluster ions reduce damage to sub-surface layers, minimising interlayer mixing and maximising depth resolution. With the right ion beam and sample combination, depth resolution as low as a few nanometres is possible.

Depth profile through the NIST Ni/Cr standard reference material using a C60 beam.

Depth profile through the NIST Ni/Cr standard reference material using a C60 beam, showing 5 nm depth resolution.

3D Imaging

The feature that sets ToF SIMS apart from other mass spectrometry and analytical techniques is the ability to acquire 3D data sets. Like a depth profile, a 3D analysis involves acquiring many 2D layers repeatedly over the same area, etching material with each pass, and building up a three-dimensional view of the sample. Large cluster ions are ideal for 3D analysis as they produce very little damage and can therefore be used to etch and analyse the sample simultaneously.

Unlike techniques like AFM, which capture the 3D topography of the sample, SIMS cannot distinguish 3D objects from flat objects. The technique works best for flat samples with layers of interest below the surface, as in the OLED example below. It is possible to reconstruct the topography of a non-flat sample; however, this requires prior knowledge of the material structure. 

3D ToF SIMS image of an OLED screen, showing the different components of each sub-pixel unit.
This 3D ToF SIMS image of an OLED screen is acquired on the J105 SIMS using a 70kV water cluster primary ion beam. The RGB subpixel units appear at different distances from the surface, depending on their colour.

Read more about the applications of ToF SIMS in our Application Notes section. Or, to dive deeper into more advanced topics, check out the list of publications using our equipment here. You might also like to learn more about how the J105 SIMS operates, which you can read here.

How to choose the best GCIB for every application

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How the J105 SIMS works: An introductory guide

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The J105 SIMS is a state-of-the-art 3D imaging ToF SIMS combining innovative design with cutting-edge science that has redefined ToF SIMS. Designed to exploit the benefits of cluster ion beams, the J105 delivers exceptional sensitivity to molecular ions, 3D MS imaging, and consistent performance across all samples.

In this article, we aim to give you an overview of how the J105 SIMS works, as it is quite different to other ToF SIMS. We will guide you through the various features of the instrument and explain their purpose, how they work, and what the benefits are.

How the J105 SIMS works: an introductory guide

1.      The Ion Beam

The J105 was designed to get around many of the limitations faced by traditional ToF SIMS instruments, particularly for biological samples. One of the ways this is achieved is by not pulsing the primary ion beam, but instead running it in DC, or continuous mode. This is a major advantage and is what makes the J105 a very different instrument to most other ToF SIMS.

One of the biggest advantages of having a continuous beam is that any ion beam, no matter what size, can be used as the primary source. This gives the user a lot of choice when designing their experiment. We’ll cover the intricacies of different ion beams in a different article, but for the purposes of this discussion we’ll focus on gas cluster ion beams (GCIB).

A GCIB typically consists of thousands of constituent atoms, giving it a collective molecular weight anywhere from 100,000 g/mol upwards. Under typical acceleration voltages (kV), such a large ion moves very slowly, requiring longer pulses and on a conventional ToF would result in poor mass resolution. By running in continuous (or long pulsing) mode, the J105 is able to get around this issue and take full advantage of the benefits of using GCIBs.

The other major advantage of running in DC mode is that focusing the ion beam to a fine spot can be prioritized without affecting the performance of the mass spectrometer. With our most powerful GCIB, the GCIB SM, the optics have been designed to enable spot sizes of just 1.5 µm, combining greater spatial resolution with high-sensitivity mass spectrometry. The benefits of this are clear, and have been highlighted recently by the pioneering work published in Science.

Benefits: Simultaneous high-sensitivity mass spectrometry with high-spatial resolution.

View of a sample through a window

2.      The Extraction Optics

As the primary beam is not pulsed, in order to determine a time-of-flight the secondary ion beam is pulsed instead. This is done by the Buncher, but the extraction optics play a key role is controlling the secondary ion beam prior to that step.

Secondary ions extracted from the surface contain a lot of energy making them difficult to control. In order to form the secondary ions into a controlled beam, they enter an RF quadrupole filled with N2, which slows the ions down through the process of collisional cooling.

This is a crucial step, as it decouples the effects of the primary beam and the sample from the secondary ions. By effectively wiping the memory of any interaction on the surface, this step enables the J105 to analyse samples with complex topography without any loss of mass spec performance.

Benefits: Consistent performance that is independent of ion beam or sample topography.

The analysis chamber of the J105 SIMS

3.      The Buncher

In many ways the heart of the instrument, the Buncher is what takes a continuous stream of secondary ions coming from the quad and forms them into a very short pulse. In order to measure the time-of-flight without pulsing the primary beam or the extraction, the Buncher creates an asymmetric pulse that focuses all ions of the same mass to a single time focus, T0. This is an essential step, and is what ultimately determines the mass resolution.

Benefits: High mass accuracy, high mass resolution.

J105 SIMS reflectron & mass analyser

4.      Tandem MS & Time-of-Flight

As with any form of mass spectrometry, definitively assigning peaks requires a secondary validation step. One way to do this is through tandem MS, whereby a parent ion is selected to undergo fragmentation and the resulting spectrum is used to determine the exact form of the parent. The J105 SIMS was the first SIMS instrument to introduce tandem MS, and is included as standard on all our instruments.

When a user selects an ion of interest, it is directed into a high-energy collision cell filled with N2, producing characteristic fragment ions. Whether running an MS1 or MS2 experiment, ions then enter the 1500 mm long reflectron before being detected.

Benefits: Tandem MS for accurate peak identification, high mass resolution.

The J105 SIMS contains several innovative design features that combine to produce an instrument like no other, optimized to enable both maximum sensitivity and maximum spatial resolution simultaneously from any ion beam. Consistent performance is guaranteed, as the mass spectrometer delivers high mass resolution (> 10,000) and mass accuracy (< 5 ppm) that are completely independent of the ion beam and the sample environment.

The J105 SIMS is the ideal tool for a wide range of applications and sample types, including biological research, pharmaceuticals, thin films, polymers, energy applications and many more. To find out if the J105 might be the right instrument for you, or to arrange a demonstration, please get in touch via our Contact Page.

High-resolution multi-omics profiling of individual cells

In a landmark publication, Tian et al. demonstrate the feasibility of combined GCIB/C60 SIMS imaging for multi-omics profiling in the same tissue section at the single-cell level.

A new approach

Multi-omics data are vital to understanding normal regulatory processes and are essential for designing new anti-cancer modalities. Unfortunately, sample preparation methods between different omics are typically incompatible. As such, it is nearly impossible to correlate multiple omics profiles within the same sample, let alone their spatial co-localisation at the single-cell level.

The new approach developed by Tian et al. thus represents a significant leap forward.

The study, reported in the journal Analytical Chemistry, uses a multimodal approach using the J105 SIMS to correlate different cell types. First, water cluster SIMS maps the lipids and metabolites in individual cells. Then, multiplexed SIMS imaging with a high-resolution C60 beam maps the same tissue section stained by lanthanide tagged antibodies.

Close up picture of a microscope

Multiplexed SIMS imaging involves linking specific lanthanide isotopes to antibodies and applying them to the tissue. Subsequent SIMS imaging of the lanthanides maps multiple cellular epitopes at sub-cellular resolution.

The combined approach of water cluster SIMS plus multiplexed ion beam imaging on the same tissue section enables mapping lipids, proteins, and metabolites at the single-cell level.

High-resolution multi-omics

Cryogenic water cluster SIMS was conducted on the J105 SIMS at 1.6-micron beam spot size on fresh frozen sections of invasive ductal carcinoma/ductal carcinoma in situ (IDC/DCIS) tissue. This analysis was followed by staining with lanthanide-tagged antibodies on the same frozen-hydrated tissue and imaging the same region using a C60 beam with a 1.1 µm spot size.

Workflow schematic. The frozen-hydrated IDC/DCIS sample is first analysed using water cluster SIMS with a 1.6 µm spot size. The sample is then stained with lanthanide-tagged antibodies and imaged with C60-SIMS with a 1.1 µm spot size. Image reproduced from Anal. Chem. 2021, 93, 23, 8143-8151.

The first results

Water cluster SIMS revealed the distributions and intensities of more than 150 lipids and important metabolites up to m/z 2000. HCA analysis revealed considerable variation between the location of cluster SIMS identified ions and the nine C60-SIMS cell markers.

This work represents the first successful attempt to profile proteins, lipids, and metabolites on the same tissue at the single-cell level. GCIB-SIMS, especially water clusters, has demonstrated its unique ability to detect lipids and metabolites in biological samples at unprecedented resolutions.

Picture of a person placing a sample into the J105 SIMS

Using a combined SIMS imaging approach enables the correlation of different cell types with their metabolic and lipidomic status. It offers valuable information about proteins, lipids, and metabolites on the same sample and at the same resolution.

The J105 SIMS provides a unique platform for this multimodal SIMS approach. The only instrument to offer water cluster SIMS plus multiplexed ion beam imaging, the J105 also provides high mass accuracy and tandem MS capabilities for accurate ion assignment.

Read the complete publication here.

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Cocaine metabolite imaging in fingerprints with Water Cluster SIMS

Detection of drug compounds and their metabolites in natural environments is a critical topic for both forensic and pharmaceutical applications, and requires overcoming some of the limitations in existing microscopic and analytical techniques.

Time of Flight Secondary Ion Mass Spectrometry (ToF SIMS) is a powerful analytical technique capable of providing detailed chemical and spatial information about a surface, and as such has recently been employed in a number of forensic studies for drug and metabolite detection. However, ToF SIMS can suffer from low sensitivity due to insufficient ionisation efficiency, and this is particularly true for complex biomaterials, i.e. those of most interest to forensic and medical analysts.

Recently, we have led the development of a powerful unique gas cluster ion beam (GCIB) using water clusters. The Water Cluster Source is capable of enhancing ion yields by many orders of magnitude compared to other conventional ion beams (C60+, Bi3+ etc.), and is particularly effective for biomolecular imaging and 3D analysis of organics such as tissue, cells, fingerprints, etc.

Plot displaying increase in signal intensity using water clusters
Water clusters enhance sensitivity to intact biomolecules such as lipids, even compared to current state-of-the-art GCIB technology.

In this application note, an experimental fingerprint detection approach using the Water Cluster Source identifies traces of ingested cocaine on human skin. The use of the J105 SIMS equipped with the Water Cluster Source (Water Cluster SIMS) provides both visualisation of the latent fingerprint as well as discrimination between contact-only and ingested cocaine by looking for metabolites of the drug excreted through the skin.

Detectable levels of metabolite in a fingerprint are extremely low, for instance 25 mg of ingested cocaine excretes less than 2.5 ng/mL in sweat,1 and previous attempts using other mass spectrometry imaging (MSI) techniques such as MALDI and DESI were unsuccessful. Using Water Cluster SIMS, it was possible not only to detect the metabolite, but also to generate a high-contrast chemical map of the entire fingerprint.

The fingerprint specimen, provided by University of Surrey, was collected on a piece of silicon wafer from a donor who had previously ingested cocaine,2 then a ToF-SIMS analysis was acquired on an 18×6 mm2 area with a 70 kV (H2O)29k+ primary ion beam in the J105 SIMS.

Figure 1(a) shows the chemical image of the 290.14 m/z signal, demonstrating the characteristic fingerprint features with ridges, valleys, as well as sweat pores. Due to the high mass accuracy of the J105 SIMS, this signal is confidently annotated as the cocaine metabolite benzoylecgonine (BZE, C16H20NO4+). Figure 1(b) shows a colour overlay of BZE (magenta) and the cocaine molecular ion (C17H22NO4+, 304.15 m/z – yellow). As expected, cocaine was observed in particulate form (see arrow) due to direct contact of the donor with the powder, and is not co-localised with BZE.

ToF SIMS image of cocaine metabolite BZE in a fingerprint.
Figure 1(a) Positive ion image of BZE (C16H20NO4+, 290.14 m/z) in a fingerprint. (b) Overlay positive ion image with BZE (magenta) and cocaine (C17H22NO4+, 304.15 m/z – yellow). (c) BZE peak, with high mass accuracy and high mass resolution.

These images, with the small amounts of BZE and cocaine present, demonstrate the benefits of Water Cluster SIMS for enhancing sensitivity, particularly for trace detection of organic compounds in complex sample matrixes.

The J105 SIMS is a powerful tool for 2D and 3D molecular imaging, providing high sensitivity analysis with a range of powerful features. Now featuring the new Water Cluster Source, the J105 takes another leap forward to offer even greater sensitivity and to intact molecular ions. This exciting new technology has been shown to dramatically improve the imaging of drug metabolites ingested by the body, and is a powerful tool for visualising molecular information in a wide range of applications.

To find out more about how the J105 SIMS can benefit your research, get in touch via our Contact Page.

References

  1. Kacinko, S. L., Barnes, A.J. et al. , Disposition of Cocaine and Its Metabolites in Human Sweat after Controlled Cocaine Administration, Clinical Chemistry, 51, 2085 (2005). https://doi.org/10.1373/clinchem.2005.054338
  2. Jang, M., Costa, C., Bunch, J. et al. On the relevance of cocaine detection in a fingerprint. Sci Rep 10, 1974 (2020). https://doi.org/10.1038/s41598-020-58856-0

GCIB-SEM: 3D electron microscopy with < 10nm isotropic resolution

GCIB-SEM is a new technique that combines high resolution electron microscopy with the damage free sputtering of gas cluster ions to produce incredible 3D tomography with less than 10 nm isotropic resolution.

Over the last two decades, gas cluster ion beams (GCIB) have become increasingly popular as add-on components for ultra-high vacuum techniques such as XPS, SPM, and SIMS. Due to their excellent combination of fast yet low-damage sputtering, GCIBs have been widely adopted as depth profiling ion beams, or as a means of cleaning samples in situ.

Very low impact energies, as little as 1 eV per atom, means cluster ions sputter material without modifying the surface chemistry, i.e. without breaking bonds. This makes GCIBs particularly effective for high-resolution depth profiling of soft materials such as polymers and organic matter.

GCIB 10S cluster schematic, and PET C 1s XPS spectrum comparing Ar1 and Ar2000.
The GCIB 10S is a powerful tool for damage-free depth profiling of polymers, organics, and other soft materials, delivering consistently superior results over monatomic beams.

Traditional sputter beams such as Ar1 typically have impact energies in the kilovolt range, resulting in not only large amounts of fragmentation to surface molecules, but also penetration of the ions beneath the surface causing further damage. This damage shows up in XPS and SIMS spectra, and limits the depth resolution of the technique.

Cluster beams also sputter soft, organic material much faster than hard, inorganic materials, making them extremely useful for removing adventitious carbon and other surface contamination without damaging the substrate — ideal for cleaning surfaces prior to analysis.

It is no surprise then that GCIBs have become so popular as add-on components for surface analysis instrumentation.

The GCIB 10S

  • 10 kV argon cluster ion source
  • Selectable clusters from Ar1 to > Ar3000
  • Real-time cluster measurement & adjustment
  • Sample current imaging
  • Gate valve for quick & easy servicing
  • Large spot size and wide scan field for even removal of material

The versatile nature of the GCIB makes it a useful tool in a variety of other techniques as well, beyond strictly surface science. In particular, the GCIB has recently been shown to be powerful tool in electron microscopy. A new technique pioneered by researchers at HHMI Janelia Research Campus combines high resolution electron microscopy with the damage free sputtering of gas cluster ions to produce incredible 3D tomography with less than 10 nm isotropic resolution.

Published in Nature Methods in 2019 the GCIB-SEM system developed by Hayworth et al. consists of a GCIB 10S from Ionoptika mounted on a Zeiss Ultra SEM. Using 1 µm thick serial sections of brain tissue, high-resolution electron imaging was interleaved with wide-area ion milling until the entire section was consumed. Full experimental details can be found in the paper linked above.

Figure detailing results achieved using GCIB SEM, by Hayworth et al
GCIB-SEM is a powerful technique for acquiring extremely detailed 3D maps on an unprecedented scale. Images from a GCIB-SEM run performed on three sequential 500-nm-thick sections of mouse cortex. bioRxiv: http://dx.doi.org/10.1101/563239.

The result is a 3D data set hundreds of microns in area by tens of microns deep, with less than 10 nm isotropic resolution throughout. Such a high resolution data set then allows researchers to map the brain structure in incredible detail. The figure above shows a 15 x 15 x 10 µm section of mouse brain, the detail of which is truly remarkable. Panel e shows a single spiney dentritic process with axons synapsing on it, while panel f shows various high-resolution 2D and 3D views of a single spiney synapse.

Other technologies used to perform similar experiments include FIB-SEM and diamond knife based sectioning, however both have their drawbacks. FIB provides the necessary resolution, but is thus far incompatible with the high-throughput needed for larger volumes, while diamond knife techniques are highly compatible with larger volumes, but lack the consistency needed at such thin cuts.

In contrast, the GCIB 10S mills away just the top few nanometres of the surface resulting in an improvement in depth resolution of a factor of 3 or more over other techniques, whilst simultaneously improving sectioning reliability. The rapid, wide area milling afforded by the GCIB 10S is also compatible with the new multi-beam SEM systems now on the market, which will enable even larger volumes to be analysed with no loss of resolution.

GCIB-SEM

  • Large-area and fast (up to 450 µm3 s-1).
  • Can be automated and is highly scalable.
  • Consistent performance over large volumes.
  • Simple, easy to maintain, and reliable.
  • Improves z resolution by a factor of 3 or more.

GCIB-SEM is a powerful technique for exploring complex materials and structures in three dimensions with extraordinary detail. For this application, control of the cluster size and current is critical to the result. Unlike other gas cluster beams, the GCIB 10S lets the user take complete control of the experiment. With real-time cluster measurement, cluster size can be tuned to the users’ needs and the settings saved for later use.

Real-time cluster measurement on the GCIB 10S

The GCIB 10S is easily installed on a range of instrumentation, from XPS and SIMS, to electron microscopes, Auger, and more. To speak with us and find out how the GCIB 10S might be right for your application, or to request a brochure, please get in touch via our Contact Page.

Why you shouldn’t overlook C60 beams just yet

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C60 cluster ion beams are a fantastic tool for analyzing both hard and soft materials. Composed of sixty carbon atoms arranged into a football shape, C60 ions combine several different features making it a great all-rounder ion beam. This is why we always recommend customers to consider including a C60 beam when specifying their J105.

As the C60 molecule is larger (approx. 7 Å) than the lattice constant for most materials, it does not experience channeling the way smaller ions such as bismuth do. As such C60 beams exhibit incredibly uniform sputter rates across a wide range of materials, and even on challenging poly-crystalline materials where there is a range of crystal orientations.

As a cluster ion, C60 also produces very shallow craters with very little, if any, subsurface damage, so etch cycles are not needed to remove damaged layers when performing depth profiles or 3D imaging. As the J105 samples 100% of the analysis volume, high sensitivity is guaranteed, and combined with spot sizes as low as 300 nm, C60 is a powerful beam for delivering maximum resolution in 2D and 3D.

NiCr Standard Depth Profile C60
C60 depth profile through the NIST NiCr standard showing <5 nm depth resolution. As there is no need to perform etch cycles to remove damaged layers, depth resolution on the J105 is limited only by the crater depth of the ion beam.

The figure below shows a 3D image of a semiconductor stack alongside a depth profile through the same, performed with a 40kV C60 beam in positive ion mode. The sample consists of layers of InSb, Al, and GaAs respectively, covered in a protective photoresist layer.

3D SIMS image of InSbAlGaAs Stack with depth profil
The J105 has one mode of operation, so amazing 3D images, high-resolution 2D images, as well as detailed depth profiles can all be obtained from a single data set.

The resulting 3D SIMS image shows the layers in amazing clarity, with very sharp interfaces. As the J105 always samples 100% of the analysis volume, high sensitivity is guaranteed. The detailed depth profile through the sample also shows the presence of dissolved Al within the InSb layer, as well as the presence of Sb in the pure Al layer.

The 40kV C60 beam is ideal for this type of sample or application due to the combination of soft organic, inorganic, and hard metallic layers within the same sample. Combined with spot sizes as low as 300 nm, C60 is a powerful beam for delivering maximum resolution in 2D and 3D, no matter what type of sample you have.

Drug detection with high-sensitivity using ToF SIMS

The high attrition rate of pharmaceutical drug compounds adds enormously to the cost of those that make it to market, so there is an urgent and growing need to identify failure at earlier stages of drug development.

In order to do so, researchers require as much information as possible. Specifically, there is a need to measure the concentration of a drug at the target in order to accurately predict its pharmacological effect. This then requires a means of generating label-free sub-cellular imaging, as fluorescent labels may affect drug chemistry, altering results.

Time of flight secondary ion mass spectrometry (ToF SIMS) is a powerful tool for label-free chemical imaging, having typically very high lateral resolution capable of resolving sub-cellular features with 3D analysis capabilities.

ToF SIMS is thus a potentially powerful analysis tool for the screening of new drug compounds. However, the use of high energy projectiles for ToF SIMS analysis can cause molecules to fragment, preventing the molecular ion from being detected. This can lead to a lot of ambiguity, for example distinguishing between a drug compound and its metabolites.

Another possible stumbling block is the issue of sensitivity, particularly for those compounds of most interest. In a recent study by the National Physical Laboratory (NPL), Vorng et al. demonstrate that the sensitivity in ToF SIMS is proportional to the Log P of that compound, such that compounds with low or negative Log P values are extremely difficult to detect.  

Log P, or partition coefficient, is a measure of hydrophobicity, and is a major factor used in pre-clinical assessment of a compound’s druglikeness.  It is advisable that a drug candidate be as hydrophilic as possible while still retaining adequate binding affinity to the therapeutic protein target, i.e. that the Log P be as low (or negative) as practicable. This presents an obvious problem for the use of ToF SIMS as an analytical tool in this context.

Cluster beam colliding with a surface.

We have recently led the development of a new type of ion source for ToF SIMS that provides unparalleled sensitivity particularly for organic species. Available exclusively on the J105 SIMS, the Water Cluster Source simultaneously reduces fragmentation while increasing ionization, for truly unparalleled sensitivity of drugs, metabolites, biomarkers, lipids, peptides and more.

Combining this new ion source with the already impressive sensitivity of the J105 SIMS, even low Log P compounds can be detected in tissue and cells, with direct, label-free imaging of the molecular compounds at sub-cellular resolutions.

To demonstrate this, we doped tissue homogenate with 4 different pharmaceutical compounds that span the range of Log P from -0.8 to 7.6. The relationship between sensitivity and Log P reported by NPL is observed in this data, however the slope of the line is greatly reduced, with only a factor of 40 between the highest and lowest values.

ToF SIMS sensitivity to drugs as a function of Log P
ToF SIMS sensitivity of four different drugs using the Water Source. Sensitivity shows a linear relationship to the partition coefficient, Log P, though the slope is not steep.

As a comparison, we performed the same experiments with a state-of-the-art Ar gas cluster ion beam and plotted the yield against that of the new Water Source. The Water Source increased sensitivity by an order of magnitude in most cases, with the largest increase being for those compounds with the lowest Log P values. This indicates that the improvement in sensitivity is greatest for those compounds that need it the most.

Comparing sensitivity of argon and water cluster beams for four different drugs
Comparing sensitivity of a state-of-the-art Ar cluster source with the Water Source. Sensitivity improves by roughly an order of magnitude when using water, with the largest increase for those compounds with lower Log P values.

As a final demonstration of the capabilities of the J105 with the Water Source, we performed tandem MS analysis on the homogenate samples. Tandem MS is an important step for confirming any assignment in mass spectrometry, however the inefficiency of the process often means it can only be performed on high intensity peaks. With the boost in sensitivity provided by the Water Source, tandem MS analysis is possible even on compounds with relatively low Log P values, such as ciprofloxacin.

Tandem MS analysis of the drug ciprofloxacin
Tandem MS performed on the J105 SIMS with a Water Source. Greater sensitivity allows definitive confirmation of many more peaks.

ToF SIMS is a potentially powerful analysis tool for the screening of new drug compounds, however research is hampered by the inherently low sensitivity to many drug candidates. The J105 SIMS in combination with the Water Cluster Source provides unparalleled sensitivity to drug compounds, particularly in complex matrices such as tissue and cells, even for low Log P compounds. This unprecedented sensitivity combined with sub-cellular imaging and high-resolution 3D imaging mean the J105 SIMS is a powerful tool for drug analysis.

To learn more about how the J105 SIMS can benefit your research or to set up a demonstration, get in touch via our Contact Page.

High-resolution molecular imaging ToF SIMS

Historically ToF SIMS has not been sensitive to intact molecules due to the excessive fragmentation caused by the primary ion beam. Now however, thanks to the progress in gas cluster ion beam (GCIB) technology over the last decade, sensitivity to intact molecular species in ToF SIMS has increased by several orders of magnitude, making it possible to achieve molecular imaging with the high-spatial resolution traditionally associated with SIMS.

The development of high-energy gas cluster beams with small spot sizes has dramatically altered the sensitivity to intact molecular species. This is enabled by the unique design of the J105 SIMS, which allows any ion beam to be used without impacting performance. So large gas cluster beams may be used while still maintaining high mass resolution, and thereby greatly improving molecular sensitivity.

It is now possible to map the distribution of lipids in biological tissue with higher resolution than ever before. This is illustrated in Figure 1, where two sphingolipid species and a glycerophospholipid species are imaged within rodent cerebellum tissue. The inset line scan demonstrates the sharp drop off in C24-OH signal, on the order of a few microns, giving researchers unparalleled clarity into the structure of their sample.

Figure 1. 2 μm per pixel lipid mapping in rodent brain tissue, analysed using a 40kV Ar4000 beam. Boundaries between sphingolipid species C24 (m/z 890.6 – blue), and C24-OH (m/z 906.6 – green), and the glycerophospholipid species PI(38:4) (m/z 885.6 – red) are clearly resolved. Inset: line scan drawn across the C24-OH signal. Data courtesy of the University of Gothenburg.

As damage to the sample molecules is minimised, a volume of material can be analysed, not just a static dose limit, resulting in higher signals and the ability to depth profile without wasting material. Significantly, for a given dose, larger cluster beams have been shown to produce higher signals from most large molecules, as illustrated in Figure 2.

Figure 2. High signal intensity with low fragmentation. (a) Normalised signal intensity for molecular and significant fragment signals from Irganox 1010 with an Ar4000 beam, showing much higher ion yields for higher beam energy. (b) Normalised signal ratios comparing levels of fragmentation for four different beam energies. Data courtesy of the University of Gothenburg.

Figure 3 shows the mass spectrum and corresponding image of the DG region of a rodent hippocampus. Using a 30 kV [CO2]3k+ beam, the distribution of GM1(36:1), GM1(38:1), and ST(18:0) were mapped at a pixel density of 2 μm per pixel. A wealth of information is contained within the spectrum, with detailed phospholipids, cardiolipin species, and high-mass ganglioside species all clearly present and identified.

Figure 3. Negative SIMS spectrum and corresponding image from DG region of rodent hippocampus, showing the range of phospholipid, ganglioside, and cardiolipin species detected. Analysis performed using a 30 kV [CO2]3k+ beam at 2 μm per pixel. Data courtesy of Pennsylvania State University.

Large molecular species such as lipids play an important role in basic cellular processes. As such it is crucial to have the correct tools with which to study these systems. The J105 SIMS, alongside the development of new gas cluster ion sources, is pushing the capabilities of ToF-SIMS, both in terms of the mass detection limits, and the limits of spatial resolving power, enabling researchers to probe further and discover more.

For further information about our instruments or to arrange a demonstration, please get in touch via our Contact page.

Detecting pollutants in a Cherry Blossom leaf

Plant samples such as leaves are a challenging sample for ToF SIMS. Composed of insulating materials such as cellulose (cell walls) and lipophilic coatings (cuticular layer), charge build up can affect measurement quality. Using an electron gun during analysis can alleviate the charging effect and enables 3D analysis of the surface of a leaf.

Cherry blossom leaves (Prunus serrulata) collected in a busy city were analysed on the J105 SIMS using a 40 kV C60 ion beam. Pieces of blossom leaves were mounted onto double sided tape, attached to a sample stub, and gently pressed down in the corners to ensure best possible contact without deforming the leaf surface. Overview images were acquired on both sides of the leaf surface, with a spatial resolution of 1 µm per pixel and a primary ion dose of 2.2×1013 ions/cm2.

Experimental Conditions

Ion Beam:40kV C60+
Dose:2.2×1013 ions/cm2
Spatial Resolution:1 μm
Charge Compensation:60V Electron Gun, 25V Stage Bias

Without charge compensation, no secondary ions could be detected. Applying an ever-increasing stage bias would produce secondary ions temporarily. Only a combination of charge compensation methods via a 25 V pulsed stage bias and electrons emitted at 60 V beam energy enable us to generate an image of the leaf surface as well as steady signal during depth profiling.

Figure 1. Analysing the surface of a cherry blossom leaf.

Surface analysis reveals the outline of single plant cells. The outlines of the cells contain CaOH (m/z 56.97), while inorganic compounds such as K2O+ (m/z 93.92), Na2Cl+ (m/z 80.95), and Fe+ (m/z 55.93) are unevenly dispersed on the surface of the leaf. All compounds identified across the uneven leaf surface have a mass accuracy < 5 ppm (Table 1).

Analysis also revealed the surface to be coated with an even layer of organic compounds represented by molecules containing aromatic structures, e.g. tropylium ion, C7H7+ (m/z 91.05). Wax coatings on plants take the form of long aliphatic carbon chains, so aromatic structures such as these are unexpected and may indicate the presence of gasoline pollutants such as BTX (benzene, toluene, xylene).

Analysis of complex, insulating, and uneven samples such as these is made routine on the J105 SIMS.

Depth profile analysis reveals that as the cells are etched away, the layer of aromatic compounds reappears on the underside of the sample. Additionally, potassium containing substances are detected that are not present on the surface and only occur within certain cell walls (Figure 1 inset, green).

Imaging depth profile through a leaf showing CaOH (red), K2O (green), and C7H7 (blue).

Repeating the analysis on the lower epidermis reveals a high concentration of aromatic signals surrounding the stomata (Figure 2, green). It is known that plants can absorb pollutants such as BTX, mainly through the stomata, giving further evidence to the origin of these compounds.

Analysis of lower epidermis. Concentration of aromatic signals such as C7H7+ around the stomata may indicate uptake of pollutants such as BTX.

Analysis of complex, insulating, and uneven samples such as these is made routine on the J105 SIMS. Performing high-resolution 3D analysis with high sensitivity creates a more complete picture, enabling a greater understanding of the sample and its environment.

We gratefully acknowledge NESAC/BIO and the University of Washington for the use of their data in this work.

For further information about our instruments or to arrange a demonstration, please get in touch via our Contact page.