Nanoscale secondary ion mass spectrometry

Key takeaways

Nanoscale secondary ion mass spectrometry belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Nanoscale secondary ion mass spectrometry to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Nanoscale secondary ion mass spectrometry from memory before moving on to harder problems.

Reference excerpt

NanoSIMS (nanoscale secondary ion mass spectrometry) is an analytical instrument manufactured by CAMECA which operates on the principle of secondary ion mass spectrometry. The NanoSIMS is used to acquire nanoscale resolution measurements of the elemental and isotopic composition of a sample. The NanoSIMS is able to create nanoscale maps of elemental or isotopic distribution, parallel acquisition of up to seven masses, isotopic identification, high mass resolution, subparts-per-million sensitivity with lateral resolution down to 30 nm.

The original design of the NanoSIMS instrument was conceived by Georges Slodzian at the University of Paris Sud in France and at the Office National d'Etudes et de Recherches Aérospatiales. There are currently around 60 NanoSIMS instruments worldwide.

How it works The NanoSIMS uses an ion source to produce a primary beam of ions. These primary ions erode the sample surface and produce atomic collisions, some of these collisions result in the release of secondary ion particles. These ions are transmitted through a mass spectrometer, where the masses are measured and identified. The primary ion beam is rastered across the sample surface and a ‘map’ of the element and isotope distribution is created by counting the number of ions that originated from each pixel with at best a 30 nanometer lateral resolution, 10-50 times greater than conventional SIMS. This is achieved by positioning the primary probe in close proximity to the sample using a coaxial lens assembly. The primary ion beam impacts the sample surface at 90°, with the secondary ions extracted back through the same lens assembly. This allows for the isotopic composition of individual cells to be distinguished at parts per million (ppm) or parts per billion (ppb) range. The main drawback of this set up is that the primary and secondary ion beams must be of opposite polarity which can limit which elements can be detected simultaneously. NanoSIMS can detect minute mass differences between ions at the resolution of M/dM > 5000, where M is the nominal mass of the isotope and dM is the mass difference between the isotopes of interest. The high mass resolution capabilities of NanoSIMS allows for different elements and their isotopes to be identified and spatially mapped in the sample, even if very close in mass. The mass spectrometer is capable of multicollection, meaning up to 5 (NanoSIMS 50) or 7 (NanoSIMS 50 L) masses can be simultaneously detected, from hydrogen to uranium, though with limitations. The relatively large number of masses helps eliminate measurement errors as possible changes in instrumental or sample conditions that may occur in between runs are avoided. The ion beam must either be set to detect negative or positive ions, commonly completed by using a cesium+ or oxygen- beam, respectively. The high mass resolution achievable is particularly relevant to biological applications. For example, nitrogen is one of the most common elements in organisms. However, due to the low electron affinity of the nitrogen atom, the production of secondary ions is rare. Instead, molecules such as CN can be generated and measured. However, due to isotope combinations, such as the isobars 13C14N-, and 12C15N-, nearly identical molecular weights of 27.000 and 27.006 daltons, respectively, will be generated. Unlike other imaging techniques, where 13C14N and 12C15N cannot be independently measured due to nearly identical masses, NanoSIMS can safely distinguish the differences between these molecules allowing isotopic spiking experiments to be conducted.

The physics of NanoSIMS The magnetic sector mass spectrometer causes a physical separation of ions of a different mass-to-charge ratio. The physical separation of the secondary ions is caused by the Lorentz force when the ions pass through a magnetic field that is perpendicular to the velocity vector of the secondary ions. The Lorentz force states that a particle will experience a force

F = q [ E + ( v × B ) ] {\displaystyle \mathbf {F} =q\left[\mathbf {E} +(\mathbf {v} \times \mathbf {B} )\right]}

when it maintains a charge q and travels through an electric field E and magnetic field B with a velocity v. The secondary ions that leave the surface of the sample typically have a kinetic energy of a few electron volts (eV), although a rather small portion have been found to have energy of a few keV. An electrostatic field captures the secondary ions that leave the sample surface; these extracted ions are then transferred to a mass spectrometer. In order to achieve precise isotope measurements, there is a need for high transmission and high mass resolution. High transmission refers to the low loss of secondary ions between the sample surface and the detector, and high mass resolution refers to the ability to efficiently separate the secondary ions (or molecules of interest) from other ions and/or ions of similar mass. Primary ions will collide with the surface at a specific frequency per unit of surface area. The collision that occurs causes atoms to sputter from the sample surface, and of these atoms only a small amount will undergo ionization. These become secondary ions, which are then detected after transfer through the mass spectrometer. Each primary ion generates a number of secondary ions of an isotope that will reach the detector to be counted. The count rate is determined by

I ( i M ) = d b × S × Y × X M × A i × Y i × T {\displaystyle I(^{i}M)=d_{\mathrm {b} }\times S\times Y\times X_{\mathrm {M} }\times A_{\mathrm {i} }\times Y_{\mathrm {i} }\times T}

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Illustrations

Worked examples

Example 1 — a first encounter with Nanoscale secondary ion mass spectrometry

Start with the simplest possible case. Write down what Nanoscale secondary ion mass spectrometry claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Nanoscale secondary ion mass spectrometry before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Nanoscale secondary ion mass spectrometry ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Nanoscale secondary ion mass spectrometry

In research: Nanoscale secondary ion mass spectrometry appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Nanoscale secondary ion mass spectrometry in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Nanoscale secondary ion mass spectrometry is common in secondary-school and first-year university syllabi. It links to neighbouring topics Imaging, Mass spectrometry, Semiconductor analysis, so understanding it makes those chapters shorter.
In everyday life: Look for Nanoscale secondary ion mass spectrometry outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Nanoscale secondary ion mass spectrometry in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Nanoscale secondary ion mass spectrometry means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Nanoscale secondary ion mass spectrometry out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Nanoscale secondary ion mass spectrometry in simple terms?

NanoSIMS (nanoscale secondary ion mass spectrometry) is an analytical instrument manufactured by CAMECA which operates on the principle of secondary ion mass spectrometry. The NanoSIMS is used to acquire nanoscale resolution measurements of the elemental and isotopic composition of a sample.

Why does Nanoscale secondary ion mass spectrometry matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Nanoscale secondary ion mass spectrometry?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Nanoscale secondary ion mass spectrometry.