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Imagine dissolving a single grain of salt in an Olympic swimming pool and then being asked to measure how much sodium is in the water. That’s roughly the kind of sensitivity ICP-MS routinely achieves. Inductively coupled plasma mass spectrometry can measure dozens of elements in a sample at the same time, down to parts per trillion (ppt) and below. It’s the go-to technique for trace metals in drinking water, food, blood, rocks and semiconductor materials.
What the name means
- Inductively coupled plasma (ICP): an extremely hot argon plasma used to turn the sample into ions.
- Mass spectrometry (MS): separates and counts those ions by their mass-to-charge ratio.
So ICP-MS combines a very efficient ion source with a very sensitive detector. See mass spectrometry for the general principles.
Step 1: sample introduction
Most samples are introduced as liquids. Solids (soil, food, tissue, rock) are first dissolved, usually by digesting them in concentrated acids, often in a sealed microwave vessel.
The solution is pumped into a nebuliser, which turns it into a fine aerosol. A spray chamber removes larger droplets, so only the finest mist reaches the plasma.
Other methods include laser ablation, where a laser vaporises tiny spots of a solid sample directly, useful for analysing minerals, archaeological artefacts or growth rings in teeth and shells.
Step 2: the plasma
A plasma is a gas that’s so hot that many of its atoms have lost electrons, leaving a mixture of ions and free electrons. See states of matter for how plasma compares with solids, liquids and gases.
In ICP, argon gas flows through a quartz torch surrounded by a copper coil carrying a radio-frequency current (typically 27 or 40 MHz). The changing magnetic field accelerates electrons, which collide with argon atoms and ionise them. This creates a self-sustaining plasma with temperatures of about 6000–10,000 K, hotter than the surface of the Sun.
When the sample aerosol enters the plasma, it’s dried, vaporised, broken into atoms and then ionised, almost completely for most elements. Most elements form singly charged positive ions (M⁺).
Argon is used because it’s inert and has a high ionisation energy (about 1521 kJ/mol), which means it can ionise almost every other element but isn’t itself easily turned into interfering molecular ions with the sample.
Step 3: the interface
The plasma is at atmospheric pressure, but the mass spectrometer needs a high vacuum. Ions pass through small holes in two metal cones (the sampler cone and skimmer cone), which extract a beam of ions into progressively lower pressures. Ion lenses then focus the beam and separate the positive ions from photons and neutral particles.
Step 4: mass analysis and detection
The ion beam enters a mass analyser, most commonly a quadrupole: four parallel rods with oscillating electric fields that allow only ions of a selected m/z to pass at any instant. By scanning rapidly across m/z values, the instrument measures many elements within seconds.
Ions that pass through hit a detector (an electron multiplier) that counts them individually. The number of counts is proportional to the concentration of each isotope.
More advanced instruments use high-resolution magnetic sector or time-of-flight analysers, or tandem (triple quadrupole) designs, for difficult samples.
Calibration and quantification
Like other analytical methods, ICP-MS uses calibration standards containing known concentrations of the elements being measured. An internal standard (an element not present in the sample, such as indium or rhodium) is added to every solution to correct for drift and matrix effects. See calibration curves.
Interferences
The biggest challenge in ICP-MS is that different species can have the same nominal m/z:
- Isobaric interference: isotopes of different elements with the same mass number, such as ⁵⁸Fe and ⁵⁸Ni. Choosing a different isotope of the target element usually solves this.
- Polyatomic interference: molecular ions formed from argon, the solvent or the sample matrix. For example, ⁴⁰Ar³⁵Cl⁺ has m/z 75, the same as arsenic-75, a problem when samples contain chloride. Another example: ⁴⁰Ar¹⁶O⁺ at m/z 56 interferes with iron-56.
- Doubly charged ions: some elements form M²⁺ ions that appear at half their mass.
Modern instruments reduce polyatomic interferences with a collision/reaction cell: ions pass through a cell containing helium or a reactive gas, which breaks up or removes the interfering molecular ions while letting the target ions through.
Isotope ratios
Because ICP-MS measures individual isotopes, it can also determine isotope ratios. Uses include:
- tracing the source of lead pollution (lead from different ore deposits has different isotope ratios)
- dating rocks and minerals (for example, uranium–lead methods)
- checking food and drink provenance
- nuclear safeguards and forensics
- tracing nutrients or drugs labelled with stable isotopes in the body
Typical detection limits
| Technique | Typical detection limits |
|---|---|
| Flame AAS | parts per million (mg/dm³) |
| ICP-OES (optical emission) | parts per billion (μg/dm³) |
| Graphite furnace AAS | parts per billion |
| ICP-MS | parts per trillion (ng/dm³) or lower |
Uses of ICP-MS
- Drinking water: checking regulated elements such as lead, arsenic, cadmium, chromium, uranium and selenium in one run.
- Food safety: metals in rice (arsenic), seafood (mercury) and baby food.
- Clinical and toxicology: trace elements and toxic metals in blood and urine; monitoring patients with metal joint replacements.
- Pharmaceuticals: international guidelines limit elemental impurities (such as lead, cadmium, arsenic and mercury) in medicines, and ICP-MS is the main method for measuring them.
- Semiconductors: ultra-pure chemicals and silicon wafers must contain almost no metal contamination.
- Geology and mining: rare earth elements and precious metals in rocks.
- Environmental science: metals in rivers, sediments and air filters.
Keeping contamination out
At parts-per-trillion levels, contamination is the enemy. Dust, sweat, ordinary glassware and even the air in a normal lab contain far more of many metals than the samples being measured. ICP-MS labs therefore use high-purity acids, plastic (not glass) containers that have been acid-washed, ultrapure water and, for the most demanding work, clean rooms with filtered air. Every batch of samples is run alongside procedural blanks, which go through the whole preparation without any sample, to reveal any contamination introduced along the way.
ICP-OES: the optical cousin
ICP-optical emission spectroscopy (ICP-OES) uses the same plasma but measures the light emitted by excited atoms and ions rather than counting ions by mass. It’s less sensitive than ICP-MS but more tolerant of high salt content and cheaper to run, so many labs use both.
Key takeaways
- ICP-MS turns samples into ions in an argon plasma at 6000–10,000 K, then separates and counts them by m/z.
- It measures dozens of elements at once, down to parts per trillion.
- Calibration standards and internal standards give accurate concentrations.
- Isobaric and polyatomic interferences are managed with isotope choice and collision/reaction cells.
- It’s used for water, food, clinical, pharmaceutical, semiconductor and geological analysis; compare with atomic absorption spectroscopy.
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