ICP-MS trace metal analysis: how the technique works and when to use it

ICP-MS appears on a lot of laboratory reports without much explanation of what it actually does. For clients who need to understand why a particular technique was used, or who are comparing methods across laboratories, a basic understanding of the technique is useful.

What ICP-MS does, in plain terms ¶

Inductively coupled plasma mass spectrometry works by converting a liquid sample into a fine aerosol, passing it through an argon plasma at roughly 6,000 to 8,000 degrees Celsius, and then separating the resulting ions by their mass-to-charge ratio. The detector counts ions at each mass, which corresponds to a specific element and isotope. The result is a concentration for each element, expressed in micrograms per litre (µg/L) for water or micrograms per kilogram (µg/kg) for solids.

The technique is particularly well-suited to trace-level analysis because the plasma ionises nearly all elements efficiently, and the mass spectrometer can distinguish between elements that would overlap in optical techniques. Detection limits for most elements are in the range of 0.001 to 0.1 µg/L in water, which is well below the Maximum Acceptable Concentrations in Health Canada's drinking water guidelines.

When ICP-MS is the right choice versus ICP-OES ¶

ICP-OES (optical emission spectrometry) is a related technique that is faster and less expensive per sample, but has detection limits roughly 10 to 100 times higher than ICP-MS. For most environmental compliance work where the guideline values are in the µg/L range (lead, arsenic, cadmium, mercury), ICP-MS is necessary to achieve the required detection limits.

For samples where the elements of interest are present at higher concentrations. Major cations in a soil extract, for example. ICP-OES is often sufficient and more cost-effective. A good laboratory will recommend the appropriate technique based on the guideline values you need to compare against, not simply default to the more expensive option.

Interferences and how they are managed ¶

ICP-MS is susceptible to polyatomic interferences. Cases where two or more ions combine to produce a signal at the same mass as the element of interest. The classic example is the interference of ArCl on arsenic at mass 75. Modern instruments manage this with a collision/reaction cell that breaks up polyatomic ions before they reach the detector.

The laboratory's method should document which interferences are relevant for the analyte list and how they are controlled. If you are reviewing a report and want to understand the interference management approach, the method citation will point you to the relevant standard.

If you are comparing quotes from multiple laboratories and ICP-MS is listed on both, ask each lab for the detection limits they achieve for the specific elements you need. Detection limits vary between instruments and sample matrices, and they matter when the guideline value is close to the detection limit.