ICP-OES stands for inductively coupled plasma optical emission spectrometry. It is the standard method for measuring metal and metalloid concentrations in water, soil, food, and several other matrices. Understanding roughly how it works helps clients interpret their results and ask better questions about the analysis.

The basic principle

A liquid sample is introduced into a stream of argon gas and converted into a fine aerosol. The aerosol passes into a plasma torch, where temperatures reach around 6,000 to 10,000 Kelvin. At these temperatures, atoms in the sample are excited and emit light at characteristic wavelengths. The instrument measures the intensity of light at each wavelength and compares it to calibration standards of known concentration. The result is a concentration value for each element, typically expressed in mg/L for liquids or mg/kg for solids after digestion.

What it can and cannot measure

ICP-OES can measure most metals and metalloids across a wide concentration range, typically from around 0.01 mg/L up to several thousand mg/L depending on the element. It is well suited to calcium, magnesium, sodium, potassium, iron, manganese, zinc, copper, lead, arsenic, cadmium, chromium, and nickel, among others. It cannot distinguish between different chemical forms of an element: total chromium is not the same as hexavalent chromium, and total arsenic is not the same as inorganic arsenic. Speciation analysis requires a different approach, typically HPLC coupled to ICP-MS.

Sample preparation and its importance

For water samples, preparation is relatively straightforward: acidification, filtration if required, and dilution to bring the sample within the calibration range. For solid matrices (soil, food, tissue), the sample must first be digested to bring the metals into solution. We use microwave-assisted acid digestion for most solid matrices, which gives complete dissolution of most matrices and minimises contamination risk. The quality of the digestion is as important as the quality of the instrument measurement, which is why we include certified reference materials in every batch.

If you have questions about whether ICP-OES is the right technique for your specific analytes and matrix, that is exactly the kind of question to raise in a scoping call before you submit samples.