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Short answer
ICP spectroscopy is elemental analysis in which a liquid sample is nebulized into a radio-frequency inductively coupled plasma. The elements are then read in one of two ways: by the light they emit (ICP optical emission, called ICP-AES in EPA Method 200.7) or by their mass-to-charge ratio (ICP-MS, EPA Method 200.8). Both determine many elements from one solution.
Definition and scope
EPA Method 200.7 describes inductively coupled plasma-atomic emission spectrometry (ICP-AES) as a technique used "to determine metals and some nonmetals in solution." EPA Method 200.8 covers dissolved elements in ground water, surface water and drinking water, and total recoverable elements in those waters plus wastewaters, sludges and soils. In both, the plasma is the common element: it is the source that turns a nebulized solution into excited atoms or ions.
The umbrella term "ICP spectroscopy" therefore covers two instrument families that share a front end but differ at the detector. This page explains the shared concept and the optical-emission side in practice; for the mass-spectrometry side see the ICP-MS guide.
How a sample becomes a signal
Method 200.7 describes the optical route in four steps: samples are nebulized, the aerosol is carried to the plasma torch, element-specific emission spectra are produced by a radio-frequency inductively coupled plasma, and a grating spectrometer disperses the spectra so a photosensitive device can monitor line intensities at specific wavelengths. The instruments can be sequential or simultaneous.
Method 200.8 describes the mass route: the solution is introduced by pneumatic nebulization into a radiofrequency plasma, where energy transfer causes desolvation, atomization and ionization. The ions are extracted through a differentially pumped vacuum interface, separated by a quadrupole mass spectrometer on mass-to-charge ratio, and detected by an electron multiplier or Faraday detector.
The two routes side by side
| Stage | Optical emission (Method 200.7) | Mass spectrometry (Method 200.8) |
|---|---|---|
| Sample introduction | Nebulized; aerosol transported to the plasma torch | Pneumatic nebulization into a radiofrequency plasma |
| What the plasma does | Produces element-specific emission spectra | Desolvation, atomization and ionization |
| Separation | Grating spectrometer disperses the spectra | Quadrupole separates ions by mass-to-charge ratio |
| Detection | Photosensitive device monitors line intensities at specific wavelengths | Electron multiplier or Faraday detector |
Interferences to expect, by technique
The two methods name different problems, which is the practical reason the routes are not interchangeable at the bench.
Optical emission
- Spectral: background emission (continuous and recombination), stray light, overlap of a line from another element, and unresolved overlap from molecular bands (Method 200.7, section 4.1).
- Physical: changes in viscosity and surface tension that affect nebulization. The method lists a high-solids nebulizer, dilution, a peristaltic pump and an internal standard as remedies, and notes that salt building up at the nebulizer tip causes drift (section 4.2).
- Background correction is required: the background must be measured adjacent to an analyte wavelength during analysis (section 2.2).
Mass spectrometry
- Corrections must compensate for isobaric elemental interferences and interferences from polyatomic ions; drift and matrix suppression or enhancement are corrected with internal standards (Method 200.8, section 2.2). The polyatomic interference guide goes deeper.
Where it matters for labs: a pre-result checklist from Method 200.7
Method 200.7 states several conditions that bear on the quality of an optical-emission result. Each item below is taken from the method text. The copy of Method 200.7 cited here (Revision 5.0, January 2001) is marked "Draft", so confirm which revision your program requires before relying on it for compliance.
- Solids: for dissolved analytes, dissolved solids should be below 0.2% (w/v) to reduce interferences (section 1.3).
- Direct analysis: where the method is approved for certain metal and metalloid contaminants in drinking water, and with the exception of silver, aqueous samples may be analyzed without acid digestion if they are properly acid-preserved and have turbidity below 1 NTU at the time of analysis (section 1.4).
- Total recoverable preparation: gentle refluxing with nitric and hydrochloric acids (section 2.1). For digestion practice on the mass-spectrometry side, see acid digestion methods.
- Quality control: the method is performance based. Laboratories may modify steps if performance requirements are met, but may not omit any quality control analyses (preface).
- Compliance use: the method points to 40 CFR Part 136 Table 1B (NPDES), Part 141 section 141.23 (drinking water) and the latest Federal Register notices (section 1.2). The EPA Method 200.8 vs 200.7 vs 6020 guide compares the methods.
- View and wavelength: detection limits and linear ranges vary with the wavelength selected, the spectrometer and the matrix (section 1.12). The method records difficulties analyzing molybdenum in biosolids with a radial ICP, which may call for an axial ICP; the method says this is subject to a validation study (section 1.11).
- Safety: the plasma should only be viewed with proper eye protection from ultraviolet emissions (section 5.4).
Common misunderstandings
ICP-AES and ICP-OES are different techniques
They are not. EPA Method 200.7 uses the name ICP-AES; many laboratories say ICP-OES for the same optical-emission measurement. The ICP-MS vs ICP-OES comparison covers the naming and how the two routes differ.
One ICP method fits every compliance need
Method 200.7 describes optical emission only. The mass-spectrometry route is a separate method (200.8) with its own scope and its own interference corrections, and for compliance work Method 200.7 directs users to the 40 CFR references and Federal Register notices listed above.
FAQ
What does ICP stand for in spectroscopy?
Inductively coupled plasma. It is the radio-frequency plasma that atomizes and excites or ionizes the sample.
What is the difference between the two detection routes?
Optical emission measures characteristic atomic-line light dispersed by a grating spectrometer; mass spectrometry separates ions by mass-to-charge ratio with a quadrupole.
Does the sample have to be a liquid?
Both methods determine elements in solution. For total recoverable analysis, Method 200.7 prepares the sample by gentle refluxing with nitric and hydrochloric acids.
Sources
- U.S. EPA, Method 200.7, Revision 5.0 (EPA-821-R-01-010, January 2001), https://19january2021snapshot.epa.gov/sites/static/files/2015-08/documents/method_200-7_rev_5_2001.pdf. Checked 2026-10-29.
- U.S. EPA, Method 200.8, Revision 5.4 (1994), https://www.epa.gov/sites/default/files/2015-08/documents/method_200-8_rev_5-4_1994.pdf. Checked 2026-10-29.
