# Optical emission spectrometers

Elemental analysis by spark and plasma emission, with sample preparation, matrix calibration and reporting limits.

Reviewed: 2026-11-01

Tags: inspection, instruments, chemical-analysis

## What OES measures

Spark optical emission spectrometry (OES) excites a small area of metal with an electrical discharge. Emitted light identifies elements and supports concentration estimates under the applicable calibration. [Shimadzu, Principle of Optical Emission Spectrometry](https://www.shimadzu.com/an/products/elemental-analysis/oes-app/principle-of-optical-emission-spectrometry/index.html)

In its February 2021 Q&A, Hitachi states an in-house **OE750 aluminum-alloy scandium calibration of 0.00005–0.06%**. That is approximately **0.5–600 ppm by mass**, using 1% = 10,000 ppm. It is an example of a stated calibration range, not a guaranteed detection limit, quantitation limit or uncertainty for every aluminum specimen. Confirm the supplied method and demonstrated performance with representative material. [Hitachi, non-ferrous melt-control Q&A](https://hha.hitachi-hightech.com/en/blogs-events/blogs/2021/02/15/non-ferrous-melt-control-q-and-a/)

Specify the excitation method, not just “OES.” The broader term also includes **ICP-OES**, using an inductively coupled plasma, and methods based on arc or glow discharge. Their samples, preparation and analytical capabilities differ. This reference focuses on spark analysis of solid metals, while identifying the solution-analysis distinction. [Shimadzu, OES method terminology](https://www.shimadzu.com/an/products/elemental-analysis/oes-app/principle-of-optical-emission-spectrometry/index.html)

OES reports chemistry. A composition consistent with a grade does not independently establish heat treatment, hardness, strength, dimensional conformance or traceable heat identity. Define whether the task is alloy sorting, quantitative acceptance against a composition requirement, process monitoring or comparison of specimens; the required evidence is different.

## Spark-OES versus ICP-OES

| Arrangement | Sample path | Main selection question |
| --- | --- | --- |
| Spark-OES | Prepared solid metal is excited at the spark stand or suitable probe | Can the specimen geometry and alloy-specific method support representative analysis? |
| ICP-OES / ICP-AES | A solution is introduced into an inductively coupled plasma | Does sample preparation bring the required analytes into a valid solution and calibration matrix? |

EPA **Method 200.7, revision 4.4 (1994)** is an example of ICP atomic-emission analysis of metals and some nonmetals in solution. It addresses water and waste samples, with preparation and interference controls appropriate to that scope. It is not a spark-OES method for directly accepting a finished alloy part. The PDF's 1994 revision is printed in its front matter. [EPA Method 200.7, sections 1–2](https://www.epa.gov/sites/default/files/2015-08/documents/method_200-7_rev_4-4_1994.pdf)

A request for “total composition” needs further definition: which elements, concentration ranges, reporting basis and specimen population? Dissolution or digestion, where used, must recover the analytes of interest; an instrument's optical coverage does not establish sample-preparation recovery.

## Sampling and surface preparation

A spark result characterizes the prepared analysis area. Establish how that area represents the part or heat, especially where casting segregation, surface treatment or heterogeneous inclusions are possible. Plan destructive sampling and the resulting burn marks before testing a finished component. The FBI's spark-discharge procedure explicitly describes SDAR-OES as destructive. [FBI METAL-400-03, issued 15 August 2024, page 2](https://fbilabqsd.fbi.gov/file-repository/chemistry/metal-400-03-compositional-analysis-by-spark-discharge-in-argon-optical-emission-spectroscopy-sdar-oes.pdf/)

Preparation must remove the unwanted surface without introducing analytes. Shimadzu explains that grinder particles can contaminate soft samples and describes lathe or milling preparation for materials such as white metal. Its guidance allows grinding where trace elements are not being investigated and discusses changing abrasive media to minimize contamination. Those choices depend on the analytical task; one preparation recipe is not suitable for every alloy and trace-element requirement. [Shimadzu, Sample Pretreatment for Optical Emission Spectrometry](https://www.shimadzu.com.sg/an/products/elemental-analysis/oes-app/sample-pretreatment-for-optical-emission-spectrometry/index.html)

Check specimen flatness, coverage of the spark opening, accessible area, thickness and porosity against the actual instrument method. Hitachi's **1 July 2021** Q&A explains that thin-sample behavior depends on alloy and spark parameters; it declines to give one universal minimum thickness. It also identifies different preparation approaches for ferrous and non-ferrous materials. Do not convert a successful thin-sheet demonstration into a general specimen-size guarantee. [Hitachi, aluminum and steel OES Q&A](https://hha.hitachi-hightech.com/en/blogs-events/blogs/2021/07/01/oes-live-demo-q-and-a/)

## Argon, excitation and maintenance

For Hitachi's stationary spark instruments, argon provides the inert spark atmosphere. Air ingress, poor gas quality and incorrect preparation can produce anomalous results. Electrode and spark-stand maintenance are part of the analytical method, with details assigned to the instrument manuals. [Hitachi, OES Q&A: inert gas and maintenance](https://hha.hitachi-hightech.com/en/blogs-events/blogs/2021/07/01/oes-live-demo-q-and-a/)

Specify gas purity and impurity limits for the analytes and concentration range, together with supply, purge and leak-control requirements. Hitachi's **15 February 2021** discussion specifically requires Ar 6.0 for gas analysis on its OE750 and FOUNDRY-MASTER Pro2 and highlights the sensitivity of low-level nitrogen to air and surface preparation. This is a named application requirement, not a universal purity prescription for every OES instrument. [Hitachi, non-ferrous melt-control Q&A](https://hha.hitachi-hightech.com/en/blogs-events/blogs/2021/02/15/non-ferrous-melt-control-q-and-a/)

Follow the instrument's site and operating requirements for extraction, electrical safety, gas handling, interlocks and cleaning. Include the actual alloy and preparation debris in that review rather than treating all metal specimens as equivalent.

## Calibration, standardization and verification

Select the correct base-metal matrix and analytical program before interpreting concentrations. Standardization or drift correction restores an established response; it does not validate a new alloy family or extend a calibration range. Verification uses suitable reference material to check that the current analytical sequence performs acceptably.

A concrete laboratory example is **FBI METAL-400-03, issue 03, 15 August 2024**. It separates optical re-profiling, alloy-class standardization, CRM verification and specimen analysis, then checks reference material again after testing. It also retains individual measurements and their statistics. These controls illustrate an analytical sequence; its specified reading counts and forensic comparison rules are requirements of that procedure, not universal OES operating instructions. [FBI procedure, sections 6.3–6.5](https://fbilabqsd.fbi.gov/file-repository/chemistry/metal-400-03-compositional-analysis-by-spark-discharge-in-argon-optical-emission-spectroscopy-sdar-oes.pdf/)

Reference materials should support the relevant matrix, elements and concentration range. Repeated burns with close agreement demonstrate a component of precision, but can all share preparation bias or the wrong calibration. Define what happens when verification fails: investigate, correct and determine which intervening results need repetition.

## Named analytical capability and reporting limits

Keep these quantities separate:

- **Detection limit:** evidence that an analyte signal can be distinguished under the stated method conditions.
- **Quantitation limit:** the lower region where quantitative reporting meets the method's criteria.
- **Repeatability:** dispersion under specified repeat conditions.
- **Measurement uncertainty:** uncertainty attached to the reported result for the actual method and sample.

The FBI procedure treats quantitation limits as alloy-class dependent and says uncertainty can depend on element concentration and other constituents. Its limitations caution against unqualified quantitative use near or below the LOQ. Do not report an undetected element as an exact zero. [FBI procedure, sections 9–10](https://fbilabqsd.fbi.gov/file-repository/chemistry/metal-400-03-compositional-analysis-by-spark-discharge-in-argon-optical-emission-spectroscopy-sdar-oes.pdf/)

Spectral overlap, background and matrix effects also need method controls. For ICP-OES, Thermo Fisher distinguishes physical, chemical and spectral interferences and discusses selecting suitable analytical wavelengths and corrections. A large optical wavelength range alone does not eliminate these effects. [Thermo Fisher, ICP-OES Data Analysis](https://www.thermofisher.com/ar/en/home/industrial/spectroscopy-elemental-isotope-analysis/spectroscopy-elemental-isotope-analysis-learning-center/trace-elemental-analysis-tea-information/icp-oes-information/icp-oes-data-analysis.html)

## Specify and retain the analytical context

For a purchase or test request, state specimen form and matrix, required elements and ranges, applicable method, preparation, allowed damage, throughput and acceptance criteria. Request application data rather than one instrument-wide “accuracy” number.

For results, retain sample identity and location, preparation, instrument/program, calibration or standardization references, verification results, individual analyses and exclusions, reporting units, limits and applicable uncertainty. Compare chemistry with the correct specification revision using the agreed decision rule. Keep a software grade suggestion distinguishable from a documented acceptance conclusion.

## Related reference

[Inspection and quality documents](../../sourcing/documents/index.md)