Mobile Optical Emission Spectrometers (OES): Everything You Need to Know

By Chris Carolan, Consultant at Metal Analysis Group

The manufacturing industry has little to no tolerance for error. That means suppliers often need to know the exact composition of the metallic alloys they’re dealing with. Mobile optical emission spectrometers (OES) have gained popularity in PMI testing. This is likely due to their portability and ability to provide the full chemistry of metal including true carbon measurement.

In this post, we’ll cover questions around mobile OES including:

What are mobile optical emission spectrometers (OES)? A mobile optical emission spectrometer, or mobile OES, is a spark- or laser-based system created to accurately determine the elemental composition of metals.

Let’s dive in!

Psst: This post is based on a podcast with co-host Chris Carolan. To hear this episode (and more like it), subscribe to The Manufacturing Show on Apple Podcasts, Spotify, or wherever you listen to podcasts.

What is optical emission spectroscopy?

First thing’s first: If you’re unfamiliar with positive material identification (PMI), take a gander at this post. Then come right back! Reviewing PMI testing will help you better understand optical emission spectroscopy.

Now, what exactly is optical emission spectroscopy (OES)?

Excited atoms

OES starts with an electrical or laser-based source to charge atoms within a metallic sample. When the atoms within a particular metal get excited, they emit a trademark light.

This light, or optical emission, requires a small piece of the sample to be heated up to thousands of degrees Celsius. What’s good at heating stuff up? Lightning and lasers!

Essentially, OES uses electricity or lasers to vaporize the sample’s surface, charging up its atoms. Then, the metallic sample shows its true colors. The light a sample emits is consistent with its chemical composition.

Measuring intensity

So, you’ve seen the colors that the metallic sample produces when heated. Now, it’s time to measure the intensity of each of those colors.

Fun fact: Iron emits over 8,000 different wavelengths!

The emitted light passes into the spectrometer. The OES tool is then able to grade each separate light into element-specific wavelengths. A corresponding detector measures the intensity of each wavelength (aka, each light’s intensity).

Subsequently, the intensity of each wavelength is proportional to the concentration of their respective elements.

In other words, the nickel wavelength measures very intensely? There’s a lot of nickel in the sample. The wavelength corresponding with copper is weak? There’s not much copper in there.

Overall composition

Thirdly in OES testing, a computer system processes the intensity data. Through a predefined calibration, the system is able to generate elemental concentration levels.

Ta-da! You now know the overall composition of the sample. That means you can accurately compare it to a metallic grade library to make sure it’s the right stuff.

History of mobile optical emission spectrometers

As the knowledge around positive material identification began to expand, so did OES methods. Here’s a quick history of where the mobile OES got its start.

Using a flame to test metal

In the early 20th century, manufacturers used a flame as the light source for OES testing. The flame would heat up the sample’s atoms, producing a light consistent with its components.

Ever heated up a penny before? You probably saw a greenish-blue light emitting from it due to the copper. Using a flame for OES testing is the same concept.

From benchtop to mobile

Of course, there had to be a more accurate way of measuring the composition of metals.

Enter, arc spark OES (or, spark OES). This metal testing method came about in the 1950s and 60s because of its increased excitation capabilities.

Gotta keep those atoms excited, right?

In its early days, spark OES was primarily used in metal production labs where they needed to know the full chemistry and composition. The only spark OES tool available was rather stationary. But, it got the job done.

As PMI testing became prevalent outside of the lab, mobile OES was developed to support the need for testing materials on the shop floor or out in the field.

“The laser can replace the spark as the source that’s vaporizing the metal.” — Chris Carolan

Recently, the advancement of laser technology has allowed lasers to take the place of sparks in the vaporizing process.

Pros and cons of mobile optical emission spectrometers for PMI

Considering a mobile optical emission spectrometer for your business? Take note of the following pros and cons I’ve collected through my experiences in the metals space.

Pros of using mobile OES for PMI

With a mobile optical emission spectrometer, you can…

1. Measure a broader elemental range.
2. Determine the material’s full chemistry.
3. Measure the intensities of elements such as carbon, boron, sulfur, phosphorus, and nitrogen.
4. It’s the go-to technology to acquire full metal chemistry outside of the lab.

Sounds pretty great, huh? Well, there are also a few drawbacks to think about.

Cons of using a traditional, spark-based mobile OES for PMI

Drawbacks of using a traditional, spark-based mobile OES include...

1. It’s sample preparation intensive — not just point-and-shoot testing.
2. You have to know what the base material is first.
3. It requires a sufficient amount of knowledge and training to use.
4. It requires a high level of maintenance.
5. Although it’s no longer stationary, a traditional, spark-based mobile OES is still pretty big and bulky.
6. A traditional, spark-based mobile OES requires argon.

Sounds pretty great, huh? Well, there are also a few drawbacks to think about.

“Mobile OES is the go-to technology when you need full metal chemistry outside of the lab.”— Chris Carolan

Pro tip: Laser-based OES provides all the chemistry and sensitivity you need with almost none of the cons of traditional, spark-based OES.

Next, we’ll discuss the costs of using a mobile OES for positive material identification.

Costs of using mobile optical emission spectrometers for PMI

It’s safe to say purchasing your business’s own mobile optical emission spectrometer isn’t going to be cheap. Even so, the consequences of failing to conduct positive material identification are even costlier.

It’s a necessary evil, some would say, to gain access to a mobile OES.

Initial costs

Purchasing a mobile OES will put you out anywhere from $35-$60K, sometimes more. You may be able to budget in the initial cost; however, traditional, spark-based mobile optical emission spectrometers require more maintenance than most other machines.

Upkeep costs

Depending on the repair, plan on spending $7-$20K for the maintenance of spark-based analyzers. A few more factors you should plan on include

1. Transportation to and from the maintenance provider — these things aren’t easy to move.
2. The cost of the manufacturer service rep visit.
3. The fact that owning a mobile OES doesn’t lead to many opportunities when you’re ready to sell it.

To keep costs down and increase convenience, consider renting your spark-based mobile optical emission spectrometer (or, switch to a laser-based system).

Mistakes to avoid when using a mobile optical emission spectrometer

To attain the best PMI results, there are several mistakes to avoid when using OES to test.

1. Failing to do proper sample preparation. To achieve the cleanest burn possible, the metallic sample should be flat, free of corrosion, and in direct contact with the OES electrode or laser.
2. Poor maintenance of the electrode. Along with many other parts on a traditional, spark-based mobile OES, the electrode absolutely needs to be cleaned routinely in order to work properly.
3. Impure argon gas. Ensuring the argon gas is ultra grade will result in more efficient burns.
4. Not allowing the analyzer to adjust to its environment. Any time a traditional, spark-based OES is moved or is paired with a new system, it needs time to adjust.

“If you don’t have strong enough light signals, you’re not going to be able to read the elements.” — Chris Carolan

Avoid making these mistakes when using a mobile OES and there should be smooth sailing.

Mobile optical emission spectrometer brands

Brands that supply mobile optical emission spectrometers include

1. Hitachi (formerly Oxford) for spark-based OES systems
2. SPECTRO for spark-based OES systems
3. QuantoLux for laser-based OES systems

In addition to these listed brands, there are more European OES providers entering the market.

Master your metal testing

Choosing the right tool for PMI testing is crucial for numerous reasons. OES is a cornerstone technique for determining the complete chemistry of a metallic alloy.

In an industry that requires little to no margin of error, it’s worth it to fully understand all the PMI tools at your disposal.

For more expert insights on PMI testing, subscribe to The Manufacturing Show on Apple Podcasts, Spotify, or wherever you listen to podcasts.

By Chris Carolan, Consultant at Metal Analysis Group

Chris serves as the Inside Sales and Marketing Consultant for MAG. He has worked in Elemental Analysis for Oil & Gas and related industries for over 12 years connecting with customers at each point in the supply chain. Spending most of his career in reference material manufacturing, Chris has learned a great deal about troubleshooting specific customer scenarios and applications.

Gaining experience in production, quality control, marketing, and sales, Chris is eager to put that experience and knowledge to use by helping customers create unique solutions that save money, reduce risk, and increase profits.