LIBS vs. OES vs. XRF: PMI Testing Techniques

Boots are best for working outside. Tennis shoes are best for the gym. Sandals are best for the beach.

Sometimes, though, footwear styles overlap each other. Maybe you need tennis shoes for working outside or sandals in the gym’s locker room. It all depends on the application.

The same can be said about PMI (positive material identification) testing techniques. LIBS, OES, and XRF all have their pros and cons. But, when you get down to it, it all depends on the scenario. The application.

When should you use LIBS vs. OES vs. XRF for PMI testing? To choose the correct PMI testing technique (LIBS vs. OES vs. XRF), ask yourself these 4 questions:

1. Can you allow for marks on the samples?
2. Which elements or alloys are you needing to positively identify and measure?
3. How quickly do you need to complete testing?
4. Is safety liability and/or government oversight a grave concern?

We’ll unpack these questions so you can better identify which PMI testing technique to use. But first, let’s review how LIBS, OES, and XRF work.

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.

Handheld XRF overview

X-ray fluorescence, or XRF, is a widely used PMI technique. XRF technology has been around for over 70 years and is more recently available in a handheld format.

How X-ray fluorescence works

All XRF instruments, including handheld and benchtop, consist of an X-ray source and a detector.

XRF technology uses X-ray beams to excite the atoms in a metal sample. A secondary X-ray is generated from the unstable atoms. The secondary X-ray is specific to the elemental composition of the metal.

The detector, then, recognizes the secondary X-ray beam and is processed by the analyzer.

Next, the analyzer’s results are compared to a metal grade library and an alloy grade ID is provided based on the chemistry.

Pros of using XRF for PMI

The benefits of using XRF technology for positive material identification include:

1. XRF is truly non-destructive (no marks on the samples).
2. XRF can test samples within seconds.
3. The handheld XRF is portable.
4. The handheld XRF is agile when it comes to application.
5. XRF is more sensitive to lower amounts of elements than other PMI techniques (except for light elements).
6. The software that comes with an XRF device is easy to use.

Cons of using XRF for PMI

There are also drawbacks when using XRF analyzers for PMI. These include:

1. You’re using radiation that can be very harmful to humans if precautions are not taken.
2. Most states require you to register your XRF device because it uses dangerous radiation.
3. The handheld XRF tends to be fragile and expensive to repair ($7k-$10k).
4. Handheld XRF cannot test for light elements like lithium, beryllium, and carbon.
5. Handheld XRF has trouble testing for light elements like magnesium, aluminum, and silicon.

"The more you understand the ins and outs of the technology, and how it's used for your application, the better prepared you'll be to make the right decision for you, your team, and your customers."

Chris Carolan

OES overview

OES, or optical emission spectroscopy, comes in two forms:

1. Traditional spark OES
2. LIBS laser OES

In the 1960s, metal production labs had evolved from using a flame to using a spark for PMI testing. And, although it’s not widely adopted yet, advances in technology have made it possible for the spark to be replaced by a maintenance-free laser.

How traditional spark OES works

The traditional spark OES system uses an electrical source to charge the atoms in a sample. The light that the sample subsequently emits corresponds to its elemental makeup.

The intensity of the light’s wavelength is picked up by a detector and measured by the computer system. Lastly, the software uses the information from the detector to determine each element’s intensity to provide a full chemistry and composition of the sample.

Pros of using traditional spark OES

The pros of using spark OES include:

1. It’s a trusted PMI technique for identifying elements including carbon, boron, phosphorus, sulfur, and nitrogen.
2. It provides full chemistry and composition of the metal sample at trace levels.
3. Spark OES systems are widely available.

Cons of using traditional spark OES

The disadvantages of using spark OES for PMI are:

1. It takes a long time to test metals.
2. It’s tricky to keep the probe in place during long test times.
3. The mobile spark OES is 35+ lbs, making it bulky and difficult to move.
4. There are potential safety hazards to moving a traditional spark OES.
5. Restocking the high purity argon gas tank is rather laborious and needs to be done routinely.
6. The stability of spark OES systems is easily compromised by their environment.
7. Spark OES systems need time to reacclimate to new environments.
8. Prepping samples for spark OES is arduous.
9. The electrode requires regular maintenance.
10. There are no small sample sizes because of the size of the burn site.
11. Long recalibration and standardization processes are required.

How LIBS laser OES works

LIBS laser OES uses high-energy laser pulses to charge the atoms within a sample. The plasma light that is subsequently emitted shows spectral signatures of the present elements.

Similarly to spark OES, laser OES provides a computer software system with light intensity data. The system, then, makes it easier for the user to understand the elemental composition of the sample.

Pros of using LIBS laser OES for PMI

The benefits of using LIBS laser OES technology are:

1. It can take 10 measurements per minute and 300-600 measurements per hour.
2. The device is <18 lbs including its argon tank, so it’s easy to handle and transport.
3. There are no safety hazards when transporting a laser OES system.
4. It only requires a very light bottle of argon that lasts for over 1,000 measurements.
5. The argon tank is small and easy to refill.
6. The internal battery is lightweight and rechargeable.
7. It doesn’t need any time to reacclimate when environments change.
8. It doesn’t use high voltages or dangerously high temperatures to test.
9. There isn’t much sample prep time needed.
10. Laser OES systems do not have electrodes that require regular maintenance or replacement.
11. Laser OES microspot can test small samples such as nuts and bolts.
12. It can recalibrate using a single sample in just seconds.
13. The data management system and software are well beyond those of the traditional spark OES systems.

Cons of using laser OES for PMI

There are a few drawbacks of deciding to use laser OES for PMI:

1. It’s not widely available yet.
2. There’s already a waiting list for laser OES systems.
3. It’s a relatively new technology requiring some more feedback from the field.

Handheld LIBS overview

LIBS (aka, laser-induced breakdown spectroscopy) was established 50 years ago, but LIBS handheld technology has only been around for 7 or 8 years.

Advancements in technology have made it possible to miniaturize the laser while maintaining quality performance. So, instead of a benchtop LIBS system, manufacturers now have access to the more portable LIBS device.

How LIBS works

LIBS technology uses a laser source to vaporize the sample, thus exciting the atoms within it.

This results in a signature light source given off by the elements in the material. The detector recognizes the light signatures and transfers the data to the analyzer. The analyzer determines the chemistry and makeup of the metal.

Lastly, the analyzer compares the elemental percentages, chemistry, and composition against a grade library to provide an alloy ID.

Pros of using handheld LIBS for PMI

The advantages of using handheld LIBS for PMI include:

1. It can test a sample within 1-3 seconds.
2. There’s no radiation being used to test.
3. It takes the same amount of time to test no matter the type of metal alloy.
4. Most states do not have regulations concerning the use of handheld LIBS.
5. The cost of ownership is the lowest of all PMI testing techniques.
6. It’s sturdy.

"If you're looking to sort aluminum, LIBS is the way to go."

Chris Carolan

Cons of using handheld LIBS for PMI

Cons of using handheld LIBS for PMI

The disadvantages of using handheld LIBS are:

1. Perfect contact with the sample is required.
2. The surface of the sample must be completely clean.
3. There can’t be any rust, corrosion, or coating on the sample.
4. Handheld LIBS has trouble with the heaviest metals like tungsten.

4 questions for choosing the right PMI testing technique (LIBS vs. OES vs. XRF)

Now that we’ve covered the backgrounds of these three PMI techniques, let’s explore the questions that need to be asked before picking one.

1. Can you allow for marks on the sample?

If the answer is NO…

Choose XRF. Because it uses X-rays, it’s the only truly non-destructive PMI technique.

Keep in mind, however, that if the sample has any plating or coating applied, it will affect your results. Shop around for a handheld XRF device that allows you to see the outer layer as well as the base layer.

If the answer is YES…

Consider using a laser-based analyzer so you can avoid the radiation and expensive repairs of XRF.

2. Which elements or alloys are you needing to positively identify and measure?

For measuring carbon, you’ll want…

LIBS or OES

"If you have to positively identify based on carbon, you have to have a carbon number, and you can't use XRF."

Chris Carolan

For L-grade and H-grade steel at <800 ppm carbon, you’ll want…

OES

For measuring boron, phosphorus, sulfur, and nitrogen, you’ll want…

OES

For measuring tungsten, you’ll want…

XRF

To sort aluminum in the yard, you’ll want…

LIBS

3. How quickly do you need to complete testing?

As quickly as possible, choose…

LIBS. Its speed is unmatched in the applications it’s able to do. Plus, since it doesn’t use radiation, you can simply hold the sample in your hand to test. This can lead to greater efficiency and productivity.

PMI techniques from fastest to slowest (with a clean sample and direct contact):

LIBS, laser OES, XRF, spark OES

4. Is safety liability and/or government oversight a grave concern?

Choose LIBS. You’ll have minimal government paperwork if any.

Nonetheless, make sure your employees are properly trained in using LIBS. Although LIBS doesn’t use any dangerous radiation like XRF, it’s still important that users have a complete understanding of how the tool works.

Choosing between LIBS, OES, and XRF

Refer to this table for a bird’s eye view of LIBS, OES, and XRF:

LIBS vs. OES vs. XRF takeaways

The main takeaway I hope you walk away from this article with is to do your homework when it comes to choosing the best PMI testing technique for you and your organization.

There are negative and positive stories about each of these three PMI techniques. It’s up to you to understand the ins and outs of your application and to determine the features you need in a PMI system.

And, as always, keep on mastering your metals!

To learn more about PMI techniques, subscribe to The Manufacturing Show on Apple Podcasts, Spotify, or wherever you listen to podcasts.