The techniques Laser-OES and Spark-OES (Optical Emission Spectrometry) are both used for elemental analysis, but they differ in terms of the way they excite the sample and detect the emitted light, as well as in their operating principles. In this article we will discuss main differences between these two methods and its applications.

Principle of Operation

In Spark-OES, spark is the excitation source. Once excited, the atoms return to lower energy states and emit light at specific wavelengths. Spark is created using spark generator with electrode, therefore it is problematic to analyse non-conductive samples using this method. 

Laser-OES uses a focused laser pulse to strike the surface of a sample, creating a plasma with excited atoms at the surface. When the plasma cools, it emits light (photons), specific for the elements present in the sample. The laser can be finely controlled, making it ideal for rapid, localized analysis without needing sample preparation.

To put it simply, for the end-user: laser-OES is very similar to spark-OES with the only difference in excitation. The laser replaces the spark generator with electrode. Laser-OES can read almost all elements of the periodic table and has no limitation to sample type, surface or shape.

Sample Preparation

Spark-OES: OES is typically used for solid or liquid samples, and it may require more preparation, such as melting the sample, creating a slurry, or preparing a specific geometry for analysis.

Laser-OES: used to analyze solid, liquid, or gaseous samples with little to no preparation required. It is a  non-destructive technique, and analysis can often be performed directly on the sample, as will be shown further in text.

Argon consumption

Spark-OES requires much higher argon consumption, compared to laser-OES. For example, laser-OES device analytical flow is 10 l/h, compared to 250-300 l/h in Spark-OES. Additionally, some laser-OES applications do not require argon at all!

Burnspot

Compared to spark-OES, during Laser-OES measurement the laser generates a small plasma plume and it’s only leaves a very small burnspot. Typically it is around 1 mm in diameter with excited plasma area of <0.1mm. Excited sample mass depends on various factors, like sample material (Fe, Al, Pb) or laser pulse settings, but typically it is 0.6 µg.

The sample surface will not be burned deep and appeared burnspot can be easily polished away without trace. That’s why laser-OES can be referred to NDT (non-destructive testing) techniques, which is a huge advantage, if NDT required, e.g. by the API standards.

Non-destructive testing is essential in industrial operations to ensure safety, quality, and reliability while minimizing downtime and reducing costs associated with failures or damages with destructive testing methods.

Detection and Resolution

Laser-OES can have a high spatial resolution since it can focus on a small area (down to micrometer scale) and analyze it directly, allowing for detailed mapping of elemental composition.

Spark-OES generally has a lower spatial resolution, due to using larger sample areas. However, it's well-suited for bulk elemental analysis and is often more sensitive for certain elements in specific sample types (e.g., metals). 

Long-Term Stability and Maintenance

Compared to Spark-OES the standardization intervals for Laser-OES are significantly longer. Additionally, there is no downtime and no routinely maintenance from user side. Since there is no spark stand and electrode, there is no need to clean it from condensate or change it.

Taking QLX1 device as example, during annual or semi-annual maintenance, the service engineer will check and clean sample stage as well as lenses and windows in the light path.

Precision 

Precision of intensities and concentrations with laser-OES is similar to spark-OES, if sample preparation is properly done, since laser-OES is more sensitive to surface contaminations and effects. Also, micro-structures in metals have a larger influence on the result than in spark-OES, but this effect can be compensated using pre-processing software tools. So, it is possible to compare achievable detection limits with laser-OES to ones in spark-OES.

Accuracy

Overall, accuracy of laser-OES is expected to be similar as in spark-OES (correlation coefficients are typically between 0.98 and 1). 

Why don't other manufacturers use laser?

There are two main reasons: higher laser price, resulting in higher instrument price, and different technological approach in laser-OES.

Lasers are still much more expensive than a digital spark source. Laser-OES, would, thus, have a much higher instrument price. Also, change to laser only works in newly designed instruments. You need to re-design spark stand, optics, argon block, trigger, read-out, software. Only replacing the source with a laser in an existing instrument is not possible. 

Partially, these are also reasons why some potential users feel an apprehension before introducing a new technology to their sites. 

Even though it is true, that initial cost of laser-OES system might be higher, compared to the spark one, it will be compensated over the long-term use due to extremely low operational expenses. Laser-OES system has reduced argon consumption and no disposables, almost no maintenance costs and downtime.