When delving into the realm of chemical testing, one method that often comes up is X-ray fluorescence (XRF). XRF is an analytical technique that is popular in the electronics and metal industries, especially for testing heavy metals like lead, mercury, or brominated compounds, under RoHS, REACH, POP, or Prop. 65 regulations.
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XRF is famous for its non-destructive sample requirements. The primary requirement is for the sample’s surface to be flat, which is already the case for most machined parts. However, when dealing with non-flat objects like rocks, grinding them into a powder and pressing them into pellets is often necessary. The person doing the analysis should also clean the sample to remove dirt or oxidation layers from interfering with the analysis.
After being placed in the sample chamber, an X-ray beam hits the sample and induces fluorescence.
Simply put, fluorescence is a process that occurs when an electron moves from a higher energy state to a lower energy state (represented by n=3 and n=2). This results in the electron losing energy, which is then released as a particle of light (also called a photon). The energy of the photon is equal to the energy gap between the two electron states, which is represented in the image as a blue arrow. Since the amount of energy emitted is unique to each element, analyzing the photons will allow us to determine which elements are present in the sample.
How fluorescence occurs in XRF: when the X-ray strikes an atom, it causes the atom to eject an electron from the lowest energy level – also called ionization. There is now a hole in the atom, which is unstable. To stabilize itself, an electron from a higher energy level will fall into the hole and release a photon. The instrument then directs the photons to a detector, which measures the energy of each photon and creates an energy spectrum. We can then analyze the spectrum to determine which elements are present.
Below is an XRF spectrum of multiple elements.
On the X-axis, you can see the amount of energy emitted by the atom, which we use to characterize the substance (qualitative analysis). On the y-axis, you can see the relative intensity, which we use to determine the amount of the substance that is present (quantitative analysis).
Unfortunately, XRF is limited in the compounds it can test for. It is only capable of detecting individual elements, such as lead (Pb), mercury (Hg), and nickel (Ni), rather than compounds that are made up of multiple elements, such as bisphenol A (BPA) or di(2-ethylhexyl) phthalate (DEHP). This is because X-ray fluorescence affects individual atoms rather than molecules as a whole. As a result, XRF cannot give information on how atoms are bonded together.
Furthermore, detecting light elements (such as carbon, oxygen, and fluorine) can also be difficult. The relatively lower intensity of X-rays that light elements emit makes their detection more challenging than heavier elements.
Since XRF excels with heavy elements, it’s a great choice to analyze lead (Pb) mercury (Hg), and cadmium (Cd), restricted under RoHS. It can also detect chromium but cannot differentiate between hexavalent chromium and its other forms. Therefore, if it does detect chromium, additional testing would be necessary to determine if it is in the hexavalent state.
XRF cannot identify the remaining RoHS substances – PBB, PBDE, and the RoHS phthalates – since they are compounds.
Gas chromatography mass spectrometry (GC-MS) is a more suitable analysis method.
Similarly, XRF would be helpful if you wanted to test for EU REACH substances that contain heavy metals. Some examples are nickel (Ni), arsenic (As), and lead. However, many EU REACH substances are organic compounds, rendering XRF ineffective. The US TSCA 5-PBT and EU POP regulations also restrict organic compounds.
In contrast, XRF is an ideal choice for compliance testing with the EU Battery Regulation, which restricts cadmium, lead, and mercury. XRF can also be used to test for halogens.
Below is a summary of how you can use XRF for various environmental compliance regulations.
Environmental Regulation | Substances that can be analyzed |
|---|---|
RoHS |
|
EU REACH (SVHC list or Annex XVII) |
|
EU Battery Directive |
|
Halogen-free requirements |
|
US TSCA 5-PBT | None |
EU POP | None |
In addition to regulatory compliance, XRF is widely used in the electronics industry. One of the main ways is for quality control. Because of its non-destructive nature, quality assurance specialists often use XRF to verify the quality of finished products. For instance, you can employ XRF to validate the chemical composition of a steel alloy, ensuring its adherence to ASTM standards. You can also use the method for failure analysis since XRF can identify unexpected substances contributing to device malfunction.
In addition, professionals use XRF to monitor the condition of hydraulics systems. This is because, over time, small amounts of pipe material will dissolve into the oil as the machinery wears down (similar to how lead pipes gradually leach lead into water). XRF can quantify the mineral content in the oil, indicating the extent of pipe degradation. They often use this method as a preventive measure, allowing for ongoing assessment of the system’s condition.
It’s worth noting that XRF is very popular in the electronics industry because, in most cases, it doesn’t require the sample to be destroyed. In addition, it requires little sample preparation. With many other analytical techniques, workers must dissolve solid samples in an organic solvent – handling strong chemicals often requires technical training due to the hazards they pose. In contrast, the sample preparation for XRF saves time and reduces laboratory costs. This being said, XRF still requires training in most countries.
There’s a wide variety of handheld XRF instruments on the market. These portable devices are beneficial for on-site analysis, especially in environmental research, where samples can quickly degrade. They also offer increased speed and efficiency, with some devices capable of analyzing a sample in a matter of seconds. Additionally, since it’s non-destructive, industry workers can use handheld XRF instruments to analyze large pieces of equipment.
The major disadvantage of using portable XRF devices is that they are less powerful and are often incapable of detecting elements lighter than magnesium (atomic number 12).
If you have any questions about XRF, GC-MS, or any other testing method, please contact Enviropass for a free consultation!
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