Portability and nondestructive analysis are indisputable advantages of a handheld XRF spectrometer. These features make it a perfect screening tool.
By screening with XRF analysis, assemblers can determine whether more expensive and labor-intensive testing is required. For example, if we test a material with an XRF analyzer and find that lead content is 2,800 milligrams per kilogram, we can safely reject it as noncompliant without additional testing. Similarly, should the screening test detect a chromium content of 200 milligrams per kilogram, we can deem the material compliant. Without XRF screening, we would have to test the sample specifically for hexavalent chromium, only to learn after much more time and expense that the material is compliant.
If a sample is homogeneous, it can be analyzed with a portable XRF spectrometer. Based on the results, engineers can then decide if the sample meets ROHS requirements, or whether the results are inconclusive and further testing is necessary. To account for measurement error, acceptance threshold values for ROHS elements should be set lower than those set forth in the regulation: 100 parts per million for cadmium and 1,000 parts per million for the other five substances.
For example, if the lead concentration in a sample is 910 parts per million, and the one sigma error associated with the result is ±50 parts per million, we cannot decide if the lead content is below the regulatory threshold of 1,000 parts per million. At the two sigma level, the true lead concentration may be anywhere between 810 and 1,010 parts per million. At the three sigma level, the band of uncertainty stretches from 760 to 1,060 parts per million. To make a positive decision, we will need to repeat the test for lead with greater precision.
On the other hand, if the lead content is measured at, say, 750 parts per million, we would deem it compliant since even at the three sigma error level, it would not exceed the 1,000 parts per million threshold. Alternatively, a lead concentration of, say, 1,200 parts per million, would mean noncompliance. For bromine, the threshold value is much lower-350 parts per million-because the threshold for brominated flame retardants refers to the total amount of the compound, and not just the bromine content. In addition, for chromium and bromine, there’s only one threshold value rather than a range of inconclusive readings. This is because the analyzer determines elemental rather then chemical composition of the sample. If the chromium content exceeds the threshold, it must be retested with a method specific for hexavalent chromium. Similarly, if the bromine concentration exceeds the threshold, a follow-up analysis is mandatory with a method specific for PBD or PBDE.
XRF analysis can be applied in almost every phase in the life cycle of plastic parts, from compounding the polymer, to collecting and separating used parts for recycling. The most beneficial time for XRF analysis is before materials and components are introduced into the manufacturing process for the final product, and after the product is fully assembled. These two points are the domain of ROHS.
Here is a list of the best ways to apply portable XRF analyzers: * Incoming inspection of compounded plastics, such as granulate. * Incoming inspection of components and subassemblies, such as cables, wires and enclosures. * Screening plastics for brominated flame retardants and other elements, such as antimony and tin. * Screening circuit boards for brominated flame retardants. * Quantitative analysis of metals in plastics. * Screening components for forbidden substances. * Preliminary screening of finished products.
One plastics molder recently learned a lesson in just how important screening can be. The molder used a portable XRF analyzer to test an incoming shipment of polyvinyl chloride pellets. The analyzer was inserted into the bin of pellets and a 5-second test was run. The analyzer revealed that the pellets contained lead at a prohibitively high concentration of 8,600 parts per million. By preventing the material from entering production, this data saved the manufacturer from a very costly loss.
The advantage of portable XRF spectrometers cannot be overestimated. The ability to bring the “laboratory” to the object to be analyzed results in improvements on many fronts. First, the cost of testing is incomparably smaller. Additionally, on-site XRF analysis yields results in real-time. Consequently, more extensive testing will be performed, and many objects will be tested that otherwise might not be. More testing translates into better compliance.
Portability is not synonymous with inferior results. The portable XRF spectrometer analyzes samples without destroying them. If the sample is heterogeneous, the results will reflect that. However, if the sample is homogeneous or if it is prepared in the same way as a laboratory specimen, there is virtually no performance difference between the handheld instrument and the laboratory version.
Portable XRF spectrometry is not a panacea. As always, engineers should use the best possible tool to solve the problem at hand, rather than find a problem for the tool we happen to have. Portable XRF is such a tool.