Olympus: Each has its place
According to Ted Shields, portable products manager, analytical instruments division, Olympus Scientific Solutions Americas, LIBS is best viewed as a complimentary technology to more traditional handheld XRF, and each type of technology does certain things well.
"The most intriguing thing about LIBS is the ability to measure elements that you cannot measure with XRF," says Shields. "Carbon, beryllium and lithium are all possible, and recent advances are making carbon in L-grades achievable for some LIBS analyzers."
He says however that LIBS technology struggles with some elements that XRF excels at. Most notably, he says, these include refractory metals, such as Cr, Zr, Mo, Ta and other common and commercially important alloying elements. "LIBS has a much smaller spot size than XRF," continues Shields. "This is good in the sense that the burn mark left behind is small. But it makes it much harder to get the same answer when you test twice in a row. This is because the sample varies across the small scale of the LIBS spot."
He says some LIBS analyzer manufacturers compensate for this by recommending users take several shots at different places (called rastering the laser). But this adds to the test time. "Variations in sample geometry effect the results to a much larger degree for LIBS," he continues.
"LIBS is also more sensitive to moisture and surface contamination. Some manufacturers try to address this by recommending users perform a cleaning pre-burn with the laser, again adding time to the analysis."
Shields continues by adding that while LIBS calibrations are empirical, XRF metal calibrations are Fundamental Parameters (FP). "FP lets the XRF instrument calculate the effect of every element on every other element," he says. "This calculation lets one XRF calibration be remarkably accurate for a wide range of samples. On the other hand, empirical calibrations are only as broad as the samples used to create them. If you analyze something outside of the range of calibration samples, the results can be dramatically wrong. This is why LIBS analyzers require users to choose among several calibrations, and why making the right choice is critical to achieving reasonable results."
Shields also emphasizes that knowledge about what materials are being dealt with is critical to achieving higher purity standards, especially in light of increased global standards for end materials, including those recently established by China.
"One sample with a high concentration of an undesirable element can contaminate an entire batch of metal," he says. "The only way to know is to test. And the test needs to be quick and portable - like it is with handheld XRF technology," which he adds "are faster and more sensitive than ever before, making it possible for recyclers to meet higher purity standards."