As we know, pXRF technology works by bombarding samples with X-rays and causing their atoms to fluoresce characteristic X-rays of the elements that they contain. The energy of the fluorescent X-rays corresponds directly to the atomic number of the element. Light elements have energy levels that are low enough that they struggle to escape from the sample without being absorbed. For the fluorescent X-rays that do escape the sample, some of them will not be able to penetrate the air between the sample and the instrument to reach the detector.
For the light elements that have fluorescent X-rays that do reach the detector, these need to be detected with enough significance to quantitatively estimate the element concentration. To do this, there must be a recognisable peak within the background noise for the element concentration to be calculated, and with the low energy light elements this is a challenge.
So essentially, light elements are difficult to measure by portable XRF because their fluorescence struggles to reach the instrument for it to calculate how much of that element is present. This also explains why the lightest elements that the instruments measure have higher detection limits, often 0.5-1%, because they need that higher concentration to produce enough energy that can be recognised.
There have been certain recent improvements in portable XRF technology that enable us to detect more of the light elements than we have previously, like Mg for example. Think about what we have just said about why we cannot measure some of the light elements, their signal is too weak to estimate their concentration. What if we just apply more energy to boost the signal? That is what the introduction of the detectors used in the current instruments has allowed. These SDD (silicon drift detectors) allow for more energy in total to be received and measured by the portable XRF, improving detection limits and the number of lighter elements that can be measured.
Measuring light elements with pXRF has been topical in recent months due to the recent boom in exploration for lithium around the world. No, lithium cannot be measured with pXRF, but as discussed in last month’s article, there are plenty of pathfinder elements that can.
And by the way, if anyone tells you portable XRF instruments cannot measure heavy elements, like gold (Au), well, they most certainly can. They just might not be able to measure them to the levels that you need. It’s all about detection limits (and heterogeneity), but more on that next time.