![]() ![]() Confirmatory XRF readings provided a wide range of results that wouldn’t likely be observed in a paint film. The stairwell may have had fly ash contamination of the substrate from lead (a Z! error) or perhaps from mercury (Z-2) or bismuth (Z+1). I hypothesized at the time that the ceilings had electrical wire, roofing flashing debris or other leaded materials sitting on the top surface presenting an interference. Confirmatory paint chips samples yielded essentially non-detectable results. It didn’t make sense that only these surfaces would be painted with LBP. Aside from the exceptions, all the readings were very low to non-detect (0.0-0.1 mg/cm2). There was no LBP found except for a few closet drywall ceilings, a stairwell underside, and a cinderblock wall of a stairwell. My first experience with false positives was testing a late 1960s apartment complex. Evaluation of these types of errors require an advanced understanding spectral physics and are not further considered here. Testing of a curved surface is less accurate than a flat one. Fortunately, false negatives for XRF tend to be associated with the heavy and uncommon elements such as thorium and uranium. A false negative or a biased result can occur when the silicon inside the detector absorbs some of the energies before counted. Aluminum is one metal that can cause a broad range of interferences with other metals when present at large concentrations. Such “sum peak” occurrences may include iron (K alpha=6.4) doubling up to look like lead (6.4x2= 12.8, closely resembling one of the lead peaks at 12.61). ![]() Artifact peaks can also occur from random circumstances when two photons arrive in the detector at the same time. Certain types of cellulose can cause a broad “backscattering” of energy. Z: A broad catch-all term for various interferences caused when a broad band of peaks or instrument or other matrix noise hides a smaller peak.The figure of the light element K-shell spectra below should be helpful in understanding this sequential progression of spectral lines as shown in Figure 1 below. For heavier metals, there are also gamma (tertiary lines) that are smaller and less distinct but can be useful in some circumstances (such as a “tiebreaker” if there are spectral overlaps). The middle weight elements therefore tend to give the cleanest detections with both K and L spectra lines available for use. At the same time, the K-shell peaks continue to have increasingly higher energies and therefore become less resistant to excitation and start to become less relevant, being eliminated from use at the atomic weight of barium. When we pass one complete period of the Periodic Table and arrive at calcium (immediately below magnesium), L-shell energies become detectable, and increase incrementally about 0.05-0.1 KeV per element. As one proceeds across the periodic table for increasingly heavier elements, these distinct emission lines increase steadily in increments of roughly 0.20 to 0.30 KeV. For example, starting at magnesium, the lightest element detected, the electron K-shell alpha (primary) emission line and beta (secondary) emission line are 1.25 and 1.30 KeV. ![]()
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