Scope and Limitations
In summary, most metals and elements starting with Titanium (Z = 22) exhibit detection limits in the 10-150 ppm range if they are present in a material consisting mainly of light elements.
In the case of a heavy element mixture, such as an alloy, the LODs can be higher. For example, while lead in soil can be detected below 20 ppm in the absence of interferences, lead in tin becomes hard to detect below 500 ppm (0.05%), based on experiments with standards in Analytical Mode. For standard Analytical Mode, a number of metals in alloys appear to have LODs in the 200 ppm range (acquisition time of 180 s) based on analyses of certified standards, but there is significant variability depending on the specific metal and alloy. However, the RoHS-WEEE Mode has incorporated a special method for detecting Cd in alloys slightly below its 100 ppm limit in the regulations, involving an acquisition time of several minutes. This method is also more suitable for trace Pb in alloys.
Light elements such as phosphorus and sulfur are hard to detect in a matrix of heavy elements owing to the relatively low intensity of their fluorescence. Consequently, for metals, one must use caution in interpreting high purity readings. A reading of “100% pure” from the instrument just means that everything else was below the LODs for the experiment conditions. For instance, this could be consistent with a 95wt% pure material containing 5wt% light elements (e.g., Al, P, S, Si), or a 99.9% material containing 10-20 heavy elements, each in the ~50-100 ppm (0.005-0.01%) range. Even with some assumptions (e.g., the sample is composed of only heavy elements, and the number of impurities limited), the upper limit for estimation of purity would be approximately three nines.
Quantitation and Alloy ID. The software in Analytical Mode determines % composition, then matches the values to tables in a grade library to identify unknown metals and alloys. The instrumentation delivers good performance in identifying most alloys. In the case of major elements that cannot be observed directly, such as aluminum, the grades can be usually be identified by the relative concentration of minor alloying elements.
It is important to note that the software delivers the best match based on the elements that are detectable and included in the quantitation table. Alloys that differ only in the amount of undetectable light elements may look the same.
Likewise, the wt% composition reflects the relative amounts of the heavy (detectable) elements, typically beginning with titanium (Z = 22, Group 4 in periodic table). For most alloys the difference would be small, but in some cases the actual wt% of the major element would appear higher than it is in reality if light elements are present.