A few years ago, when I was hired head of my research group and did not have much time to go to the lab, I encountered some analytical difficulties. I trusted a chemistry colleague who had never analyzed before rare earth elements (REE) by ICP-MS. Even if I gave him my previous protocols and interferences to correct he did not make the job.

Since the 1990s, the development of ICP-MS (Inductively Coupled Plasma Mass Spectrometer) allows direct determination of REE and thus became widely used technique. This method is used to introduce solutions with weak concentrations of REE while providing a low detection limit and is able to measure all REE simultaneously without any separation from the matrix. Ions produced in high temperature plasma are identified on the basis of the mass to charge ratio, m/z, which is characteristic of a given isotope. Each REE has at least one isotope that is free from isobaric overlap and the sensitivity is relatively uniform from 139La to 175Lu. The major analytical problem encountered with ICP-MS is the level of oxide formation in the plasma. REE form a continuous group from 139 to 175 m/z, and the formation of light REE and barium oxide can produce significant middle REE interferences. Refractory oxide ions are influenced by the plasma operating parameters. The oxide production level is close to 1 to 2%. Mathematical corrections are required in order to suppress these spectroscopic interferences; these corrections are calculated as a function of the oxide level. Previous studies have shown the importance of these corrections for accurate REE quantification. In the case of Eu, Ba that forms the BaO+ species and those of isotopes 135Ba (135+16=151) and 137Ba (137+16=153) isobarically interfere with 151Eu and 153Eu, respectively.

I started looking at REE in plants and decided to present some preliminary results at EGU in April 2014. Even if i know possible Eu interference, my results show huge anomalies (Figure 1) and i was questionned by a few colleagues.

Figure 1 Example of A. chinensis biomass analysis without Ba interference correction.

Back in the lab, i decided to re-analyzed myself the whole samples, and did not get the same results, let’s check by yourself (Figure 2). I have submitted those results to Chemical Geology this summer, you can check the preprint here.

Figure 2 Same example with Ba interference correction.

Take home message : False positive anomaly!

The presence of enormous positive Eu anomalies as displayed in Figure 1 is extremely suspicious and has we show an analytical artifact due to insufficient correction for a large BaO interference. We may have some concerns on the analytical quality of some data published where they found positive Eu anomalies in some of their analyzed samples. In those papers we often couldn’t find any information on REE preconcentration in the methods, so we assume that wasn’t done. Europium determinations in low-REE samples suffer from interferences of BaO on Eu, often leading to false positives in normalized patterns. The authors unfortunately don not provide Ba concentrations. The authors should make sure that the Eu-signal is real (rule of thumb: Ba/Eu<1000), and that Eu is not a false positive