In the first part of this blog, I covered the most prominent problems with selenium analysis when using single quadrupole ICP-MS, namely the Ar2 and ArCl interferences. I ended that post with the observation that there are still two more problems to be aware of, which I’ll explore now.
Carbon enhancement of the selenium ion signal
It turns out that when your samples contain carbon (either as dissolved CO2, carbonates or organic solvents), something peculiar happens in that the sensitivity of Se jumps significantly. For example, in the presence of 2% (v/v) methanol, the sensitivity of Se climbs by typically about a factor of 3. The result is false positive results, unless you matrix match the carbon content of your calibration standards (and blanks) with that of your samples. The cause of this enhancement is not well understood, but as it also happens (albeit to a lower extent) with Se atomic emission, it may be related to a releasing of Se from anionic species in solution by carbon ions in the plasma. Freed Se atoms may excite and ionize more easily than Se bound in e.g. selenate ions, leading to enhanced signals. Others have suggested that the cause of the enhancement could be that an increased population of carbon ions or carbon-containing ions in the plasma enables a more complete ionization of analytes with ionization potentials lower than carbon. It’s useful to note that the enhancement effect is not limited to just Se – it also occurs to varying degrees with other, high ionization potential elements such as As and Be. Fortunately, this effect tails off with increasing carbon content such that you can usually compensate easily for it by adding methanol (or any other water soluble organic solvent) at a concentration of 2% to 3% (v/v) to all blanks, calibration standards and samples. A simple way to do this is to add the organic solvent to your internal standard and add the internal standard on-line using a T-piece.
Doubly charged ion interference
An additional spectral interference on Se arises from an unexpected source. Rare earth elements in the samples pose a problem as they form a certain amount of doubly charged ions in the high temperature of the plasma. As quadrupole mass spectrometers separate ions on the basis of their mass to charge ratio, doubly charged ions appear at half the mass of the parent ion. For Se, the problem is gadolinium (Gd), which has 7 isotopes ranging from mass 152 to mass 160. Table 3 shows the interferences Gd2+ generates on the Se isotopes.
Table 3. Interferences on Se from Gd2+
No Gd interference
No Gd interference
Interference on Se from Gd2+ is quite common in environmental sample analysis as a result of the occurrence of rare earth elements in soils, so if you have been getting a positive bias on Se in your analysis that isn’t fully corrected by addition of carbon as described above, then Gd may be to blame. More surprising is that Gd2+ can also be a problem for Se determination in clinical samples. The cause is injection of large amounts of a Gd based compound (usually gadoteric acid) into patients undergoing MRI scans, which is done to improve the MRI image contrast to aid in diagnosis. To avoid this problem, clinicians have to take care to take blood samples for Se analysis prior to injecting gadoteric acid.
Unfortunately, collision cell operation with He doesn’t help, as it cannot remove this interference, so the most practical solution for routine analysis is mathematical correction (as described above) using the instruments software. You can achieve this by adding 156Gd to your analyte list, running a pure Gd solution at the start of the run (at, say, 20 ng/mL concentration), calculating the Gd2+/Gd ratio for this solution and then applying this correction ratio to all samples in the run to subtract the contribution of 156Gd2+ from 78Se. Use of certain reactive gases in the cell can remove this interference, but they cause limitations on the measurement of other analytes, so are not ideal for routine, multi-element analysis where Se is just part of a wider suite of elements. Finally, it is also possible, as my colleague Michael Paul (Thermo Fisher Scientific, Germany) showed at this year’s Winter Plasma Conference, to resolve the Gd2+ interference from the isotopes of Se using a quadrupole mass resolution of 0.4 mass units. With this resolution, both 155Gd2+ and 156Gd2+ can be resolved from the signals at 77Se and 78Se respectively. Measuring Se using the high quadrupole resolution setting of the instrument is, then, a simple way to yield Gd2+ interference-free results, but keep in mind that use of higher quadrupole resolution comes at the price of a loss of sensitivity.
Other interferences to consider
There are a couple of other potential sources of trouble for Se analysis using ICP-MS, such as NiO and NiOH interference on Se, but at the Ni levels encountered in most environmental, food and clinical analyses, collision cell operation is effective in removing these interferences.
I hope that you have found my above meanderings through the complexities of Se analysis using single quadrupole ICP-MS useful. If you’d like to learn about how HR-ICP-MS handles Se analysis, look out for part 3 of this blog.
In the meantime, if you have any questions about measurement of Se (or any other element) using ICP-MS or if you’d like to learn more about how Thermo Scientific’s ICP-MS instruments can help meet your needs for trace element analysis, just let us know via the comments box below!
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