In part 1 of this blog post series, I covered the most notable problems encountered when analysing selenium using single quadrupole ICP-MS, namely Ar2+ and ArCl+ interferences, as well as the trouble caused by the smaller BrH+ and Kr+ interferences. In part 2, I discussed the additional problems of carbon enhancement of the selenium ion signal and rare earth doubly charged ion interference on the main selenium isotopes and in part 3, I explored the use of high resolution ICP-MS for improving the accuracy of selenium analysis. In this final part of the series, I’ll describe how the development of triple quadrupole ICP-MS has revolutionised selenium analysis and taken it to new levels of accuracy.
A Quick Review of the Challenges of Se Analysis Using ICP-MS
Further to the details discussed in the earlier blog posts, Table 1 below shows a combined list of the main interferences encountered on Se when using ICP-MS detection.
Table 1. Primary interferences observed on Se when using ICP-MS.
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As mentioned in part 2 of this blog post series, there are other possible interferences on Se such as NiO+, NiOH+, CuOH+, ZnO+ and ZnOH+. In most cases, these are very small and don’t affect the Se measurement accuracy. However, if your analytical task involves measurement of trace Se in Ni, Cu and Zn pure metals and alloy digests, these interferences become a significant problem. Applying collision cell single quadrupole ICP-MS and high resolution ICP-MS technologies help, but are not always able to fully remove the interferences. This is where triple quadrupole ICP-MS comes in.
How does triple quadrupole ICP-MS work?
Triple quadrupole ICP-MS instruments are an adaption of single quadrupole systems, with an additional quadrupole being inserted prior to the collision cell. The result is the ability to selectively filter target masses with the first quadrupole, thereby dramatically cleaning up the ion beam before it enters the collision cell. This both improves the performance of the instrument in conventional helium collision cell mode and allows the use of reactive gases in the cell to carry out selective ion chemistry to achieve outstanding removal of some very tricky interferences.
Triple quadrupole ICP-MS uses two primary modes of operation: on mass and mass-shift analysis.
With on-mass analysis, the isotopes of interest are transmitted through the first quadrupole (Q1) together with the associated interference and the collision cell (Q2) is used to remove the interference (either with helium or a reactive gas such as oxygen) while allowing the isotopes of interest to continue on to be analysed at their original mass in the third quadrupole (Q3).
In contrast, with mass-shift analysis, Q1 filters out the isotopes of interest in the same way as with on-mass analysis, but this time, Q2 is used to react the isotopes of interest with a gas such as oxygen or ammonia, in order to convert the isotopes to higher mass product ions. At the same time, the corresponding interferences don’t react, allowing the isotopes of interest to be very efficiently separated and measured interference-free.
If you’d like to learn more about on-mass and mass-shift analysis, along with some other ICP-MS related musings, take a look at this article.
So, how does triple quadrupole ICP-MS get better results for Se?
Inside the collision cell of an ICP-MS, selenium reacts very efficiently with oxygen in a mass-shift reaction to produce SeO+, whereas the difficult Ni, Cu and Zn based polyatomic interferences don’t (see figure 1). In addition, on collision with O2, Gd2+ does not produce an ion that overlaps with SeO+ so this interference is also effectively eliminated. Using mass-shift with oxygen in this way for Se analysis has the added benefit that the most abundant 80Se isotope can be used, leading to higher sensitivity and lower detection limits. With helium cell gas, 80Se can’t be used because the corresponding 40Ar2+ interference is much too large, leading to 78Se being the preferred isotope to measure. However, as shown in Table 1, 78Se is interfered by 156Gd2+, so still requires triple quadrupole, mass-shift operation with oxygen to get the best results.
Figure 1. Se determination using triple quadrupole ICP-MS mass-shift interference removal.
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If you’d like to see how our triple quadrupole instrument, the Thermo Scientific™ iCAP™ TQ ICP-MS enhances Se (and other elements such as P, S and As) measurement in practice, you can download our iCAP TQ ICP-MS Applications Compendium (accessing this Compendium requires you to provide your contact details).
To discover more about the complete Thermo Scientific™ ICP-MS instrument portfolio, take a look here. 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!