In Parts 1 and 2 of this blog, I explored the various challenges of measuring selenium using single quadrupole ICP-MS. Here in Part 3, I’ll cover how high resolution ICP-MS deals with this analysis.
What is high resolution (HR) ICP-MS and how does it work?
HR-ICP-MS is a technique in which the different mass ions generated in the ICP are separated using a magnetic field instead of the electric fields used in quadrupole ICP-MS. The separation in the magnet occurs along what’s called the focal plane which basically means that the ions become spatially separated from each other as they pass through the magnet. For a given applied magnetic field, heavier ions are steered less by the magnetic field than lighter ones are. By incrementally increasing the applied magnetic field strength and/or by scanning a voltage prior to the magnet (called the acceleration voltage) to alter the ion energy, progressively heavier ions can be steered onto the detector. In order to achieve high resolution, today’s instruments include narrow slits before and after the magnet, to filter the transmitted ions in such a way as to improve the separation of neighboring masses. Additionally, a device called an electrostatic analyzer (ESA) helps to focus ions of the same mass by slightly different energy before they are detected. The resolution provided by these components mean that interferences from the plasma gas and sample components, such as Ar2+ and ArCl+, can be separated from the analyte with which they interfere. A schematic of our HR-ICP-MS instrument, the ELEMENT 2 is shown in Figure 1 below. The ELEMENT XR instrument is very similar, but has an additional Faraday detector for increasing the linear dynamic range to 1012 and an additional lens for improving abundance sensitivity (i.e. the contribution of the peak tail of a major isotope (with a certain mass value) to an adjacent (M-1 or M+1) mass). In Figure 2, an example of a mass spectrum from the ELEMENT 2 showing high resolution separation of the ion signals around selenium at mass 78 is shown.
Figure 1. Schematic of the ELEMENT 2 HR-ICP-MS.
Figure 2. High resolution separation of the ions detected in a 1:10 diluted seawater sample spiked with 100 pg/mL Se, around mass 78 (resolution = 10,000).
In Parts 1 and 2 of his blog series, Dr. Simon Nelms explored the various challenges of measuring selenium using single quadrupole ICP-MS. Here in Part 3, he covers how high resolution ICP-MS deals with this analysis. #AnalyteGuru
By changing the width of the slits before and after the magnet (well, actually after the ESA, which is located after the magnet), you can change the resolution from low, to medium, to high. ‘Why would you want to do that?’, I hear you ask. The answer is that some interferences, such as 40Ar16O+ on 56Fe only require medium resolution while others such as 38Ar40Ar+ on 78Se require high resolution. ‘Why would you not measure everything at high resolution to keep things simple then?’, I now hear you ask. The answer to that is that increasing the resolution leads to a decrease in sensitivity, which affects the detection limits you can achieve. With all this talk of resolution, I see that I haven’t explained what resolution actually is. Resolution (R) of two masses is defined by the International Union of Pure and Applied Chemistry (IUPAC) as:
R = M/ΔM
where M is the mass of the second ion and ΔM is the difference in mass between the two ions you are intending to resolve from each other. The closer in mass the two ions you wish to separate, the higher the resolution you need. The ELEMENT 2 HR-ICP-MS has three resolution settings; low (R = 300, for non-interfered isotopes), medium (R = 4000, for relatively easily resolved interferences) and high (R = 10,000 for more difficult to resolve interferences).
Interferences on selenium and the resolution required to separate them using HR-ICP-MS.
As mentioned in Part 1 of this blog, all conventional ICP-MS instruments use argon (Ar) gas to generate the plasma. In the high temperature environment of the plasma, Ar forms a range of interferences by combining with itself to form Ar2 and the components of the sample, to form, for example, ArN, ArO and ArCl. As the 40Ar2+ interference is very large, 80Se is usually not selected and, as with single quadrupole ICP-MS, the much less interfered 78Se is used instead. Although 76Se and 82Se are slightly more abundant than 77Se, these two isotopes are also not generally used due to the isobaric (i.e. isotopic overlap) of 76Ge with 76Se and 82Kr (present as an impurity in the Ar gas) with 82Se.
In Part 2 of this blog, I showed that further interference can also be generated by the production of doubly charged ions, such as Gd2+. In Table 1 below, I’ve listed the most common interferences on the two isotopes of selenium that are generally used with HR-ICP-MS — namely 77Se (7.6% abundance) and 78Se (23.8% abundance), as explained above — and the resolution required to separate them.
Table 1. Interferences on 77Se and 78Se and the resolution required to separate them
From Table 1, it’s clear that with a mass resolution of 10,000 (the high resolution setting on the ELEMENT 2) you can resolve the Ar2+, ArCl and CaCl interferences on 77Se and 78Se, while the Gd2+ interference on both these isotopes can be resolved using the medium resolution setting of 4000. In practice, the more abundant 78Se is usually the preferred isotope and high resolution is used to ensure that all the above interferences (as well as a variety of others, including some unidentified species that may not be completely removed using quadrupole based, collision cell ICP-MS, as shown in Figure 2 above) are completely resolved from it.
As with single quadrupole (SQ) ICP-MS, if your samples contain carbon (either as dissolved CO2, carbonates or organic solvents), the sensitivity of Se jumps significantly, but, also as with SQ-ICP-MS, you can compensate 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. Alternatively, a methane in argon gas mixture (e.g. 75 mL/min of 2% (v/v) CH4 in Ar) can be added via a gas port into the plasma to achieve the same effect.
I hope that you have found my discussion of the challenges of Se analysis using ICP-MS in this three part blog series useful. If you have any questions about the analysis of Se, or any other element for that matter, just let me know via the comments box below!
To learn more about the Thermo Scientific ELEMENT 2 HR-ICP-MS, see here.
Also, check out our video: http://www.youtube.com/watch?v=ycdFbS6yBuU.
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