Selenium was first discovered by Baron Jöns Jacob Berzelius in Stockholm, Sweden, in 1817. Apparently, he had interests in a sulphuric acid works and was curious about a reddish-brown sediment that precipitated in the vessels in which the acid was manufactured. Initially, he believed that the sediment was a compound of the previously discovered element tellurium, but on subsequent investigation he eventually discovered, in 1818, that it did in fact contain a new element. Due to its similar properties to tellurium, named after the Latin ‘tellus’ meaning earth, Berzelius decided to call the new element selenium, from the Greek mythological goddess of the Moon, Selene.
Selenium in the Body, the Environment and Industry
Selenium is an essential trace element nutrient that functions as a cofactor for enzymes that catalyze reactions in the body that remove harmful reactive oxygen species, such as hydrogen peroxide and organic hydroperoxides. Selenium is also found (as selenocysteine) in three deiodinase enzymes, which convert one thyroid hormone to another. In contrast, at high levels selenium becomes toxic, causing selenosis, symptoms of which include a garlic odor on the breath, gastrointestinal disorders, hair loss and neurological damage. In extreme cases, selenosis can result in cirrhosis of the liver, pulmonary oedema and death.
In the environment, selenium is found in inorganic form as selenide, selenate, and selenite in various minerals. It is present in soil at concentrations ranging from < 0.1 µg/g to > 5 µg/g, and enters our diet via sources such as wheat, nuts and vegetables.
Aside from its biological importance, selenium has uses in glass making, production of certain alloys and as copper indium gallium selenide in solar cell manufacture.
All of the above means that being able to measure Se accurately in a wide range of sample types is highly important.
Using ICP-MS for Selenium Analysis
Selenium can be measured using a variety of techniques, but furnace AA, ICP-OES and ICP-MS are most commonly used. As the level of Se in environmental and clinical samples is typically in the low to sub-µg/g (or µg/mL), once the samples are prepared for analysis (e.g. digested and diluted) the actual level to be measured is in the ng/mL range. This leads to ICP-MS generally being the preferred technique of choice.
There are five basic types of ICP-MS, namely single quadrupole (SQ)-ICP-MS, triple quadrupole ICP-MS, magnetic sector high resolution (HR) ICP-MS, multi-collector HR-ICP-MS and ICP time of flight (TOF) MS. Of these SQ-ICP-MS is by far the most commonly used, but when it comes to Se analysis, this technique faces some challenges.
Interferences on Selenium in ICP-MS
All conventional ICP-MS instruments use argon gas to generate the plasma. Argon has isotopes at mass 36, 38 and 40, which at first sight would seem to pose no problem for Se analysis, with its isotopes ranging from mass 74 to mass 82 (80Se being the most abundant). However, Ar, as noble a gas as it is, becomes somewhat less noble in the extremely hot (> 5000ºC) environment of the plasma. At these temperatures Ar forms a plethora of interferences by combining with itself to form Ar2 and the components of the sample to form, for example, ArN, ArO and ArCl. The isotopes of Ar2 (the so-called argon dimer) overlap with the isotopes of Se, as shown in Table 1 below. Table 1 also shows the relative abundance of the Se isotopes and the corresponding Ar2 interference (in brackets; the larger the number, the larger the relative interference).
Table 1. Interferences on Se from Ar2
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|Se isotope||Ar2 interference|
|74Se (0. 9%)||36Ar38Ar (0.0004%)|
|76Se (9.4%)||36Ar40Ar (0.67%)|
|77Se (7.6%)||No Ar2+ interference|
|78Se (23.8%)||38Ar40Ar (0.13%)|
|80Se (49.6%)||40Ar40Ar (99.2%)|
|82Se (8.7%)||No Ar2+ interference|
From Table 1, it’s clear that all but 74Se are sufficiently abundant to be potentially useful for low level Se analysis, with 80Se having the highest abundance and therefore being the most sensitive. However, Table 1 also shows that the large relative Ar2 interference on 80Se poses a potential problem, depending, of course, on how large the interference actually is. Unfortunately, the mass 80 Ar2 interference is huge – typically millions of counts per second. The interference is so large that it is also significant on 76Se and 78Se as well, even though the relative abundance of Ar2 on these isotopes is low. This leaves 77Se and 82Se as potential candidates. Unfortunately, nature is not on our side here either, as these two isotopes also suffer from interferences derived from other components present in most samples and impurities in the Ar plasma gas, as shown in Table 2.
Table 2. Interferences on 77Se and 82Se from typical sample components and Ar impurities
|82Se (8.8%)||81Br1H, 82Kr|
Dealing with Selenium Interferences in ICP-MS
So, what can we do to solve these problems? Well, we have three options. Firstly, we can make a mathematical correction to subtract the contribution of the interference from the selected Se isotope. This works well enough when the interference is fairly small compared to the Se signal from the sample, so is often an effective solution for correcting the 40Ar37Cl interference on 77Se for example. However, the lower the Se concentration and the higher the signal from the interference, the less accurate mathematical correction becomes.
Secondly, we can use collision cell operation to reduce/eliminate the interferences. The details of collision cell operation will be covered in a later blog. In practice, the outcome of using collision cell technology is that 77Se and 78Se come out as the best isotopes to measure, as the lower intensity 40Ar37Cl and 38Ar40Ar interferences coupled with the reasonable abundance of 77Se and 78Se means that the best signal to interference ratios (and hence lowest detection limits) are achieved with these isotopes. As 78Se is about 3x more abundant than 77Se, it is usually the most preferred isotope.
The third option is HR-ICP-MS – more on that in part 3 of this blog.
So far so good. Single quadrupole ICP-MS with collision cell operation and 78Se as the isotope of choice. Seems like we have a good solution for accurately measuring Se, doesn’t it? Unfortunately not. There are still two more problems to be aware of, which I will cover in part 2 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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