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simon-nelms
Team TFS
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selenium analysis

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 selenideselenate, 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

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

Se isotope Interference
77Se (7.5%)  40Ar37Cl
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!

Additional Resources

    • To learn more about the iCAP Q ICP-MS, see here.
    • For further applications information for the iCAP Q ICP-MS, see here.
    • Visit our Community pages at www.thermoscientific.com to discover more about our solutions for Food and Beverage, Environmental, Pharmaceutical , Biopharmaceutical analysis and other application areas.

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24 Comments
Not applicable
Very interesting.
simon-nelms
Team TFS
Team TFS
Thanks Andrew; good to hear that you found the post interesting.
Not applicable
Simon, are you able to provide references for the tables in this blog?
simon-nelms
Team TFS
Team TFS
Hi Nick, I generated the tables myself using rounded up isotope and molecule abundance data from our ICP-MS software. A good source of isotope abundance data is IUPAC and data for each element from this source can be found at http://www.ciaaw.org/. Kind regards, Simon
Not applicable
Thank you. We are coming to terms with selenium interferences and how collision cell technology works. This was helpful.
simon-nelms
Team TFS
Team TFS
Thank you for your comment, James. I'm glad it was helpful.
Not applicable
I would like any blogs about ICP-MS.
AnalyteGuru_KB
Team TFS
Team TFS
Thank you for the feedback, Jessica! We always want to know what our readers would like to see on Analyte Guru. Thanks for following us!
Not applicable
I'm having trouble with Se matrix spikes when analyzing drinking water. The spike typically recovers at 135-150% when I spike a tap water sample. I have tried making my spikes with both multi element standards and a single element Se standard with similar results. I have no problem with Se spike recovery when I spike distilled water, and I was wondering why I'm having high recoveries with tap water?
simon-nelms
Team TFS
Team TFS
Which isotope are you using for measuring Se? If you are using 82Se you may be getting interference from 81Br1H. It could also be that the tap water, depending on its source, contains a relatively high carbon content (from dissolved humic substances or high carbonate levels) which can cause enhancement of the Se signal in the sample compared to the calibration standards. If the problem is BrH interference, you’ll need to apply an interference correction equation in the instrument software after determining the BrH formation rate using a bromide standard solution (say at 100ppb Br concentration). If carbon enhancement is the problem, add 1% methanol to all your blanks, calibration standards and samples, either off-line or on-line (e.g. by adding it to the internal standard solution and adding the internal standard via a T-piece) to compensate for the enhancement effect. A less likely case could be doubly charged rare earth element interference, in which case you’ll need to either apply an interference correction equation or use pure H2 (if your instrument allows safe use of this gas) as the collision cell gas for Se analysis. Good luck in solving the problem; let me know how it goes.
Not applicable
Hi
Thanks.it was usefull.
Dear simon, how can you test a sample of se74 isotope?
Its purity and chemical isotope amount is what i need to be tested.
simon-nelms
Team TFS
Team TFS
Hi Sara, Natural 74Se is tricky to measure, as it has very low abundance (< 1%) and is interfered with by both 36Ar38Ar and 74Ge (which has an abundance of around 36%). Triple quadrupole ICP-MS offers some potential for solving these interference problems, but I believe that the usual approach of mass shifting Se to SeO using O2 as the collision / reaction cell gas may not work in this case as Ge also reacts with O2 (although I don’t yet know how efficient this reaction is). There may be other cell gases that selectively react with Se and not with Ge, since Se and Ge are in different Periodic Table groups, so it is probable that there is a triple quadrupole ICP-MS solution to this problem. Alternatively, separation of Ge and Se prior to analysis may be possible using either a solid phase extraction method or an on-line chromatographic separation technique (e.g. ion chromatography) coupled directly to an ICP-MS detector.
Not applicable
Hi Simon. Thanks for the informative blog post, but there is something I'm confused about. Why not use Se(82) with He gas mode? It seems like the collision cell would assist with interference from BrH, and an interference equation could be used for Kr interference. It seems like both of those interference would be minimal provided that Ar purity is high and no seawater samples.
I'm working with soil and tap/fresh water samples. Thanks!
simon-nelms
Team TFS
Team TFS
Hi Fred,

Thanks for your question. There are a few reasons not to use 82Se. Firstly, the abundance of 82Se is only 8.8% compared to 23.8% for 78Se, so the latter is almost 3x more sensitive. Secondly, as 82[BrH] is close in cross section area to 82Se, collision cell operation with pure He and kinetic energy discrimination isn’t actually that effective in removing BrH. Thirdly, the presence of Kr is often a significant problem, as sourcing higher purity argon is prohibitively expensive for routine ICP-MS use. In addition, with fresh water samples you could get a contribution on 82Se from 164Er and 164Dy doubly charged ions, depending on the source of the water. Since I wrote the Se blog series, we have launched a triple quadrupole ICP-MS, the Thermo Scientific iCAP TQ ICP-MS, which solves each of the interference problems I mention in the these blogs. Triple quadrupole operation allows the Se isotope masses to be selected by the first quadrupole then reacted with O2 cell gas in the collision cell (the second quadrupole) to form SeO, before measurement of SeO, free from Ar2, ArCl, BrH and Gd++ interference using the third quadrupole. I think perhaps I should now write a part 4 of this Se blog series…

Cheers,
Simon
Not applicable
Instructive and concise. Only read part I of the Selenium Analysis Using ICP-MS, Part 1. Looking forward to reading part II.
Not applicable
Hi Simon, thanks for the information through the blog. I wish to confirm with you that He gas in KED mode is sufficient to remove intereferences on 78Se from Ar2? Or must H2 gas be used? Because I was told otherwise that He in KED mode is limited in Ar2 interference elimination for 78Se. I understand that 78Se has interference from 83Kr too? I am working with samples with high Mn level and possibly pure Mn metal/derivative digests. Hope to hear from you soon. Thanks!
AnalyteGuru_KB
Team TFS
Team TFS
Yes, generally, with single quadrupole ICP-MS He is sufficient for removing the Ar2 interference on 78Se as this particular Ar2 interference has relatively low abundance. Kr has a minor isotope at mass 78 (just 0.35% abundance) so this is not usually a problem for Se determination on mass 78. As regards Mn, there is no interference from this element on mass 78 as the most notable problems of MnO and MnOH only range from mass 71 to mass 75. If, however, your samples contain significant amounts of Ni, Co or Fe, these could cause interference on mass 78, and if rare earth elements are present, the Gd++ becomes a problem too. Pure hydrogen as a cell gas can help with these interferences, but the best way to completely resolve them on a quadrupole based instrument is to use triple quadrupole ICP-MS with O2 as the cell gas as Se measured as SeO+.
Not applicable
I have implemented EPA Method 200.8 using Agilent 7800 ICP-MS. We order the PT aqueous sample containing Selenium at 760 ppb. We reported 1060 ppb and we failed. I have read your article and I think there was interference, but my question is, if this is truth, why the 1:5 dilution was correct?. Results from the diluted sample was 769 ppb. Could you please give me your opinion?.

Best regards
Not applicable
Nice articles on Se. Can you please advise which Se isotope would be suitable for its analysis in the Blood in the DRC mode with O2 as the gas? We are using internal standard mixture Bi, Ge, In, Li6, Sc, Tb, Y; calculation based on Tb. Blood dilutions with 0.5% nitric acid, 1% isopropanol add 0.05% Triton X 100.

Regards
AnalyteGuru_KB
Team TFS
Team TFS
Hi Juan, Which isotope are you measuring? Since you get a result close to the expected result when you dilute the sample 1:5 there are two possible explanations that initially spring to mind. Firstly, the error in the undiluted sample result could be caused by a calibration linearity issue. Secondly, it could be that the PT sample contains enough carbon to bias the undiluted result significantly but cause less of an enhancement when the sample is diluted 1:5. Interference from BrH on 82Se is also a possibility as this may induce a less positive bias in the result when the sample is diluted. Interferences such as doubly charged rare earth elements might be expected to have a more linear effect on selenium leading to the diluted sample showing a similar result to the undiluted sample, so this is probably less likely to be the cause of the problem.
AnalyteGuru_KB
Team TFS
Team TFS
Hi Meenu, Thanks for your positive responses to my articles. To measure Se in blood in DRC mode with O2 as the cell gas, I would suggest trying 80Se (this will provide the highest sensitivity as Se converts to SeO) and then measuring Se as 80Se16O at mass 96. Be careful to examine the background at mass 96 to ensure that you are not getting interference contributions on this mass from your samples when you operate in DRC O2 mode. Using reactive cell gases such as O2 works best on triple quad ICP-MS systems as you can remove potential interferences at mass 96 before the ion beam enters the collision cell, as well as removing pre-cursor ions from the ion beam that could form interferences at mass 96 inside the cell.
egarcia72
Involved Contributor
Involved Contributor

What is the mathematical correction equation to subtract out the interference for Se? If argon is used. 

 

Thanks!

GeorgeTFS
Community Manager
Community Manager

Thanks for your question. Dr. Nelms is in a different time zone but will respond to your query as soon as he can.