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Selenium Analysis and Speciation

Selenium (Se) is both an essential nutrient as well as a toxic chemical.  This dual effect has been recognized for many naturally occurring chemicals.  Trace amounts are beneficial, while larger amounts are harmful.  Of course the toxicity of selenium depends greatly on the chemical form or species.

Selenium is used in food supplements, photographic processes, and electronics.  It is regulated by both EPA and FDA.  It is also on the California Prop. 65 list as the disulfide; however, no "safe harbor limit" has been established.

FYI

Selenium, Web Elements

Selenium as an Essential Nutrient

EPA Drinking Water Fact Sheet

EPA: Selenium Hazard Information

CA Prop 65


Selenium by ICPMS

Traditional tests for trace Se include hydride generation-atomic absorption (HG-AA) and graphite furnace atomic absorption (GFAA).  Modern techniques include ICPMS and ICPMS-Cell.  

Selenium has 6 naturally occurring isotopes, 3 of which are commonly used in ICPMS: 77 (7.6%), 78 (23.8%), 80 (49.7%), and 82 (9.2%).  The major isotope 80Se is isobaric with the 40Ar2+ from the argon plasma.  A reaction or collision cell is needed to reduce this interference.  Normally the 78Se or 82Se has a slightly better signal to noise than either the 77Se or  80Se.

Interferences include 40Ar37Cl+ on 77Se and Br+ on 80Se and 82Se.  These interferences can easily be removed in either of two ways: (1) using hydride generation to introduce the sample into the ICPMS or (2) using a collision or reaction cell to remove the interfering ions from the plasma.  Both methods are very efficient, so much so that sea water can be analyzed without any interference.  Detection limits for Se are generally 2 ug/L with normal ICPMS.  With hydride or reaction cell, the detection limit is generally <0.5 ug/L.


Selenium Speciation

Selenium can exist in various ionic and covalent, organic and inorganic forms.  Some of the more chemically important forms are listed in the adjoining table.  Various forms of liquid chromatography (HPLC) have been used to separate these ions and compounds.  The chromatogram below shows the ion chromatographic separation of selenite and selenate ions at 1 ug/mL using a Hamilton PRP X-100 column with 10 mM ammonium nitrate and phosphate at pH 6.3.  The 82Se isotope was monitored.

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Se Species

Inorganic forms: 

  • selenite (SeO3-2) 

  • selenate (SeO4-2)

  •  selenocyanate (SeCN-)

  • disulfide (SeS2, antidandruff)

  • oxide (SeO2)

  • various selenides (e.g. PbSe).

Organic forms:  

  • seleno-amino acids (selenomethionine) 

  • seleno-proteins

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Selenite  Se(IV) and selenate Se(VI) by IC-ICPMS


Selenocyanate (update 10/04)

Selenocyanate is a concern in wastewaters from petroleum refineries.  Much of the selenium has been characterized as selenocyanate ion (SeCN-).  With our new Agilent 7500ce and HPLC interface, we have implemented this analysis following the work of Wallschlager and Roehl, JAAS, 2001, 16, 922-925 and Miekeley, et al., Spectrochimica Acta B, 60 (2005), 633-641.

 

While selenite (Se+4) and selenate (Se+6) separate by ion chromatography under mild buffer conditions, selenocyanate requires a much stronger eluent.

The chromatogram below is a 10 ug/L standard showing the separation of the three species of concern.  Detection limits are approximately 1 ug/L.  Two characteristic isotopes of selenium (77 and 78) are monitored in the chromatogram.

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