Page 7: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Selenium
Part II. Science and Technical Considerations (continued)
The United States Environmental Protection Agency (U.S. EPA) has three approved analytical methods (Method 200.5 Rev. 4.2, Method 200.8 Rev. 5.4 and Method 200.9 Rev. 2.2) for the analysis of dissolved and total selenium in drinking water (U.S. EPA, 2011). Total selenium is defined as the sum concentration of the dissolved ([selenite-Se(IV)] and [selenate-Se(VI)]) and the suspended fractions of a water sample. The following methods, developed by voluntary consensus standard organizations, are approved by the U.S. EPA and available for the analysis of selenium: SM 3113 B and SM 3114 B (APHA et al., 1992, 1995, 2005), online version SM 3113 B-04, 99 and SM 3114 B-09, 97 (U.S. EPA, 2011) and ASTM - D3859-98 A,B, D3859-03 A,B and D3859-08 A,B (ASTM, 1998, 2003, 2008).
Method 200.5 Rev. 4.2, which employs axially viewed inductively coupled plasma atomic emission spectrometry, has a method detection limit (MDL) of 1.3 µg/L. When this method is used, sample preparation procedures, such as preliminary recoverable digestion and preconcentration prior to analysis, are required. The preconcentration step prior to the analysis increases the analytical sensitivity (U.S. EPA, 2003). Possible interferences that may occur include: 1) spectral interferences, caused by background emission, stray light from the line emission of high-concentration elements or overlap of a spectral line from another element; 2) chemical interferences, such as molecular compound formation, ionization effects and solute vaporization effects; 3) physical interferences associated with the sample nebulization and transport processes; and 4) memory interferences, when analytes in a previous sample contribute to the signals measured in a new sample.
Both U.S. EPA methods 200.8 Rev. 5.4 and 200.9 Rev. 2.2 provide procedures for the determination of dissolved and total recoverable selenium. The methods applied use the same preservation and/or pretreatment steps, depending on the types of data required. The differences between these methods are in the equipment used for the measurement.
Method 200.8 Rev. 5.4, based on inductively coupled plasma mass spectrometry, has an MDL of 7.9 µg/L. The sample is atomized and ionized into radio-frequency plasma. The ions are extracted from the plasma by a vacuum interface and separated on the basis of their mass-to-charge ratio by a mass spectrometer. Separated ions are detected by an electron multiplier or Faraday detector (U.S. EPA, 1994). Interferences may be caused by: 1) equal mass isotopes of different elements present in the sample; and 2) ions consisting of more than one atom that have the same nominal mass-to-charge ratio as the isotope of interest. Physical interferences associated with the transport and conversion of the sample into the plasma and memory interferences, when isotopes of elements in previous samples contribute to the signals measured in a new sample, may also occur.
Method 200.9 Rev. 2.2, which uses stabilized temperature platform graphite furnace atomic absorption, has an MDL of 0.6 µg/L. The technique includes a series of three heating steps: 1) a drying step, 2) a charring step designed to reduce interferences caused by concomitant ions; and 3) a final step in which the temperature of the furnace is raised and selenium is atomized from the pyrolytic graphite surface into an atmosphere of high-purity argon. The light of a specific wavelength is passed through the atomic cloud, and the measurement is made of the attenuation of the intensity of the light (U.S. EPA, 1994). The interference sources include: 1) spectral interferences caused by the absorbance of light by a molecule or atom different from the analyte of interest; 2) matrix interference inhibiting the atomization cycle; 3) specific element interference; and 4) memory interference resulting from analysis of a sample containing a high concentration of an element that is not quantitatively removed in the furnace step.
Standard method SM 3113 B has also been approved for analysis of selenium using electrothermal atomic absorption (APHA et al. 1992, 1995, 2005). The optimum selenium concentration range reported for SM 3113 B is 5-100 µg/L, and the estimated detection level is 2 µg/L (APHA et al., 2005).
Standard method SM 3114 B, a manual hydride generation atomic absorption spectrometry method, is applicable for an optimum selenium concentration range of 2-20 µg/L and has an MDL of 2 µg/L (APHA et al., 2005). This method is applicable to the determination of selenium by conversion to its hydride form by a sodium borohydride reagent and transported into an atomic absorption atomizer. Se(VI) is not measurably reduced by sodium borohydride. To determine selenium, the method reduces Se(VI) to Se(IV) by an acid digestion procedure. Se(IV) in the filtered water sample is converted to volatile selenium hydride and transported to an atomic adsorption atomizer where it can be analysed. By using this preparation step to convert Se(VI) to Se(IV), it is possible to distinguish the selenium species in the sample (APHA et al., 2005).
The methods cited in the 22nd edition of Standard Methods for the Examination of Water and Wastewater (APHA et al., 2012) are also available for the analysis of selenium.
The ASTM International methods approved by the U.S. EPA are the 1998, 2003 and 2008 versions of ASTM D3859 A and ASTM D3859 B. Both methods utilize atomic absorption procedures. Method A (gaseous hydride atomic absorption spectrometry) is equivalent to SM 3114 B and is applicable within the range from 1 to 20 µg/L. The range reported for method B (graphite furnace atomic absorption spectrometry) is from 2 to 100 µg/L, and this method is equivalent to SM 3113 B.
The current practical quantitation level (PQL), based on the capability of laboratories to measure the concentration of selenium within reasonable limits of precision and accuracy, is 10 µg/L (U.S. EPA, 1991c). Recently, as part of the U.S. EPA's second 6-year review, an assessment of the analytical data for selenium from the Proficiency Testing Program was conducted. The U.S. EPA reported high passing rates for laboratories (greater than 75%) analysing samples at the current PQL. However, because of a lack of analytical performance data below the current value of 10 µg/L, the U.S. EPA has not recommended lowering the current PQL (U.S. EPA, 2009).
A recent experimental method reported an MDL of 0.01 µg/L each for Se(IV) and for Se(VI) in tap water using microwave-induced nitrogen plasma mass spectrometry (Minami et al., 2003).
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