In recent years, selenium has become a hot issue within the environmental community after studies linked its presence in water to mutations in aquatic life. This has led to selenium being regulated via state and federal permitting programs, and in some cases resulted in legal action being taken by environmentalists in efforts to limit selenium discharges.
Selenium is contained in soil and rock strata and when these are exposed in the course of mineral extraction it can be released into receiving streams. In order to report a result on a NPDES Discharge Monitoring Report (DMR), the sample must be analyzed by a certified laboratory using a method approved by the U.S. Environmen-tal Protection Agency (EPA), which is covered under 40CRF136. Among these currently approved methods are graphite furnace atomic absorbance (GFAA), inductively coupled plasma atomic emission spectroscopy (ICP/AES), inductively coupled plasma mass spectroscopy (ICP/MS), and gaseous hydride atomic absorbance (GHAA).
Bio-Chem’s ICP mass spectrometer.
Bio-Chem Testing has always strived to provide customers with the best available analytical data and based on West Virginia Department of Environmental Protection’s (WVDEP) Quality Assurance Office recommendations, Bio-Chem began studying the GHAA instruments on the market. Since approval of analytical methods often lags behind the development of new technologies, Bio-Chem looked at not only GHAA instruments but also gaseous hydride atomic fluorescence (GHAF) instruments. Based on a manufacturer’s touted claims of lower detection levels for GHAF technology and on existing customer testimonials, Bio-Chem decided to purchase a GHAF instrument instead of a GHAA instrument even though its use for NPDES reporting would require obtaining an Alternate Test Procedure (ATP) approval from the EPA Region III. On February 11, 2011, after completing the required comparability studies for GHAF versus ICP and ICP/MS for an ATP, Bio-Chem was granted approval to report NPDES results for selenium by GHAF by the EPA Region III Environmental Assessment and Innovation Division.
Since 2011, Bio-Chem has analyzed thousands of selenium samples by GHAF. This includes samples from coal companies, environmental consultants, and engineering firms building and operating selenium reduction and remediation pilot plants.
Some advantages, disadvantages and interferences of the various approved methods for analyzing selenium follow:
Graphite Furnace Atomic Absorption (GFAA):
Advantages: Low detection levels.
Disadvantages: Low sample throughput, expensive consumables.
Interferences: Molecular absorption — occurs when components in the sample matrix volatilize during the atomization process resulting in broadband absorption. Background correction, matrix modification, temperature ramping, method of standard additions, and the use of a graphite platform can be used to compensate.
Bio-Chem’s GFAA spectrometer.
Inductively Coupled Plasma (ICP):
Advantages: High sample throughput.
Disadvantages: Detection level may not be as low as required.
Interferences: Spectral interference — caused by the emission of light from spectral sources other than the element of interest. Judicious use of background correction positions and alternate wavelengths can be used to compensate.
Physical interferences — caused by the physical properties of the sample such as viscosity, surface tension or dissolved solids. Sample dilution, matching the calibration standards with the sample, or method of standard additions can be used to compensate.
Chemical interferences — caused by molecular compound formation, ionization effects, and thermochemical effects associated with sample vaporization and atomization into the plasma. The same techniques used for physical interferences apply along with the addition of careful selection of instrument operating conditions.
Memory interference — caused by the analyte from a previous sample being measured in the current sample and can be minimized by a sufficiently long rinse time between samples.
Inductively Coupled Plasma/Mass Spectrometry (ICP/MS):
Advantages: High sample throughput, low detection levels.
Disadvantages: In the case of selenium, disadvantages are mainly a polyatomic interference (see below).