National Water Quality Laboratory

Sunday July 22, 2018

U.S. GEOLOGICAL SURVEY Open-File Report 97-198

Methods of Analysis by the U.S. Geological Survey National Water Quality Laboratory - Determination of Molybdenum in Water by Graphite Furnace Atomic Absorption Spectrophotometry

By Sandra R. Jones and Betty J. McLain

Graphite furnace atomic absorption spectrophotometry (GF-AAS) is a sensitive, precise, and accurate method that can be used to determine molybdenum in water samples. This method has been developed to replace the chelation extraction method by flame atomic absorption spectrophotometry (F-AAS), reducing sample preparation time and safety hazards. The reporting range is 1 to 50 micrograms per liter (µg/L), with a method detection limit of 0.9 µg/L and a linear working limit of 50 µg/L.

Although sulfate interferes in the determination of molybdenum, the use of magnesium nitrate in conjunction with ammonium phosphate as a matrix modifier minimizes this interference. Numerous water samples containing various concentrations of sulfate were spiked with 25 µg/L molybdenum and analyzed for a mean spike recovery of 91 percent.

Molybdenum tends to form carbides, resulting in memory-effect (carry-over) interferences. Carry-over interferences of approximately 2 µg/L may result following a molybdenum concentration of 50 µg/L. These carry-over interferences are eliminated by routine intermittent blank-sample analysis, a multistep high-temperature cleanout program, and the use of pyrolytically coated graphite tubes.

Precision and accuracy studies demonstrate that the GF-AAS method is comparatively accurate and more precise than the F-AAS method. Eleven standards tested with concentrations from 0.6 to 50 µg/L display a median relative standard deviation of 6.6 percent. Fourteen reference samples were tested to demonstrate precision and accuracy of the GF-AAS method; three of the reference samples show an improvement in precision when compared with the F-AAS method. Statistical comparison studies of dissolved and whole water recoverable analysis results on water samples demonstrated an insignificant difference between the GF-AAS and F-AAS methods. The p-values indicated the differences between methods to be statistically different from zero; however, in both cases, the median differences are less than the method reporting limit of 1.0 µg/L. Precision and accuracy by the graphite furnace method are further defined when compared with inductively coupled plasma-mass spectrometry. Using samples with low concentrations (0 - 3.0 µg/L), the paired t-est demonstrates the differences between methods to be statistically different from zero; however, the median difference is well below the method reporting limit.

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