USGS-NWQL: I-1492-96: Molybdenum in Water by Graphite Furnace Atomic Absorption Spectrophotometry, Filtered

Summary
Analytes
Revision
Data and Sites

Official Method Name

Molybdenum, dissolved

Current Revision

1997

Media

WATER

Instrumentation

Graphite Furnace-Atomic Absorption Spectrometer

Method Subcategory

Inorganic

Method Source

USGS-NWQL

Citation

Jones, S.R., and McLain, B.J., 1997, Methods of analysis by the U.S. Geological Survey National Water Quality Laboratory -- Determination of molybdenum in water by graphite furnace atomic absorption spectophotometry: U.S. Geological Survey Open-File Report 97-198.

Brief Method Summary

Molybdenum determination by GF-AAS requires placing a small volume of sample in a graphite tube, which is held between two graphite rings with quartz windows at each end. The tube is pyrolytically coated with high-density carbon to reduce the formation of nonvolatile carbides and prevent surface adsorption of the sample onto the wall of the graphite tube, resulting in longer tube life (Ghe and others, 1983, p. 711). The sample is evaporated to dryness, pyrolized, and atomized using specified temperatures and high-temperature ramping. The absorbance second signal is produced and compared to standards. Background noise is corrected using Zeeman-effect background correction, that is, a magnetic field is turned on and off at approximately 60 hertz (cycles per second) during atomization, causing the sample signal to split into polarized and nonpolarized components that correct for background interference.

Scope and Application

This method is used to analyze filtered and nonfiltered water samples for dissolved and whole water recoverable (WWR) analysis. Using a 20 uL sample and Zeeman background correction, the method is applicable in the range from 1 to 50 ug/L. Sample solutions that contain molybdenum concentrations exceeding the upper limit of the analytical range must be diluted and reanalyzed or analyzed by an alternate method. Furnace temperature programs, volumes, matrix modifiers, and other instrumental settings may be modified provided that characteristic mass (plus or minus 20 percent) is maintained, and the method detection limit (MDL) is met or improved. Characteristic mass (MO) best describes instrumental and operational performance; it is defined as the mass of an analyte, in picograms, required to produce a signal of 0.0044 absorbance-seconds (a-s), so that instrument performance and optimization can be evaluated (Beaty, 1988, p. 5-1).

Applicable Concentration Range

1 - 50 (undiluted)

Interferences

"Sulfate interferes with the GF-AAS determination of molybdenum in aqueous solutions with concentrations of only 0.5 percent weight per volume (w/v) sodium sulfate (Na₂SO₄), causing complete elimination of the molybdenum absorbance peak in solutions free of other salts" (Emerick, 1987, p. 69). To reduce the sulfate interference, a matrix modifier is used. The use of magnesium nitrate in conjunction with ammonium phosphate as a matrix modifier reduces the sulfate interference considerably. To rule out matrix interferences, all samples with specific conductances greater than or equal to 5,000 uS/cm should be diluted or spiked, or both. Carbide may form in the furnace environment because molybdenum binds with carbon to form MoC, resulting in some signal loss. "During the drying step, MoO₃ is formed; it melts at 1,340^oC and is further converted by carbon to Mo₂C with pyrolysis temperatures up to 2,150^oC. Above 2,250^oC, the Mo₂C is converted to MoC and Mo. The absorbance signal is larger if ashing [pyrolysis] does not exceed 2,100^oC. This minimizes the formation of MoC, some of which remains after the atomization step" (Slavin, 1984, p. 123), causing memory effects. The use of pyrolytically coated tubes helps to reduce the formation of carbides. The high temperature (approximately 2,700^oC) for cleanout is necessary to remove the MoC found on the wall of the graphite tube, but it reduces tube life considerably. To decrease memory effects and increase tube life, multiple, short, high-temperature clean-out steps are suggested rather than one long clean-out step. Additional interference information can be found in USGS OFR 97-198.

Quality Control Requirements

Quality-control samples area analyzed at a minimum of one in every ten samples. These QC samples include at least one of each of the following: blanks, quality control samples, third party check solutions, replicates, and spikes. Correlation coefficients for calibration curves must be at least 0.99. QC samples must fall within 1.5 standard deviations of the mean value. If all of the data-acceptance criteria in the SOPs are met, then the analytical data are acceptable.

Sample Handling

Description: 250 mL Polyethylene bottle, acid-rinsed. Treatment and Preservation: Filter through 0.45-um filter, use filtered sample to rinse containers and acidify sample with nitric acid (HNO₃) to pH < 2.

Maximum Holding Time

180 days from sampling

Relative Cost

Less than $50

Sample Preparation Methods

This method has 1 analyte associated with it.

Analyte	Detection Level	Bias	Precision	Pct False Positive	Pct False Negative	Spiking Level
Molybdenum(7439-98-7) Amperit 105.054 Amperit 106.2 EINECS 231-107-2 HSDB 5032 MChVL Metco 63 Mo Molybdenum Molybdenum, metallic TsM1	0.900 ug/L	N/A	N/A

Precision Descriptor Notes:	Various studies using a variety of filtered quality-control and water samples for dissolved analysis were performed to measure the precision and accuracy of the graphite furnace method for determining molybdenum. Samples were analyzed over a period of 35 days to demonstrate acceptable precision. The mean relative standard deviation ranged from about 58 percent at 0.6 ug/L to 5 percent at 33 ug/L; the median relative standard deviation was 6.6 percent for molybdenum concentrations ranging from 0.6 to 50.1 ug/L. The accuracy of molybdenum determinations by GF-AAS was verified by analyzing USGS Standard Reference Water Samples (SRWS) and commercially prepared and certified standards (SPEX). All results were well within the established norms for the means. Additional precision information can be found in USGS OFR 97-198.
Detection Level Note:	Method detection limits (MDLs) were determined for this method using the procedures of the U.S. Environmental Protection Agency (1997).

Precision Descriptor Notes:

Various studies using a variety of filtered quality-control and water samples for dissolved analysis were performed to measure the precision and accuracy of the graphite furnace method for determining molybdenum. Samples were analyzed over a period of 35 days to demonstrate acceptable precision. The mean relative standard deviation ranged from about 58 percent at 0.6 ug/L to 5 percent at 33 ug/L; the median relative standard deviation was 6.6 percent for molybdenum concentrations ranging from 0.6 to 50.1 ug/L. The accuracy of molybdenum determinations by GF-AAS was verified by analyzing USGS Standard Reference Water Samples (SRWS) and commercially prepared and certified standards (SPEX). All results were well within the established norms for the means. Additional precision information can be found in USGS OFR 97-198.

Detection Level Note:

Method detection limits (MDLs) were determined for this method using the procedures of the U.S. Environmental Protection Agency (1997).

Revision	PDF/Link
1997

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USGS-NWQL: I-1492-96: Molybdenum in Water by Graphite Furnace Atomic Absorption Spectrophotometry, Filtered

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