USGS Test Method I-2063-98


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Source:
USGS

Method Name:
Arsenic in water by graphite furnace-atomic absorption spectrometry, dissolved. Official Name: Determination of Arsenic and Selenium in Water and Sediment by Graphite Furnace-Atomic Absorption Spectrometry

Description:
The determination of arsenic by GF-AAS requires that a small (microliter) volume of sample be pipetted into a graphite tube. The tube is held between two graphite rings with quartz windows at each end, which produces a nearly closed environment to enhance the absorbance signal. The tube is pyrolytically coated with high-density carbon to reduce the formation of nonvolatile carbides to prevent surface adsorption of the sample onto the walls of the graphite tube, and to increase tube life (Ghe and others, 1983). The sample is evaporated to dryness, charred, and atomized by using specified temperatures and high-temperature ramping. The absorbance signal is measured and compared to standards.

Scope:
This method is used to analyze filtered water samples for the determination of dissolved arsenic. Using a 30-uL sample injection, the linear analytical range is 1 to 50 ug/L Samples that contain arsenic concentrations exceeding the upper limit of the analytical range need to be diluted and re-analyzed or analyzed by an alternate method.

Citation:
Jones, S.R., and Garbarino, J.R., 1998, Methods of analysis by the U.S. Geological Survey National Water Quality Laboratory-- Determination of arsenic and selenium in water and sediment by graphite furnace-atomic absorption spectrometry: U.S. Geological Survey Open-File Report 98-639.

Interferences:
Two types of interferences affect GF-AAS analyses: spectral and nonspectral. The most common type of spectral interference is background absorption where undissociated molecular forms of matrix materials produce broadband absorption spectra. Zeeman background correction compensates for broadband absorption by using a strong magnetic field to shift the electronic energy levels of an element's atom. The shifted atomic spectrum can then be differentiated from the background spectrum (Beaty, 1988). Atoms having absorption wavelengths unresolved from the elemental absorption wavelength cause either positive or negative errors when measuring the element concentration profile (Flajnik-Rivera and Delles, 1996).Sufficient energy must be available during atomization to dissociate the element and create free atoms. The composition of the sample matrix interferes with this process and results in either positive or negative errors. Matrix vapor condensation takes place not only in end-heated graphite atomizer-type furnaces, but also in spatially isothermal, transverse heated graphite atomizer-type furnaces (Frech and L'Vov, 1993). It has also been shown that atoms can be trapped on the surface of condensed matrix particles and deposited at the cooler tube ends, resulting in a depression of the elemental signal. Introducing a low-flow of inert gas during the atomization step can eliminate this type of interference. However, using low-flow causes some loss of sensitivity because some of the atoms are swept from the chamber (Frech and L'Vov, 1993). Nevertheless, low-flow conditions are recommended for routine sample analysis at the NWQL.Arsenic measurements are generally unaffected using stop-flow or low-flow conditions for all metal salts added except for aluminum sulfate. All recovery measurements are within the acceptance criteria of 25 +/- 7.5 ug/L (+/- 30 percent). However, although recoveries in aluminum sulfate are acceptable, the measurements were not reliable because of spectral interference. Aluminum interferes with arsenic at 193.7 nm, causing false positive results (Flajnik-Rivera and Delles, 1996).

QC Requirements:
Analyze a quality-control sample immediately following calibration and after every tenth sample. Minimum quality-control requirements per set must include analysis of a laboratory reagent blank and quality-control samples, such as SRWS, and may include check standards, sample duplicates, and sample spikes. Field spikes and duplicates are suggested as additional checks. Correlation coefficients for calibration curves must be at least 0.999. 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.

Maximum Holding Time:
180 days

Media:
WATER

Subcategory:
Inorganic

Concentration:
1 to 50 ug/L

Precision:
The bias and variability of the method are determined by comparing results from the new method and a former official method with standard reference materials and natural-water samples.

Detection:
The MDL was determined using the procedure described by the USEPA Appendix B, Part 136.

Revision Number:
1998

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