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R9 Laboratory SOP 505
Determination of Trace Elements in Water by Inductively Coupled Plasma - Atomic Emission Spectrometry
Summary
This SOP provides procedures for the determination of dissolved and total recoverable elements by Inductively Coupled Plasma - Atomic Emission Spectrometry (ICP-AES) in environmental samples. It is applicable to water and wastewater. This SOP is based on EPA Method 200.7.
Dissolved elements are determined after suitable filtration and acid preservation. In order to reduce potential interferences, dissolved solids should not exceed 0.2% (w/v) (}{\fs26 Section 6.2}{\fs24 ). Where this method is approved for the determination of metal and metalloid contaminants in drinking water, samples are analyzed directly by pneumatic nebulization without acid digestion if the samples have been properly acid-preserved and have turbidity of < 1 NTU at the time of analysis. This total recoverable determination procedure is referred to as "direct analysis". Note the exception for silver discussed in Section 1.6.
For the determination of total recoverable analytes in aqueous samples containing particulate and suspended solids a digestion step is required prior to analysis. Aqueous samples containing suspended or particulate material > 1% should be digested as a solid type sample (Section 9.1.4). When determining boron in aqueous samples, only plastic, PTFE or quartz labware should be used from time of collection to completion of analysis. When possible, borosilicate glass should be avoided to prevent contamination of boron.
Silver is only slightly soluble in the presence of chloride unless there is a sufficient chloride concentration to form the soluble chloride complex. Therefore, low recoveries of silver may occur in samples, fortified sample matrices and even fortified blanks if determined as a dissolved analyte or by "direct analysis" where the sample has not been processed using the total recoverable mixed acid digestion. For this reason samples are digested prior to the determination of silver. The total recoverable sample digestion procedure is suitable for the determination of silver in aqueous samples containing concentrations up to 0.1 mg/L. For the analysis of wastewater samples containing higher concentrations of silver, succeeding smaller volume, well mixed sample aliquots must be digested until the analysis solution contains < 0.1 mg/L silver.
The total recoverable sample digestion procedure given in this method will solubilize and hold in solution only minimal concentrations of barium in the presence of free sulfate. For the analysis of barium in samples having varying and unknown concentrations of sulfate, analysis should be completed as soon as possible after sample preparation.
An aliquot of a well-mixed, homogeneous aqueous sample is accurately measured for sample processing. For total recoverable analysis of an aqueous sample containing undissolved material, analytes are first solubilized by gentle refluxing with nitric and hydrochloric acids. After cooling, the sample is made up to volume, mixed and allowed to settle overnight prior to analysis. For the determination of dissolved analytes in a filtered aqueous sample aliquot, or for the "direct analysis" total recoverable determination of analytes in drinking water where sample turbidity is <1 NTU, the sample is made ready for analysis by the appropriate addition of nitric acid, and then mixed before analysis.
The method describes the multi-elemental determination of trace elements in aqueous solutions by ICP-AES using a simultaneous instrument. The instrument measures the characteristic atomic-line emission spectra by optical spectrometry. Samples are nebulized and the resulting aerosol is transported to the plasma torch. Element specific emission spectra are produced by a radio-frequency inductively coupled plasma. The spectra are dispersed by a grating spectrometer, and the intensities of the line spectra are monitored at specific wavelengths by a photosensitive device. Photo currents from the photosensitive device are processed and controlled by a computer system. A background correction technique is required to compensate for variable background contribution to the determination of the analytes. Background must be measured adjacent to the analyte wavelength during analysis.