WATER QUALITY: Analytical Methods - Determination of selenium in water-suspended sediment mixtures (3667-74) and bottom materials (5667-74). August 6, 1974 QUALITY OF WATER BRANCH TECHNICAL MEMORANDUM NO. 75.02 Subject: WATER QUALITY: Analytical Methods - Determination of selenium in water-suspended sediment mixtures (3667-74) and bottom materials (5667-74). Enclosed for immediate application are provisional methods for determination of selenium in water-suspended sediment mixtures and bottom materials. These procedures are similar to the method for selenium in water (1667-74) that was distributed by Quality of Water Branch Technical Memorandum No. 74.14 (April 8, 1974). Procedure 3667-74 provides for withdrawal of a sample aliquot while the sample is stirred vigorously in a modified mixer- blender. Procedure 5667-74 provides for the analysis of a sample aliquot of air-dried bottom material. All Division laboratories are requested to adopt the subject provisional methods. Questions or comments pertaining to the methods should be directed to the Chief, Quality of Water Branch (Mail Stop 412, Reston, Virginia). R. J. Pickering Enclosure WRD Distribution: A, B, FO-L, PO Water-Suspended Sediment Mixture Selenium (3667-74) Atomic-absorption spectrophotometric method 1. Summary of method 1.1 The entire sample is homogenized in a blender and an appropriate volume, while mixing, is withdrawn for analysis by the atomicabsorption spectrophotometric method. 1.2 For additional information on the principles of the method, see "Selenium (1667)." 2 Application 2.1 This method may be used to analyze samples of water-suspended sediment mixtures containing at least 2 ug selenium per liter. 3. Interferences 3.1 See "Selenium (1667)." 4. Apparatus 4.1 Stirrer, blender,with glass container, lOOO-ml capacity or larger, modified as described in the attachment to this method. (Van Waters & Rogers Scientific Co. No. 58977-227 has been found to be satisfactory.) 4.2 For additional items of required apparatus, see "Selenium (1667)." 5. Reagents 5.1 See "Selenium (1667)." 6. Procedure 6.1 Collect approximately 900 ml of sample in a l-liter plastic bottle. To facilitate collection of the proper amount of sample, place a mark on the bottle at the 900-ml level. Transfer the entire contents of the sample bottle to the blender. Mix at approx 4500 rpm (setting 2) for approximately 15 sec and, while mixing, cautiously and rapidly pipet a volume of sample containing less than 2.5 ug Se and 100 mg suspended sediment (100 ml max) into a 250-ml reagent bottle. Rinse the pipet with demineralized water to remove adhering particles and combine with sample. Dilute sample to 100 ml (Notes 1 and 2). 2 Selenium (3667-74) NOTE 1. Be sure that the tip of the pipet does not get near the blades in the blender when withdrawing the sample. NOTE 2. The tip of the pipet must be cut off in order to withdraw and drain the water-sediment mixture. 6.2 Proceed as directed in "Selenium (1667)," beginning with paragraph 6.2 7. Calculations 7.1 See "Selenium (1667)." 8. Report 8.1 Report Se determined by this method as "Selenium (Se), total recoverable." 8.2 Report total recoverable Se concentrations as follows: Less than 100 ug/l, nearest ug/l; 100 ug/l and above, two significant figures. 9. Precision 9.1 See "Selenium (1667)." Water-suspended sediment mixtures Mixer-blender modification The withdrawal of a portion of a sample of a water-suspended sediment mixture requires great care to ensure a proper representation of the suspended particulate matter in the portion taken. This can best be accomplished by using a pipet to withdraw a measured portion of the sample while the sample is being vigorously agitated in a highspeed mixer-blender. The mixing action of the blender ensures uniform distribution of sediment particles throughout the sample; withdrawal of a portion for analysis while the sample is being agitated offers greatest assurance that the portion analyzed will contain the same distribution of sediment particles as is present in the bulk sample. Using a mixer-blender having a mixing container of about one liter capacity, it is possible to withdraw up to 800 ml of sample (eight lOO-ml portions) before running the risk of inadequate mixing, or the risk of the sampling pipet being struck by the revolving blade of the mixer. In order to ensure rapid transfer of the sample and to minimize settling of sediment in the pipet during transfer, it is necessary to enlarge slightly the delivery orifice of the transfer pipet. Admittedly, this introduces a slight error into the measurement of the sample volume. It is preferable, however, to allow a small error in sample volume measurement in order to ensure proper distribution of suspended particulate matter in the portion taken for analysis. The pipet orifice may be enlarged by cutting off about 5 mm of the delivery tip. The abrasive action of the water-suspended sediment mixture on the mixer blades during vigorous agitation introduces traces of iron, chromium, and nickel into the sample solution. To avoid this, the stainless steel blades must be replaced with a plastic blade and blade support assembly (Figures 1 and 2). This can be accomplished by any well equipped machine shop. The plastic blade is as effective as the original steel blades for maintaining adequate mixing of the sample during withdrawal of portions for analysis. Furthermore, the plastic blade minimizes the risk of adding trace metals to the sample during mixing. Bottom Material Selenium (5667-74) Atomic-absorption spectrophotometric method 1. Summary of method 1.1 An accurately weighed portion of air-dried sample is mixed with demineralized water and analyzed by tke atomic-absorption spectrophotometric method. 1.2 For additional information on the principles of the method, see "Selenium (1667)." 2. Application 2.1 This method may be used to analyze samples of bottom material containing at least 2 ug selenium per gram of air-dried material. 3. Interferences 3.1 See "Selenium (1667)." 4. Apparatus 4.1 See "Selenium (1667)." 5. Reagents 5.1 See "Selenium (1667)." 6. Procedure 6.1 Weigh, to the nearest milligram, an amount of sample containing less than 2.5 ug Se (100 mg max). Use sample material prepared as directed in "Sample preparation (0520)." 6.2 Quantitatively transfer the weighed sample to a 250-ml reagent bottle. Add approximately 100 ml demineralized water and mix. 6.3 Proceed as directed in "Selenium (1667)," beginning with paragraph 6.2. 7. Calculations 7.1 Determine the ug Se in each sample from a plot of absorbances of standards. 7.2 Determine the concentration of Se in ug/g of air-dried sample as follows: ug Se in sample Se in ug/g = ____________________ sample weight, grams 8. Report 8.1 Report Se determined by this method as "Selenium (Se), total in bottom material." Parameter code 01148. 8.2 Report Se concentrations in units of ug/g (micrograms per gram) of air-dried sample as follows: Less than 100 ug/g, to the nearest ug/g; 100 ug/g and above, two significant figures. 9 Precision 9.1 Precision data are not available.