A Convenient, Facile Method for Detecting Hexavalent Chromium in Drinking Water. By: Derek Colwill

Transcription

1 A Convenient, Facile Method for Detecting Hexavalent Chromium in Drinking Water. By: Derek Colwill

2 Overview Problem: Chrome platting factory had an accident that released hexavalent chromium into a nearby river. The river, which is used by several towns downstream as a source for drinking water, must be tested to assure the levels of Cr(VI) are safe for consumption. Samples will be taken every five miles until safe level is reached. Method: Ion Chromatography inductively coupled plasma mass spectrometry. Cr(III) complexed with Ethylene diamine tetraacetic acid (EDTA) to form a ligand that can be separated from Cr(VI). If Cl - and C are not separated out before ICP, they can form ClOH and ArC when reacted with the plasma.

3 Sources of Hexavalent Chromium Hexavalent chromium can be found naturally in rocks, but is mostly produced from industrial procedures such as chrome plating, making dyes, leather tanning and for the production of stainless steel. Because Cr(VI) is a strong oxidizer (readily accepts electrons) it is also very suitable for making hard paint coatings that can be used on boats, cars and airplanes. Other names for Cr(VI) include Chromium 6, Hex Chrom and HexChrome [1-2].

4 Health Hazards The safe limit of hexavalent chromium in drinking water, set by the EPA, is 0.1mg Cr(VI)/1L water [4]. While trivalent chromium (Cr(III)) is a nutrient used for sugar metabolism, Cr(VI) is a known carcinogen even in small doses. In the human body, hexavalent chromium enters cells via the sulfate transport mechanism (note: trivalent chromium can not pass through cell walls) and once inside, it is reduced to Cr(III) which can bind to proteins and cause an immune response [3]. Furthermore Cr(VI) can cause chromosomal breaks inside cells, leading to DNA damage.

5 Method In the detection of Cr(VI) in drinking water, there are three interferences that must be separated before an acceptable resolution can be achieved by using ICP-MS. Cr(III) must be separated from Cr(VI) by forming an anionic ligand complex with EDTA. Cr(III) will complex with this ligand, but Cr(VI) will not. The two can then be separated using anion-exchange chromatography. C and Cl - can be separated from the target analytes by first using an ion chromatograph. In anion exchange chromatography, the retention time of the anions is dependent on factors such as the PH of the mobile phase and the anion s concentration in solution. By changing these factors, C and Cl - can be separated from Cr(VI) [5].

6 Equipment Figure 1 - Agilent 1100 liquid chromatography module Figure 2 - Agilent 7500c ICP-MS Photos courtesy of:

7 Equipment Figure 3 - Schematic representation of IC-ICP-MS (HPLC being used as ion chromatograph) [6].

8 Sample Analysis Separation Preparation Prepare samples by adding EDTA and heating for 20 minutes at 80 C. To separate out complexed Cr(III), ArC and Cl -, run samples through IC (Agilent 1100 with a G3154A/102 (Agilent) column). Samples then run through ICP-MS (Agilent 7500c) monitoring at m/z = 52 (the most common isotope). Experimental Procedure Overview Data Quantification Calibration curves (1µg/L µg/l concentration standards) are used to quantify data.

9 Experimental Table 1 IC-ICP-MS conditions needed to run samples [5].

10 Data Figure 4 - Chromatogram of Cr(III) complexed with EDTA, Cr(VI) and Cl - [5].

11 Data To quantify the data, calibration curves are obtained by graphing peak area vs. the concentration of Cr(VI) for many samples with known concentrations ranging from 1 µg/l to 1000 µg/l. The regression lines are linear with correlation coefficients close to The limit of detection for this method is 0.1µg/L for Cr(VI) with a 91.5 to 104.3% recovery range [5].

12 Conclusion Ion chromatography inductively coupled plasma mass spectrometry can very accurately and efficiently detect small amounts of hexavalent chromium in waste water while ignoring interferences such as trivalent chromium, C and Cl -. The limit of detection of 0.1 µg/l is well under the EPA limits, making it ideal for testing water samples with very little analyte present. Because of the steep price tag (around $100,000 for both machines), and the bulkiness of the equipment, this is not an ideal method to be used on a small scale or in the field.

13 References [1] 3M. Highlights of the New Hexavalent Chromium Standard. Brochure ed. St. Paul, MN: Author, [2] Small Entity Compliance Guide for the Hexavalent Chromium Standards. United States of America. U.S. Department of Labor. OSHA. Vol. OSHA N [3] Finley, Brent L., Brent D. Kerger, Melanie W. Katona, Michael L. Gargas, Gwen C. Corben, and Dennis J. Paustenbach. "Human Ingestion of Chromium (VI) in Drinking Water: Pharmacokinetics Following Repeated Exposure." Toxicology and Applied Pharmacology 142 (1997): [4] "Consumer Factsheet on: CHROMIUM." Ground Water and Drinking Water. 26 Nov U.S. Enviromental Protection Agency. 11 Oct < [5] Chen, Zu L., Mallavarapu Megharaj, and Ravendra Naidu. "Speciation of Chromium in waste water using ion chromatography inductively coupled plasma mass spectrometry." Talanta 72 (2006): [6] Schilling, Jean-Guy, and Richard Kingsley. "ICP-MS Laboratory." University of Rhode Island. 24 Nov <