Mercury Determination in Bottled Water from World Sources


Jeff Forsberg, John McQuatters and Paul Goble, CETAC Technologies Omaha, NE, USA Tel: +1 (402) 733-2829 • Email: jforsberg@cetac.com • Web: www.cetac.com/mercury_analyzers


Measuring mercury in water and food is a crucial part of environmental monitoring. With a growing number of people relying solely on bottled water as their source


of drinking water there is a concern that bottled water could be a source of mercury contamination. There is also a concern that the production process may be a source of contamination rather than the supplied water. In the production process contamination may come from bottle production or the process of filling and packaging the bottles. Mercury as a contaminant in water does not make water cloudy, give it a noticeable odour or change the taste but it may still be present.


The United States Food and Drug Administration (FDA) and Environmental Protection Agency (EPA) are both responsible for the safety of drinking water in the United States. The EPA regulates public drinking water (tap water), while the FDA regulates bottled drinking water. The FDA samples and tests both the source water and the final product for contaminants. The FDA “Bottled Water Final Rule” (published in the US Federal Register; 70 FR 33694 June 9, 2005) sets the allowable levels of total mercury at less than 0.002mg/L. The allowable level is easily distinguished above background because today’s instrumentation can easily detect levels 1000 times lower than the current allowable level. Trace to ultra-trace mercury analysis can present many obstacles for the analyst. The major obstacles are background contamination, interferences, and ultra-trace sample preparation. Mercury is prevalent through point source contamination that often comes from industry and bio-accumulation. Commercially- purchased bottled water from around the world was obtained, fortified, and analysed using US EPA method 1631.


Figure 1: CETAC QuickTrace™ M-8000 Mercury Analyser


Sources of bottled water were collected from 11 countries from the global regions of Europe, South America, Australia, Africa, Asia, and the Middle East. Water sources included natural mineral water, spring water, and purified water. Analyses were performed on the CETAC QuickTrace™ M-8000 Cold Vapor Atomic Fluorescence Mercury Analyser in gold trap mode.


Instrumentation and Principle of Operation The working range for the QuickTrace™ M-8000 Mercury Analyser (see Figure 1) is from < 0.05ng/L (ppt) to > 400µg/L (ppb). The QuickTrace™ M-8000 is a stand-alone analyser that uses Cold Vapor Atomic Fluorescence (CVAF)


spectrometry for obtaining reliable quantitative data. The QuickTrace™ M-8000 is accompanied with an autosampler which enables unattended sample batch analysis. The QuickTrace™ M-8000 has a four-channel peristaltic pump that ensures consistent sample uptake into the analyser and allows for sample/reagent reduction online in a closed system. The reduced sample then flows into the patented non-foaming Gas-Liquid Separator (GLS), and argon gas is passed across a thin film of the sample as it flows down a frosted glass post within the GLS. Vapor-phase elemental mercury is liberated into the argon stream and enters into the analyser.


The elemental mercury may optionally be collected for a set time period on a gold amalgam trap composed of gold-coated glass beads. This has the effect of pre-concentrating the mercury, enhancing detection power. The gold trap is heated in an automated furnace, driving off the mercury which is carried by argon to the detector cell. The sample concentration measurement is carried out by a filtered photomultiplier tube in which the intensity of fluorescence from irradiation with a perpendicular mercury lamp is measured at a wavelength of 253.7nm.


Fluorescence intensity is recorded in real-time in the QuickTrace™ software. The measured intensity of fluorescence is proportional to the concentration of mercury in the sample. Software instrument controls include but are not limited to argon flow, lamp, photomultiplier automatic voltage select, pump control and smart rinse threshold. Adjusting these parameters enables increased or decreased sensitivity.


Preparing for the Experiment


The bottled drinking water was digested in pre-cleaned 50mL polypropylene digestion tubes. The digestion tubes were pre-cleaned by soaking with 10% trace metal grade nitric acid, rinsed with ultrapure water in triplicate, soaked overnight in 0.5% bromine solution (v/v; 0.1N potassium bromide / potassium bromate in DI water), and rinsed again in triplicate with ultrapure water.


The cleaned sample vials were stored in polyethylene zipper storage bags until use. Before each lot of pre-cleaned tubes was used, the lot was tested at a 10% rate to validate the tube cleaning process.


Procedure


Figure 2: Washing of the sample bottles for contamination elimination


After logging, the samples were individually cleaned, bagged, and stored in an argon-purged enclosure. The bottles were cleaned by rinsing the outside of the bottle with 3% HCl followed by purged DI water (see Figure 2). Bottles were dried, labeled and logged, and stored in zipper storage bags until analysis.


The bottled water samples were digested in 40mL aliquots by pouring directly into pre-cleaned digestion tubes. The sample digestion was accomplished by the addition of 1.2mL of ultra-pure hydrochloric acid and 0.200mL of 0.1N potassium bromide / potassium bromate.


The vial was sealed and inverted to homogenise the sample and digested for 12 hours at room temperature, ensuring that the solution remained yellow. The oxidation process was followed by the addition of 0.060mL of 12% hydroxylamine hydrochloride solution to reduce the excess potassium bromide / potassium bromate which releases halogens, reducing the risk of gold trap destruction. The sample was then sealed and inverted, and allowed to sit for five minutes. Prior to concentration on the gold trap and subsequent measurement, the oxidised mercury in the calibration standards, quality control solutions, samples, spikes and duplicates was reduced to elemental mercury with online excess addition of 10% tin (II) chloride in 7% hydrochloric acid.


AET October / November 2011 www.envirotech-online.com


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44