The previous part of this article (Světlo 4/2014 pp. 44-46) was a mosaic of known and lesser known properties of mercury. This section reviews several methods for measuring mercury in different environments. Specialized co-authors have taken up the approach to most of them.

RNDr. Alice Dvorská, Ph.D.

Measurement of atmospheric mercury concentrations

Unique measurements of atmospheric mercury concentrations are carried out at the Atmospheric Station Křešín near Pacov in Vysočina, which is operated by the Centre for Global Change Research of the CAS, v. v. i. This station consists of a 250 m high mast on which the vertical concentration gradient of elemental mercury gas is measured (see Figure 1). Measurements at high altitudes are particularly important for the study of long-distance transport of mercury, as mercury is able to 'travel' over long distances, often between continents. The automatic TEKRAN 2537 B instruments allow measurements of mercury concentrations every 5 min. The principle is to extract mercury from the air sample into amalgam on gold cartridges. From these, the mercury is desorbed at high temperature and detected on a fluorescence spectrophotometer. The data from Křešín u Pacov are transmitted to the international database Global Mercury Observation System. The average surface concentration of elemental gaseous mercury in the period December 2012 to June 2013 was 1.50 ng/m3, which is consistent with values measured at other European stations.

Mercury measurements in the field

One method suitable for the design of portable mercury analysers is the atomic absorption spectrometry (AAS) method. It is based on the Lambert-Beer law: the negative logarithm of the attenuation of the flux of monochromatic radiation passing through a substance is directly proportional to the concentration of the substance and the thickness of its layer. The most appropriate wavelength for mercury is its strongest emission line, 253.7 nm. The radiation source is a low pressure mercury lamp. The absorption occurs inside the measuring cuvette into which the measured gas is pumped. By means of mirrors, the beam path in the cuvette is multiplied and thus the sensitivity of the system is improved. After passing through the monochromator, the beam is detected by the photomultiplier. However, attenuation of the measuring beam also occurs in the background, i.e. outside the sample. An interesting correction is used in the Russian portable instrument LUMEX RA-915 M (see Figure 3). Thanks to Zeeman splitting of the emission lines and fast polarization modulation, it allows the cuvette to be alternately illuminated by both side lines. With the smaller wavelength, the attenuation in the sample including the background is measured. The longer wavelength does not interact with mercury and only measures the background attenuation. The detection limit is 0.5 ng Hg per 1 m3 of air. The instrument is also equipped with a liquid cuvette compartment and detects 0,5 ng Hg per 1 l of water. With the thermal decomposition accessory, solids, tissue samples or body fluids can also be analysed. This instrument is a popular choice for environmental and medical measurements.

Ing. Libor Valenta

Measurement of mercury in solids and liquids

Electrotechnical Testing Institute, s. p., is an independent state-accredited testing and certification body, providing services since 1926. Among other things, it has been working in the area covered by the RoHS Directive since 2006. Mercury and other hazardous substances according to European Directive 2011/65/EU (RoHS 2) are measured at the institute using energy dispersive X-ray fluorescence spectrometry in accordance with EN 62321-3-1:2014. The detection limit for the total concentration of each element is 2 ppm for Cd, Pb, Hg, Br and 5 ppm for Cr. The Directive and Government Regulation No 481/2012 Coll. set a limit of 100 ppm for cadmium and 1 000 ppm for the other elements. For hexavalent chromium, polybrominated biphenyls and diphenyl ethers, this measurement is preliminary and the valence states or specific compounds are mainly distinguished by chemical methods and subsequently by gas chromatography. The test area of the X-ray analyser (see Figure 2) is 460 × 360 × 150 mm and the testing itself is completely non-destructive. For all measurement methods, sample preparation and correct interpretation of the results are very important. Sample preparation for measuring mercury content in fluorescent lamps The methodology is covered by EN 62554, which provides procedures for different types of fluorescent lamps and uses the solubility of mercury in nitric acid. It works with unused fluorescent lamps. For example, for double-pot fluorescent lamps, the method of injecting or aspirating concentrated nitric acid in an amount equal to 1/30 of the volume of the lamp is specified. After rinsing, the tube is cut into pieces and crushed. All the fragments are again dipped in nitric acid and the other metals are subsequently dissolved in hydrochloric and hydrofluoric acid. The output is several containers of liquid that can be tested with an X-ray analyser. The uncertainty of the measurement is 5 % when the preparation procedures are followed.

An interesting improvement is the condensation of mercury on the cooled spot (Cold spotting), patented by the General Electric Company. A flow cuvette is mounted on the fluorescent lamp in a horizontal position halfway along its length, through which flows a mixture of alcohol and water cooled to 0°C. The fluorescent lamp is lit and after a few days its light changes to a faint orange-pink (Ar+Ne). All the mercury is then collected in a cooled place. The advantage is minimal mercury leakage and its concentration in a small section of the tube. Small cold cathode tubes (e.g. for LCD displays) are analysed by a mass spectrometer after crushing.

RNDr. Ladislav Viererbl, CSc.

Measurement of mercury content in fluorescent lamps by neutron activation analysis

The neutron activation analysis (NAA) method is one of the few methods that allows the determination of relatively small amounts of mercury in fluorescent lamps. It is well known to the public, e.g. due to the exclusion of Tycho Brahe's mercury poisoning. The sample under examination (a fluorescent lamp without electronics) is irradiated in a neutron field in the irradiation channel of a research reactor. During the irradiation process, the sample produces a number of different radionuclides depending on the content of each element. After irradiation, the activated sample is transported to the spectrometry laboratory where, after a certain period of time, the spectrum of the emitted gamma radiation is measured using a germanium semiconductor detector and the activity of the individual radionuclides is determined. The mass of each element in the sample is then calculated from these values. In the case of fluorescent tubes, which should contain milligrams of mercury, an irradiation time of minutes in the reactor is appropriate. Several radionuclides are produced from mercury, of which the isotope 203Hg is suitable for use in NAA. It has a half-life of 46,6 days and emits gamma radiation with an energy of 279 keV during decay. This radiation is used to identify the 203Hg radionuclide and measure its activity. To determine the mass of mercury in the sample, a standard is also determined, which is a sample with a known mass of mercury that is simultaneously irradiated with the measured fluorescent lamp (mercuric chloride was used because of its low toxicity). The luminophore of fluorescent lamps usually contains europium, which is strongly activated to produce the radionuclide 152Eu with a half-life of 13.5 years. Therefore, after the NAA measurement, the fluorescent lamp remains slightly radioactive in the long term and the piece should be disposed of as radioactive material. Gadolinium, which is sometimes used as an activator in phosphors, has the largest effective cross section of all the elements for trapping thermal neutrons and acting as a shielding. It also shields boron, which is often present in glass. The result for 203Hg must be corrected for this shielding. Experiments performed successively on about twenty samples were used to determine the most appropriate neutron field fluence (dose) and exposure time. For none of the samples tested did the measured value exceed the mercury level stated by the manufacturer. The channel allows irradiation of samples with a maximum diameter of 78 mm and a maximum length of 300 mm, and is therefore mainly suitable for compact fluorescent lamps or high-pressure discharge lamps. The method allows the determination of total mercury even in used fluorescent lamps where the standard method cannot be used. The measurement uncertainty is ≤ 20 %.

Questions about the previous section

On the methylmercury cycle in ocean fish, there was a question about what happens to it in fish that have not become food for any larger fish. Dimethylmercury (extremely toxic) is released into the water from the smoldering tissues, which rises to the surface and flows into the air where it changes back to methylmercury. Interestingly, the concentration of mercury decreases with depth in the ocean. A possible explanation is the presence of selenium at greater depths. Selenoenzymes play a similar role in the human body. As our territory is one of the areas poorer in selenium, it is sometimes recommended to consume dietary supplements containing selenomethionine.

Historical uses of mercury

Mercury has played an important role in the history of science and technology. It was the key to the discovery of atmospheric pressure, oxygen and noble gases, as well as the first electric motor or ECG. Mercury vapor lamps made possible the production of light bulbs and later of tubes and discharge lamps. Mercury discharge rectifiers have long been indispensable in power, industry and traction. Also the polarographic method of Prof. Heyrovsky (Nobel Prize in Chemistry in 1959) would not have been possible without the mercury electrode. For readers interested in further details, the author would like to suggest visiting the discussion site http://edu.nasli.net/rtut/, where texts beyond the scope of this article have found application.

Acknowledgements

The author would like to thank the company NBB Bohemia s. r. o. for providing free samples of NARVA fluorescent lamps with low mercury content. He would also like to thank OSRAM Česká republika, s. r. o. for the tip on the best selling models of the range. Special thanks go to RNDr. Viererbl and his team from CVJ Řež, who put the idea of measuring mercury using NAA into practice with their own enthusiasm.

Fig. 1. TEKRAN 2537 B and other equipment at an altitude of 230 m, on the right the mast of the atmospheric station (photo: Vlastimil Hanuš, www.czechglobe.cz)

Fig. 2. XRF workstation, HORIBA XGT-1000WR in the foreground (photo: Libor Valenta, www.ezu.cz)

Fig. 3. Portable mercury analyzer LUMEX RA-915 M (promotional materials www.lumex.biz and www.rmi.cz)

Fig. 4. The first and unsuccessful attempt - too high exposure in the reactor channel (photo: Ladislav Viererbl, www.cvrez.cz)

Author. Antonín Fuksa, NASLI & Blue Step, spol. s r. o.,
RNDr. Alice Dvorská, Ph.D., Centre for Global Change Research, CAS,
Ing. Libor Valenta, Electrotechnical Testing Institute, s.p.,
RNDr. Ladislav Viererbl, CSc., Centrum výzkumu Řež, s.r.o.
Published in Světlo 6/2014