The availability of adequate techniques for the sampling and analysis of mercury in air and precipitation has grown during the last decade. Today, monitoring of mercury in air and precipitation is routinely done by a number of institutes throughout Europe and North America. From 1999, mercury has been part of the EMEP program; but at present, monitoring of the atmospheric content of mercury is carried out only at a minor number of EMEP stations.
Important anthropogenic sources of mercury are burning of fossil fuels, waste incineration plants and crematoriums.
Sampling of particulate phase mercury in air is one of the difficult steps in measurements of atmospheric mercury. This is mainly due to errors that may occur during both sampling and analysis. In ambient air the particulate fraction of mercury usually is less than 5%, with volatile mercury making up the reminder. This increases the risk of gas to particle conversion during sampling. For this reason, sampling and analysis of particulate phase mercury should be considered as an operationally defined method. The presence of particulate mercury in air will greatly influence the overall atmospheric deposition of mercury and further development of these techniques is highly desirable, and is needed before any recommended procedure can be given.
We are very grateful to OSPARCOM and John Munthe at IVL to allow us to use their guidelines on sampling and analysis of mercury. The text is with minor changes taken from their report (Munthe, 1996; OSPAR, 1997).
Mercury is collected in special precipitation samplers. Two alternative materials may be used for funnels and collection bottles: borosilicate glass and a halocarbon such as Teflon or PFA. Borosilicate glass is often preferred due to lower costs and general availability. Quartz glass may also be used but is generally avoided due to high costs.
Precipitation can be sampled using either wide-mouthed jars or funnels and bottles. The sampling vessels can be bulk samplers which are open at all times or wet-only samplers which are open only during precipitation events. For monitoring purposes, bulk sampling using funnels and bottles is normally adequate (Iverfeldt, 1991a,b; Jensen and Iverfeldt, 1994). Wet-only samplers are used by the German national monitoring programme as well as by research groups working in the Great Lakes area (Landis and Keeler, 1996) and in the US National Atmospheric Deposition Programme (Vermette et al., 1995). Wet-only samplers have the advantage that they avoid particle dry deposition, although the contribution of gaseous or particulate mercury species to the wet deposition fluxes in non-industrialised or non-urban areas is probably not large (Iverfeldt and Sjöberg, 1992; Iverfeldt and Munthe, 1993).
For extended sampling periods it is necessary to prevent the diffusion of Hg0 into the precipitation sample collected, since it could contribute to the mercury content of the sample via oxidation to water-soluble forms. This can be done easily by using a capillary tube between the funnel and the bottle. It is also necessary to shield the sample bottles from light to avoid photo-induced reduction of the mercury in the precipitation sample.
Samplers should be designed for sampling during all seasons and all climatic conditions. Thus a heating device should be included for melting snow and to prevent the formation of ice in the funnel and bottles during winter and, depending on climatic conditions, it may be useful to cool the samples in locations where high temperatures are expected during summer. Funnel area and bottle sizes should be modified to suit the sampling period used.
The bulk sampler design used in the Swedish national monitoring program for mercury is shown in Figure 3.13.1.
Figure 3.4: Schematic
of bulk sampler for sampling of precipitation for mercury analysis from
IVL-Gothenburg.
In the air pollution network of the German Federal
Environmental Agency (Umweltbundesamt) a commercially available automatic
(wet-only) precipitation collector (type: ARS 721) is used for of mercury
(Bieber, 1995). The funnel of this sampler is made of borosilicate glass while
the collection bottle is made of halocarbon polymer (PFA) and has a heating
system with a thermostat.
All glass equipment used for sampling or for storing water samples for mercury analysis must be cleaned extensively before use. The glassware is washed extensively according to the following procedure.
Plastic gloves should be used during all steps of the washing procedure. Glassware that is not being leached in tanks should always be stored in double plastic bags.
Bottles that are believed to be contaminated are baked at 500oC for 5 hours and then washed according to the procedure above, but excluding step 1.
This procedure is generally sufficient for containers used for the sampling and analysis of precipitation. After analysing samples with extremely low concentrations of mercury the BrCl treatment steps according to steps 4-6 above is generally sufficient.
Before sending the precipitation bottle to the field they should be filled with 5 ml/L concentrated HCl. One should therefore be extra careful when handling these bottles.
The following procedure is recommended for bulk samplers of the design seen in Figure 3.13.1. For alternative sampling devices, this procedure can be adapted.
Samples for storage must be refrigerated and kept in the dark. They may be stored up to 6 months provided that the long-term stability is checked. This includes the testing of sample blanks stored for corresponding periods.
A critical step when evaluating wet deposition of mercury is the availability of correct data on precipitation amounts. Different sampler designs have different precipitation sampling efficiencies and this may lead to incomparable results when calculating the wet deposition of mercury, even if the analysis are harmonised. For all techniques, a parallel measurement of precipitation amounts should be made in order to identify discrepancies. A standard rain gauge should be used in parallel with the sampling equipment for mercury and the WMO recommendations should be followed. If systematic errors are found, the sampler design should be reconsidered.
Written instructions for personnel carrying out the sampling are necessary to avoid contamination. All routine handling of samples and sampling equipment should take place according to specified procedures. Furthermore, replacement parts should be easily available so that glassware can be exchanged if contamination is suspected.
The risks of contamination when using bulk samplers and extended sampling times are controlled by using two or three samplers in parallel. Contaminated samples can then be identified and discarded and the corresponding data excluded.
Field blanks should be taken at least four times each year.
Two extra sampling bottles should be brought to the site; one containing dilute HCl (pH 3 to 4) and one empty. After removing the regular sample bottle the empty bottle should be installed and the dilute HCl poured through the sampling device (e.g. funnel and capillary). The bottle should be stoppered, double bagged and brought to the laboratory for analysis. The mercury content of the dilute HCl should be compared to that of samples stored in a clean laboratory environment. If the blank values exceed 20% of the concentrations normally measured at the site, measures should be taken to reduce the blanks (for example, by exchanging or by cleaning the sampling devices).
The yearly average blank value is used to determine the detection limit and should be reported to CCC.
When first starting to sample precipitation for mercury analysis numerous problems can arise, mostly associated with high blanks. Weekly sampling should be undertaken initially, even if monthly sampling is planned. In this way the number of sampling periods without results can be reduced.
The most common causes of sample contamination are insects, bird droppings or other material in the sampling vessels. This is the major drawback to bulk sampling. In areas with large numbers of birds in the vicinity, it may be necessary to install devices preventing birds from perching on the samplers.
Using two or three samplers in parallel controls the risks of contamination when using extended sampling periods. Contaminated samples can then be identified and the results discarded thus minimising the loss of information. Contamination of two samplers in parallel is very rare.
When temperatures are high, it may be necessary to cool the sample bottle to prevent the evaporation of mercury from the sample.
|
Recommendation |
Acceptable alternative |
Material |
Borosilicate glass. |
Halocarbon materials, quartz. |
Sampler design |
Bulk samplers or wet-only samplers with gaseous Hg prevention and light shield. Heating and/or cooling of sample bottle depending on climatic conditions. |
Event sampling using funnels and bottles or jars. |
Sampling time |
1 week to 1 month. |
|
Preservation of samples |
Monthly sampling 5 ml/l HCl (Suprapur) prior to sampling. |
Adding 10 ml/l HCl after sampling in sampling periods of <2 weeks and samples are cooled if necessary. |
QA/QC |
Field blanks. Written instructions for field personnel. |
|
The sampling of total gaseous mercury (TGM) in air is usually done using gold traps with gasmeters or mass flow controllers for air volume measurements. During the last few years, automated instruments for the sampling and analysis of mercury in air have been made available. The automated method applies the same basic principles as the manual method and has been shown to generate comparable results (Schroeder et al., 1995; Ebinghaus et al., 1999).
Gold traps comprise 10-12 cm quartz glass tubes filled with gold adsorbent. The gold adsorbent can either be small pieces (1-2 mm) of 1 mm solid gold wire mixed with a crushed-quartz glass bearer or, alternatively, sand, glass beads or quartz glass coated with a thin layer of gold. The latter trap type usually generates lower blank values.
The gaseous mercury collected with the gold trap in the field is transferred by heating to a calibrated Au trap, using Hg-free argon with a purity of >99.998% (at a flow rate of 30 ml/min) as the carrier gas. This is known as the dual amalgamation technique. The sampling and transfer lines are made of Teflon tubing. Glass to glass connections (i.e. between the sampling trap and the analytical trap) are made with silicone tubing. The inlet filter is a quartztube 75 mm in length (with an inner diameter of 4 mm and an outer diameter of 6 mm) with a quartzwool plug 15 mm in length. A brief summary of the equipment and cleaning procedure is as follows:
The sampling of total gaseous mercury in air is relatively straightforward and without major difficulties. The site should be chosen with great care to avoid contamination or non-representative results. Sampling should be performed >1.5 m above the ground or other surfaces (walls etc.) in order to avoid the influence of local fluxes. The sampling system contains 1 protective quartz wool plug followed by 2 gold traps in series.
The analytical method for sampling total gaseous mercury (including elemental, organic and inorganic mercury) is based on the amalgamation of mercury with gold. Total gaseous mercury is collected on the surface of the gold. For sample collection two of these traps are placed in series. With this arrangement a breakthrough of mercury is detected with a significant mercury amount on the second trap.
The sampling time and air volumes should be sufficient to collect enough mercury for analysis but not so large as to cause a breakthrough of mercury. Sampling flow rates in the range 0,1-0,5 l/min up to a maximum volume of 100-1000 litres is normally adequate.
The general configuration for the set-up of the sampling system for total gaseous mercury in air:
Field traps should be exchanged according to a fixed procedure taking great care to avoid contamination. Before and after sampling the ends of the traps are closed with plastic caps and the traps are stored in a firmly closed glass bottle. To prevent contamination during storage 1 g of silver wool should be kept in the bottle to bind gaseous mercury diffusing into the storage vessel.
To convert the sample volume to a volume with standard conditions (0ºC and 1 atm or 273.16ºK and 1013.25 mB) it is necessary to multiply the pump volume with correction factors (calculated from the Ideal Gas Law).
V (Std.) = V (current) *273.16 K/T (inlet) and V (Std). = V (current) * P (inlet) / 1013.25 mB.
The necessary quality control steps are primarily associated with gold trap collection and analytical instrument reliability. All gold traps must be individually calibrated at regular intervals. This is most conveniently done using a source of gaseous mercury, i.e. a thermostated vessel containing liquid mercury from which gaseous samples can be drawn with a gas tight syringe. Gold traps with low recovery must be discarded.
Alternative methods for sampling mercury in air are not generally available. Commercially available iodated carbon traps have, however, been successfully used for sampling over extended periods, i.e. days (Bloom et al., 1995).
Under certain conditions, breakthrough of mercury can occur at air volumes considerably smaller than those recommended above. This is usually due to the presence of trace constituents in the air which block the gold surface. Possible contaminants are sulphur-containing volatile organic compounds and volatile inorganic species capable of forming solid salts on the gold surface via atmospheric reactions (e.g. (NH4)2SO4). If this happens, considerably smaller sampling volumes should be used, i.e. <100 litres.
When using automated systems frequent re-calibration is necessary and this frequency will vary according to the temporal resolution of the sampling. Daily re-calibration is a minimum.
|
Recommendation |
Acceptable alternative |
Material |
Solid gold traps. |
Coated gold traps. |
Sampling design |
1 protective quartz wool plug followed by 2 gold traps in series. |
Teflon filter or quartz fibre filter followed by 2 gold traps in series. |
Air flow rate |
200 to 500 ml/min. |
|
Sample volume |
100 to 800 litres. |
|
QA/QC |
2 gold traps in series to check mercury breakthrough. |
|
Within any monitoring network where data are reported from different institutes, regular intercomparisons are necessary. The intercomparisons should be performed for all steps in the measurement procedure. The recently completed intercomparisons on the sampling and analysis of heavy metals organised by Umweltbundesamt in Germany together with EMEP/CCC, HELCOM, PARCOM and AMAP, is a good example of a successfully managed exercise with encouraging results (Winkler and Roider, 1997).
In 1991 an intercomparison exercise for mercury deposition was held at Rörvik, Sweden (Iverfeldt and Sjöberg, 1992). The conclusions from that exercise was that the measured fluxes varied within a factor of 2 to 4 which was explained as systematic errors in the methods used by several of the participants. More encouraging results were found in the Mace Head intercomparison in 1995 (Ebinghaus et al., 1999) where relatively good agreement between different methods for measurements of mercury in air and precipitation were obtained.
The equipment list below shows examples of known good
samplers. However there might be several others excellent products on the
marked, but if used they must anyhow also have been proven to give comparable
and reliable results.
Sampler |
Name |
Manufacture |
Materials |
Wet only |
MDN 1 sampler
modified Aerochem Mmetric sampler used in |
|
Double system: glass funnel, glass capillary and bottle for Hg sampling the other PE or Teflon funnel and bottle for trace metals |
ARS 721 |
Eigenbrodt |
Borosilicate glass funnel, PFA bottle, heating |
|
Bulk |
NSA 181 KD |
Eigenbrodt |
Quartz glass funnel, teflon tube teflon bottles, heating and cooling |
IVL |
IVL, P.O.Box 47086 |
Borosilicate glass funnel, glass filter, capillary tube and glass bottle, heating in the modified version |
|
GKSS |
International bureau
of GKSS, Germany |
Teflon funnel, brown glass bottle |
|
Hg(g) sampler |
Gold traps |
Brooks Rand, US |
|
Hg-monitor |
Tekran 2537A |
Tekran Inc,
Toronto, |
Automatic, 5 min time resolution |
Automated water |
P.S. Analytical |
P.S. Analytical, Kent, UK, http://www.psanalytical.com |
|
Perkin Elmer |
|
||
Tekran 2600 |
Tekran Inc,
Toronto, |
|
|
Detectors |
P.S. Analytical |
P.S. Analytical, Kent, UK, http://www.psanalytical.com |
CV-AFS |
Tekran 2500 |
Tekran Inc,
Toronto, |
CV-AFS |
Bieber, E. and Althoff, S. (1995) Methods for sampling and analysis of total mercury in precipitation in the air pollution network of the German Federal Environment Agency (Umweltbundesamt). In: JAMP Guidelines for the sampling and analysis of mercury in air and precipitation. London, OSPAR (Technical Annex 2, 20-23).
Bloom, N.S., Prestbo, E.M., Hall, B. and Von der Geest, E.J. (1995) Determination of atmospheric mercury by collection on iodated carbon, acid digestion and CVAFS detection. Water, Air, Soil Pollut. 80, 1315-1318.
Ebinghaus, R., Jennings, S.G., Schroeder, W.H., Berg, T., Donaghy, T., Ferrara, R., Guentzel, J., Kenny, D., Kock. H.H., Kvietkus, K., Landing, W., Mazzolai, B., Mühleck, Munthe, J., Prestbo, E.M., Schneeberger, D. Slemr. F., Sommar, J., Urba, A. Wallschläger, D. and Xiao, Z. (1999) International field intercomparison measurements of atmospheric mercury species at Mace Head, Ireland. Atmos. Environ., 33, 3063-3073.
Fitzgerald, W.F. and Gill, G.A. (1979) Subnanogram determination of mercury by two-stage gold amalgamation and gas-phase detection applied to atmospheric analysis. Anal. Chem., 51, 1714-1720.
Iverfeldt, Å. (1991a) Occurrence and turnover of atmospheric mercury over the Nordic countries. Water, Air, Soil Pollut., 56, 251-265.
Iverfeldt, Å. (1991b) Mercury in canopy throughfall water and its relation to atmospheric deposition. Water, Air, Soil Pollut., 56, 553-542.
Iverfeldt, Å. and Munthe, J. (1993) In: Proceedings from the EPA/A&WMA symposium measurement of toxic and related air pollutants, Durham, NC.
Iverfeldt, Å. and Sjöberg, K. (1992) Intercomparison of methods for the determination of mercury deposition to convention waters. Göteborg, Swedish Environmental Research Institute (IVL Report B 1082).
Jensen, A. and Iverfeldt, Å. (1994) Atmospheric bulk deposition of mercury to the southern Baltic sea area. In: Mercury pollution Integration and Synthesis. Watras, C.J., Huckabee, J.W. (Editors), Boca Raton, Lewis Publ., pp. 221-229.
Landis, M.S. and Keeler, G.J. (1996) A critical evaluation of a wet only precipitation collector designed for network operation for mercury and trace elements. Presented at the Fourth International Conference on mercury as a global pollutant, Hamburg.
Munthe, J. (1996) Guidelines for the sampling and analysis of mercury in air and precipitation. Gothenburg (IVL-report L 96/204).
OSPAR (1997) JAMP Guidelines for the sampling and analysis of mercury in air and precipitation. London.
Schroeder, W.H., Keeler, G., Kock, H., Roussel, P., Schneeberger, D. and Schaedlion, F. (1995) International field intercomparison of atmospheric mercury measurement methods. Water, Air Soil Pollut., 80, 611-620.
Vermette, S., Lindberg, S. and Bloom, N. (1995) Field tests for a regional mercury deposition network - sampling design and preliminary test results. Atmos. Environ., 29, 1247-1251.
Winkler, P. and Roider G. (1997) HELCOM-EMEP-PARCOM-AMAP field intercomparison of methods for the determination of heavy metals in precipitation 1995. Berlin, Umweltbundesamt (Report 104 08 540).