Testing for a suite of metals is one of the most common requests received by environmental laboratories, and is generally perceived as straightforward and simple to understand. This is actually a complex area, however, with some misconceptions relating to the analysis and how data should be interpreted.
Environmental consultants are generally concerned with respect to the most toxic metals, such as arsenic, cadmium, mercury and lead, but many other common metals will impact on human health at relatively low levels, particularly when present dissolved in water.
Soils and waters are routinely tested for metals, but with different methods and different detection limits – generally much lower for waters, as water soluble metals are of much greater concern.
These generally fall into suites: • ICRCL (Interdepartmental Committee for the Redevelopment of Contaminated Land), now superseded, but still frequently requested: Arsenic (As), boron (B or WSB for water soluble), cadmium (Cd), chromium (Cr), copper (Cu), mercury (Hg), nickel (Ni), lead (Pb), selenium (Se), zinc (Zn) • CLEA (Contaminated Land Exposure Assessment): As above, and also beryllium (Be), barium (Ba), vanadium (V) • USEPA (United States Environmental Protection Agency): Similar to ICRCL above, but also includes antimony (Sb), silver (Ag) and thallium (Tl) • Dutch suite: Similar to ICRCL above, but also includes molybdenum (Mo) and cobalt (Co) • Akaline metals – Most commonly requested in water samples, and are a necessity for ionic balance calculations: Sodium (Na), calcium (Ca), magnesium (Mg) and potassium (K) • Speciated metals – Due to differing toxicities, it is sometimes important to understand the form of the metal as it exists in the soil or water. Common examples of these are hexavalent chromium, or elemental, organic and inorganic mercury. With respect to other organically bound metals, common parameters include tributyl and triphenyl tin, and the tetraethyl/tetramethyl lead species • Other metals sometimes requested include iron (Fe), phosphorus (P), bismuth (Bi), aluminium (Al), manganese (Mn), titanium (Ti), strontium (Sr), silicon (Si) and tellurium (Te)
There may be some additional metals not listed above, which are site specific due to a particular industrial process.
In environmental laboratories, the two most common methods for metals analysis are ICP-OES and ICP-MS, acronyms for Inductively Coupled Plasma – Optical Emission Spectroscopy, or Mass Spectroscopy. The beauty of the ICP is that it will measure many wavelengths at once, so more than 30+ metals can be analysed in about four minutes, ensuring this is a very efficient method.
The method of preparation of a soil sample is critical in assessing the concentration of metals in soil – is the laboratory using the whole sample, or is this sieved prior to analysis of the fines only? What sieve size is used? Some laboratories just hand pick out the larger lumps. All of these protocols will give widely differing results (for further information, see the full article on this topic in the June 2011 issue of AWE).
The soil is mixed, dried and crushed, and weighed out (usually 5g of sample), and then digested on a hot block with aqua regia (a 3:1 mix of concentrated hydrochloric and nitric acids) for up to two hours. The acid extract is filtered and then loaded into the autosampler rack on the ICP – OES, where it will be aspirated and pumped into the plasma (a very hot, ionised gas at 10,000o C).
Here, energy causes electron excitation, and then as the sample passes out of the plasma, this energy will be emitted as the electrons return to their ground state. The energy emitted will be at specific wavelengths, according to the metals present in the sample, and this energy is detected by the spectrophotometer.
One common myth is that this method provides a total metals result, but this is not the case, as many silicate minerals are not soluble in aqua regia, and therefore will not be included. Hydrofluoric acid would be required to dissolve the silicate matrices, but this presents significant health and safety handling problems, and is therefore not permitted in most laboratories. The aqua regia method is, however, now the industry standard, but should really be reported as ‘aqua regia soluble’.
Metals in waters can be analysed either on the filtered or unfiltered samples. If samples are filtered, then only the dissolved metals will be analysed by the ICP, but if they are not filtered, then any sediment may be included in the analysis. For the majority of samples, most contractors need the filtered (dissolved) metals, but if an effluent discharge is being monitored, it may be necessary to monitor the total loading into a water course, and this will include metals in the sediment.
It is very important for clients to specify if they require filtered (dissolved) or unfiltered metals when submitting water samples. Sometimes ‘total metals’ are requested, but this can mean ‘total dissolved’ to some clients, and is therefore ambiguous – it is preferable to use the terms filtered or unfiltered.
For best practice, water samples should be filtered on site through a 0.45 micron filter, but this does not always happen, due to time constraints for the site operatives. If samples are not filtered on site, it is possible for sediment to dissolve further into the sample, or with heavily contaminated waters, some salts may precipitate out of solution. Filtration with this size of filter will also remove bacteria, and so prevent further microbial degradation.
If samples are filtered on site, then preserved bottles can be used and these will ‘fix’ the analyte of interest to prevent further changes. If preserved bottles are used without filtering, then some sediment may dissolve in the preservative, leading to falsely high values.
Filtered waters are acidified with nitric acid prior to analysis by ICP-MS, and unfiltered waters are digested with a more aggressive mix of nitric and hydrochloric acids to dissolve any sediment, prior to analysis.
A requirement to know the form of mercury in soil is specified by the Environment Agency (EA) in their SGV (Soil Guideline Value) publication for mercury, which can be found on their website. This stipulates the following values for mercury in residential soil:
Some practitioners are not aware of this requirement, or may not consider it a site specific risk, and will only request the standard aqua regia soluble mercury. As this is usually performed on a dried and crushed subsample of the soil, there may be significant losses of both elemental and methylmercury.
There are now laboratories who have developed the speciated method and can provide the breakdown of mercury into the component parts to satisfy the EA requirement. This method involves heating a portion of the soil to release the elemental and the methylmercury as vapours, which are then analysed by atomic fluorescence or LCMS (liquid chromatography mass spectroscopy) respectively.
Most chromium is usually present in the trivalent form, which is much less toxic than the hexavalent form, with GACs for residential use of 3000 mg/kg and 4.3 mg/kg respectively (Generic Assessment Criteria, acknowledgement to Land Quality Management). Consequently, it may be necessary to check if the hexavalent form is present. The method currently used is spectrophotometric, depending upon a colour reaction occurring in an aqueous or alkaline soil extract, or in a water sample. It is important that an acid extraction is not used, as this breaks down the hexavalent form to the trivalent form.
The tributyl and triphenyl tin (TBT, TPT) are analysed using GCMS, and can reach very low detection limits. These compounds were used as anti-fouling agents in the paint used on shipping. The tetramethyl and tetraethyl lead (TEL, TML) are analysed using GCFID or GCMS, usually with a headspace system of sample introduction, as they are quite volatile compounds found in the now superseded leaded petrol.
An alternative method to ICP is XRF (X-ray fluorescence), whereby a solid pellet of the soil is prepared and analysed, either by a portable or a laboratory based instrument. The benefit of XRF is that the result is a true total of the metals present in the soil, regardless of their form or species, so this may not therefore be comparable with the aqua regia method.
The downside is that the detection limits are often not as low as with the ICP methods. This method is, however, gaining in popularity for field measurements, whereby a rapid screen using a hand held meter is of value in mapping out hotspot areas, giving rapid data and allowing savings on overall project costs.
It should be remembered that field measurements are performed on the wet soil, and therefore the moisture content needs to be taken into account before comparing with dried and crushed laboratory samples.
XRD (X-ray diffraction) is a method whereby the crystalline form of a mineral can be analysed, e.g. iron oxide, calcium sulphate, magnesium carbonate, and the percentage composition of each mineral present in the sample can be reported. This is very useful when the form of the metal is required, but is only successful on crystalline formations and will not work with amorphous species.
Atomic absorption spectroscopy (AAS) is a classic method for the analysis of metals, and is still widely used in many laboratories, depending upon the application. The downside to AAS is that only one metal can be measured at once, and therefore if a suite of metals is required, this can be a very slow alternative.
Atomic fluorescence is often the method of choice for the analysis of mercury, as it is the most sensitive, and can reach 10 ppt (parts per trillion), or 0.01 micrograms per litre.
There may be some samples which are more difficult for the laboratory to analyse, due to other compounds present in the sample which suppress or enhance the concentration of the analytes of interest. Iron can sometimes cause problems, as it has a number of strong spectral lines which can interfere with less abundant metals, but modern day ICPs usually have systems to compensate for this, or another wavelength can be selected which is less prone to interference. The addition of a surrogate or internal standard to the samples will usually highlight if this problem exists, as the standards will also be affected by a matrix effect.
A highly concentrated matrix, such as a saline sample, can also cause issues, and various preparative procedures may need to be implemented to reduce the level of sodium chloride in the sample. Diluting the sample with large volumes of water may be adequate, but this will cause the limits of detection to be significantly raised.
Examples of detection limits available for metals by ICP are listed in the following tables, although these can vary, depending upon the laboratory, site specific requirements or legislative guidelines – note the soils are in milligrams and the waters in micrograms (apart from hexavalent chromium).
If a lower limit of detection is required, it may be possible to achieve this, but the preparative methods may be different, and this may invalidate the accreditation status of the method – clients need to consult with their laboratory if this is necessary.
Soil Guideline Values as published by the Environment Agency, are only available for five metals at the current time, although lead is under consideration.
These metals are arsenic, cadmium, mercury, nickel, and selenium, and they are expressed for three site usages – residential, allotments, and commercial use.
There are GAC (Generic Assessment Criteria) published by LQM/CIEH, and also by CL:AIRE/EIC, and these are also provided for the three site usages, and also at three differing TOC (total organic carbon) values. All these values are to be used as guidelines only, however, and the individual risk assessment on any site must be performed specifically for that site, based on its history and its end usage.
Guideline values for waters are more extensive, but these may be different for drinking water abstraction, groundwaters, surface waters or other water bodies, and also the regulatory organisation. A selection is provided in the table below.
This overview of metals analysis demonstrates the wide range of possibilities in the choice of metals reported, the method of analysis, and the regulatory requirements. Some of the critical issues are highlighted, such as soil preparation methods, filtering or not filtering waters, analytical methods, and the desired limits of detection. It is recommended that practitioners should discuss their requirements with their laboratory of choice prior to implementing a site investigation or monitoring programme.
• Interdepartmental Committee on the Redevelopment of Contaminated Land, Guidance Notes, DETR, 1983 • Hydrogeological Risk Assessments for Landfills, Environment Agency, 2003 • The LQM/CIEH Generic Assessment Criteria for Human Health Risk Assessment, 2009 • WHO Drinking Water Standard, 2006 • EC Drinking Water Directive, 98/83/EC
Hazel Davidson, Technical Marketing Manager, ALcontrol Laboratories Hazel Davidson has worked for ALcontrol Laboratories for 30 years, initially as an analyst, but then in a series of managerial roles. Special projects included the integration of several laboratory acquisitions, relocation of the laboratories from Chester to Hawarden, a Phare project in Bulgaria and Romania (implementing quality systems), and a UN project involving training for Iraqi environmental scientists in Jordan. Hazel participates on several industry committees (BSi, MCERTS, SCA and EIC), is a frequent speaker at conferences and runs several seminars each year for ALcontrol clients, as well as providing general technical support, both internally and externally.
Published: 01st Mar 2012 in AWE International