Heavy water analysis

Trace metals occur naturally but are potential contaminants because human activities influence their levels in the environment.

Marine, estuarine, and waters of a similar nature are generally not suitable for direct determinations of trace metals because of the large amounts of dissolved solids that they contain.

Electrochemical techniques such as voltammetry offer a viable alternative approach for quantification of trace metals. The advances made in recent years in terms of voltammetric instrumentation and method development ensure that often an unrivalled sensitivity can be obtained, usually for a fraction of the cost of spectroscopic techniques and without the need for separation of the metals either using solvent extraction or co-precipitation.

Generally, concentrations of heavy metals are comparatively low in open sea areas where human activities have not yet had a major effect, however in coastal areas accumulation of trace metals are often evident.

These higher concentrations tend to be found in offshore sediments close to discharge points of municipal and industrial waste run-off.

Trace metals can also reach marine waters by air from industrial smoke discharges and traffic exhausts.

The matrix effects of saline waters vary in severity with regard to analysis by spectroscopic techniques.

The effects are minor for lead and copper but can impart substantial matrix interference upon cadmium where it is often necessary to employ a matrix modifier.

In the past fifteen years research organisation from the United Kingdom have conducted marine studies in diverse environments such as the English Channel, North Sea, Adriatic Sea, Indian and Pacific Oceans to study the effects that trace metals pose upon species resident in the marine environment.

Voltammetry - a brief overview.

Heyrovsky first introduced polarography in 1922.

The term voltammetry is applied to designate the current-voltage measurement obtained at a given electrode.

Polarography is a special case of voltammetry referring to the current-voltage measurement acquired using a dropping mercury electrode with a constant flow of mercury drops. Stripping voltammetry uses the same instrumentation as traditional polarography but a stationary electrode such as the hanging mercury dropping electrode (HMDE) is used.

The voltammetric measurement is performed on a stationary mercury drop allowing one to achieve considerable increases in sensitivity.

The jump in sensitivity is possible by electrochemical preconcentration of the metals in question at the surface of the stationary electrode before the current-voltage curve is recorded.

The recorded current is the redissolution (reoxidation) current of the preconcentrated metal traces. Voltammetry today represents a refined, clean, simple technique that offers outstanding limits of detection and is now the fastest growing analytical technique for trace analysis.

With the Metrohm Multi Mode Electrode (MME) the mercury is hermetically sealed in the reservoir and suffices for around 200,000 drops, ensuring low laboratory running costs.

Impact of trace metals in marine waters.

The trace elements found in marine waters can have a profound influence on life in the ocean, for example zinc can affect the phytoplankton growth rate.

As well as the direct effects on marine life one should also be aware that any possible contamination of the marine environment could possibly make its way into the human food chain and have a real impact on human life.

From studies conducted in the North Pacific, it has been observed that the trace metal concentrations of cadmium, lead and copper show vertical concentration profiles (ie, they increase with depth).

These are related to such parameters as temperature, salinity, dissolved oxygen and dissolved inorganic carbon.

This implies that the metals are directly involved like macronutrients in the biological cycle of algal uptake in the surface water and heterotrophic remineralisation at depths typical of oceanic ecosystems.

From previous work, it has been revealed that there is a close correlation between zinc and silicate that shows as the amount of silicate increases so does the zinc.

This seemingly implies that zinc is taken up in the mineral parts of phytoplankton such as calcium carbonate or silicate exoskeleton of coccolithophores and diatoms respectively.

Zinc is a recognised as an essential trace element for plankton growth and is a necessary cofactor for several important enzyme systems required for photosynthesis.

The toxic elements lead and cadmium present in the sea contaminate marine life and food, which can result in a decrease in a particular species population or in the worst case possible extinction.

Cadmium is produced commercially as a by-product of zinc and lead mining and since the 1950s has been used as a stabiliser and pigment in many plastics and solders, as well as its use in electroplating and batteries. Metal production and refining as well as waste disposal are major sources of cadmium emissions.

Natural emissions occur from volcanoes and windblown dust and together with man-made emissions contribute somewhere in the order of 8000 tonnes per year to the world's oceans.

Cadmium is thought to be probably the most bio-toxic element and is regarded as a primary pollutant.

In previous studies conducted on estuarine environments, crustaceans appear to be particularly sensitive to the effects of cadmium followed by molluscs and polychaetes.

In marine environments, the organisms appear to be less sensitive to the toxicity of cadmium then estuarine and freshwater animals.

Studies on shrimp species have shown that species in regions with high cadmium concentrations are able to both handle and survive the high body loads of cadmium, although interestingly the female species were found to have a higher tolerance than their male counterparts. Studies have shown there to be an interesting relationship between cadmium and phosphate in deeper sections of the global oceans.

In these waters, the cadmium concentration increases linearly with that of phosphate whereas in shallow shelf waters one sees a departure from this correlation.

Despite some fluctuations from the cadmium-phosphate concentration model, it is clear that cadmium is involved in organic matter formation through photosynthesis before remineralisation deeper in the ocean.

It is not clearly understood at present why cadmium is so closely involved as it has no known or specific biochemical function.

One theory that has been proposed for its occurrence is based on the close similarity of the ionic radius of the divalent cadmium and calcium, that hypothesises that cadmium substitutes into the crystal lattice of calcium carbonate minerals that are formed by marine species.

Cadmium tends to be absorbed through the body via food and water intake and can adversely injure the renal, pulmonary, skeletal, testicular and nervous systems in addition to it being a recognised carcinogen.

Renal failure tends to be the earliest cause of death due to the sensitive nature of the kidneys where the cadmium becomes concentrated.

There has not been as much research work carried out on lead as for other trace metals upon the effects that contamination presents to a marine environment.

It is known that lead is a cumulative poison and the vast majority retained in the body enters the bones from where it can be remobilised.

Lead can bind to a number of molecules within the body such as amino acids, haemoglobin, enzymes, RNA and DNA and can disrupt many of the metabolic pathways.

Some of the effects of lead toxicity include impaired blood synthesis, hypertension, hyperactivity, brain damage and cancer.

Such effects can occur at levels equivalent to 20-200mg l-1 in soft tissue, this is the order of lead normally found in human adults so one should view any factor that increases lead intake with a degree of concern.

Copper is an essential trace element for both humans and animals being a critical functional component of a number of essential enzymes, it is found in the environment as a naturally occurring mineral.

Copper is part of the oxygen carrier in snails, crabs and some crustaceans.

Excessively high levels of copper found in the marine environment are likely to be as a result of emissions and discharges caused by man-made activities. Toxicity from copper contamination is extremely rare in the general population although acute copper toxicity can cause liver damage, kidney failure and ultimately death.

Method for analysis of zinc, cadmium, lead and copper in marine waters.

Metrohm has developed a method whereby it is possible to determine the trace metals zinc, cadmium, lead and copper in a single voltammetric sweep.

8ml of deionised water, 2ml of marine water sample, 1ml acetate buffer and 200ml of molar nitric acid were added to the reaction vessel in the Metrohm 757 VA Computrace.

The role of the electrolyte and additional solutions in voltammetry is crucial. Many determinations are pH dependent and the electrolyte can increase the conductivity and selectivity of the solution.

The solution was then degassed with nitrogen for a period of five minutes to remove the electrochemically active oxygen, before the four metals were determined with two standard additions using the HMDE.

The HMDE is an electrode mode of the MME.

Four mercury drops of a defined size are formed in succession at the MME and the last drop remains suspended on the end of the capillary.

The entire voltage sweep is then performed on the single stationary drop.

As a final point in terms of sample methodology, it should be noted that samples of marine waters often contain large amounts of organic material that may require additional sample treatment.

The Metrohm 705 UV Digester is eminently suited to this task and can eliminate moderate to high (after dilution) amounts of dissolved organic matter that can disturb the voltammetric analysis of trace metals.

The digestion of organic matter is achieved through photolytic generation of OH radicals that in turn react with the organic compounds and decompose them. Hydrogen peroxide serves as an initiator of the radical reaction and a mercury lamp provides radiant energy that is converted to heat accelerating the digestion process.

The advantage of UV photolysis over other sample digestion techniques is that only a little hydrogen peroxide has to be added ensuring that blank values can be kept low.

This is crucially important for analysis of marine waters as the low content of trace metals means that the samples are very sensitive to contamination.

Conclusion. Toxic concentrations of trace metals interfere with the normal metabolic processes of fish and other species of life in the marine environment, and these metals can then be passed onto humans who consume them and pose adverse effects on the human metabolic processes.

Voltammetry is an increasingly popular technique that in many instances offers unrivalled detection limits even when compared to vastly more expensive analytical techniques.

Often voltammetry requires little or no sample preparation and the result, of up to four metals in a single voltammetric sweep, determined by standard addition obtained in less than ten minutes.

The advantage of using standard addition as a means of calibration and quantification is that matrix effects present in the sample are taken into account.

The portability of the Metrohm voltammetric equipment means that it can easily be installed on, for example, a research vessel that may be used to collect samples away from the shoreline. Voltammetry requires no specialist laboratory infrastructure like expensive fume extraction, all that is required is a sturdy bench top on which to mount the instrument and a regulated flow of an inert gas.

The running and maintenance costs of voltammetry are minimal ensuring a cost effective analytical solution to surpass the demands required by those organisations interested in quantifying trace metals in marine waters.