Heavy water analysis
Metrohm (UK)contact supplier
The analysis of zinc, cadmium, lead and copper in marine waters using voltammetry is outlined by Jonathan Bruce, applications manager for Metrohm UK
Trace metals occur naturally but are potential contaminantsbecause human activities influence their levels in theenvironment.
Marine, estuarine, and waters of a similar natureare generally not suitable for direct determinations of tracemetals because of the large amounts of dissolved solids that theycontain.
Electrochemical techniques such as voltammetry offer aviable alternative approach for quantification of trace metals.The advances made in recent years in terms of voltammetricinstrumentation and method development ensure that often anunrivalled sensitivity can be obtained, usually for a fraction ofthe cost of spectroscopic techniques and without the need forseparation of the metals either using solvent extraction orco-precipitation.
Generally, concentrations of heavy metals arecomparatively low in open sea areas where human activities havenot yet had a major effect, however in coastal areas accumulationof trace metals are often evident.
These higher concentrationstend to be found in offshore sediments close to discharge pointsof municipal and industrial waste run-off.
Trace metals can alsoreach marine waters by air from industrial smoke discharges andtraffic exhausts.
The matrix effects of saline waters vary inseverity with regard to analysis by spectroscopic techniques.
Theeffects are minor for lead and copper but can impart substantialmatrix interference upon cadmium where it is often necessary toemploy a matrix modifier.
In the past fifteen years researchorganisation from the United Kingdom have conducted marinestudies in diverse environments such as the English Channel,North Sea, Adriatic Sea, Indian and Pacific Oceans to study theeffects that trace metals pose upon species resident in themarine environment.
Voltammetry - a brief overview.
Heyrovskyfirst introduced polarography in 1922.
The term voltammetry isapplied to designate the current-voltage measurement obtained ata given electrode.
Polarography is a special case of voltammetryreferring to the current-voltage measurement acquired using adropping mercury electrode with a constant flow of mercury drops.Stripping voltammetry uses the same instrumentation astraditional polarography but a stationary electrode such as thehanging mercury dropping electrode (HMDE) is used.
Thevoltammetric measurement is performed on a stationary mercurydrop allowing one to achieve considerable increases insensitivity.
The jump in sensitivity is possible byelectrochemical preconcentration of the metals in question at thesurface of the stationary electrode before the current-voltagecurve is recorded.
The recorded current is the redissolution(reoxidation) current of the preconcentrated metal traces.Voltammetry today represents a refined, clean, simple techniquethat offers outstanding limits of detection and is now thefastest growing analytical technique for trace analysis.
With theMetrohm Multi Mode Electrode (MME) the mercury is hermeticallysealed in the reservoir and suffices for around 200,000 drops,ensuring low laboratory running costs.
Impact of trace metals inmarine waters.
The trace elements found in marine waters can havea profound influence on life in the ocean, for example zinc canaffect the phytoplankton growth rate.
As well as the directeffects on marine life one should also be aware that any possiblecontamination of the marine environment could possibly make itsway into the human food chain and have a real impact on humanlife.
From studies conducted in the North Pacific, it has beenobserved that the trace metal concentrations of cadmium, lead andcopper show vertical concentration profiles (ie, they increasewith depth).
These are related to such parameters as temperature,salinity, dissolved oxygen and dissolved inorganic carbon.
Thisimplies that the metals are directly involved like macronutrientsin the biological cycle of algal uptake in the surface water andheterotrophic remineralisation at depths typical of oceanicecosystems.
From previous work, it has been revealed that thereis a close correlation between zinc and silicate that shows asthe amount of silicate increases so does the zinc.
This seeminglyimplies that zinc is taken up in the mineral parts ofphytoplankton such as calcium carbonate or silicate exoskeletonof coccolithophores and diatoms respectively.
Zinc is arecognised as an essential trace element for plankton growth andis a necessary cofactor for several important enzyme systemsrequired for photosynthesis.
The toxic elements lead and cadmiumpresent in the sea contaminate marine life and food, which canresult in a decrease in a particular species population or in theworst case possible extinction.
Cadmium is produced commerciallyas a by-product of zinc and lead mining and since the 1950s hasbeen used as a stabiliser and pigment in many plastics andsolders, as well as its use in electroplating and batteries.Metal production and refining as well as waste disposal are majorsources of cadmium emissions.
Natural emissions occur fromvolcanoes and windblown dust and together with man-made emissionscontribute somewhere in the order of 8000 tonnes per year to theworld's oceans.
Cadmium is thought to be probably the mostbio-toxic element and is regarded as a primary pollutant.
Inprevious studies conducted on estuarine environments, crustaceansappear to be particularly sensitive to the effects of cadmiumfollowed by molluscs and polychaetes.
In marine environments, theorganisms appear to be less sensitive to the toxicity of cadmiumthen estuarine and freshwater animals.
Studies on shrimp specieshave shown that species in regions with high cadmiumconcentrations are able to both handle and survive the high bodyloads of cadmium, although interestingly the female species werefound to have a higher tolerance than their male counterparts.Studies have shown there to be an interesting relationshipbetween cadmium and phosphate in deeper sections of the globaloceans.
In these waters, the cadmium concentration increaseslinearly with that of phosphate whereas in shallow shelf watersone sees a departure from this correlation.
Despite somefluctuations from the cadmium-phosphate concentration model, itis clear that cadmium is involved in organic matter formationthrough photosynthesis before remineralisation deeper in theocean.
It is not clearly understood at present why cadmium is soclosely involved as it has no known or specific biochemicalfunction.
One theory that has been proposed for its occurrence isbased on the close similarity of the ionic radius of the divalentcadmium and calcium, that hypothesises that cadmium substitutesinto the crystal lattice of calcium carbonate minerals that areformed by marine species.
Cadmium tends to be absorbed throughthe body via food and water intake and can adversely injure therenal, pulmonary, skeletal, testicular and nervous systems inaddition to it being a recognised carcinogen.
Renal failure tendsto be the earliest cause of death due to the sensitive nature ofthe kidneys where the cadmium becomes concentrated.
There has notbeen as much research work carried out on lead as for other tracemetals upon the effects that contamination presents to a marineenvironment.
It is known that lead is a cumulative poison and thevast majority retained in the body enters the bones from where itcan be remobilised.
Lead can bind to a number of molecules withinthe body such as amino acids, haemoglobin, enzymes, RNA and DNAand can disrupt many of the metabolic pathways.
Some of theeffects of lead toxicity include impaired blood synthesis,hypertension, hyperactivity, brain damage and cancer.
Sucheffects can occur at levels equivalent to 20-200mg l-1 in softtissue, this is the order of lead normally found in human adultsso one should view any factor that increases lead intake with adegree of concern.
Copper is an essential trace element for bothhumans and animals being a critical functional component of anumber of essential enzymes, it is found in the environment as anaturally occurring mineral.
Copper is part of the oxygen carrierin snails, crabs and some crustaceans.
Excessively high levels ofcopper found in the marine environment are likely to be as aresult of emissions and discharges caused by man-made activities.Toxicity from copper contamination is extremely rare in thegeneral population although acute copper toxicity can cause liverdamage, kidney failure and ultimately death.
Method for analysisof zinc, cadmium, lead and copper in marine waters.
Metrohm hasdeveloped a method whereby it is possible to determine the tracemetals zinc, cadmium, lead and copper in a single voltammetricsweep.
8ml of deionised water, 2ml of marine water sample, 1mlacetate buffer and 200ml of molar nitric acid were added to thereaction vessel in the Metrohm 757 VA Computrace.
The role of theelectrolyte and additional solutions in voltammetry is crucial.Many determinations are pH dependent and the electrolyte canincrease the conductivity and selectivity of the solution.
Thesolution was then degassed with nitrogen for a period of fiveminutes to remove the electrochemically active oxygen, before thefour metals were determined with two standard additions using theHMDE.
The HMDE is an electrode mode of the MME.
Four mercurydrops of a defined size are formed in succession at the MME andthe last drop remains suspended on the end of the capillary.
Theentire voltage sweep is then performed on the single stationarydrop.
As a final point in terms of sample methodology, it shouldbe noted that samples of marine waters often contain largeamounts of organic material that may require additional sampletreatment.
The Metrohm 705 UV Digester is eminently suited tothis task and can eliminate moderate to high (after dilution)amounts of dissolved organic matter that can disturb thevoltammetric analysis of trace metals.
The digestion of organicmatter is achieved through photolytic generation of OH radicalsthat in turn react with the organic compounds and decompose them.Hydrogen peroxide serves as an initiator of the radical reactionand a mercury lamp provides radiant energy that is converted toheat accelerating the digestion process.
The advantage of UVphotolysis over other sample digestion techniques is that only alittle hydrogen peroxide has to be added ensuring that blankvalues can be kept low.
This is crucially important for analysisof marine waters as the low content of trace metals means thatthe samples are very sensitive to contamination.
Conclusion.Toxic concentrations of trace metals interfere with the normalmetabolic processes of fish and other species of life in themarine environment, and these metals can then be passed ontohumans who consume them and pose adverse effects on the humanmetabolic processes.
Voltammetry is an increasingly populartechnique that in many instances offers unrivalled detectionlimits even when compared to vastly more expensive analyticaltechniques.
Often voltammetry requires little or no samplepreparation and the result, of up to four metals in a singlevoltammetric sweep, determined by standard addition obtained inless than ten minutes.
The advantage of using standard additionas a means of calibration and quantification is that matrixeffects present in the sample are taken into account.
Theportability of the Metrohm voltammetric equipment means that itcan easily be installed on, for example, a research vessel thatmay be used to collect samples away from the shoreline.Voltammetry requires no specialist laboratory infrastructure likeexpensive fume extraction, all that is required is a sturdy benchtop on which to mount the instrument and a regulated flow of aninert gas.
The running and maintenance costs of voltammetry areminimal ensuring a cost effective analytical solution to surpassthe demands required by those organisations interested inquantifying trace metals in marine waters.
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