LITERATURE REVIEW

2.1 INTRODUCTION

This chapter gives an insight into various studies conducted by outstanding researchers, as well as explained terminologies with regards to assessing the contamination level by heavy metals in five different brands of baby foods. The chapter also gives a resume of the history and present status of the problem delineated by a concise review of previous studies into closely related problems.

2.2 CONCEPTUAL FRAMEWORK

Heavy Metals

A metal of relatively high density (specific gravity greater than about 5) or of high relative atomic weight is defined as a heavy metal. The term "Heavy metals" is used to describe more than a dpzen elements that are metals or metalloids (Vogel's; 1989); e.g. chromium, arsenic, cadmium, lead, mercury, manganese, etc. Heavy metals are natural constituents of the Earth's crust. Because they cannot be degraded or destroyed, heavy metals are persistent in all parts of the environment. In small amounts, they enter the human body via food, drinking water and air. Living organisms require varying amounts of "heavy metals". Iron, cobalt, copper, manganese, molybdenum, and zinc are required by humans. Excessive levels can be damaging to the organisms. Therefore, heavy metals can be described as any metallic element that has a relatively high density and is toxic or poisonous at low concentrations.

Human activities affect the natural geological and biological distribution of heavy metals throügh pollution of air, water, and soil. Humans are also responsible for altering the chemical forms of heavy metals released to the environment. Such alterations often affect a heavy metalfs toxicity by vallowing it to bioaccumulate in plants and animals, bioconcentrate in the food chain, or attack specific organs of the body. Bioaccumulation refers to an increase in the concentration of a metal in a biological organism over time, compared to the normal concentration in {he environment. Many metals and other chemicals accumulate in living things aiiy time they are taken up and stored faster than they are broken down' (metabolized) or excreted.

Some heavy metals such as mercury and lead are toxic metals that have no known vital or beneficial effect on organisms, and their accumulation over time in the bodies of animals can cause serious illness. Certain elements that are normally toxic are, for certain organisms or under certain conditions, may be beneficial. Examples include vanadium, tungsten, and even cadmium. Heavy metals are stable and persistent environmental contaminants since they cannot be degraded or destroyed* Therefore, they tend to accumulate in the soil, seawater, freshwater, and sediments,

In small quantities, certain heavy metals are nutritionally essential for a healthy life (e.g., iron, copper, manganese, and zinc). Some of these are referred to as the trace elements. These elements, or some form of them, are commonly found naturally in foodstuffs, in fruits and vegetables, and in commercially available multivitamin products.

The Occurrence of Heavy Metals in Nature

Metals in the environment may be present in the solid, liquid or gaseous state. They may be present as individual elements, and as organic and inorganic compounds. The movement of metals between environmental reservoirs may or may not inVolVe changes of state.

The geosphere is the original source of all metals (except those that enter the atmosphere from space in the form of meteorites and cosmic dust). Within the geosphere, metals may be present in minerals, glasses, and melts. In the hydrosphere, metals occur as dissolved ions and complexes, colloids, and suspended sOlids.

In the atmosphere, metals may be present as gaseous elements and •compounds and as particulates and aerosols (Nriagu; 1989). Gaseous and particulate metals may be inhaled and solid and liquid (aqueous-phase) metals may be ingested or absorbed, thereby entering the biosphere. In addition to being the original source of all terrestrial metals, the géosphere may represent a sink for metals. The aånosphere and hydrosphere also constitute sinks for metals; however, from a geological perspective, they are more likely to be considered as agents of transport.

The movement of metals from one site to another depends on the linear and temporal scales of observation. For example, the oceans are a vast reservoir for a variety of chemical elements. They also serve as a conduit for elements derived from weathering of rocks to return to the geosphere through sedimentation. A reservoir may act as a catalyst for changes of state of metals and metal compounds, without actually having incorporated those metals, as in the case of some biologically mediated reactions (Adrienne et.al.2008).

Behaviour of heavy metals in the Environment

The main source for metal input to plants and soils is atrnospheric deposition. Volatile metalloids such as As, Hg, Se, and Sb can be transported over long distances in gaseous forms or enriched in particles, while trace metals such as Cu, Pb, and Zn are transported in particulate phases (Adriano, 2001 ; Adriano et al. 2005). In terrestrial ecosystems, soils are the major recipient of metal contaminants, while in aquatic systems sediments are the major sink for metals (Sparks, 2005). Freshwater syStems are contaminated due to runoff and drainage via sediments or disposal, while groundwater is impacted thiough leaching or transport via mobile colloids (Adrian02001).

A number of biogeochemical processes take place at the heterogeneous interfaee between the rock, soil, water, air and living organisms (Spark,' 2005). These processes or interactions in turn confrol the solubility, mobility, bioavailability and toxicity of metals (Sparks, 2005). Metals are found in soil solution as free ions or complexed to inorganic or organic ligåfids: Both the free ions and the metal—ligand complexes can be (i) taken up by plånts, (ii) retained on mineral surfaces, natural organic matter, and microbes, (iii) transported through the soil profile into groundwater via leaching or by colloid-facilitated transport, (iv) precipitated as solid phases, and (v) diffused in porous media such as soils.

Microorganisms can fransform metals such as Hg, Se, Sn, As and Cr by means of oxidation-reduction and methylation (the process of replacing an atom, usually a H atom, with a methyl group) mechanisms and dimethylation reactions. These processes affect transport or mobility and solubility or toxicity of metals (Adriano 2001; Sparks 2005). For example, methylated (organic) forms of Hg are more toxic than inorganic forms of the element and they bioaccumulate in organisms. Methylation is favoured in environments characterized by low oxygen levels, low pH, and high soil organic matter (SOM) contents.

Heavy metal contamination of soil is a far more serious problem than air or water pollution because heavy metals are usually tightly bound by the organic components in the surfaee layers of the soil. Consequently, the soil is an important geochemical sink which accumulates heavy metals quickly and usually depletes them very slowly by leaching into groundwater aquifers or bioaccumulating into plants (Infotox, 2000).

Heavy metals can also be very quickly translocated through the environment by erosion of the soil particles to which they are adsorbed or bound and re-deposited elsewhere. The transport, cycling, fate, bioavailability and toxicity of heavy metals are markedly influenced by their physico-chemical forms in water, sediments and soil. Whenever a heavy metal or its compound is introduced into an aquatic environment, it is subjected to a wide variety of physical, chemical and biological processes. These include hydrolysis, chelation, complekation, redox, biomethYlation, precipitation and adsorption reactions. Often, heavy metals experience a change in the chemical form as a result of these processes and so their distribution, bioavailability and other interactions in the environment are also affected. They can leach into living systems from natural ore deposits and other sources such as waste disposal of heavy metal containing waste. In fact, waste disposal accounts for higher percentage of most heavy metals including manganese in the environment.

Uses of Heavy Metals

Heavy metals are important components of building materials, vehicles, appliances, tools and computers; and are essential in the infrasfructure including highwayé, bridges, railroads, airports,' electrical utilities and food production and distribution (Sparks, 2005). Civilization was founded upon the metals of antiquity, gold, copper, silver, lead, tin, iron and mercury (Sparks, 2005). Natural and consumer products contain small concentrations of different heavy metals. For example, cadmium is mainly used in batteries, plastics and it is also found in cigarette smoke, in shellfish and vegetables (ATSDR, 1999). Mercury is found in batteries, dental amalgam, vacuum pumps and valves. Airborne mercury comes from the combustion of diesel, jet fuel and heating oil. Arsenic is high in seafood and may also be found as a contaminant in animal feeds. It is also present in wood preservatives, herbicides, corrosion inhibitors, in lead and copper alloys.

Cadmium is used industially as an anti-friction agent, as a rust-proofer, in plastics manufacture, in alloys and in alkaline storage batteries. Chromium is found in fresh foods, copy machine toner and nickel in coins, kitchen utensils and milk (ATSDR, 1997). Copper is essential to all living organisms and has a wide range of effects depending on concentration and chemical formulation. It is used in the electrical industry in alloys such as brass, in chemical catalysts and in wood-preservatives. Lead has been used in batteries, electronic equipments, in petrol, toys, paint, etc. Lead has been used as fuel additive in many countries for several years, although this practice has since stopped in most of the countries of the world, because of the health implications of lead.

Manganese compounds are used in manufacturing of products such as batteries, steel and unleaded petrol. Manganese dioxide is commonly used in the production of dry-cell batteries, matches, fireworks, porcelain and glass-bonding materials. It is also used as the starting material for the production of other manganese compounds (US EPA, 1984). Manganese chloride is a precursor of other manganese compounds, It is used as a catalyst in the chlorination of organic compounds, in animal feed to supply essential frace minerals and in dry-cell batteries (US EPA, 1984). Manganese sulfate is used as a fertilizer, livestock nutritional Supplement and in ceramics (US EPA, 1984 As potassium permanganate, it is used as ah oxidizing agent and disinfectant in water purification and in waste-treatment plants. It is used in metal cleaning, bleaching and as a preservative for fresh flowers and fruits.

Currently, methylcyclo-pentadienyl manganese tricarbonyl (MMT) is the most popular lead replacement compound due to its lower production costs (Chevron. 2002; Graboski, 2003). The manganese based additive is added in smaller amounts than TEL (tetraethyllead), with a maximum concentration of 18 mg/L (Chevron. 2002; DEAT & DME, 2003; Goosen, 2003).

The anthropogenic sources of Heavy Metals

Much research has been conducted on heavy metal contamination in soil from various anthropogenic sources such as industrial wastes (Haines and Pocock, 1980; Parry et al., 1981; Culbard et al., 1983; Gibson and Farmer, 1983), automobile emissions (Lagerwerff and Specht, 1970; Fergusson et al., 1980; Garcia-Miragaya, 1984), mining activity (Davies and Ginnever, 1979; Culbard and Johnson, 1984), and agricultural practices (Colboum and Thomton, 1978).

"Ihe anthropogenic heavy metals are believed to easily accumulate in the top soil, causing potential problems such as toxicity to plants and animals, accumulation in food chain, perturbation of the ecosystem and adverse health effects.

Human Exposure and Health Hazards associated with Heavy Metals

HumariS are' always exposed to the natural levels of trace elements. Under normal circumstances; the body is able to control some of these amounts. However, continuous exposure to elevated levels of metals could cause serious illness or death (Okonkwo, 2005; Rollin et al., 2005; Moja, 2007). Increased exposure may occur through inhalation of airborne particles or through ingestion of contaminated soil by children or by absorption through the skin (WHO, 1981).

Metals and their compounds can accumulate in the bodyes tissues, such as bones or nerves. "Ihey can cross the placenta and harm an unborn child in pregnant women. Children are the most susceptible to health problems caused by heavy metals, because their bodies are smaller and still developing (Fergusson, 1991; Thomson, 2007). The health hazards presented by heavy metals depend on the level and the length of exposure (Thomson, 2005). In some cases, the health effects are immediately apparent; in others, the effects are delayed.

High levels of toxic metals deposited in body tissues and subsequently in the brain, may cause significant developmental and neurological damage, including depression, increased irritability, anxiety, insomnia, hallucination, memory loss, aggression and many other disorders.

Aluminium

Rare in its free form, Aluminium is one of the abundant elements in Earth's crust. It does not appear to have any beneficial function for living cells; rather it has some toxic effect at higher level. If consumed in excessive amounts, it may have toxic effects. However, the use of aluminum cookware, popular because of its corrosion resistance and good heat conduction, has not been observed to have toxicity (Sparks et al., 1999). Excessive consumption of antacids containing aluminum compounds and excessive use of aluminumcontaining antiperspirants are more likely causes of toxicity. Aluminum has been linked to Alzheimer's disease by some workers. Increase in aluminum content in human plasma from

0.005 mg/L to 0.25 mg/L leads to osteodystrophy (adverse neurological conditions affecting speech and memory, eventually leading to dementia).

Arsenic

Arsenic (As) is a ubiquitous element found in the atmosphere, soils and rocks, natural waters and organisms. It is mobilized in the environment through a combination of natural processes such as weathering reactions, biological activity and volcanic emissions as well as through a range of anthropogenic activities. Background concentrations of arsenic in soil range from 1 to 40 mg/kg, with a mean value of 5 mg/kg (Beyer and Cromartie, 1987; Bowen, 1979). Soils overlying naturally arsenic-rich geological deposits, such as sulphide ores, may have significantly higher concentrations, in some cases up to two orders of magnitude higher (NAS, 1977). Most environmental arsenic problems are the result of mobilization under natural conditions as well as activities associated with mining activity, combustion of fossil fuels, use of arsenical pesticides(US EPA, 1982; Chatterjee & Mukherjee, 1999), herbicides and crop desiccants and use of arsenic as an additive to livestock feed, particularly for poultry(Ng and Moore ; 1996). Although the use of arsenical products such as pesticides and herbicides has decreased significantly in the last few decades, their use for wood preservation is still common.

Arsenic has been classified as a human carcinogen, with chronic ingestion associated with skin cancer while inhalation is associated with lung cancer (Taylor et al., 1989). In several countries, the exposure of population to arsenic derived from mining and smelting activities has caused health problems. More recently, studies have shown that the burning of arseiiic-rich Coal in homes in the Guizhou Province of China has resulted in serious health effeOts ( Smedley and Kinniburgh, 2002).

Drinking water containing elevated levels of As poses the real problem (Das et al., 1996; Pandey et al., 2002; Chandra Sekhar et al., 2003a; Wang andWai, 2004). Ingestion of arsenic-rich water leads to skin lesions in exposed populations which are also subject to increased incidence of cancers of the skin (NAS, 1999). It has also been suggested that cancers of internal organs may result from this exposure (Tseng, 1977; Mandal et al., 1998). In addition, it has been suggested that exposure to arsenic-rich water increases the risk of vascular disease, hypertension and diabetes mellitus.

Cadmium

Cadmium is a toxic trace element which may accumulate in soils from various human activities (Sauvea et al., 2000). Natural source of cadmium is weathering of rocks while some cadmium enters air through forest fires and volcanoes. No cadmium ore is mined for the metal, because more than enough is produced as a byproduct of the smelting of zinc from its ore, sphalerite (ZnS), in which CdS is a significant impurity, making up as much as 3%. It is similar in many respects to zinc but it forms more complex compounds. About three-fourths of cadmium is used in Ni-Cd batteries, most of the remaining one-fourth is used mainly for pigments, coatings and plating, and as stabilizers for plastics (Low and Lee, 1991). Cadmium has been used particularly to electroplate steel where a film of cadmium only 0.05 mm thick will provide complete protection against the sea. Cadmium has the ability to absorb neutrons, so it is used as a barrier to contol nuclear fission. Metal plating and tire rubber are considered the likely sources of Cd in urban soil and street dust (Hewitt and Rashed, 1988).

Cadmium (Cd) and its compounds are extremely toxic at all levels and tend to bioaccumulate in organisms and ecosystems (ATSDR, 1999).Cadmium derives its toxicological properties from its chemical similarity to zinc (Xian, X. 1989b). Cadmium is biopersistent and, once absorbed by an organism, remains resident for many years (over decades for humans) although it is eventually excreted. In humans, long-term exposure is associated with renal dysfunction. High exposure can lead to obstructive lung disease and has been linked to lung cancer, although data concerning the latter are difficult to interpret due to compounding factors. Cadmium may also produce bone defects (osteomalacia, osteoporosis) in humans and animals.

Cadmium is strongly adsorbed to organic matter in soils. When cadmium is present in soils it can be extremely dangerous, as the uptake through food will increase (Xian, X. 1989a). Soils that are acidified enhance the cadmium uptake by plants. Human uptake of cadmium takes place mainly through food. Foodstuffs that are rich in cadmium (liver, mushrooms, shellfish, mussels, cocoa powder and dried seaweed) can greatly increase the cadmium concentration in human body. Tobacco smoke transports cadmium into the lungs. Blood will transport it to the rest of the body. Other high exposures can occur with people who live near hazardous waste sites or factories that release cadmium into the air and people that work in the metal refining indusfry. When people breathe in cadmium, it can severely damage the lungs. This may even cause death.

Cadmium transported to the liver binds with proteins to form complexes that are transported to the kidneys where it is likely to damage the filtering mechanism. This causes the excretion of essential proteins and sugars from the body damaging kidney further. The highest concentration of cadmium is found in liver and kidney through its strong binding with cystine residue of metallothionin, with somewhat lower concentration in pancreas and spleen. Laws (1981) has stated that ingestion of small amount of cadmium over a period of a few years may lead to the accumulation of chronically or even acutely toxic level of cadmium in body. It takes a very long time before cadmium that has accumulated in kidneys is excreted from human body. Other health effects that can be caused by cadmium are: diarrhea, stomach pains and severe vomiting, bone fracture, reproductive failure and possibly even infertility, damage to the central nervous system, damage to the immune system, psychological disorders, and - possibly DNA damage or cancer development.

Cadmium poisoning causes softening of the bones and kidney failures and was responsible for the "itai-itai" disease (a name derived from the painful screams in Japanese language) due to the severe pain in the joints and the spine (ATSDR, 1999). Cadmium was released into the rivers by mining companies in the mountains of Japan in the late 1940's. The disease arose from increased uptake of cadmium in locally consumed rice grown in paddy fields irrigated with cadmium-contaminated river water.

Chromium

Chromium is used in metal alloys and pigments for paints, cement, paper, rubber, and other materials. Low-level exposure to Cr can irritate the skin and cause ulceration. Longterm exposure can cause kidney and liver damage, and also circulatory and nerve tissues (ATSDR, 2000). Chromium often accumulates in aquatic life, adding to the danger of eating fish that may have been exposed to high levels of chromium.

The level of chromium in air and water is generally lowe In drinking water the level of chromium is usually low as well, but contaminated well water may contain the dangerous chromium(IV) or hexavalent chromium. For most people eating food that contains chromium(lll) is the main route of chromium uptake, as chromium(lll) occurs naturally in many vegetables, fruits, meats, yeasts and grains. Various ways of food preparation and storage may alter the chromium contents of food. When food is stored in steel tanks or cans chromium concentrations may rise.

Chromium(lll) is an essential nutrient for humans and shortages may cause heart conditions, disruptions of metabolisms and diabetes. But the uptake of too much ChromiUm(III) can cause health effects as well, for instance skin rashes. Chromium(VI) is very toxic and is the real danger to human health, mainly for people who work in the steel and textile industry. People who smoke tobacco also have a higher chance of exposure to chromium (Katz and Salem, 1994).

Chromium(VI) in leather products can cause allergic reactions, such as skin rash. After breathing chromium(VI) I air, nose irritations and nose bleeds are quite common. Other health problems that are caused by chromium(VI) are: skin rashes, upset stomachs and ulcers, respiratory problems, weakened immune systems, kidney and liver damage, alteration of genetic material and lung cancer.

Cobalt

Cobalt is of relatively low abundance in the Earth's crust and in natural waters, from which it is precipitated as the highly insoluble cobalt sulfide, COS. Cobalt is used in many alloys (super alloys for parts in gas turbine aircraft engines, corrosion resistant alloys, highspeed steels, cemented carbides), in maglets and magnetic recording media, as catalysts for the petroleum and chemical industries, as drying agents for paints and inks.

Human exposure to Co takes place through air, drinking water and food (ATSDR, 2001). Cobalt is not freely available in the environment, but when cobalt particles are not bound to soil or sediment particles the uptake by plants and animals is higher and accumulation in plants and animals may occur.

Cobalt is contained in vitamin BE, essential for human health. Cobalt is used to treat anemia in pregnant women, because it stimulates the production of red blood cells. The total daily intake of cobalt may be as high as I mg, but almost all will pass through the body unadsorbed, except that in vitamine BIX

Health effects from Co may also arise due to exposure to radiations from radioactive cobalt isotopes. This can cause sterility, hair loss, vomiting, bleeding, diarrhea, coma and even death. When this radiation is used in cancer-patients to destroy tumors, the patients suffer from hair loss, diarrhea and vomiting,

Copper

Presence of Cu in water imparts taste and colour (Leeper, 1978). Cu content of soil ranges from 10 — 80 ppm, depending upon the nature of the parent material and the pedologic processes. It exists in soils mostly as cupric (Cu2+) and less frequently as cuprous (Cu+) ions. Cu is a component of many plant enzymes (oxidase for example) and is involved in many electron fransfer processes. Plants absorb copper in the form of cupric ions. The most common soil mineral with copper is chalcopyrite (CuFeS2) with Cu in the Cu+ form. Cu exists in both exchangeable and less exchangeable forms, and also in the form of soluble organic complexes or chelates. Copper is strongly adsorbed to many solids (Clays, alumina, iron hydrous oxides, and manganese oxides). Copper, generally forms stronger bonds with organic ligands than most other metal ions.

Copper is an essential element for human life, but excessive intake results in its accumulation in the liver and produces gastrointestinal problems (Nuhoglu and Oguz; 2003), anemia, liver and kidney damage. Continued inhalation of copper-containing sprays is linked with an increase in lung cancer (Yu et.al.2000). People with Wilson's disease are at greater risk for health effects from overexposure to copper (ATSDR, 2002).

Copper can be released into the environment by both natural sources (e.g. wind-blown dust, decaying vegetation, forest fires and sea spray) and human activities (mining, metal production, wood production and phosphate fertilizer production). Because copper is released both naturally and through human activities, it is very widespread in the environment. Copper is often found nearmines, industrial settings, landfills and waste disposals.

Long-term exposure to copper can cause irritation of the nose, mouth and eyes and headaches, stomachaches, dizziness, vomiting and diarrhea, Intentionally high uptakes of copper may cause liver and kidney damage and even death. Industrial exposure to copper fumes, dusts, or mists may result in metal fume fever with atrophic changes in nasal mucous membranes. Chronic copper poisoning results in Wilson's disease, characterized by hepatic cirrhosis, brain damage, demyelination, renal disease, and copper deposition in the cornea.

Iron

Most of the rocks contain iron as one of the common elements and it is also an important component of many soils especially clay soil where it is usually a major constituent. Iron occurs as particulate ferric hydroxide or in the form of organometallic compound in natural system. Fe (Ill) oxide and Fe (Il) are ubiquitous in anoxic environments and they affect the distribution, transport, and biogeochemistry of chemical contaminants by sorption onto Fe(lll) oxides and by control of oxidation and reduction reactions (Fendorf and Li; 1996).

Iron is regarded as one of the essential elements for humans. Approximately 3000 to 5000 mg of iron exists in the human body (Landis and Yu, 1995). Therefore, as long as the quantity of iron in the environment is not too large, it may not be harmful to the human body.

However, iron can cause undesirable problems in industrial processes or ecosystems if its concentration in water is not managed properly. For example, the precipitates of iron hydroxide may block pipes create turbidity problems, and iron deposits may act as a source material for an unpleasant taste or odor in water. Thus, excessive iron in water limits the usage of water for drinking or indusfrial processes. WHO has recommended a value of 0.3 mg/L as permissible limit for drinking water. For fresh water aquatic life, the limit is 1 mg/L.

Chronic excessive intake of iron may lead to hemosioderosis or hemochromatosiss

Iron constitutes about 4.7 % of the earth's crust. It is the second most important metallic element in the terrestrial environment. Iron is extremely useful, but can also be highly toxic to cellular constituents when present in excess. Iron is an important part of the plant's oxidation- reduction reactions. As much as 75 % of the cell iron is associated with chloroplast. Iron is a stuctural component of cytochromes, hemes, and numerous other elecfron-transfer systems, including nitrogenase enzymes necessary for the fixation of dinitrogen gas. The major problem with iron availability is how to keep iron sufficiently soluble for plants to absorb enough of it. In strongly acidic solutions (pH < 5), iron becomes increasingly soluble, and is rarely deficient. It is essential for the physiological processes of all living organisms.

Manganese

Manganese is considered an essential frace element for plants and animals. Mn concentration amounts to approximately 0.1 % in the earth's crust. Dust and smoke containing manganese oxides result from mining, crushing and smelting of ores. Mn (from dust deposits and rainfall) usually accumulates in the upper layers of the soiL

Manganese is involved in many enzyme systems and in electron transport. In solution, it occurs as the Mn2+ ion. Under oxidizing conditions, most of the manganese precipitates as

insoluble Mn02. Mn in soil exists in three valence states, Mn2+, Mn3+ and Mn4+. The Mn2+ is considered to be the easily accessible form to the plant, while Mn3+ is accessible depending upon the soil conditions. The tetravalent manganese (Mn4+) is considered as inaccessible to the plant. These three forms exist in a state of dynamic equilibrium.

Organic matter decomposition aids manganese solubility. Toxic concentration of manganese is more likely than that for Zn, Fe or Cu. Toxic levels occur only in strongly acidic soils.

Prolonged inhalation of high levels of manganese negatively affects the central nervous system, visual reaction time, hand steadiness and eye-hand coordination (EPA, 2003).

Mercury

Mercury poisoning symptoms include blindness, deafress, brain damage, digestive problems, kidney damage, lack of coordination and mental retardation. One of the most famous cases of mercury poisoning resulting from chronic exposure was the disaster that occurred in Minamata, Japan, where methylmercury was discharged from a plastics manufacturing plant into the waters of Minamata Bay in the 50s and 60s. Fish in the harbor were contaminated and those who ate the contaminated fish were gradually poisoned (Harada, 1996).

Nickel

Nickel is one of the ubiquitous elements and ranks 23 in order of abundance. Its average concentration in the earth's crust is 75 mg/kg (Levinson, 1974). Nickel is relatively toxic and widespread in the environment (Forstner and Wittmann, 1979). Nickel enters the environment through two main pathways: natural- such as weathering of minerals and rocks, and geochemical emission, and anthropogenic such as industrial and vehicular emissions. The total world emission of nickel to the aånosphere has been estimated at tones/annum with the natural and anthropogenic sources contributing about 150000 and 180000 tones per year respectively.

Nickel particles in the air settle to the ground or are taken out of the air in rain. Much of the nickel in the environment is found in soil and sediments because nickel attaches to particles that contain iron or manganese, which are often present in soil and sediments. It is considered as a borderline element between hard and soft acid acceptors in chemical interactions towards donor atoms. This is reflected in its abundance in the earth's crust as oxides, carbonates, silicates with iron, magnesium and as sulphides, arsenides and teluridese Nickel salts are soluble and can occur as a leachate from nickel bearing rocks.

The affects of nickel exposure vary from skin irritation to damage to lungs, the nervous system, and mucous membranes (Nicholas et al. 2003). It is also a known carcinogen (ATSDR, 2003).

Lead

Metallic lead does not dissolve in water and does not burn, however, lead can combine with other chemicals to form lead compounds or lead salts. Some lead salts dissolve in water better than others. Although lead itself cannot be broken down, lead compounds in water may combine with different chemicals depending on the acidity and temperature of the water,

Most of the lead used by industry comes from mined ores (primary) or from recycled scrap metal or batteries (secondary). The main sources of lead pollution are lead smelters, battery manufacturers, paper and pulp industries, boat and ship fuels and ammunition industries. In addition, the production of television picture tubes, pigments, petroleum fuels, printing, glass industries, photoyaphic materials, etc., also adds Pb(ll) to the environment (Kiff, 1987). People living near hazardous waste sites may be exposed to lead by breathing air, drinking water, eating foods, or swallowing or touching dust or dirt that contains lead (ATSDR, 1999a). For others (people who do not live near hazardous waste sites), exposure to lead may occur by eating foods or drinking water that contain lead, by spending time in areas where leaded paints have been used, by working in jobs where lead is used, by having hobbies in which lead may be used such as sculpturing (lead solder) and staining glass. Cigarette smoke also contains small amounts of lead. Lead may enter foods if they are put into improperly glazed pottery or ceramic dishes and from leaded-crystal glassware.

The effects of lead exposure are the same whether it is breathed or swallowed. Low levels of lead have been identified with anaemia as it causes injury to the blood forming systems while high levels cause severe dysfunction of the kidneys, liver, the central and peripheral nervous system (Jain et al., 1989), and high blood pressure (ATSDR, 1999a). Hypertension has also been associated with lead exposure in the general population. At the typical levels to which individuals are exposed, lead can cross the placenta and damage developing fatal nervous systems. High level exposure to lead may cause miscarriage in pregnant woman and can also damage the organs responsible for sperm production in male. The most severe neurological effect of lead in adults is lead encephalopathy, which is a general term to describe various diseases that affect brain function. Lead exposure may cause weakness in fingers, wrists, or ankles.

Children are more sensitive to the effects of lead than adults(Linton et al., 1980). A child who swallows large amounts of lead may develop blood anemia, kidney damage, severe stomachache, muscle weakness, and brain damage (Thornton et al., 1989). The lower IQ levels and other neuropsychological deficiencies among the children exposed to higher lead levels have been well documented (Kotok et al., 1977). Lead acetate and lead phosphate have been shown to be potential carcinogens based on studies in animals. However, there is inadequate evidence to clearly determine lead's carcinogenicity in humans,

Lead poisoning damages the nervous system, kidneys, liver and cause sterility, growth inhibition, developmental retardation (Von Schirnding & Fuggle, 1996). Lead is toxic at all levels, hence lead based petrol, toys and paints have been banned (DEAT & DME, 2003).

Tin

Tin (Latin: Stannum) the 49th most abundant element and is obtained chiefly from the mineral cassiterite, where it occurs as tin dioxide, Sn02. Stannum (tin) as such, is known to be essential to the normal growth of plants. Tin is available in small amounts from virtually all fruits and vegetables. This silvery, malleable metal is not easily oxidized in air and is used to coat other metals to prevent corrosion.

Tin does not occur naturally by itself, and must be extracted from a base compound, usually cassiterite (Sn02). Minerals with tin are almost always in association with granite rock, which, when they contain the mineral, have a 1% tin oxide cåntent. Tin itself is not toxic but most tin salts are toxic.

Organotin compounds have the widest range of uses of all main-group organometallic compounds, with an annual worldwide industrial production of probably exceeding 50,000 tomes. Organotin compounds have relatively high toxicity, and for this they have been used for their biocidal effects Was fungicides, pesticides, algacides, wood preservatives, ånd antifouling agents: Tributyltin oxide is used as a wood preservative. Tributyltin was used as additive for ship paint to prevent growth of marine organisms on ships. The use declined after organotin compounds were recognized as persistent organic pollutants with an extremely high toxicity for some marine organisms. The uptake of organotin compounds can cause acute effects as well as long-tenn effects. Acute effects are eye and skin irritations, headaches, stomachaches, sickness and dizziness, severe sweating, breathlessness and urination problems. Long-term effects are depressions, liver damage, malfunctioning of immune systems, chromosomal damage, and shortage of red blood cells, brain damage (causing anger, sleeping disorders, forgetfulness and headaches).

Vanadium

Vanadium is present in many foods like mushrooms, shellfish, black pepper, parsley, dill weed, beer, wine, grain and grain products, buckwheat, soya beans, olive oil, sunflower oil, apples and eggs and artificially sweetened drinks as a trace element and may be essential, in small amounts, in the body. It is believed to be involved in normal bone growth. The main sources of vanadium are the minerals patronite (VS4), vanadinite and carnotite Vanadium is usually produced as a by-product of refining

other ores and from Venezuelan oils. Annual world wide production is around 7,000 tons. However, anthropogenic releases to water and sediments are far smaller than natural sources (Van Zinderen Bakker and Jaworski 1980).

Vanadium is considered as the marker element of air pollution emitted from the combustion of fossil fuels, particularly residual fuel oils, which constitute the single largest overall release of vanadium to the atmosphere (Wang et al., 1999).Vanadium compounds are not regarded as serious hazards, however, workers exposed to vanadium peroxide dust were found to suffer severe eye, nose and throat irritation (Gulson et al., 1995; De Miguel et al., 1999). It can have a number of effects on human health, when the uptake is very high.

Vanadium in air can cause bronchitis and pneumonia.

The acute effects of vanadium are irritation of lungs, throat, eyes and nasal cavities. Other health effects of vanadium uptake are cardiac and vascular disease, inflammation of stomach and intestines, damage to the nervous system, bleeding of livers and kidneys, skin rashes, severe trembling and paralyses, nose bleeds and throat pains, sickness and headaches, dizziness and behavioural changes.

The health hazards associated with exposure to vanadium are dependent on its oxidation state. Vanadium oxide (V205) is used as a catalyst in manufacturing sulfuric acid and maleic anhydride and in making ceramics. The pentoxide form is more toxic than the elemental form. Chronic exposure to vanadium pentoxide dust and fumes may cause severe irritation of the eyes, skin, upper respiratory tract, persistent inflammations of the trachea and bronchi, pulmonary edema, and systemic poisoning. Signs and symptoms of overexposure include conjunctivitis, nasopharyngitis, cough, labored breathing, rapid heart beat, lung changes, chronic bronchitis, skin pallor, greenish-black tongue and an allergic skin rash.

Zinc

Zinc is an essential trace element for humans. Its deficiency as well as excess is harmful. Recent research has shown that zinc is extremely important especially in fetal development and the nufrition of infants. The adult human body contains about 2.3 g of zinc which occurs mostly in over 100 enzymes. The normal daily requirement for zinc is 15 mg for adult and 5 mg for children. Zinc plays a role in carbohydrate, lipid and protein metabolism and in the synthesis and breakdown of DNA. Because of these functions, zinc deficiency in the fetus will result in retarded growth; malformation of body, and chromosomal abnormalities, A zinc deficiency after birth may result in dwarfism, poor appetite, mental lethargy, etc.

Excess amount of zinc on the other hand can cause stomach cramps, nausea, vomiting, central nervous system disorder. Nriagu (1980) stated that zinc is toxic also for aquatic biota.

Most of the rocks in the earth's crust contain zinc in varying concentrations. Depending on the type of the rock, highest concentration is found in basic eruptive rocks (e.g. basalt, 70-130 mg/kg). An average concentration of 50-60 mg/kg is generally found in acid eruptive (granite; rhyolite) rocks. Zinc is found in relatively high concentration in soils (50 mg/kg on an average) (Aubert and Pinta, 1980). Zn Content in soil is much higher in the vicinity of ore deposits and smelters. Atmospheric deposition increases Zn concentration in surface soil in Zn2+ form and in complexes. The concentration is low in the surface soil, but increases in greater depth. Zn is an essential growth element for plants and animals, but at elevated levels, it is toxic to some aquatic species (APHA, 1998). Excessive Zn in soil may cause damage to plants and at lower pH, the yield is reduced (Leeper, 1978).

Bioavailability of Heavy Metals and chemical speciation

Bioavailability is defined as the fraction of the element from an ingested matrix such as soil, water or food that can be absorbed by an organism (Ng et al., 1998). The bioavailability and environmental mobility of the metals are dependent upon the form in which the metal is associated with the soil (Jaradat and Momani, 1998). However, determining the total metal contents of soil provides little insight into their bioavailability. To determine bioavailability and the actual dangers posed by the heavy metals in soil, one approach is based on sequential extraction of the metals into phases so that the labile fractions could be estimated in atmospheric particulate matter, sediment and soils (Tack and Verloo 1995; Filgueiras et al. 2002; Gleyzes et al. 2002; Ross, 1994, Smichowski et al. 2005). Bioavailability of metals from contaminated sites is a very important aspect of health risk assessment prog•ams (Ng et al., 1998).

The distribution of trace elements in the environment, their accumulation by organisms, their bioavailability and their toxicity to organisms (including humans) can be understood only in terms of trace element species (Bernhard et al., 1986). Therefore, it is very important to identify elemental species so that environmental processes can be fully understood.

Soils consist of heterogeneous mixtures of organic and inorganic solid components as well as a variety of soluble substances (Xian, X. 1989b). Therefore, metal distribution among specific forms varies widely based on the metal's chemical properties and soil characteristics (Soon and Bates, 1982). Thus, it is important to evaluate the availability and mobility of heavy metals to establish environmental guidelines for potential toxic hazards and to understand chemical behavior and fate of heavy metal contaminants in soils (Davies, 1980).

Chemical Speciation

Information about the physicochemical forms in which the metals occur helps in understanding their environmental behaviour (mobility, pathways, bioavailability). The disfribution of frace elements in the environment, their accumulation by organisms, their bioavailability and their toxicity to organisms (including humans) can be understood only in terms of chemical species to which the elements are combined (Bernhard et al., 1986). The environmental behaviour and toxicity of a metal can only be understood in terms of its actual chemical from and how it is bound to the same. This has led to the introduction of the term "speciation" used to describe the distribution of the metals in various chemical forms (IUPAC 2000). Although no generally accepted definition of the term exists, speciation can broadly be defined as the identification and quantification of the different, defined species, forms or phases in which an element occurs.

Since the behaviour of the elements in a soil-water-plant system depends on their chemical forms, single or sequential extraction is most useful in separating the different bound states. The procedures involve subjecting a solid sample (soil or sediment) to successive extraction with reagents possessing different chemical properties (acidity, redox potential, or complexing behaviour) in which each extract includes a part of the trace metals associated with the sample.

The analytical procedures used for separating the metal species are referred to also as "fractionation", The term "fractionation" is frequently used interchangeably with speciation but emphasizes the concept of subdividing a "total content' (IUPAC 2000).

The behaviour and fate of metals are governed by a range of different physicochemical processes, which dictate their availability and mobility in the soil or sediment system. In the water phase, the chemical form of a metal determines the biological availability and chemical reactivity (sorption/desorption, precipitation/dissolution) towards other components of the system. The binding form in the solid phase is related to the kinetics and equilibrium of metal release to the liquid phase and hence the likelihood of remobilization and bioavailability.

Selective sequential extraction procedures have been commonly used for studying metal mobility and availability in soils because in soil, metals are associated with several chemical fractions. Consequently, soil sequential extractions have been employed to isolate and quantify metals associated with different fractions (Tessier et al., 1979; Kot and Namiesnik, 2000).

Numerous extraction schemes have been described in the literature (Gupta and Chen 1975; Stover et al., 1976; Tessier et al.. 1979; Tessier and Campbell, 1988; Miller and McFee, 1983; Welte et al., 1983; Shuman, 1985; Rauret at al., 1989) to determine the partitioning of heavy metals in environmental samples especially for soils and sediments. The

procedure of Tessier et al. (1979) is recognized as one of the most thoroughly researched and widely used procedures to evaluate the efficacy of decontamination treatments (Pardo et al., 1990; Rauret et al., 1988; Spevackova and Kucera, 1989; Calvet et al., 1990; Vicente-Beckett etal., 1991).

The extraction scheme used in this work is based on operationally defined fractions: water soluble and exchangeable, carbonate, Fe„Mn oxides, organic, and residual fractions. These procedures are not entirely specific, and there are often overlapping between the fractions. Despite uncertainties as to the selectivity of the various extractants and the possible problems due to readsorption, extraction procedures provide qualitative evidence regarding the forms of association of trace metals and, indirectly, of their bioavailability (Harrison, 1981, Xian, 1987, 1989). Such assessments assume that metal bioavailability decreases with each successive extraction step in the procedure for extracting the fractions (i) water soluble and exchangeable, (ii) carbonate, (iii) Fe-Mn oxides, (iv) organic and (v) residual fractions. Thus, metals in the water soluble and exchangeable fraction would be readily bioavailable to the environment, whereas the metals in the residual fraction are tightly bound and would not be expected to be released under natural conditions (Xian 1989; Clevenger and Mullins, 1982). It is also to be noted that the five chemical fractions follow the order of decreasing solubility (Xian, 1989; CleVehger and Mullins, 1982; Soon and Bates, 1982; Lake et al., 1984) and bioavailability may be related to solubility.

Interestingly, all the fractionation schemes reported in the literature have been used to characterize pollution sources, to evaluate metal mobility and bioavailability, and to identify binding sites of metals in order to assess metal accumulation, pollution and transport mechanisms. Despite uncertainties as to the selectivity of the various extractants and to possible problems due to readsorption, extraction procedures provide qualitative evidence regarding the forms of association of trace metals and, indirectly, of their bioavailability (Harrison, 1981).

The sequential extraction procedure reported by Tessier et al. in 1979 and the BCR procedure elaborated in 1993 by the European Community's Bureau of References (now The Standards, Measurements and Testing Programme, SM&T) are the most representative procedures.

Methods based on Tessier's scheme

The original five-stage sequential extraction method was developed and applied by Tessier (Tessier et al., 1979) for speciation studies with respect to river sediment. The method selectively partitions tace metals into chemical forms that are likely to be released in solution under various environmental conditions. 'Ihe method has been recognized as the best known and the most widely used sequential exfraction procedure. It consists of five steps in which heavy metal distribution into five different fractions is achieved (Table 3.3 in Chapter 3).

In their pioneering paper, Tessier et al. (1979) developed and examined the merits of sequential chemical extraction for partitioning trace metals (Cd, Co, Cu, Fe, Mn, Ni, Pb and Zn) from bottom sediments into chemical forms likely to be released in solution under various environmental conditions. Five fractions were separated as follows: exchangeable fraction (1.0 mol/L magnesium chloride), bound to carbonates (1.0 mol/L sodium acetate), bound to Fe and Mn oxides (0.04 mol/L hydroxylamine chloride in 25% acetic acid), bound to organic matter (0.02 mol/L nitric acid + 30% hydrogen peroxide + 3.2 mol/L ammonium acetate in 20% nitric acid), and residual (hydrofluoric and perchloric acids).

The BCR scheme

In order to harmonize and validate different fractionation schemes, a group of experts as part of the European Community Measurement and Testing Program has developed a common scheme for sequential extractions based on three stages (Ure et al., 1993).

The BCR scheme allows the separation of extractable metals into three fractions: acid soluble (using 0.1 mol/L acetic acid), reducible (using 0.1 mol/L hydroxylamine hydrochloride), and oxidizable (using 8.8 mol/L hydrogen peroxide), It was proposed in an attempt to harmonize the different schemes and was first applied to the fractionation of Cd, Cr, Cu, Ni, Pb and Zn in sediments.

Tessier's scheme vs the BCR scheme

In general terms, the Tessier and BCR schemes use the same extractants for the acid soluble and reducible fractions, but different concentrations and experimental conditions are applied, and consequently, the metal contents found are different. Tessier's scheme has been subjected to different modifications, optimizations and improvements over the years in order that it can be used to exfract metals from different environment samples like soil, sediments, sludge and aerosols. These modifications have contributed to speeding up the conventional method and therefore improving sample throughput.

Drawbacks of Extraction

Even though sequential chemical extraction procedures provide useful information in environmental studies, they also have some limitations and drawbacks. The selectivities of the chemical reagents toward specific forms of binding were not taken into account for most of the schemes reported. Also, the influence of the operational conditions should be critically evaluated when an extractant is selected, mainly because each extraction step could change the original distibution of species in the sample.

Problems in the speciation procedures are mainly related to the limited selectivity of the extractants, the trace element redistributions among phases during extraction, overloading of the chemical system if the metal content was too high and possible changes in the chemical species during sampling and pre-treatment of samples.

REVIEW OF INSTRUMENTAL TECHNIQUES FOR METAL ANALYSIS

2.8.1 Atomic Spectroscopy

Atomic specfroscopy is the determination of elemental composition by its electromagnetic or mass spectrum. The study of the electromagnetic spectrum of elements is called Optical Atomic Spectroscopy. Electrons exist in energy levels within an atom. These levels have well defined energies and electrons moving between them must absorb or emit energy equal to the difference between them, In optical spectoscopy, the energy absorbed to move an elecfron to a more energetic level and/or the energy emitted as the electron moves to a less energetic energy level is in the form of a photon (Vogel's; 1989). The wavelength of the emitted radiant energy is directly related to the electronic transition which has occurred. Since every element has a unique electronic structure, the wavelength of light emitted is a unique property of each individual element. As the orbital configuration of a large atom may be complex, there are many electronic transitions which can occur, each transition resulting in the emission of a characteristic wavelength of light.

The process of excitation and decay to the ground state has yielded three techniques in the fields of atomic spectroscopy (Atomic Absorption, Atomic Emission, Atomic FluOiescence), which are now widely used for analytical purposes (Dockery et al; 2008).

In the atomic absorption process, if light of just the right wavelength impinges on a free, ground state atom, the atom may absorb the light as it enters an excited state. Atomic absorption measures the amount of light at the resonant wavelength which is absorbed as it passes through a cloud of atoms. As the number of atoms in the light path increases, the amount of light absorbed increases in a predictable way. By measuring the amount of light absorbed, a quantitative determination of the amount of analyte element present can be made.

In atomic emission, a sample is subjected to a high energy, thermal environment in order to produce excited state atoms, capable of emitting light. The energy source can be an electrical arc, a flame, or plasma. The emission spectrum of an element exposed to such an energy source consists of a collection of the allowable emission wavelengths, commonly called emission lines, because of the discrete nature of the emitted wavelengths. This emission spectrum can be used both for qualitative and quantitative identification of the element. For a "quantitative" analysis, the intensity of light emitted at the wavelength of the element to be determined is measured. The technique of flame photometry is an application of atomic emission for quantitative analysis.

Atomic fluorescence technique incorporates aspects of both atomic absorption and atomic emission. Like atomic absorption, ground state atoms created in a flame are excited by focusing a beam of light into the atomic vapor. In the process, instead of looking at the amount of light absorbed in the process, however, the emission resulting from the decay of the atoms excited by the source light is measured. The intensity of this "fluorescence" increases with increasing atom concentration, providing the basis for quantitative determination. The source lamp for atomic fluorescence is mounted at an angle to the rest of the optical system, so that the light detector sees only the fluorescence in the flame and not the light from the lamp itself. It is advantageous to maximize lamp intensity since sensitivity is directly related to the number of excited atoms which in turn is a function of the intensity of the exciting radiation.

The ease and speed at which precise and accurate determinations can be made have made atomic absorption one of the most popular methods for the determination of metals among the three techniques.

2.8.2 Flame Atomic Absorption Spectrometry (FAAS)

Flame atomic absorption is a very common technique for detecting metals and metalloids in environmental samples. It is very reliable and simple to use. The technique is based on the fact that ground state metals absorb light at specific wavelengths. Metal ions in a solution are converted to atomic state by means of a flame. Light of the appropriate wavelength is supplied and the amount of light absorbed can be measured against a standard curve.

Basic Principle

The technique of flame atomic absorption spectroscopy (FAAS) requires a liquid sample to be aspirated, aerosolized, and mixed with combustible gases, such as acetylene and air or acetylene and nitrous oxide. The mixture is ignited in a flame whose temperature ranges from 2100 to 2800 Oc.

During combustion, atoms of the element of interest in the sample are reduced to free, unexcited ground state atoms, which absorb light at characteristic wavelengths. The characteristic wavelengths are element specific and accurate to 0.01 0.1 nm. To provide element specific wavelengths, a light beam from a lamp whose cathode is made of the element being determined is passed through the flame. A device such as photomultiplier can detect the amount of reduction of the light intensity due to absorption by the analyte, and this can be directly related to the amount of the element in the sample.

This instrument in particular is designed to operate either with a flame or with a graphite furnace. The graphite furnace is additionally equipped with an auto sampler. Flame atomic absorption hardware is divided into six fundamental groups (light source, atom cell, monochromator, detector, amplifier, and signal display-data station) that have two major functions: generating atomic signals and signal processing. Signal processing is a growing additional feature to be integrated or externally fitted to the instrument (Figure given below).

Simpli(ieå diqgcaM of hAS ectvipment

A cathode lamp (light source), shown in the figure, is a stable light source, which is necessary to emit the sharp characteristic spectrum of the element to be determined. A different cathode lamp is needed for each element, although there are some lamps that can be used to determine three or four different elements if the cathode contains all of them. Each time, a lamp is changed; proper alignment is needed in order to get as much light as possible through the flame, where the analyte is being atomized, and into the monochromator.

The atom cell is the part with two major functions: nebulization of sample solution into a fine aerosol solution, and dissociation of the analyte elements into free gaseous ground state form. Not all the analyte goes through the flame, part of it is disposed. As the sample passes through the flame, the beam of light passes through it into the monochromator. The monochromator isolates the specific spectrum line emitted by the light source through spectral dispersion, and focuses it upon a photomultiplier detector, whose ftnction is to convert the light signal into an electrical sigial (Haswell, 1991).

The processing of electrical signal is fulfilled by a signal amplifier. The signal could be displayed for readout, or further fed into a data station for printout in the requested fonnat.

Types of flame

Different flames can be achieved using different mixtures of gases, depending on the desired temperature and burning velocity. Some elements can only be converted to atoms at high temperatures. Even at high temperatures, if excess oxygen is present, some metals form oxides that do not redissociate into atoms. To inhibit their formation, conditions of the flame may be modified to achieve a reducing, non-oxidizing flame. Following table shows the characteristics of various flames (Source Reynolds et al., 1970.).

Table: Characteristics of different flames

Flame

Max. flame speed (cm/s)

Max. Temp. (oc)

Air-Coal gas

55

1840

Air-propane

82

1925

Air-hydrogen

320

2050

Air-50% oxygen-acetylene

160

2300

Oxygen-nitrogen-acetylene

640

2815

Oxygen-acetylene

1130

3060

Oxygen-cyanogen

140

4640

Nitrous oxide-acetylene

180

2955

Nitric oxide-acetylene

90

3095

Nitrogen dioxyde-hydrogen

150

2660

Nitrous oxide-hydrogen

390

2650

2.8.3 Electro Thermal Atomic Absorption (ET-AAS)

Flame atomic absorption is very convenient and widespread, and has an acceptable level of accuracy for most analytes. However, for the determination of the metals like arsenic, mercury, selenium which are present in very minute level (parts per billion, ppb) in environmental samples, electrothennal atomization device is used, which allow for better sensitivity and more control over the chemical environment of the analyte.

This type of atomization requires a graphite furnace, where after thermal pretreatment the sample is rapidly atomized. To maintain a dense fraction of free ground state atoms in the optical path, an inert gas atmosphere is used. Since the dilution and expansion effects of flame cells are avoided, and the atoms have a longer residence time in the optical path, a higher peak concentration of atoms is obtained.

Carbon rod analyzer is other type of device in atomic absorption analysis used to convert a powdered sample into atomic vapour. In this process a current is applied to a very thin, heated carbon rod that contains the solid sample in order to vaporize it.

Inductively Coupled Plasma (ICP)

Inductively Coupled Plasma Mass Spectrometry or ICP-MS is an analytical technique used for elemental determinations (Vogel's; 1989). The technique was commercially introduced in 1983 and has gained general acceptance in many types of laboratories. Geochemical analysis labs were early adopters of ICP-MS technology because of its superior detection capabilities, particularly for the rare-earth elements (REEs). ICP-MS has many advantages over other elemental analysis techniques such as atomic absorption and optical emission spectrometry, including ICP Atomic Emission Spectroscopy (ICP-AES), as föllows:

Detection limits for most elements equal to or better than those obtained by Graphite

Furnace Atomic Absorption Spectroscopy (GFAAS)

Higher throughput than GFAAS

The ability to handle both simple and complex matrices with a minimum of matrix interferences due to the high-temperature of the ICP source

Superior detection capability to ICP-AES with the same sample throughput

The ability to obtain isotopic information.

An ICP-MS combines a high-temperature ICP (Inductively Coupled Plasma) source with a mass spectrometer. The ICP source converts the atoms of the elements in the sample to ions. These ions are then separated and detected by the mass spectrometer. The schematic flow chart given below illustrates this sequence of processes.

Mass discriminator and Detector

Figure: Schematic of ICP-MS main processes

ICP technology was built upon the same principles used in atomic emission spectrometry. Samples are decomposed to neutral elements in high temperature argon plasma and analyzed based on their mass to charge ratios. An ICP-MS can be thought of as consisting of four main processes, including sample introduction and aerosol generation, ionization by an argon plasma source, mass discrimination, and the detection system.

Flame photometry

Flame photometry (flame atomic emission spectrometry) is a branch of atomic spectroscopy in which the species examined in the spectrometer are in the form of atoms (Vogel's; 1989). Flame photometry is cost effective compared to the other two branches of atomic spectroscopy (AAS and ICP-MS) which are not used in Standard base experiments. In all cases the atoms under investigation are excited by light. Absorption techniques measure the absorbance of light due to the electrons going to a higher energy level. Emission techniques measure the intensity of light that is emitted as electrons return to the lower energy levels. Flame photometry is suitable for qualitative and quantitative determination of several cations, especially for metals that are easily excited to higher energy levels at a relatively low flame temperature (mainly Na, K, Rb, Cs, Ca, Ba, and Cu). This technique uses a flame that evaporates the solvent and also sublimates and atomizes the metal and then excites a valence electron to an upper energy state. Light is emitted at characteristic wavelengths for each metal as the electron returns to the ground state that makes qualitative determination possible. Flame photometers use optical filters to monitor for the selected emission wavelength produced by the analyte species. Comparison of emission intensities of unknowns to either that of standard solutions (plotting calibration curve), or to those of an intemal standard (standard addition method), allows quantitative analysis of the analyte metal in the sample solution.

Principles

Flame photomefry relies upon the fact that the compounds of the alkali and alkaline earth metals can be thermally dissociated in a flame and that some of the atoms produced will be further excited to a higher energy level. When these atoms return to the ground state they emit radiation which lies mainly in the visible region of the spectrum. Each element will emit radiation at a wavelength specific for that element. The table below gives details of the measurable atomic flame emissions of the alkali and alkaline earth metals in terms of the emission wavelength and the colour produced.

Element

Emission Wavelength (nm)

Flame Colour

Sodium (Na)

589

Yellow

Potassium (K)

766

Violet

Barium (Ba)

554

Lime Green

Calcium (Ca)

622*

Orange

Lithium (Li)

670

Red

Calcium is measured by using the calcium hydroxide band emission at 622 nm as the Calcium main atomic emission occurs at 423 nm. Over certain ranges of concentration the intensity of the emission is directly proportional to the number of atoms returning to the ground state. This is in turn proportional to the absolute quantity of the species volatized in the flame, i.e. light emitted is proportional to sample concentration.

It can be seen that if the light emitted by the element at the characteristic wavelength is isolated by an optical filter and the intensity of that light measured by a photo-detector, then an electrical signal can be obtained proportional to sample concentration. Such an electrical signal can be processed and the readout obtained in an analogue or digital form. A simple flame photometer consists ofthe following basic components:

The burner: a flame that can be maintained in a constant form and at a constant temperature,

Nebulizer and mixing chamber: a means of transporting a homogeneous solution into the flame at a steady rate.

Simple colour filters (interference type): a means of isolating light of the wavelength to be measured from that of extraneous emissions.

Photo-detector: a means of measuring the intensity of radiation emitted by the flame.

The analysis of alkali and alkaline earth metals by flame photometry has two major advantages: (i) their atoms reach the excited state at a temperature lower than that at which most other elements are excited and (ii) their characteristic wavelengths are easily isolated from those of most other elements due to wide spectral separation. The analysis of Na, K, Li, Ba and Ca are typically determined at low i.e. 1500* 20000 C, therefore suitable fuel mixtures are propane/air, butane/air and natural gas/air.