This paper seeks to identify the best way of determining trace metals in water samples between atomic absorption spectroscopy, anodic stripping voltametry and potentiometric stripping analysis. To achieve this purpose, prior studies conducted by other researchers will be critically analysed, evaluated and compared. Depending on the findings, the best method will be established for determination of trace metals in water.
Atomic Absorption Spectroscopy (AAS)
Atomic absorption measures the amount of light that is absorbed as it passes through a patchcloud of atoms at the resonant wavelength. By using special light sources and specific wavelengths, individual elements can specifically be quantitatively determined in the presence of others (Yasemin, Su leyman Niyazi 2003). Determination is usually done using calibration curve method, standard addition method and internal standard method depending on the methods appropriateness (Yasemin, Su leyman, Niyazi 2003). It also advised that interference as well as background is considered during determination to give results that are precise and accurate.
In their study of determination of metallic elements in crude oil-water emulsions using flame AAS, Platteau Carrillo (1995), prepared the samples using a dry ashing procedure which entails dehydration in the presence of ethyl alcohol instead of the most frequently used time consuming wet ashing. V, Fe, Ni, Na and Mg were then determined using FAAS. A retention agent was added to the samples to minimize sample loss during the high temperature burning process. Recoveries for the elements was good ranging between 95.8 and 104. This method was found to be reliable and simple hence can be handled by any technician with basic skills (Platteau Carrillo 1995).
Divrikli, Elci (2002) acknowledge that the detection limits of AAs limit its application in trace analysis of metals in natural water as they usually are in g1-1. They explain the need for preconcentration to enhance the selectivity as well as sensitivity of methods used in determination of trace metals. Mahmoud et al., (2010) have used chemically modified silica gel N-(1-carboxy-6-hydroxy) benzylidenepropylamine ion exchanger to determine trace metals using AAS, however, Divrikli, Elci (2002) have used coprecipitation with cerium (IV) hydroxide as a preconcentration procedure to determine trace metals including Cd, Cu, Co, Fe, Pb, Ni and Mn in water and sediment samples by FAAS. The results obtained by Divrikli, Elci (2002), show that some metals such as Cd did not however agree with reference materials as the metal ions were affected by complex matrix elements interference (Divrikli Elci 2002). Co and Mn could also not be detected because of their very low concentration in the sample. The results were otherwise found to be accurate (Divrikli Elci 2002). Mahmud et al., (2010) found that maximum recovery of all the metals present in the sample was possible. By comparing the results with those a standard solvent extraction technique, the results obtained by Mahmoud et al., (2010) were found to be reliable implying that AAS was a reliable, easy method to use in determination of trace metals as long as preconcentration is conducted. Narin et al., (2000) also carried out a study in which they sought to determine trace metal ions (Co, Mn, Pb, Cd, Cr and Ni) in natural water samples using AAS but after preconcentrating the sample with pyrocatechol violet complexes on activated carbon column. The concentration of the detected metal ions after preconcontration was found to agree with the amount that was added. This method was found to be applicable to preconcentration of Mn, Cu, Cd, Ni, Pb, Cr and Co in natural water samples.
AAS is easier and faster to conduct as compared to other methods (Eletta 2007). It also gives accurate and precise determinations. Flame atomization also allows reproducibility of the results increasing precision. According to Yasemin, Su leyman, Niyazi (2003), AAS gives excellent performance as it is a precise as well as rapid analytical tool. It allows fast acquisition of data on samples as they are introduced into a flame. These authors attribute the attractiveness of this tool to its ease of operation.
AAS is however quite expensive as compared to other analytical methods. It is also not suitable for determining metals such as the alkali metals with low IP. This method is mostly limited to determination of heavy metals. Low concentration as well as matrix interference limit the direct determination of trace concentrations of metals in samples resulting to low sensitivity (d1-1) levels hence the need to employ techniques for pre-concentration and separation (Narin, Soylak, Elci, Dogan 2000).
Anodic Stripping Voltametry (ASV)
This analytical technique involves two major steps pre-concentration of a metal phase into liquid mercury or onto a solid electrode surface at negative potentials and onto an electrode surface and secondly, selective oxidation of each of the metal phase species through an anodic potential sweep.
According to Sanna et al., (2000), stripping methods and in particular ASV are preferred to spectroscopic techniques in determination of trace because of the ability to quantitatively analyse concentrations up to trace levels using instrumentation that is relatively cheaper. These methods also allow metal speciation studies (partial or full) depending on how complex the system is. In their study on determination of heavy metals in honey using ASV at microelectrodes, Sanna et al., (2000) use differential pulse anodic stripping voltametry (DPASV). They found that it when Cu, Zn, Cd and Pb, Cd were simultaneously quantified in the mineralised matrix, they prevented cu-Zn intermetallic species from forming during the deposition step. Addition of Ga (II) ions and setting the potential at the metal deposition lower than that required for reduction of Zn however allowed the four metals to be determined by two DPASV experiments consequently conducted on the same sample. The results gave more metal and were unbiased hence accurate.
Lau, Cheng (1998), argue that ASV is advantageous in trace metal analysis as it allows species characterization and low instrumentation as well as operation costs. Instruments used in anodic stripping analysis are also small in size, demand lower power and do not require any special installation including ventilation or cooling as it is in atomic absorption spectroscopy. These features make this technique most appropriate for in situ measure where sample loss and contamination from adhesion to the sample bottle during transportation and storage can be reduced. The authors used ASV method to determine Zinc in environmental samples. To eliminate Cu (II) ion interference in ZN determination by ASV, the authors added sulphide. This method was found to be applicable to various environmental samples including water. The resultant calibration graph was linear ranging from 0.3-11g of Zn. Recovery of Zn from the samples using the proposed technique was found to be 98-102 percent. It was found that most of metal ions as well as anions present in the sample except Co and Ni, did not interfere with the analysis (determination).
Qingguo et al., (1997) argue that linear sweep ASV is limited is characterised by insufficient sensitivity. They use an ASV method whose sensitivity and selectivity has been improved by application of Osteryoung square with a gold disc electrode to determine the capacity of natural waters to form mercury complexes. To eliminate interferences during stripping, medium exchange (0.1M perchloric acid2.5mM Hcl) was used. The method was found to be effective in conducting speciation studies the findings indicated that complexation with natural organic matter dominated speciation of Hg in natural waters. Nanomolar (parts per trillion) is achievable in ASV (Hoshika, Takimura, Shiozawa, 1977).
Potentiometric Stripping Analysis (PSA)
Potentiometric stripping analysis is based on the phenemena of electrodeposition of metals at a potential that is constant onto a mercury film which is then followed by stripping (redosolution) of metals. In their analysis of Zn, Cd and Pb in environmental water by PSA using a carbon nanotube electrode that is multiwalled, Tarley et al., (2009), found that PSA is a simple as well as highly selective electrochemical method for carrying out simultaneous determination of ZN, Cd and Pb in water. The analytical curves for all the analytes were within the linear range varying between 58.4 to 646 gl-1. According to these authors, the proposed method is the simplest and very effective method of determining traces metals in water. They identified the following advantages of the technique reduced analysis time, excellent measurements that are reproducible, simple instrumentations as the electrode is simple to build, high sensitivity and allows simultaneous monitoring of metal ions in a sample.
Stankovica, Cicbkariia Markoviib (2007) explain that electrochemical techniques are the cheapest and quickest techniques in determination of trace metals in water. These techniques can be highly sensitive and very specific. They consider electrochemical stripping methods as the most suitable in the analysis of trace as well as ultra-trace heavy metals and that PSA is one such technique. The instrumentation for PSA is small and requires no special installation requirement such as and gas supply or servicing. It also has very low detection limits of up to 10-10moldm3. In their determination of PB and Cd (simultaneous) in water using computerised PSA, Stankovica, Cicbkariia Markoviib (2007), found that PSA was a highly sensitive and selective electroanalytical technique. The method was applied to water samples and the results compared with Graphite AAS method. It was found that by changing the conditions of the water samples (adjusting pH) the metal ions under study could be simultaneously determined by PSA.
Electrochemical stripping methods are indensible because of their high sensitivity and the fact that several metal elements can be determined at a time unlike spectroscopic methods in which only one only one element can be determined at a time and lack sensitivity for some elements including Cu, Ti, Pb and Cd (Riso, Corre, Chaumery (1997). These authors use PSA and TraceLab (PSU22) to conduct rapid and simultaneous determination of trace metals (PB, Cd and Cu) in sea water. This method was found to give lower detection limits as compared to Tracelab PSU20 reducing the time required for analysis. Tracelab PSU22 unit was also found to have a higher sampling rate. The technique gave results that were precise, accurate and reproducible and compared well with other techniques.
Based on the above arguments, it is obvious that electrochemical techniques are the best to employ in determination of trace metals in water because of the several advantages they have over spectroscopic techniques. Electrochemical techniques are the cheapest and quickest techniques for conducting determination. They are also very specific and selective. Of the two electrochemical techniques, potentiometric stripping analysis is the best. This is because it is much cheaper in terms of instrumentation as compared to anodic stripping voltametry, has higher sensitivity and selectivity. PSA also has lower detection limits as compared to ASV and gives less erroneous results as it is not affected by substances that are surface active. PSV is the best method to employ when sensitivity per invested money is put into account.
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