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During the past decades, with the rapid development of economy and industry, there have been a increasing demand and consumption of water resource. However, these commercial and industrial activities may lead to different contaminants, such as chemicals. Heavy metals and pathogens, discharged into natural water bodies and catchments. It would potentially pose environment and human health related risks. In order to ensure the water quality and safety of water consumption, the monitoring of contaminants within drinking water and wastewater effluent is essential. There is a number of contaminants recommend in different water quality guidelines. In this paper, it will focus on the monitoring and analysis of cadmium. This will be demonstrated by following sections: briefly introduction of cadmium and related risks, water sample collection and pretreatment, sample extraction techniques, introduction and evaluation of currently used sample analysis techniques, accurate quantitation of the analyte, quality control procedures and discussion of analytical detection limits could be achieved.
1 Cadmium and its effect on human health
Cd is a soft, bluish-white metal and could present in natural environment such as mineral rocks, soil and surface water in small quantities and in the combination with other elements. In relation to human activities, cadmium is produced mainly as a byproduct from mining, smelting, and refining sulfide ores of zinc, lead and copper. Cd could be utilized in many industries: cadmium is mainly used in batteries, predominantly for rechargeable nickel-cadmium batteries; it was used as pigment for paints and for corrosion resistant plating on steel for a long time; stabilizer in plastic product such as PVC; as a component in various alloys. Additionally, the unintentional utilizations of Cd as component in other products involve phosphate fertilizer, phosphogypsum, lime, gypsum (Water UK 2001). However, most utilizations of Cd have been decreasing due to that it is bioaccumulative toxic.
Besides the Cd present in natural environment, it is mainly present as Cd(â…¡)form , the main pathways for Cd enter water include: domestic sewerage system, due to Cd is contained by many household chemical products such as detergents and bleach; runoff and leaching , including both urban runoff and associated with agricultural activity such as use of fertilizer; industrial discharge and illegal discharge. Once Cd enter water bodies, it could pose human health risks via food chain and water consumption activities. Even at a low level concentration, after long term exposure and accumulation, it could lead to numerous diseases such as erothrocyte destruction, nausea, salivation, diarrhea and muscular cramps, renal degradation, chronic pulmonary problems, skeletal deformity and cancer (Lalor 2008).
2 sample collection and pretreatment.
Water sampling may involve the collection, storage, transport and pretreatment of water samples. For the water quality monitoring and analysis, it should be regarded that the significance of analysis is restricted by the adequacy of the sampling programme. It requires that after these steps, the water sample still could accurately representing the part of the environment sampled. However, the major problem is that the concentrations may change with depth, salinity, proximity to discharge point and time. Thus, the selection of sampling techniques should be appropriate.
2.1 techniques for sample collection.
Grab sampling technique is the most common used sampling technique that samples are taken in a a single vessel. It is easy to operate and require low capital cost and less staff training. Thus it might be suitable for surface water sampling. However, grab samples cannot be extrapolated to other times or to other parts of the water bodies without additional monitoring. Moreover, due to the concentrations may vary with the change of water condition such as flow, temperature and depth, the location, time, frequency and sample numbers of grab sampling should be considered carefully.
In order to describe the variable concentration of determinants, it could be necessary to obtain an average or the short term peak concentrations. The possible and simple approach is to take a continuous samples or to generate a composite sample by automatic sampling system (Nollet c2000). It could take sample in a fixed pace, typically can obtain 24 samples without changeover and possible to be extended. Sampling can be triggered externally, such as water level, turbidity and temperature. This technique is suitable for effluent or stream monitoring, however, it could not provide chemical speciation information. Additionally, it requires relatively high capital cost, maintenance and flow characteristics for trace metal flux (lecture notes 2010).
There are also a range of semi-continuous and continuous monitoring approaches available that can be programmed to take samples at fixed or variable time. The use of ion selective electrodes could enable broader range of analytes to be monitored continuously. For instance, a single solid surface fluorescence based flow-through optosensor has been developed for the determination of trace amounts of cadmium in drinking water samples. Under optimized conditions, the proposed optosensor was calibrated in the range 2-60 Î¼g/ l, obtaining a detection limit of 0.48 Î¼g/ l, and a R.S.D of 1.9%, with a sampling frequency of 18 hâˆ’ 1 (García-Reyes, Ortega-Barrales & Molina-Díaz 2006). Some other methods such as passive sampling seems to be less utilized in normal practice and will not be considered in this paper.
2.2 sample storage.
Preservation of the sample is one of the major difficulties of sampling. To ensure the accuracy of analysis, the initial composition of sample must be maintained from sampling process to analysis.
The possible change of sample composition could be attributed to following reasons: contamination, which could be either from external source or from contaminated equipment. Loss of target element, which can occur due to biological process, hydrolysis, evaporation/volatilization and other chemical reactions. Sorption to the wall of sample container could also reduce the concentration in the water phase (Madrid & Zayas 2007). Thus, in order to avoid and minimize these undesirable effect, methods of sample conservation have to be applied.
Regarding that cadmium is the target element in this paper, thus firstly it is necessary to choose the Polyethylene and Teflon sample container to reduce adsorptive loss. Moreover, acidification (HNO3) to pH 1-2 followed by distilled water rinse could be optimal for the prevention of metabolism by microorganisms, hydrolysis and precipitation. For the collection, it is necessary to overfill sample containers and then seal with no entrapped air. If necessary, sample could be filtered. Then cooling/freezing sample (in field or soon later) could limit microorganisms activity and oxidation kinetics (Nollet 2000). Meanwhile, bulk chemical parameters of water samples could be measured in the field. In Australia drinking water guidelines, the guideline value of Cd is 0.002 mg/l and suggested monitoring frequency is 'quarterly' (NRMMC 2004). However, the frequency could be modified depending on the variation of concentration or events.
3. sample extraction techniques
Metals could be determined by various spectroscopic or chromatographic methods for trace quantities, thus the sample preparation for determination could serve several fundamental functions including: to solubilize the matrix and release metals; to extract metals from sample matrix into solvent; to concentrate metals into a suitable range; to separate single analyte(s) of other species; to dilute the matrix sufficiently; to separate different chemical forms of analyte(s). These purposes may vary with the type of sample and determination techniques (Mitra 2003). There are a number of preparation and extraction procedures could be available, such as electrochemical deposition, co-precipitation and precipitation, liquid-liquid extraction, cloud point extraction and solid phase extraction and could be assisted by other quipment. Here it will focus on the three common methods which are: liquid-liquid extraction (LLE), cloud point extraction (CPE) and solid phase extraction (SPE).
3.1 Liquid-liquid extraction:
The LLE procedures LLE has also been referred to as immiscible solvent extraction, which is based on the relative solubility of the elements in two immiscible phases objecting both to improve the selectivity by separation of analyte and increase the sensitivity of the method. To isolate the analyte, the analyte should be quantitatively removed from aqueous matrix sample by introducing immiscible solvents while the interferent species must remain in the aqueous phase. Due to the different solvents may present various solubility and recovery ratio, the efficiency of this process depends on the affinity of analytes with the extracting solvent, ratio between the phases and number of extractions. However, LLE is relatively expensive, slow and would consume toxic organic compounds (Ferreira et al. 2007). Typical liquid-liquid extraction flow analysis system diagram is shown below:
Fig. 1: Typical liquid-liquid extraction system.
C: carrier; R: reagent; P: propulsion unit; S: sample; IV: injection valve; MC: mixing coil; DB: displacement bottle; ORG: organic solvent; SG: segmenter; EC: extraction coil; PS: phase separator; D: detector; RC: restriction coil; W: waste. (Silvestre et al. 2009)
3.2. Cloud point extraction
The separations and preconcentrations of metal ions, based on cloud point extraction (CPE) have been largely employed in analytical chemistry. The procedure is based on the properties of non-ionic or amphoteric surfactants at levels upper to their critical micellar concentrations (CMC). Above CMC, a system composed by a unique phase will be separated into two isotropic phases (surfactant-rich phase and aqueous phase) when some condition such as temperature and pressure is changed or appropriate substance is added to the solution. Ligands such as 1-(2-pyridylaso)-2-naphthol (PAN) and 1-(2-thiazolylazo)-2-naphthol (TAN) have been used in cloud point extraction of cadmium. Micelles formed from surfactant molecules act as organic solvents in liquid-liquid extraction and the analytes are partitioned between the micellar and aqueous phases. When compared with LLE, CPE is more convenient and simple to operate, including higher extraction and preconcentration factors, lower cost and lower toxicity for the analyst and environment (Pinto et al. 1996)
3.3 solid phase extraction
SPE refers to the nonequilibrium, exhaustive removal of chemical constituents from a flowing liquid sample via retention on a contained solid sorbent and subsequent recovery of selected constituents by elution from the sorbent. Thus the selection of sorbent is critical. Numerous sorbents are employed in cadmium preconcentration procedures based on solid-liquid extraction, including synthetic and natural materials. Synthetic sorbents, such as divinylbenzene polymers, zeolites, fullerenes and polyurethane foam could be employed for preconcentration of cadmium. Meanwhile, chelating resins, such as Dithizone and PAN, could be introduced due to higher selectivity when compared with single polymeric matrices. Some natural sorbents such as bark/tannin-rich materials, lignin, chitin/chitosan, dead biomass have been used due to their extensive surface area and porosity for retaining cations or anions. However, natural materials present poor selectivity and low uniformity composition which may negatively affect the accuracy of results.
Fig. 2: solid phase extraction.
4. techniques and instrumentation for sample analysis
A number of analytical techniques could be utilized for the determination and quantification of trace metal including Cd. This section will focus on Atomic Absorption and Emission Spectroscopy, Inductively Coupled Plasma atomization technique and Inductively Coupled Plasma-Mass Spectrometry.
4.1 Atomic Absorption and Emission Spectroscopy
The principal of Atomic Absorption Spectroscopy (AAS) determining the metals in liquid sample is: Each metal has a characteristic wavelength that will be absorbed. The sample is aspirated into the flame where it is atomized. When the analyte absorbs the light thus reducing its intensity, the instruments could measure the change in intensity and quantify the target metal (Beaty & Kerber 1993).
The typical basic components of AAS include hollow cathode lamp, nebulizer, flame, monochromator and Photomultiplier tube (PMT). Both AAS and AES could apply a continous atomization procedure. The aqueous sample stream will be taken into a nebulisation chamber at a controlled rate. The nebulizer servers to transform sample stream into fine aerosol spray thus introduce analyte into flame. Most instrumentation allow nebulisation chamber configuration to be adjusted for efficient atomization. The atomization would occur after aerosol spray enter flame, which could break down complexes and form atoms of target element. To achieve better atomization efficiency, the temperature of flame could be adjusted by variation of fuel types and fuel to oxidant ration. Following table shows the variation of premix flames (Beaty & Kerber 1993).
Table.1 Temperatures of Premix Flames
As the light resource, the hollow cathode lamp could provide an emission spectrum which contains the wavelengths the target atoms could absorb. The analyte could absorb some amount of light, proportional to its concentration. Then the monochromator would isolate the analytical line photons and remove scattered light of other wavelengths from flame. Thus the PMT (detector) could register the degree of absorbance and quantify based on calibration curve and the Beer-Lambert law.
Fig. 3 single beam AA spectrometer (Beaty & Kerber 1993).
The AES technique is similar to AAS, the major difference is the flame serves as the radiation source. For AES, the ground state atoms could be excited and could emit energy at certain wavelengths specific to its chemical character as relaxation (back to ground state) take place. Thus the radiative deactivation (degree of emission, proportional to the concentration) could be detected and quantified.
The advantages of AAS/AES are that: they are relatively simple and inexpensive to operate. Since be introduced for analysis, they have been widely utilized in different filed for a long time and many research and data have been established. There are some improved configurations such as graphite furnace AAS (furnace equipment is expensive) are available to increase sensitivity. However, they are still not very suitable for trace element analysis due to the drawbacks and limitations. For atomic absorption, it requires a certain wavelength corresponding to appropriate electron transition. Thus it have to address the process of creating ground state atoms and directing them to spectrometer. For AAS, the nebulization process may limit sample introduction rate and the flame temperatures might be not high enough to induce complete atomization of some refractory elements. Finally, it is a relatively slow technique and require large volumes of sample (Beaty & Kerber 1993). All these factors would affect the sensitivity. Thus, AAS/AES (generally in mg/L level) might be applied for concentrated wastewater, but less preferable for Cd analysis, especially for drinking water.
4.2. Inductively Coupled Plasma (ICP)
ICP is another analytical technique available for the detection of trace metals. The technology of ICP method was first applied in the early 1960's, had not been routinely utilized until the advent of plasma spectroscopy in the 1970's. Since then, ICP has been improved and utilized in conjunction with many other procedures for quantitative analysis. Two types of plasma have been used for excitation: the RF plasma and the DC plasma (direct current plasma, will not discussed in this paper) . ICP is a very high temperature (>6000oC) excitation source (typically with Argon gas), which could efficiently vaporize, excite an ionize atoms. An typical ICP mainly consists of sample introduction system (nebulizer), ICP torch, high frequency generator, transfer optics and spectrometer, and computer interface (Evans 2005). The more detailed working scheme of ICP will be discussed in further section.
Fig.4. schematic cross-section of ICP (adapt from lecture notes)
When compared with flame based techniques (AES OES typically combined as detection system), the major advantages of ICP could be: generally increased sensitivity (lower detection limits) and range of elements could be determined, especially for phosphorus and sulfur. Multi-elemental analysis could be accomplished simultaneously and in a short period. In addition, atomization is more complete while greatly reducing chemical interference and Ionization interference effects though cannot eliminate completely. Furthermore, ICP could enhance the lifetime of the atomized analyte which will not be propelled upwards through the flame. However, the typical detection limit of ICP-AES or ICP-OES is in mg/L range and can reach 10 - 100 ug/L, this technique may not suitable for Cd analysis of drinking water.
5. Inductively Coupled Plasma - Mass Spectrometry
Currently, in order to achieve higher sensitivity for determining trace element (both metal and non-metal), ICP system combined with sensitive mass spectrometry (ICP-MS) detection system has been widely utilized. ICP-MS system could measure analyte concentration by mass spectra rather than by emission spectra, and may lead to trace metal detection limits three to four orders than flame techniques. The principle of ICP-MS system is that: ICP acts as the ion source, when single positively charged analyte ions (for metals) are generated in Ar plasma, they will be extracted from high temperature and atmospheric pressure environment. Then the ions could enter a high vacuum enclosure via interface region. Then ionized isotopes of the elements are separated according to their mass to charge ratio (m/z) by a mass spectrometer. Subsequently it could be detected and measured by the detector, which typically is an electron multiplier (Robinson 1995). In this paper, it is considered as the most preferable technique.
5.1 the sample introduction
The sample introduction system is mainly functioned to generate fine aerosol of sample, which achieved with nebulizer and a spry chamber. Normally the sample is pumped into nebulizer via a peristaltic pump. The constant motion and pressure of the roller on the pump tubing could feed sample through to the nebulizer as a constant flow and thus the flow rate is adjustable. Once the ample enters nebulizer, it is then broken up into fine aerosol by the pneumatic action of gas flow. Then the aerosol will enter plasma with a low flow carrier gas also consisting of Ar. The spray chamber could ensure that only small droplests could enter plasma and smooth out pulses that result from peristaltic pump (Robert 2008).
5.2 Argon Plasma
argon plasma with high temperatures (> 6000 °C) is sustained at the end of a quartz glass torch that located within water-cooled copper coils. A radiofrequency (RF) potential is applied to the copper coils and producing Ar+ ions and free electrons. The sample is vapourised, atomized and ionized once introduced into the centre of the plasma. However, it may also occur in the cooler part of plasma, thus the formation of various molecular species would cause interference.
5.3 ion focusing system
The ion optics are positioned between skimmer cone and mass separation device, which consist of one or more electrostatically controlled lens components. The main function of ion optics system is to extract ions from hostile environment of the plasma at atmospheric pressure and steer them into mass spectrometer. The ion optic system could also prevent particulates, neutral species and photons plasma entering mass analyzer and detector. These species may cause signal instability and contribute to background levels (Robert 2008).
5.4 mass spectrometers
Mass discrimination is then typically undertaken with a mass spectrometer, commonly quadrupole mass spectrometer, which can act as a filter to separate ions based on their mass/charge ratio. Mechanically the quadrupole consists of four parallel surface, which could be accomplished with four longitudinally parallel round rods. Opposing surface s are connected electrically and to a RF and DC power source. A complex electromagnetic field is generated between the rods and this controls the trajectories of the ions entering the mass spectrometer. A mass spectrum could be obtained by changing the magnitude of the RF amplitude and DC potentials. Thus for a certain combination of RF and DC potentials, only ions with a specific mass/charge ratio can pass through (Watson & Sparkman 2007). Additionally, most recent ICP-MS instruments have a collision cell positioned before the quadropole thus could remove certain molecular interferences.
Fig. 5 quadrupole mass spectrometer
5.5 Detection System
The purpose of detector is to detect, amplify and measure the ions that have been filtered by the mass spectrometer. The most commonly used type of detector in ICP-MS is an electron multiplier though there are many different configurations available. Take discrete dynode electron multiplier for example, the detector is positioned off-axis to minimize the background noise. When positively charged ions emerges from quadrupole, they move through a curved path. When hitting the first negatively charged dynode, a number of electrons would be emitted. Once these electrons are attracted to the second dynode, which is positively charged, emission of further electrons occur. This process would be repeated at each dynode, thus generate a pulse of electrons that are finally capture by multiplier collector and greatly amplify the signal (Robert 2008).
Fig. 6 basic components of ICP-MS
the major advantages of ICP-MS include: the analysis by using ICP-MS could be more accurate, and could be utilized for multi-element in a broader range. Additionally, it could be used for measure the element's individual isotopes. The detection process of ICP-MS is less time consuming. However, it is relatively expensive and requires skillful staff to operate (Robinson 1995)
There are many analytical methods could be utilized for using ICP-MS determining trace elements such as isotope ratio, semi-quantitative routines and standard addition. Here it will focus on the common strategies include internal standards, external standard calibration, and isotope dilution.
The internal standard is a nonanalyte isotope which is added to the blank, standard and sample solutions before analysis. Typically three or four internal standard elements will be added to samples thus cover the target analyte. The software could adjust the analyte concentration in unknown samples by comparing the intensity value of the internal standard intensities in the unknown samples. Isotope dilution is the optimum method of calibration. This method requires the sample to be spiked with a solution of known element concentration, and isotopic composition has been enriched in one of the isotopes of the elements being analyzed. Thus isotope dilution can only be used when the analyte has two or more stable isotopes (lecture notes 2010). For most analytical methods used in water and wastewater analyses external calibration is a fundamental tool and typically used to quantify trace metals in ICP-MS. This method involves measuring a blank solution followed by a set of standard solutions. Thus a calibration curve could be created over the anticipated concentration range. Typically the blank and three standards which contain different analyte is selected. After the standards have been measured, the unknown samples could be analyzed and their analyte intensities could be obtained against the calibration curve (Robert 2008). The following protocol shows a typical calibration based on this method:
sample. N+1, etc
5.7 Typical analytical detection limits t can be achieved
Generally ICP-MS technique is more sensitive, however, the limit of detection (LOD) for determining Cd could be achieved various from different established literature. Some of them will be illustrated as follow:
Relative sensitivities detection limits (Î¼g/L): (Robinson 1995)
Detection limits and precision for ICP-MS analysis of water samples, average composition of water in streams, and maximum concentrations levels in waters in different regulations (Î¼g /L): (Fernandez-Turiel 2000)
(Brown & Milton 2005)
(Silva, Frescura & Curtius 2000)
LOD (Î¼g /L)
The variation of LODs for determining Cd might be attributed to the differences of methodology, initial sample quality and equipment. However, it clearly reflect that the ICP-MS could be utilized to quantify Cd in waters accurately and exceed the requirement of Australian Drinking Water Guidelines.
6. other available techniques
Though there is no specific requirement in Australian Drinking Water Guidelines to quantification the speciation of Cd, following methods could be combined with ICP-MS to detect the particular chemical forms of Cd in the waters. For instance, for the concentrated wastewater monitoring, hazard events or other purposes.
Firstly, Numerous speciation modeling programs exist with names such as Visual MINTEQ, PHREEQC, GEOCHEM-PC and JCHESS could be used to predict, simulate and determine the chemical form of trace metal, especially oxidation state. These programs employ quantitative mathematical models and some assumptions. However, these computational methods are limited by the other bulk parameters of water sample and comprehensiveness and availability of data.
Chromatography is also be widely used to assist ICP-MS system. It is a procedure to separate analyte element, molecule or molecules. Thus they could be analyzed subsequently and separately by different detectors. It could also be utilized for the removal of interference or as a concentration method. Chromatography covers a range of separation technology, such as gas chromatography (GC), liquid chromatography (LC) and High Pressure Liquid Chromatography (HPLC). The most common form of HPLC applied for trace metal speciation analyses is ion chromatography (IC).
Most application of colourimetric methods is used for the determination of trace metal oxidation states. This method involves the addition of reagent, which may only form a coloured compound with one oxidation state of trace metal. The concentration of this oxidation state could be measured and then another reagent is added to change the other oxidation state. Thus the total concentration of target element could be measured.
7. approaches to ensure the analysis quality
First of all, the analysis has to be done under control standards. For instance, Laboratory control standards (LCSs) are certified standards obtained from an outside agency or commercial source to check whether the data being generated are comparable to those obtained elsewhere. Calibration control standards (CCSs) are used to check calibration. Additionally, contamination may occur during any step of analysis, thus the contamination control is essential. Blanks could be used to assess the degree of contamination in any step of the measurement process, and to correct relatively constant, unavoidable contamination. The common used blanks are shown in table. 2 (Mitra 2003).
In addition, ICP-MS technique is generally more complex thanother atomic spectroscopic techniques. Thus, the routine maintenance is essential to ensure the performance of instrument. The regular cleaning and replacing of components are requied.
In conclusion, this paper focusing on the detection of trace element-Cd. Cd is a bio-accumulative toxic heavy metal and could pose numerous disease to humans. It might be present within waters at trace level. Consequently, the sampling process should be implemented carefully. For the sample extraction, three techniques, namely liquid-liquid extraction (LLE), cloud point extraction (CPE) and solid phase extraction (SPE), are introduced. After the comparison and evaluation of AAS/ AES, ICP and ICP-MS strategy, ICP-MS method is proposed as the suitable approach to be taken for accurate quantitation of Cd. The detailed instrumentation, typical detection limits and quantification methods are illustrated. In the final parts, it provide some available mechanisms to determine the speciation of Cd and procedures to ensure the quality of determination.
Table. 2 types of blanks