ISSN 1450-216X / 1450-202X Vol. 153 No 1 May, 2019, pp. 31-41 http://www. europeanjournalofscientificresearch.com
A Simplified Method for the Determination and Speciation of Vanadium within Environmental Matrices in UAE
(Examples from Dubai)
Iman Boukhobza
Corresponding Author, Dept. Interdisciplinary Studies Zayed University, Dubai, United Arab Emirates. PoBox: 19282
Tel: +00971-4-4021-342; FAX: +00971-4-4021-008 E-mail: Iman [email protected]
Zahra Saadatmand
Department of Interdisciplinary Studies Zayed University, Dubai, UAE
Robert Boldi
Department of Environmental Sciences Zayed University, Dubai, UAE
Abstract
Vanadium toxicity depends on the oxidation states of the metal ions, thus increasing the importance of speciation studies that evaluate the most abundant vanadium species known as vanadate (VVO3-) and vanadyl (VIVO2+) within a variety of matrices. In this work, a simplified method that uses HPLC- UV/Vis was developed to separate and quantify V(IV) and V(V) within sea water and soil samples from a public park in Dubai, United Arab Emirates (UAE). Different months’ sample collection and sample preparation have been undertaken, and results have shown that vanadium speciation and quantification within environmental samples are possible although challenging. The separation and speciation studies were performed by using an eluent containing Ethylenediaminetetraacetic acid (EDTA) and Tetrabutylammoniumperchlorate (TBAP).
Detection limits were found to be 0.1628 µg L-1 and 0.3132 µg L-1 of V(IV) and V(V), respectively. For both types of samples (sea and soil), V(IV) was found to be the most dominant species. The results of this work are aimed to increase awareness about the quality of the UAE environment as per pollutants’ concentration. And therefore, spread the culture of “protecting versus curing”.
Keywords: Vanadium, vanadium characterization, vanadium species, HPLC, environmental contamination, seawater samples, soil samples, UAE.
1. Introduction
Vanadium is a transition metal that is among the 20 most abundant elements in the earth’s crust. It can exist in different oxidation states ranging from +2 to +5. In the presence of oxidizing agents, it is always found in the +5 oxidation state, while in the presence of reducing agents, it can exist in the +4 oxidation state [1-3]. The known average concentration of vanadium in the earth’s crust is 150 µg g-1
[3]. Specifically in soil, this concentration is estimated to be in the range 3-310 µg g-1 [4], 0.2-100 µg L-1 in fresh water [3], 0.2-29 µg L-1 is sea water [4]. Environmental contamination, due to the presence of pollutants including vanadium, has emerged as a potential concern to public health in modern societies. In fact, vanadium is known to be anthropogenically released to the environment through industrial and other related activities. Vanadium released into the environment from natural resources has been estimated to be around 65,000 tonnes per year, and that released from anthropogenic sources has been estimated to be around 200,000 tonnes per year [5,6]. Previous work has presented the total amount of vanadium in soil within contaminated areas in South Africa, and the concentrations were quite high and varied from 1570 to 3600 µg g-1 [7]. Also, in studies conducted in Egypt, the amount of vanadium within sediments was reported to be 575 µg g-1 in the eastern Mediterranean Sea and 214 µg g-1 in the red sea [8]. Within golf region (GCC), total vanadium amount in marine sediments were found to be 21.9 µg g-1 in Bahrain [9], and 30.19 µg g-1 in Kuwait [10]. Within sea water, examples of vanadium concentration have been published and values have ranged from 1.6 µg L-1 in US coastal marine [11], 2.08- 2.60 µg L-1 in Saudi Coast of the Arabian Gulf [12]. Many studies have shown that vanadium has anti-carcinogenic and anti-diabetic effects [13-20], yet, the effect of higher concentration of vanadium on human health must be carefully considered. Similarly to other known trace elements [4,21,22], vanadium toxicity has been studied and compared [23-29]. In large amounts, vanadium can become toxic, and vanadium poisoning can lead to coughing, vomiting, diarrhea, anemia and higher risk of getting lung cancer [30]. Furthermore, chronic exposure to vanadium revealed problems in the respiratory system. Few studies have demonstrated the concentration-effect connection at exposures of low level and the increased prevalence of observed irritative symptoms [31,32]. In case of exposure, the lungs are the site of vanadium absorption and vanadium gets accumulated over years [4]. After ingestion, the absorption of vanadium compounds has shown to be dependent on their solubility and their chemical structures [33]. Finally, human skin didn’t show a major absorption of vanadium metal ions [34].
In the past, several spectroscopic [35,36], and separation [35] methods have been reported for the characterization and quantification of vanadium. The first category includes many techniques such as atomic absorbance spectrometry (AAS) [37,38], inductively coupled plasma optical emission spectrometry (ICP-OES) [35,38,39], inductively coupled plasma mass spectrometry (ICP-MS) [35], and in some cases X-ray fluorescence spectrometry (XRF) [38,40,41]. These techniques measure total vanadium, with no distinction between vanadium atoms in different oxidation states. For the second category, a variety of methods have been used including capillary electrophoresis (CE) and liquid chromatography (LC and HPLC) [38,40,41]. Other methods that are based on spectrophotometric [35,39,42,43] techniques and potentiometry have as well been utilized for the determination of vanadium compounds [39,44-46]. High performance liquid chromatography (HPLC) is one of the known, less expensive and easy to maintain techniques that facilitates separation, characterization and quantification of different metal ions within a variety of matrices. It allows the choice between reversed phase (RP) and normal phase (NP) [47,48]. The RP-HPLC methods are utilized by choosing the reversed phase stationary systems and have gained a lot of popularity as it allows adaptation to other modes such as ion pair (IP) and ion exchange (IE: anion exchange and cation exchange) and micellar [47-49]. The other final aspect that made HPLC a special technique for separation, characterization and quantification of many compounds is the possibility of using a variety of detectors.
Both major species of vanadium (V(IV) and V(V)) may be released into the environment as pollutants, thus the determination of these vanadium forms rather than the total vanadium is important during the evaluation of toxicity and various risks to humans, as well as its role within environmental biological and clinical samples [1,2,25,27,50-52]. For this reason, speciation analysis of vanadium within a matrix can yield information on the individual concentration of vanadium in different oxidation states. Furthermore, vanadium(IV) is known to be susceptible to oxidation with air and high
pH, however when bound to an organic ligand such as EDTA, they are known to form very stable vanadium complexes [VIVO(EDTA)]2- and [VVO2(EDTA)]3- (Figure1). Wann and Jiang have used HPLC- ICPMS to measure vanadium species within sea water samples [53]. On the other hand, Colina et al. have applied similar procedures to characterize and identify different species within sediments, mussels and fish muscle tissues [54]. Others, have reported the use of 2,6-pyridinedicarboxilic acid chelating reagent for vanadium speciation [55]. All these methods have shown high detection limits, low sensitivity and lengthy procedures [56-58].
This work aims to understand the distribution and fate of vanadium species within the UAE environment for the first time. To the best of our knowledge, these metal species have not or scarcely been investigated in the UAE. We provide here a simplified method, based on the HPLC technic, for monitoring vanadium species pollutants within surface soil and sea water collected at Jebel Ali Public Park in Dubai (UAE). The environmental samples hot spots are found within the narrow costal region located in the south of Dubai. The area is at the proximity to the Jebel Ali region hosting the major industrial activities in the city. We also demonstrate the applicability of the simplified preparation procedure to the analysis of vanadium for certified reference material HISS-1 using ICP-OES technique. The investigation also aims to verify whether or not the amount of vanadium species pollutants is time dependent and whether or not the park at the vicinity of the industrial zone is at high pollution risk.
Figure 1: Structures of [VIVO(EDTA)]2- and [VVO2(EDTA)]3-
2. Experimental 2.1. Instrumentation
The HPLC system utilized in this study was Agilent 1100 series with an isocratic pump (Agilent technology, Waldbronn, Germany), and a promosil C18 column (4.6 mm x 250 mm x 5 µm id). The mobile phase was chosen to be 0.50 mM TBAP (Tetrabutylammoniumperchlorate) in 4.0 % methanol at a pH around 8.00, and 5.0 mM EDTA (Ethylenediaminetetraacetic acid disodium salt dihydrate) at a pH around 7.00. The temperature of the HPLC column was ambient (22 °C), and the injection volume was 20 µL. The Agilent 1100 series variable wavelength detector was used for detection (UV/Vis) at 200 nm. For data acquisition and analysis, the online Agilent ChemStation software was used. Prior to that, and for the identification of the proper wavelength for the vanadium complexes’ maximum absorption, the Agilent technologies Cary 100 UV-Vis spectrophotometer was utilized. The pH meter used was 370 Jenway model, calibrated using buffer solution at pH 4.01 and 6.86. For the soil samples, an open digestion system was used and a bain-marie was utilized to control the temperature.
Centrifugation was undertaken using Hettich EBA 20.
The ICP-OES measurements were performed with a 700 series ICP-OES (Agilent Technologies, USA). The radiofrequency power was set up 1550 W. The gas used was Argon. The plasma flow was 15 L/min, the nebulizer flow was 0.85 L/min and the sample flow rate was 1.5 ml/min. Peristaltic pump tubing was employed to carry the sample. 51V was set for detection as it has the highest abundance. And the wave length used was 292 nm.
2.2. Reagents
For solution preparation, deionized distilled water was used. All solvents and chemicals were of analytical or HPLC grade. V(IV) and V(V) stock solutions were prepared by dissolving vanadium(IV) oxide sulfate hydrate, VOSO4.H2O (Aldrich) and ammonium mono vanadate NH4VO3 (Merck) respectively in a solution of 2% Hydrofluoric acid, HF (Merck) in EDTA. Mobile phases were prepared from CH3OH (Merck), TBAP (Fluka) and EDTA (Alfa Easar). Eluents were prepared fresh and pH was checked. The pH of various solutions was controlled by using hydroxide sodium solution (NaOH) and hydrochloric acid solution (HCl) at a variety of concentrations. The soil certified reference material CRM (HISS-1) was obtained from NRC (natural research council in Canada).
3. Method
3.1. Preparation of Vanadium Solutions
Firstly, a solution of HF 2% in EDTA (5.00 mM) was carefully prepared. V(IV) and V(V) stock solutions (0.01 g in 5 ml) were prepared from VOSO4.H20 and NH4VO3 in 2% v/v HF in EDTA (5.00 mM). Standard solutions were freshly prepared by dilution with water, filtered through a 40 µm filter and sonicated. The V(IV)-EDTA and V(V)-EDTA standards were stables for few months.
3.2. Sample Preparation and Extraction
The environmental matrix in this study consisted of surface soil and sea water samples. These were collected from the coastal region of Jebel Ali south Dubai, (Jebel Ali public beach park, Lattitude:
24°59’11.87” N; Longitude: 55°01’07.46” E), once a month for a period of seven months (2016-2017).
Control sea water and soil samples were collected from a region located far from Jebel Ali industrial area in the north of Dubai, (Almamzar park, Lattitude: 25°18’58.42” N; Longitude: 55°20’34.42” E).
Figure 2: Location of Jebel Ali park (up) and Almamzar park (down)
Sea water samples were collected in polyethylene bottles and kept under room temperature and normal atmospheric pressure. Sample preparation was undergone by simple filtration using Millipore filtration system (40 µm) followed by pre-concentration using rotavapor to an approximate volume of 1.00 ml of sample. Surface soil samples (sand) were collected from the same open area and stored in labeled polythene bags for analysis. These were first left to dry in room temperature in order not to alter the chemical composition that may be affected at high temperature used in ovens. Then were passed through a 2.00 mm sieve. 0.10 g of each of the soil samples were weighted in polyethylene tubes. 3.00 ml solution of 2% v/v HF in EDTA (5.00 mM) was added. Tubes were inserted into a 200.0 ml beaker containing 70.0 ml of water. A bain-marie was used to maintain the water temperature at 90°C for 40 minutes. After cooling down, samples were centrifuged for 10 minutes at 1750 rpm, then diluted 2, 5 and 10-folds. Finally filtered through Millipore 40 µm porosity before the separation and speciation process. For both sea water and soil samples, the concentration of V(IV)-EDTA and V(V)- EDTA was determined by spiking the sample with suitable quantities of V(IV) and V(V) during sample preparation.
3.3. Calibration and Detection Limits
External calibration was undertaken with five different concentrations, for each vanadium species, over the range 1-100 µg L-1. R2 coefficients were above 0.99 for both vanadium species. For all solutions, triplicate analyses have taken place and average values as well as standard deviation (SD) were calculated. Limits of detection (LOD) were calculated based on the concentrations equivalent to three times SD. They were found to be 0.1628 µg L-1 and 0.3132 µg L-1 of V(IV) and V(V), respectively. In order to validate the simplified method, a certified material (HISS-1) was utilized, and ICP-OES technique has been used to quantify the total vanadium present in the soil certified material.
4. Results and Discussion
Vanadium, like many other metals, occur naturally in the environment. In addition, these metals can be released to waters and soil by anthropogenic emissions via industrial effluents, surface run off and aerial fallout. Therefore, understanding the vanadium chemistry in terms of sorption and toxicity, speciation is very important to enable improved environmental risk assessments. Normally, the measured total vanadium concentration in environmental samples is compared with generic guideline values. However, vanadium amounts are known to vary considerably between different matrices. Thus, the probability of either over or under estimating the toxicity risk is quite high, hence the importance of vanadium speciation within environmental samples increases [59]. V(V) is known to be more toxic than V(IV), because V(V) can inhibit numerous enzymes and is a potent inhibitor of the plasma membrane sodium/potassium-ATPase [60-62].
The present work has focused on samples collected from a public park near Jebel Ali industrial area in Dubai, UAE. This zone is known to include many industries and manufacturing companies, such as metal, glass, chemical and oil industries. For collected samples HPLC-UV/Vis measurements were performed on dissolved vanadium. In both cases, trials were first undertaken to directly identify and speciate vanadium within sea water and soil samples. However as previously reported and due to low concentrations, these trials were not successful [63,64]. Therefore, chelating reagents and careful sample preparation were needed prior to separation and speciation. Characterization and quantification of V(IV) and V(V) were made possible by using a reversed phase ion pair HPLC chromatography.
Within sea water and soil samples, complexation of the metal ion with EDTA was undertaken prior to the separation process of [VIVO(EDTA)]2-and [VVO2(EDTA)]3-. Figure3 shows a chromatogram from HPLC separation using a C18 column and UV/Vis detection of these vanadium species within a standard solution. In this method, many parameters are needed to be verified and the concentration of EDTA in methanol, TBAP and the pH needed to be optimized. In this study, the method suggested by Kuo et al. was adopted with some modification at the level of sample preparation [65]. Separation and
speciation conditions were carefully chosen and both sea water and soil samples were diluted prior to HPLC analysis. In the UAE, and to the best of our knowledge, this is the first report on the amount of vanadium species in environmental matrices, using HPLC.
Figure 3: HPLC-UV/Vis chromatogram analysis of V(IV)-EDTA and V(V)-EDTA within a standard solution.
(2.4087 µg L-1 of V(IV) solution, and 3.3392 µg L-1 of V(V) solution)
4.1. Soil Samples
Soils are known to be a major sink for metals, thus vanadium characterization and speciation in soils is very important when evaluating vanadium toxic risks. Most analytical methods have shown procedural complexity, and poor results due to the low concentrations of vanadium within environmental samples.
HPLC-UV/Vis analysis of soil samples, from Jebel Ali public park, collected monthly for a period of seven months (2016-2017) were obtained and V(IV) and V(V) species were separated and quantified.
Chromatogram analysis has shown that both V(IV) and V(V) were detectable at different retention times. The concentrations vary slightly with time during a period of seven months, and in all samples, V(IV) species were the most predominant. A similar dominance of V(IV) within sediments was observed in early studies conducted in Venzuela and south Africa [7,54]. In the present work, vanadium concentrations during the month of December were found to be 72.4033 µg g-1 and 38.9228 µg g-1 for V(IV) and V(V), respectively. During the month of February, their concentration slightly increased to 77.6553 µg g-1 and 43.0931 µg g-1 whereas in April they were found to be 74.4472 µg g-1 and 45.5802 µg g-1, respectively. Table1 presents the results of the investigated vanadium species. The values are attributed to soil samples collected from the superficial part of the beach soil sand.
To further investigate whether vanadium species concentration depends on the suspected high pollution level at the collection site, soil samples were collected from Almamzar park that is expected to be less polluted than Jebel Ali park, as it is located in an urban area. The results related to the concentration of V(IV) and V(V) were calculated and have clearly shown that there is a noticeable difference in the concentrations of both vanadium species between the two parks. Results have demonstrated that in Jebel Ali park, which is close to an industrial area, there was around 20% more V(IV) and 65% more V(V) species compared to Almamzar park during the month of December.
Results from this study demonstrated that, environmental matrices that are close to industrial activities absorb more vanadium species than other matrices located far from industrial zones. This may be generalized to other metals.
Furthermore, it is well documented that metals get to the soil through a slow process of adsorption [66]. In fact, the concentrations of both V(IV) and V(V) in the collected surface soil samples did not show a great difference during a period of seven months, and thus extended and prolonged sample collection and analysis are required. In order to prove that our simplified developed method is suitable, a soil certified reference material was analyzed. Vanadium from the sample solution was subjected to ICP-OES using the same conditions utilized during HPLC analysis. As can be observed, the recovery of vanadium has been studied by spiking suitable amount of vanadium to the sample prior to extraction. The concentration of vanadium present in the certified reference material was quantified by external calibration. The amount of vanadium measured in this study was found to
be 6.78 ± 0.072 mg/kg which is in good agreement with the certified value of (6.80 ± 0.78 mg/kg), and the recovery efficiency was 99.7%.
Table 1: Concentration of vanadium species in soil samples. Values are means of triplicate measurements ± standard deviation. Control is the sample collected from a public park located in a non-industrialized zone. These concentrations were determined by spiking 0.1 g of soil samples with specific small amount of each of V(IV) and V(V).
V(IV) (µg g-1) V(V) (µg g-1)
November 67.1676 ± 0.2728 35.9559 ± 0.5834
December 72.4033 ± 0.3777 38.9228 ± 0.3302
January 61.2717 ± 0.3032 41.5118 ± 0.5182
February 77.6553 ± 0.5287 43.0931 ± 0.5770
March 79.2980 ± 0.5305 42.3615 ± 0.5474
April 74.4472 ± 0.5491 45.5802 ± 0.3333
May 74.9655 ± 0.3824 47.5416 ± 0.2814
Control (December) 57.9335 ± 0.3885 13.4552 ± 0.5311
4.2. Sea Water Samples
Vanadium metal ions abundant in water are released naturally by wet and dry deposition, erosion of soil and leaching from soils and rocks [67]. Yet, this abundance can be altered by a variety of anthropogenic activities. Coastal marine pollution with metals, including vanadium, is a major concern, because it may lead to the depletion of marine species [68]. It can also cause a high risk to human health through inhaling or eating contaminated marine products [69]. Metal pollutants tend to accumulate in the bottom sediments, yet some of it remain dissolved in water. Vanadium contamination in sea water has mostly been investigated by evaluating vanadium absorption by many marine animals such as ascidians [70]. Yet, fewer other studies have focused on the dissolved vanadium in sea water [71]. Many investigations that have been directed to the determination of vanadium in seawater were mainly focusing on the characterization of total vanadium. However, to assess the exact risk, it is important to verify the metal ion valency speciation [72].
In the present study, sea water samples were collected from Jebel Ali public park for a period of seven months. The method suggested in this paper is based on the transformation of all dissolved vanadium species in sea water into complexes by using EDTA as a chelating reagent. Sample preparations were undertaken using similar to developed and simplified method utilized for soil samples described earlier [65]. Diluted samples were prepared before HPLC-UV/Vis analysis, and both V(IV) and V(V) species were detected. Results have shown that the amount of V(IV) and V(V) dissolved in sea water have changed noticeably during the period study time. For instance, values during the month of December were 4.3978 µg L-1 and 1.7999 µg L-1 for V(IV) and V(V) respectively.
During February, these numbers have increased and were noted to be 10.2673 µg L-1 and 3.9873 µg L-1 for V(IV) and V(V) respectively. During the month of May, the HPLC-UV-Vis analysis have shown smaller concentration of V(IV) and V(V) species. These number were found to be 1.0328 µg L-1 and 0.4023 µg L-1 respectively. In contrast to soil samples where vanadium species accumulation did not show a great change during a period of seven month due to the slow process of absorption, sea water samples have shown a difference in the concentrations of vanadium species. This can be interpreted by the change in the climate. In fact, on February, in Dubai, temperatures are known to be moderate, but humidity is known to be high (an average of 25°C high and 15°C low temperature, and 65% relative humidity). Whereas, in May, temperature values are known to be high, while humidity values are moderate (an average of 38°C high and 25°C low temperature with 53% relative humidity). In the literature, few studies have demonstrated that metal uptake generally increased with increasing soil moisture (high humidity) [73]. On the other hand, temperature may also influence amounts of metal accumulation by environmental matrices. High values of temperature may affect both influx and efflux rates of metals [74,75].
Figure 4: HPLC-UV/Vis chromatogram for the separation of vanadium species in a sea water sample collected on November. The sample was spiked with 0.1675 µg L-1 of V(IV) and 0.3289 µg L-1 of V(V)
In this study, V(IV) metal ion was found to be the most predominant compared to V(V) species within sea water samples during a period of seven months. Another speciation study has shown that V(V) was the most dominant species in coastal sea water [75]. Sea water samples collected from Almamzar park were also analyzed for comparison. In December, V(IV) and V(V) concentrations were found to be 1.2482 µg L-1 and 0.4894 µg L-1, respectively. These amounts are very small compared to those of vanadium species collected during the same month from Jebel Ali park. Results have shown that there is 71.61% more of V(IV) and 72.81% more of V(V) in the Jebel Ali park samples, compared to Almamzar park that is expected to have low pollution because of location.
Finally, it is worth describing here that most of the studies have investigated metal accumulation in the sea by using marine organisms or marine sediments. Often, the metal level in the sediment and the organism is proportional to the amount of dissolved metal in sea water. Furthermore, the concentration of dissolved metals in sea water may depend on seasonal variations such as temperature and tides [76]. Thus, for a short-term analysis of the status of metal uptake in sea water, measuring dissolved metals would give a good estimation. Nevertheless, for a long-term understanding, using sediments or marine organisms would provide better results as per the status of the metal accumulation. Table 2 presents measurements of the investigated vanadium species. The values are attributed to the superficial part of the coastal sea water.
Table 2: Concentration of vanadium species in seawater samples. Values are means of triplicate measurements ± standard deviation. Control is the sample collected from a public park located in a non-industrialized zone. These concentrations were determined by spiking 1 ml of concentrated seawater samples with specific small amount of each of V(IV) and V(V)
V(IV) (µg L-1) V(V) (µg L-1)
November 3.3224 ± 0.0549 1.4010 ± 0.0333
December 4.3978 ± 0.0844 1.7999 ± 0.0212
January 6.3742 ± 0.0495 2.7737 ± 0.0311
February 10.2673 ± 0.0425 3.9873 ± 0.0327
March 4.0032 ± 0.0333 2.0456 ± 0.0268
April 1.6663 ± 0.0331 0.6760 ± 0.0377
May 1.0328 ± 0.0549 0.4023 ± 0.0415
Control 1.2482 ± 0.0333 0.4894 ± 0.0549
5. Conclusion
The purpose of this work was to estimate pollution status of vanadium species in soil and sea water samples collected from Jebel Ali public beach park during a period time of seven months (2016-2017).
Using a simplified preparation method and HPLC analysis, speciation results indicated some evidence that the studied site is polluted, suggesting inputs from anthropogenic sources located at the vicinity of
the park. For both environmental samples, vanadium species concentrations were compared with those of Almamzar park, and revealed that vanadium species are more predominant in the first park. This pollution, if left unchecked, could threaten mankind and marine life in this area. For both types of samples, HPLC analysis has shown that V(IV) species was the most abundant, as in all cases the concentration of V(IV) was much higher than that of V(V). Speciation has also shown that beach soil sand accumulates vanadium species slowly as there is no considerable change in species concentration over a time period of seven months. For sea water samples, the present study has shown that a simple method can be utilized to quantify dissolved vanadium species in their different oxidation states. Yet, the quality of these estimations is limited due to seasonal variations. Thus, using the same simple method, better estimation of the amount of vanadium species in sea water can be undertaken by studying speciation within marine sediments or organisms.
6. Acknowledgment
This research was supported by a research incentive fund (RIF grant) from the office of research in Zayed University, Dubai, UAE. The authors would like to thank Dr. Michael Allen and Ms. Ingrid Liekens, from the office of research for their unlimited support and help provided during different phases of the project. The authors are also grateful to Mr. Abdelbari L Bennani, for their feedback and input. IB thanks Ms. EL shaimaa Sakr from the library of Zayed University at the Dubai campus for her help and assistance in providing and ordering various references. Her reliable help made the preparation of this work possible.
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