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08Jul

Selenium Analysis – Chemistry Assignment

This is solution Selenium Analysis Chemistry Assignment, describes all aspects of chemical element selenium with its application in general life.

Selenium

Selenium is chemically related to sulfur and tellurium. Pure elemental state of selenium is rarely found in nature.

Chemical symbol: Se

Atomic number: 34

Atomic mass: 78.96

Chemistry of selenium compounds

Selenium dioxide (SeO2) and selenium trioxide (SeO3) are the two oxides of selenium. Elemental selenium is made to react with oxygen to give selenium dioxide (House, 2008):

Se8 + 8 O2 → 8 SeO2

It is a polymeric solid that forms monomeric SeO2 molecules in the gas phase. It dissolves in water to form selenous acidH2SeO3. Selenous acid can also be made directly by oxidizing elemental selenium with nitric acid (Wilberg, Wilberg and Holleman, 2001):

3 Se + 4 HNO3 → 3 H2SeO3 + 4 NO

Salts of selenous acid are called selenites. These include silver selenite (Ag2SeO3) and sodium selenite (Na2SeO3).

Selenium form Periodic Table of Elements for Selenium Analysis Chemistry Assignment

Hydrogen sulfide reacts with aqueous selenous acid to produce selenium disulfide:

H2SeO3 + 2 H2S → SeS2 + 3 H2O

Selenium disulfide consists of 8-membered rings of sulfur atoms with selenium replacing some of the sulfur atoms. It has an approximate composition of SeS2, with individual rings varying in composition, such as Se4S4 and Se2S6. It has various applications, including use in shampoo as an anti-dandruff agent, an inhibitor in polymer chemistry, a glass dye, and a reducing agent in fireworks (Wilberg, Wilberg and Holleman, 2001).

Unlike sulfur, which forms a stable trioxide, selenium trioxide is unstable and decomposes to the dioxide above 185 °C (House, 2008):

2 SeO3 → 2 SeO2 + O2

Selenium trioxide may be synthesized by dehydrating selenic acid, H2SeO4, which is itself produced by the oxidation of selenium dioxide with hydrogen peroxide:

SeO2 + H2O2 → H2SeO4

Hot, concentrated selenic acid is capable of dissolving gold, forming gold(III) selenate.

Selenium reacts with fluorine to form selenium hexafluoride:

Se8 + 24 F2 → 8 SeF6

Unlike its sulfur counterpart (sulfur hexafluoride) however, chemical element SeF6 is more reactive and is a toxic pulmonary irritant (Proctor and Hathaway, 2004). It can cause frostbite and severe irritation on contact with skin (Vincoli, 1997).

Other selenium halides include SeF4, Se2Cl2, SeCl4, and Se2Br2. Selenium dichloride (SeCl2) is an important reagent in the study of selenium chemistry. It is prepared in pure form by reacting elemental selenium with SO2Cl2 in THF solution (Xu and Devillanova, 2007). Some of the selenium oxyhalides, such as SeOF2, are useful as nonaqueous solvents (House, 2008).

Like oxygen and sulfur, selenium formsselenides with metals. For example, reaction withaluminium formsaluminiumselenide (House, 2008):

3 Se8 + 16 Al → 8 Al2Se3

Other selenides include mercury selenide (HgSe), lead selenide (PbSe), and zinc selenide (ZnSe). An important selenide is copper indium gallium diselenide (Cu(Ga,In)Se2), a semiconductor. { Click to understand about Chemical Engineering Assignment }

Selenium does not easily react with hydrogen, H2, although H2Se is formed when selenium vapour, mixed with oxygen-free hydrogen, is being passed through a combustion tube filled with pumice fragments and heated to 350ºC. Hydrogen selenide, H2Se, is colorless and far more poisonous than its analogue hydrogen sulfide, H2S, with a very unpleasant odor that reminds of rotten radishes. It is more water soluble than H2S and can also be prepared by first reacting selenium with a metal to produce a selenide, and then protonating the selenide anion with an acid to produce H2Se (House, 2008).

Isotopes

Selenium has six naturally occurring isotopes, five of which are stable: 74Se, 76Se, 77Se, 78Se, and 80Se. The last three also occur as fission products, along with 79Se, which has a half-life of 327,000 years. The final naturally occurring isotope, 82Se, has a very long half-life (~1020 years, decaying via double beta decay to 82Kr), which, for practical purposes, can be considered to be stable. Twenty-three other unstable isotopes have been characterized.

Organic compounds of selenium

Selenocysteineis a selenol-containing amino acid that is encoded in a special manner by DNA.Selenomethionine is a selenide-containing amino acid that also occurs naturally, but is generated by post-transcriptional modification. Glutathione oxidase is an enzyme with a diselenide at its active site.

Chemistry of organic compounds of selenium

Vinylicselenidesare organoselenium compounds that play a role in organic synthesis, especially in the development of convenientstereoselective routes to functionalized alkenes. Although various methods are mentioned for the preparation of vinylicselenides, a more useful procedure has centered on thenucleophilic or electrophilicorganoselenium addition to terminal or internal alkynes. For example, thenucleophilic addition of selenophenol to alkynes affords, preferentially, the Z-vinylicselenides after longer reaction times at room temperature.The reaction is faster at a high temperature; however, the mixture of Z- and E-vinylicselenides was obtained in an almost 1:1 ratio. Conversely, vinylicselenides can be prepared by palladium-catalyzed hydroselenation of alkynes to afford the Markownikov adduct in good yields. There are some limitations associated with the methodologies to prepare vinylicselenides illustrated above; the procedures described employ diorganoyldiselenides orselenophenol as starting materials, which are volatile and unstable and have an unpleasant odor. Also, the preparation of these compounds is complex.

Allylic oxidation is an organic oxidation converting anallylic methylene group into anallylic alcohol or a ketone. This chemical transformation is an important organic reaction. Selenium dioxide is one representative of a group of oxidizing agents that can bring about this reaction for example incycloheptatriene oxidation totropone and potassium dihydrogen phosphate (Dahnke and Paquette, 1993), β-Pinene oxidation with hydrogen peroxide (Coxon, Dansted and Hartshorn, 1977) andcyclohexanone oxidation to 1,2-cyclohexanedione (Hach, Banks and Diehl, 1952).

This type of reaction often involves free radicals but in some cases apericyclic concerted reaction FGF mechanism is found for selenium oxide oxidations. The first step is anene reaction, transferring theallylic proton to the selenium oxide, and the second step is asigmatropic reaction.

Oxidations involving selenium dioxide are often carried out with catalytic amounts of the selenium compound and in presence of a sacrificial catalystor co-oxidant such as hydrogen peroxide.Selenious acid (H2SeO3) andpyridiniumchlorochromate are other oxidizing reagents.

In presence of a β-proton, a selenide will give an elimination reaction after oxidation, to leave behind an alkene and a selenol. In the elimination reaction, all five participating reaction centers are coplanar and, therefore, the reaction stereochemistry is syn. Oxidizing agents used are hydrogen peroxide or ozone. This reaction type is often used with ketones leading toenones. An example is acetylcyclohexanone elimination withbenzeneselenylchloride and sodium hydride (Renga and Reich, 1979)

Selenium in geology

Geological samples have low amounts of selenium. Selenium concentrations are relatively high in coals and crude oils (several hundred mg/kg), while in rocks, it is similar to that found in soils and sediments and vary geographically with the parent rock (low mg/kg). Selenium concentrations in the soil vary drastically with the geological evolution of the parent rock, and change over short distances depending on differences in bedrock from which they are derived. Selenium levels are inversely related to the altitude where the soil samples are derived from, and vary from 0.01 mg/kg in deficient areas to 1,200 mg/kg in toxic areas. Guideline for Se is 1.0 mg/kg for agricultural, residential and park lands, and 3.9mg/kg forindustrial and commercial lands.Se concentration ofCanadian soils was found to range from 0.03 to 2mg/kg, with an average of 0.26mg/kg. Se sediment quality guideline in British Columbia is 2mg/kg.

Selenium in water

Typical Se concentrations in ambient waters are less than 1μg/L in the absence of direct Se sources.The background Se concentration of marine waters is 0.02 – 0.04μg/L; it has not beendetermined systematically what a corresponding background concentration would be forfresh waters, but is seems reasonable to assume that it would be comparable to the marinereference value. In waters under the impact of geogenic or anthropogenic Se emissions, Seconcentrations are typically in the range of 1-10μg/L, and can occasionally exceed 100μg/Lin exceptional cases. Se in industrial effluents can exceed 1,000μg/L under rarecircumstances, but is usually in the 10 – 100μg/L range. Current Selenium water quality regulatory guidelines include 5μg/L in the US and 1μg/L in Canada.

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Selenium in animals

Selenium is toxic to animal life when it occurs in sufficiently high concentrations. Eventhough its geological concentrations are normally quite low, its toxic threshold is also lowerthan for most other elements commonly considered as toxins in the environment. The UK reference nutrient intake (RNI) of 75μg per day for men and 60μgper day for women has been determined as the amount believed to be necessary tomaximize the activity of the antioxidant glutathione peroxidase (GPx) in plasma. The American recommended dietary allowance (RDA), set at 55μg perday for both men and women, is based on the investigations of the Se intake required toachieve plateau concentrations of plasmaGPx. The WHO/FAO/IAEA expert group recommended an intake level of only 40μg per day formen and 30 μg per day for women, assuming only two-thirds of the full expression of GPxactivity is required. However, it is generally believed then the intake levelsneeded should be approximately 80-100μg per day. (See here: Helpful Disaster Management Assignment)

Analytical methods for determination of selenium

Various analytical methods have been used in the past for determination of trace levels of selenium in a variety of biological, physiological and medical samples. Some of them from the last thirty years are: neutron activation analysis,NAA (35%); atomic absorption spectroscopy AAS(22%); gas chromatography, GLC (12%); spectrophotometry(4%); x-ray fluorescence analysis (4%); and otherfluorimetric and potentiometric methods.

Sampling

Care must be taken during biological sampling to make ensure the samples be representative. Sampling in urine should be done as soon as possible into ultrapure polyethylene vessels. The estimation of selenium could be done using neutron activation analysis, following a series of freeze-drying and irradiation operations to reduce the danger of contamination. Depending on the pH and composition of the solution, the storage of aqueous solutions of selenium could result in losses.

For water samples, only filtration and concentration are required due to low selenium content. Air sampling, however, is difficult due volatility of its compounds present either in gas, fluid or solid states. Presence of these does not allow filtration due to high vapor pressures. Due to which, cellulose filters and liquid impingers have been developed for air sampling. Absorption in water has also been recommended by authors as an efficient method of air sampling for selenium. Water is the best absorbent forselenium due to the high water solubility of selenicand selenious acids formed in the presence of moisturefrom selenium oxides. ( Explore Reconciliation Assignment Help)

Review of methods for analysis of selenium

  1. Fluorometry

Selenium is an essential part of human and animal diet, but only at low concentrations. At high concentrations it is fairly toxic. WHO has permitted a maximum Selenium concentration of 10 μg/L in water. However, due to narrow difference in permitted and toxic concentrations, it is essential to select a highly accurate method to measure on-site selenium levels of concentrations. Most general and widely used methods for determining selenium are atomic emission spectrometry, atomic emission spectrometry, atomic fluorescence spectrometry, and inductively coupled plasma-mass spectrometry. A few of them use chromatography, while others use flow techniques such as decreasing time of analysis. Although, upon further review it turns out that these techniques are only suitable for laboratory use and the equipment for instrumentation is fairly expensive.

While conventional methods use spectrophotometry and electroanalysis, the research method used in this paper is based on a sensitive and simple method which uses fluorometry. The device is simple and sensitive and can, therefore, be used for on-site screenings of selenium. The high degree of accuracy in the device allows sampling of selenium near toxicity thresholds.

The experiment involves an on-site use of a portable spectrofluorometer comprising of a LED as a light source, an LED driver, a microsyringe as a cell, an optical fibre cable, a CCD spectrometer and a personal computer. The use of LED and CCD spectrometer allows reduction in overall size and mass of the device. The device can run for 3 hours without requiring recharge of batteries. The use of microsyringe as a cell allows less consumption of reagents and sample solution. The performance of this device was compared with a standard spectrofluorometer and the applicability was tested by determining selenium in river water. The entire process only took 15 minutes and the selenium detection levels were measured to the detection limit of 0.5 μg/L (Suzuki, Hashigaya and Kawakubo, 2010).

The effect of LEDs and sensitivity evaluation was done using a fluorescin solution. For the fluorometric determination of selenium, 2,3-diaminonaphthalene (DAN) was used and the reaction resulted in formation of selenium chelate, which was later extracted using cyclohexane. Fluorescence of the sample was measured by sucking the organic phase into the microsyringe. This principle was then applied to on-site determination of selenium in river water. A pre-determined amount of selenium was added to the sample, in order to examine recovery. The recovery after the process was satisfactory and detection of selenium in the sample was measured below the detection limit

The sensitivity, reproducibility, and detection limit of the device were tested using two types of cells, which helped in investigating the dependency of the selenium concentration in cyclohexane on intensity. The evaluation of the device was done by comparing the results with a commercial spectrofluorometer. The results suggested that the sensitivity of the device was twice when cuvette was used, to that obtained from microsyringe use. However, in terms of reproducibility, microsyringe is more convenient as compared to a cuvette, which needs to be removed and holstered after each use. The detection limit measured with a microsyringe was reasonably similar to that done with a cuvette. However, the operation was simpler with a microsyringe than the cuvetter, and the volume of reagents used with a microsyringe was only 10% of that used with a cuvette. The detection limit of the device was found to be as high as 0.5μg/L as compared to 0.01 μg/L of commercialspectrofluorometer, and the difference was attributed to background fluctuation.

For measurement of selenium levels in a water sample, the paper has proposed a fluorometric method. The on-site process uses a simple and cheap device which is more efficient than commercial bench-top spectrofluorometer used for selenium determination in laboratories, on the basis of reagent and sample consumption. The process was tested and successfully applied to determine selenium in river water on-site.

  1. High performance liquid chromatography coupled to atomic fluorescencespectrometry

The paper highlights the recent applications of high performance liquid chromatography (HPLC) coupled with hydride generation or chemical vapor generation and atomic fluorescence spectrometry (HG/CVGAFS),for the determination and speciation of the selected hydride-forming selenium (Se) (Chen and Belzile, 2010). The main focus of the paper is on sample preparation, post-column treatments and applications of this technique to a variety of liquid and solid samples. The review also includes the necessity of oxidation of organo-element compounds during post-column treatments, and the limitations associated with HPLC-HG/CVG-AFS. There are two ways in which selenium speciation can be done; high performance liquid chromatography-inductively coupled plasma mass spectrometry (HPLC-ICP-MS) and HPLC coupled with hydride generation or cold vapor generation-atomic fluorescence spectrometry (HPLCHG/CVG-AFS). Although, recent studies dominantly use the MS technique, but the AFS technique exists as the most reliable, sensitive and cost-efficient approach of selenium speciation. Comparison was done for detection levels of selenium compounds using both MS and AFS techniques, and based on the obtained data; the research team recommended the AFS technique due to its high sensitivity, low costs of purchase and operation.

During recent times, speciation studies of selenium have been done mainly by HPLC-ICPMS, and therefore, there hasn’t been much research associated with HPLC-HG/CVG-AFS technique. HPLC-ICP-MS is more popular because it has a simple interface and unlike HG-CVG-AFS, it does not need oxidation or pre-reduction steps. The MS technique for speciation of selenium may be popular, but it very expensive and difficult to afford for small laboratories and universities. HPLC-HG/CVG-AFS as an alternative, on the other hand, is cheap and affordable, but the development of the process has been slow due to lack of research. This may be due to the post-column treatment requirements of the technique, that it is less attractive to the researchers.

The optimization included determination of selenium species in milk produced after feeding by different types of Se supplements. SeCys2, Se (IV)and SeMet were found in milk produced from selenised yeastfood whereas only SeCys2 and Se (IV) were found in milk obtainedfrom inorganic Se food. By using an online microwave digestion system, the author also successfully proved that the AFS technique is significantly more sensitive than the MS technique. It was noted that speciation in solid samples is more difficult than in liquid samples. This is because undesired modifications occur in the samples if it isn’t kept intact during the complicated pretreatment. Furthermore, extracting agents in solid phase extraction highly influence the column separation and detection.

The AFS technique is new and information related to the technique has only recently started developing. The focus has been on the applications of such techniques in the chemical speciation of organic compounds containing hydride-forming or chemical vapor-forming elements such as selenium. Replacing the MS with AFS has resulted in significant increases in sensitivity of the method; however, more details are needed on kinetics of UV decomposition and pre-reduction of species after column separation. Due to lack of papers on this method, no commercialized instrument is available in the market, and sensitivities vary widely with instrumental configuration and column properties.

Another thing of concern related to the technique, is sample preparation. Irrespective of the level of sophistication of the instrument, appropriate sample preparation is extremely necessary for acceptable results. The existing studies do not consider the possible artifacts produced during the processes and estimate efficiency based only on the highest total element concentration in the extract. In order to prevent a sample from degrading during the process, study needs to be done in developing the technical approach towards controlling artificial modifications.

  1. Total reflection X-ray fluorescence spectroscopy

Due to selenium’s importance in human health, it is of extreme medical need to use a technique capable of determining selenium at low concentrations but with high accuracy. On-site selenium determination is necessary in wheat production and processing sites, and selenium levels also need to be detected in dietary supplements. Some commercial techniques that are used for determining selenium levels in medical samples are electrothermal atomicabsorption spectroscopy (ETAAS) and inductively-coupled plasma mass spectrometry (ICP-MS). These techniques are simple but prove expensive during use. Small quantity of available samples is also a challenge. Therefore, testing was done for TXRF compatibility with selenium analysis in medical and dietary samples with variable concentrations and matrices (Stosnach, 2009). The results indicated that TXRF can very well be used for accurate determining of selenium levels in samples and the sensitivity is low in comparison to established techniques. However, the detectionlevels were sufficient for the physiological concentrations of selenium in the samples.

Some techniques used for selenium determination in blood samples are electrothermal atomic absorption spectroscopy (ETAAS), flow injection analysis electrothermal atomic absorption spectroscopy(FIA-ETAAS),hydride generation inductivelycoupled atomic emission spectroscopy (ICP AES)and inductively coupled plasma mass spectrometry(ICP-MS). These methods pose high operation costs and require qualified laboratory staff. Furthermore, these spectroscopy methods are limited with regard to the available sample amount. This paper focuses on how totalreflection X-ray fluorescence (TXRF) method can be used for the analysis of selenium levels in medical, biological and dietary samples. The main advantages of using this method are that small quantity of samples can be analyzed without the need of sample preparations and calibrations, that the matrix effects are negligible, and that simultaneous analysis of samples in varying concentrations is possible.

The detection limit in all sample types was below the normal concentration range. The tests done on the wheat sample showed that the detected Se levels were in good agreement with the reference values. For urine samples, the detection values were lower than those of serum and blood samples. The significant reduction was found due to sample preparation when most of the matrix was removed from the urine samples. Selenium values in dietary supplements could not be referenced with manufacturer data, although the results sufficiently indicate that selenium concentration can be accurately analyzed in this sample type.

Determination of selenium concentrations in medical, food and dietary supplements is possible by means of TXRF spectroscopy. The sensitivity of these measurements is relatively lower when compared to established methods. However, it is sufficiently proved that TXRF measurement can be used for accurately calculating detection limits in these sample types. Because of TXRF’s ability to work without calibration routines and over a wide range of concentrations, the total measurement times are reasonable, although, the actual measurement times are longer when compared to other atomic spectroscopy methods. Another advantage of this method is that it can be used for simultaneous analysis of physiological concentrations of additional elements like Cu, Fe and Zn.

  1. Sorption Spectroscopy

The experiment studied the possibility of selenium determination on the solid phase of polyacrylonitrile fibers impregnated by an AV-17 anionexchanger ([PANV-AV-17]) by virtue of diffuse reflectance spectroscopy (Dedkova,Shvoeva and Savvin, 2009).Study was also done on the resultant complex formed between selenium (IV) and organic reagents on solid phase as well as the elemental selenium sol formed both on the solid phase and in solution. In the adsorption of selenium sol formed in solution, the best parameters were observed when ascorbic acid was used. The procedure was validated by the added–found method in the analysis of river and well natural waters. The duration of analysis for 5–6 samples of the volume 100 mL was 30 min approximately.

Selenium and its compounds are highly toxic in nature. The most stable form of selenium is represented by Se (IV). The maximum permissible concentrationsof selenium(IV) species are as follows: 0.001 mg/L inwater supply for domestic use, and 0.0003 mg/L ofselenium dioxide in air. Most techniques used in determination of selenium levels are based on reduction of selenium to its elemental state. Other methods include photometry and fluorimetry, which are based on the formation of piazoselenols from the reaction between selenium species and organic reagents. This experiment was, however, done using diffuse reflectance spectroscopy on the solid surface of a polyacrylonitrilefiber filled with finely dispersed AV-17 anion-exchanger (PANV-AB-17). Fibrous ion exchangers are used because they mechanically convenient and chemical resistance helps in sorption-spectroscopic and test methods ofdetermination. Detection of selenium can be done by reacting it with an organic reagent e.g., sodium diethyldithiocarbamate, thiourea, and Dithizone. Another such method of detection is by the sol of elemental selenium formed upon reduction of selenium (IV).

Upon analysis of the results obtained it was observed that elemental selenium forms a sol best when ascorbic acid is used. The adsorption of selenium was studied using a PANV-AV-17 ion exchanger. In addition to PANV-AV-17, some other supports filled with ion-exchangers as solid phases on which selenium sol was not detected were also studied. This confirmed that selenium sol is adsorbed rather than getting filtered and extracted on the surface of the ion-exchanger. Complete sorption was observed at pH 7-9.5, and it was found that analytical signals of adsorption depend on pH of the solutions.

In order to develop a method for determining selenium levels in natural water, investigation was done on river and central supply water, and the influence of salt composition on selenium levels was studied. Samples from tap and well waters were also analyzed and the results achieved were acceptable. The matrix composition of natural water was found to cause no significant errors in the determination. The relativestandard deviation did not exceed 20%, and the duration of analysis for 5–6 samples took 30–40 min.

  1. Vapor generation atomicabsorption spectrometry

The sensitivity of on-line vapor generation atomicabsorption spectrometry of mercury and selenium wasimproved by using a new atom trap technology (Cai and Li, 2010). The residence time of the analyte atoms in the light path was increased by inner coating of a T-shaped quartz tube withSiO2.This resulted in an overall increase in the linear range of the calibration plots. Also, there was a two-fold improvement observed in the measurement of detection limits for selenium. The technique wasapplied to the determination of Se in herbs and hair.

Determination of toxic elements has been an area of attention in biological, environmental study and medical research. 70-80% of the world’s population uses herbs as home remedies and food additives. Therefore, it is highly important to monitor the toxic elements in herbs. Hair is preferred as a benchmark for metal exposure studies because the concentration levels in hair are ten times higher than those found in blood or urine samples. Furthermore, the sampling of hair is easier and less invasive than blood collection. Selenium in hair is present as selenoprotein and is an essential protein. In herbs and hair, selenium is present in very small and widely varying concentrations, and therefore, it is important to use highly sensitive technique and with extreme range of calibrations for the detection of selenium levels in herbs and hair. Some commonly used techniques are; cold vapor atomic absorptionspectrometry (CV-AAS), cold vapor atomicfluorescence spectrometry (CV-AFS), inductivelycoupled plasma optical emission spectrometry (ICP-OES), and inductively coupled plasma-mass spectrometry(ICP-MS).

Another popular technique used in determination of selenium traces is vapor generation atomic absorptionspectrometry (VGAAS). This technique is faster, cheaper and does not require high expertise in operation as compared to other traditional techniques. Also, it is easier to separate selenium species from other matrix components using vapor generation. The main drawback of this technique is that the calibration curves have a narrow linear range, and as such, dilution is required during sample preparation. VGAAS technique in this paper was done using quartz tube for atomtrap, because quartz has a low thermal expansion coefficient and a high melting point. The system was called quartz tube atom trap-vapor generation atomic absorptionspectrometry (QTAT-VGAAS). The process turns out as highly sensitive owing to the easy binding of the unsaturated surface atoms with the analyte atoms which increased the residence time of the analyte atoms in the light path. Increase in sensitivity was also due to the homogeneous temperature in quartz tube which provided a stable environment and enhanced the number of neutral atoms. For this purpose only, the technique was hyphenated with quartz tube used as an atomtrap and nanometer SiO2 for adsorption i.e., nanometer SiO2 adsorption and quartz tube atom trap-vapor generation absorption spectrometry (NSA-QTAT-VGAAS). Nanometer SiO2 was used because it provides a high surface area and facilitates easier binding.

XRD and AFM imaging of the three dimensional network was done and the selenium curves that were obtained were calibrated for practical analyses. The calibration curves and the equations of calibration were tested for different coating weight of nanometer SiO2, and were used in determination of selenium concentrations in herbs and hair. The precision and accuracy of the method were tested by spiking the samples. The results obtained for Selenium levels were in good agreement with the certified values and the recoveries of herbs and hair was sufficient for trace analysis.

The technique uses a simple AA flame spectrometer, a flow injection hydride generation system and a quartz tube coated with nanometer SiO2, and can be used in determination of trace selenium found in analytical samples. The main advantages of this technique are online, low cost and easy manufacturing. Also, there was reported a significant improvement in sensitivity of the measurement. Compared to offline enrichment techniques such as adsorption or extraction, the modified online method is much simpler, instantly applicable and requires less volume of samples. Furthermore, the concentration scope of NSA-QTAT-VGAAS is wider than QTAT-VGAAS, as was proved by the expansion in the linear range of calibration curves.

  1. Atomic Fluorescence Spectrometry

Atomic Fluorescence Spectrometry (AFS) is considered as an ideal detection technique for studiesof selenium speciation. The technique provides measurement to low detection limits and a wide range of calibration, making it useful in a variety of biological, environmental and food samples. Other techniques used in speciation are Atomic Absorption Spectrometry (AAS) and Inductively Coupled Plasma-Mass Spectrometry (ICPMS). AFS is preferred as a suitable alternative to them, as is shown in the review, which explains the instrumental couplings of chromatographic (HPLC and GC) and non-chromatographicseparations (CE) with AFS detection, with online hydride generation for thespeciation of inorganic and organic compounds of Se (Sanchez, Corns, Chen and Stockwell, 2009). Different types of samples from water, soils, air, biota and food were used for analysis and speciation was done using instrumental couplings with AFS detection. Generally, speciation is done using chromatography hyphenated with detectors based on etiheratomic absorption (AAS), atomic emission (ICP-AES), or massspectrometry (ICP-MS). For preconcentration and post-column derivatisation of the sample matrices, hydride generation (HG) is used. Pre-treatment of the analyte reduces the possibility of potential interferences in the detection, which may arise from some direct couplings with the interface, such as in LC-MS technique with electrospray ionization.

Potential advantages of AFS over other atomic and mass spectrometric techniques are being studied. AFS has been described as a technique superior to AAS, and at par with ICP-MS regarding sensitivity and linear calibration range. When applied to selenium speciation studies, AFS also serves as a simple, lower acquisition and cost-efficient technique. Speciation is possible up to low detection limits and over a wide concentration range. However, for species that are not readily volatile, additional online derivatisation steps are required (e.g. photo-oxidation, pyrolysis or microwave digestion) before HG.

Other major disadvantages of atomic fluorescence are quenching and interferences. Quenching occurs when excited atoms collide with other molecules in the atomization sources. Spectral interferences originate from source scattering and atomic emission in the atomization source. Spectral lines in the source overlap with those in the matrix elements in the atomizer and cause spectral interferences. Techniques other than AFS also suffer with loss of population of free atoms due to chemical processes during atomization.

It is important that the atomization source is simple and easy to use, has good stability and requires minimum maintenance to obtain optimum performance.Atomizers used in AFS are similar to those used in AAS or AES. For an efficient and rapid production of free atoms, it is important to increase the residence of analytein the optical path, and decreasing the quenching properties. Easy handling of the interface and cost-efficiency are also important.

Early research done for selenium speciation using AFS detection used a silica column modified with didodecyldimethylammonium bromide (DDAB) for determination of Se (IV) and Se (VI) in aqueous media. No HG step was used in between. Instead, an ultrasonic nebulizer was used as an interface between HPLC and AFS, which resulted in detection limits as low as 8.6 mg L-1. Tests were also done for a hydraulic high pressure nebulizer as interface, for separation of selenoaminoacids, and the results showed that detection limits of 50, 42, and 71 mg L-1 for SeCys, SeMet and SeEt, respectively were possible. However, when this hydraulic high pressure nebulization was applied to analysis of a real sample (edible mushrooms), the results indicated complicated background effects and matrix overlap problems.

Determination of selenoaminoacids, either with an anion exchange column or a reversed phase column modified with DDAB, has been done making the use of UV radiation (HPLC-UV-HG-AFS). UV photolysis and a UV/TiO2photocatalyst reduction device have been proposed in recent studies, as an interface between HPLC and AFS, and that is considered as an alternative to the traditional HG approach. The technique has already been successfully applied to selenium speciation study in water-soluble extracts of garlic shoots. Commonly, speciation techniques based on AFS detection use chromatographic separation in the primary stages, and most of them consider only Se (IV) and Se (VI). HG-AFS combined with flow systems, is the most commonly used non-chromatographic approach

Another technique recently used for selenium speciation is FIA-HG-AFS. The technique was applied to sewage and slurry samples, with combined extraction and reduction in a microwave oven, and determination of Se (IV) and total selenium was done. The FIA approach uses online reduction with an electrochemical HG process and allows the determination of Se (IV) and Se (VI). A homemade tubular electrolytic cell is used as hydride generator and an electromagnetic induction oven is used for reduction. The detection limits achieved were similar to those of conventional HG. Capillary Electrophoresis coupled online to HG-AFS is also being used for speciation of Se (IV) and Se (VI) in water samples. HCl is used for Se (VI) reduction and the detection limits achieved are 33 and 25mg L-1.

Another recent development in the non-chromatographic techniques for speciation of Se (IV) and Se (VI) is the use of UV photochemical vapor generator coupled with AFS detector. The analyte is made to react with an organic acid under different reaction conditions, and the approach gives good detection limits of 0.02 and 0.1 mg L-1 for Se (IV) and Se (VI), respectively. For the determination of methylated Se species (DMSE and DMDSe) for slurry sampling there have been reported references of the use of PV coupled with AFS. The technique, however, is faced with the drawback of pervaporation efficiencies ranging from 55–85%.

  1. Isotope Dilution ICPMS

The method described in the paper (Zheng, Yang, Sturgeon and Hou, 2010) for accurate determination of selenium in biological tissues is isotope dilution inductively coupled plasma mass spectrometry(ID ICPMS) based on sample introductionarising from online UV photochemical vapor generation(UV-PVG). Volatile species of selenium were exposed to a UV source in a formic acid medium. Sensitivities were enhanced 27- to 355-fold compared to those obtained using pneumatic nebulization sample introduction. The resulting precision in the measurement was decreased two-fold; however, the limits of detection of 1.7 pg g-1 were obtained which provided a 150-fold improvement over that of conventional pneumatic solution nebulization. The validity of the method was tested on biological tissue certified reference materials TORT-2 and DORM-3. The selenium concentrations measured were in good agreement with the certified values.

Use of inductivelycoupled plasma mass spectrometry (ICPMS) has been realized increasingly after its commercialization in diverse fields asnanotechnology, human health, environmental sciences, and geosciences. The technique is capable of providing rapid multi-element data and isotope specific information. The unparalleled detection power provided by the technique has made it popular in determination of trace elements. The main drawback of this technique, as is with the majority of atomic spectroscopic techniques, is sample introduction. To overcome this, ICPMS hasbeen coupled with a variety of sample introduction techniques, including gas chromatography, laser ablation, electrothermalvaporization, and vapor generation. The coupling with such alternative techniques allows to overcome the difficult sample preparation processes and spectral or matrix interferences. It also increases the sensitivity and detection power by increasing sample introductionefficiency beyond the typical 2-5% associated with conventionalpneumatic solution nebulization. Vapor generation is considered as a pioneering combination with atomic spectroscopy, since the use of Zn/HCl to generate volatile AsH3 for detection, some 40 years ago. In recent times, this has been replaced by sodium tetrahydroborate for hydride generation (HG), introduced by Braman for this purpose. The technique however, faces the problem of contamination from the NaBH4 reagent, the instability of its solutions and slow reaction rates. Furthermore, there areinterferences from transition and noble metals, and the concentration range is limited. Different species of a given element also give non-uniform responses. These obstacles lead to the introduction of a new technique, ultraviolet photochemical mediated vapor generation (UV-PVG), for generation of volatile species of selenium (and a number of other elements including mercury). This technique has all the advantages of conventional chemical VG, but further provides simpler reactions, greener chemistry, and is cost-effective. Despite increased interest in UV-PVG over the past severalyears, applications to real sample analyses remain limited. Thepurpose of the studied experimental work was to explore the further potentialadvantages of PVG as a sample introduction technique whencoupled with the high accuracy and precision offered by quantitationusing isotope dilution mass spectrometry (ID-MS). ID-MSis considered to be a primary ratio method of the highest metrological quality for trace analysis. Calibration techniquesbased on ID provide for enhanced accuracy and precision because,among other features, a ratio, rather than an absolute intensitymeasurement, is used for quantitation of analyte concentration. The use of this technique in determination of selenium serves as an example of applicationsrequiring high detection power and minimization ofisobaric interferences caused by the acidic medium.

The method was applied in the determination of selenium under normal plasma conditions. The limit of detection evaluated were 0.031 μg g-1, providing about 10-fold improvements over that realized with conventional pneumatic solution nebulization for ID calibration.This was the first application incorporating online UV-PVG with ID calibration for detection by ICPMS. Sufficiently low detection limits are well-suited for quantitation of metals in environmental samples.

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Conclusion

Review for various techniques was done for determining trace levels of selenium in a variety of biological, medical and food samples. Each technique has various advantages and disadvantages related to sensitivity measurements, cost efficiency, level of sophistication and ease of operation. For example, sensitivity and calibration range of atomic fluorescence spectrometry is superior to that of atomic absorption spectrometry, but similar to that inductively coupled mass spectrometry. Also, AFS could be chosen as a favorable technique for selenium speciation because of its simple interface and low running costs. However, online derivatisation steps are needed for complex organometallic species, and source scatter and atomic emissions also take place within the atomization sources. The presence of spectral and chemical interferences in the technique gives it some major disadvantages. Another technique reviewed was sorption spectrometry using a quartz tube coated with nanometer SiO2. The main advantage of the technique was online, low cost and easy manufacturing, and there were significant increases in sensitivity and concentration range. The volume of samples required was also relatively less.

However, the technique of choice that should be increasingly implemented in analysis of selenium levels in medical, biological and dietary samples is total reflection X-ray fluorescence (TXRF). The process doesn’t require any sample preparation or calibrations, and the technique can be used to determine a varying range of concentration in samples. Also, the matrix interferences are negligible and simultaneous analysis of samples is possible up to very low detection limits.

References

  • Cai, T. and Li, S. 2010. Sensitivity enhancement using nanometer silica particleson the surface of a quartz cell in mercury and seleniumdetermination by vapor generation atomicabsorption spectrometry. MicrochimActa, 171. pp. 11-16
  • Chen, Y. and Belzile, N. 2010.High performance liquid chromatography coupled to atomic fluorescencespectrometry for the speciation of the hydride and chemical vapour-formingelements As, Se, Sb and Hg: A critical review. AnalyticaChimicaActa671. pp. 9–26
  • Coxon, J. Dansted, P. and Hartshorn, M. 1977. Organic Syntheses, Vol. 56, pp. 25. [online] Available at: http://www.orgsyn.org/orgsyn/prep.asp?prep=cv6p0946 [Accessed 03 April 2011]
  • Dahnke, K. and Paquette, L. 1993. Organic Syntheses, Vol. 71, pp. 181. [online] Available at: http://www.orgsyn.org/orgsyn/prep.asp?prep=cv9p0396 [Accessed 03 April 2011]
  • Dedkova, V. Shvoeva, O. and Savvin.S. 2009. Sorption_Spectroscopic Determination of Selenium Using a PANV-AV-17 Fibrous Ion-Exchanger.Journal of Analytical Chemistry.65(5). pp. 455-459.
  • Hach, C. Banks, C. and Diehl, H. 1952. Organic Syntheses, Vol. 32, pp. 35. [online] Available at: http://www.orgsyn.org/orgsyn/prep.asp?prep=cv4p0229 [Accessed 03 April 2011]
  • House, J. 2008. Inorganic Chemistry, Academy Press, pp. 524.
  • Renga, J. and Reich, H. 1979. Organic Syntheses, Vol. 59, pp. 58. [online] Available at: http://www.orgsynth.org/orgsyn/prep.asp?prep=cv6p0023 [Accessed 04 April 2011]
  • Sanchez, D. Corns, W. Chen, B. and Stockwell, P. 2009. Atomic Fluorescence Spectrometry: a suitable detection technique inspeciation studies for arsenic, selenium, antimony and mercury. Journal of Analytical Atomic Spectrometry, 25. pp. 933-946.
  • Stosnach, H. 2009. Analytical determination of selenium in medical samples, staple food and dietarysupplements by means of total reflection X-ray fluorescence spectroscopy.SpectrochimicaActa Part B, 65. pp. 859-863.
  • Suzuki, Y. Hashigaya, N. and Kawakubo, S. 2010. Development of a simple and Low-cost Device for Fluorometric Determination of Selenium in Water Samples.Analytical Sciences, Vol. 26. pp. 719-722
  • Vincoli, J. 1997. Risk management for hazardous chemicals, CRC Press. pp. 2709

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