toxicity and mercury

The environment is composed of indistinguishably numerous compounds that can be either classified as organic or inorganic materials. These compounds are constantly redistributed along the different constituents of the biosphere as well as in the hemisphere.

Figure 1. Biogeochemical processes. (http://www.chemgapedia.de/vsengine /media/vsc/en/ch/16/uc/images/biogeochem.jpg)

However, these continuous and persistent redistributions or cycles of these compounds may also have noxious effects to organisms specifically to human beings. In addition to that, the entry of human activities to the naturally-occurring cycles may increase the risk of toxicity of compounds in the environment. A good example of compounds posing risks to organisms is mercury.

Mercury is said to be a naturally-existing element which may occur in its elemental, organic or inorganic forms. Normally, it is found in measurable quantities in the environment. However, once it exceeds the normal levels and bioaccumulates in living tissues, harmful and detrimental effects are expected to transpire.

Figure 2. Mercury in different forms posing risks to humans. (http://www.greenfacts.org/)

Mercury poisoning is not so prevalent nowadays. However, its toxicity should also be noted such that it can enter our biological pathways and upset them causing harmful consequences. Therefore there is a necessity to identify how the mercury poison human beings. Identifying the locations and level of exposure would increase proper precautions on the handling of such compounds. This would help scientists and laboratory technicians come up with ways of handling such compounds.

Immunobiosensors

The Element Song

Lyrics:

There's antimony, arsenic, aluminum, selenium,
And hydrogen and oxygen and nitrogen and rhenium,
And nickel, neodymium, neptunium, germanium,
And iron, americium, ruthenium, uranium,
Europium, zirconium, lutetium, vanadium,
And lanthanum and osmium and astatine and radium,
And gold and protactinium and indium and gallium, (gasp)
And iodine and thorium and thulium and thallium.

There's yttrium, ytterbium, actinium, rubidium,
And boron, gadolinium, niobium, iridium,
And strontium and silicon and silver and samarium,
And bismuth, bromine, lithium, beryllium, and barium.

There's holmium and helium and hafnium and erbium,
And phosphorus and francium and fluorine and terbium,
And manganese and mercury, molybdenum, magnesium,
Dysprosium and scandium and cerium and cesium.

And lead, praseodymium and platinum, plutonium,
Palladium, promethium, potassium, polonium,
And tantalum, technetium, titanium, tellurium, (gasp)
And cadmium and calcium and chromium and curium.

There's sulfur, californium and fermium, berkelium,
And also mendelevium, einsteinium, nobelium,
And argon, krypton, neon, radon, xenon, zinc and rhodium,
And chlorine, carbon, cobalt, copper, tungsten, tin and sodium.

These are the only ones of which the news has come to Hahvard,
And there may be many others but they haven't been discahvered.

FLAME ATOMIC ABSORPTION SPECTROMETRY

HISTORY
Aristophanes (423 B.C )- use of the lens
Euclid (300 B.C.) and Hero (100 B.C.) – studied mirrors
Seneca (40 A.D.) - observed the light scattering properties of prismS
Ptolemy (100 A.D.) - studied incidence and refraction
Alhazen (1038)- studied reflection and refraction of light
Roger Bacon (1250 ) - determined the focal points of concave mirros.
Galileo (1610) - made improvements on the telescope design
Sir Isaac Newton (1642-1727) – worked on the separation of light to obtain a spectrum
Fraunhofer (1814-15) - observed diffraction phenomena; measured wavelength instead of angles of refraction.
Herschel (1823) and Talbot (1825) - discovered atomic emission when certain atoms were placed in a flame.
Wheatstone (1835) – concluded that metals could be distinguished from one another on basis on the wavelengths of this emission
Foucault (1848) - observed atomic emission from sodium; discovered that the element would absorb the same rays from an electric arc.
Kirchoff (1800) - summarized the law which states that, "Matter absorbs light at the same wavelength at which it emits light". It is under this law that atomic absorption spectroscopy works.
Woodson - one of the first to apply this principle to the detection of mercury.
Walsh (1955) - suggested the use of cathode lamps to provide an emission of appropriate wavelength; and the use of a flame to produce neutral atoms that would absorb the emission as they crossed its path.
After 1950’s - instrumentation and applications for atomic absorption greatly expanded


BASIC PRINCIPLE
The technique of flame atomic absorption spectroscopy requires a liquid sample to be aspirated, aerosolized, and mixed with combustible gases, such as acetylene and air or acetylene and nitrous oxide. The mixture is ignited in a flame whose temperature ranges from 2100 to 2800 oC. During combustion, atoms of the element of interest in the sample are reduced to free, unexcited ground state atoms, which absorb light at characteristic wavelengths. The characteristic wavelengths are element specific and accurate to 0.01-0.1nm. To provide element specific wavelengths, a light beam from a lamp whose cathode is made of the element being determined is passed through the flame. A device such as photonmultiplier can detect the amount of reduction of the light intensity due to absorption by the analyte, and this can be directly related to the amount of the element in the sample.


THE ATOMIC ABSORPTION INSTRUMENTATION HARDWARE
Flame atomic absorption hardware is divided into six fundamental groups that have two major functions: generate atomic signals and process signal. Signal processing is a growing additional feature to be integrated or externally fitted to the instrument.
Parts:
cathode lamp - is a stable light source necessary to emit the sharp characteristic spectrum of the element to be determined.
atom cell - is the part with two major functions: nebulization of sample solution into a fine aerosol solution, and dissociation of the analyte elements into free gaseous ground state form.
monochromator – isolates the specific spectrum line emitted by the light source through spectral dispersion
photomultiplier detector - converts the light signal into an electrical signal.
signal amplifier – processes the electrical signal generated.
readout – displays the data.
data station – for data storage


ATOMIC ABSORPTION METHODS OTHER THAN FLAME
1. Electrothermal atomization requires a graphite furnace, where after thermal pre-treatment the sample is rapidly atomized. To maintain a dense fraction of free ground state elements in the optical path, an inert gas atmosphere is used. Since the dilution and expansion effects of flame cells are avoided, and the atoms have a longer residence time in the optical path, a higher peak concentration of atoms is obtained.
2. Carbon rod analyzer can be used to convert a powdered sample into atomic vapour. A current is applied to a very thin, heated carbon rod that contains the solid sample in order to vaporise it.
3. Tantalum boat analyzer is another technique that produces an atomic vapour from a solid sample. A Tantalum boat is electrically heated in a manner similar to the carbon rod system, within an inert atmosphere.


TECHNIQUES OF MEASUREMENT
1. Sample preparation
Depending on the information required, total recoverable metals, dissolved metals, suspended metals, and total metals could be obtained from a certain environmental matrix.
2. Calibration and standard curves
As with other analytical techniques, atomic absorption spectrometry requires careful calibration. Several steps include: interference check sample, calibration verification, calibration standards, bland control, linear dynamic range

Idealized calibration or standard curve is stated by Beer's law that the absorbance of an absorbing analyte is proportional to its concentration. Deviations from linearity may occur due to unabsorbed radiation, stray light or disproportionate decomposition of molecules at high concentration.

INTERFERENCES
These are factors that affect the ground state population of the analyte element since the concentration of the analyte element is considered to be proportional to the ground state atom population in the flame.
• Spectral interferences are due to radiation overlapping that of the light source. The interference radiation may be an emission line of another element or compound, or general background radiation from the flame, solvent, or analytical sample. This usually occurs when using organic solvents, but can also happen when determining sodium with magnesium present, iron with copper or iron with nickel.
• Formation of compounds that do not dissociate in the flame. The most common example is the formation of calcium and strontium phosphates.
• Ionization of the analyte reduces the signal. This is commonly happens to barium, calcium, strontium, sodium and potassium.
• Matrix interferences due to differences between surface tension and viscosity of test solutions and standards.
• Broadening of a spectral line, which can occur due to a number of factors. The most common line width broadening effects are:
1. Doppler effect arises because atoms will have different components of velocity along the line of observation.
2. Lorentz effect occurs as a result of the concentration of foreign atoms present in the environment of the emitting or absorbing atoms. The magnitude of the broadening varies with the pressure of the foreign gases and their physical properties.
3. Quenching effect occurs in flames as the result of the presence of foreign gas molecules with vibrational levels very close to the excited state of the resonance line.
4. Self absorption or self-reversal effect occurs when atoms of the same kind as that emitting radiation absorb maximum radiation at the centre of the line than at the wings, resulting in the change of shape of the line as well as its intensity. This effect becomes serious if the vapour which is absorbing radiation is considerably cooler than that which is emitting radiation.

MINIMIZING THE EFFECTS OF ERRORS
• Work in the linearity response range. The rule of thumb is that a minimum of five standards and a blank should be prepared in order to have sufficient information to fit the standard curve appropriately. Manufacturers should be consulted if a manual curvature correction function is available for a specific instrument.
• If the sample concentration is too high to permit accurate analysis in linearity response range, there are three alternatives that may help bring the absorbance into the optimum working range:
1) sample dilution
2) using an alternative wavelength having a lower absorptivity
3) reducing the path length by rotating the burner hand.

Releasing Agents - used to control chemical interferences due to stable compounds formed in the desolvation process during the sample preparation; acts by forming a stable oxysalt with the interference ion and the analyte is released for the atomic process. Lanthanum or strontium are the most frequently used, although calcium, magnesium, and rare earth elements have been used.

Ionization suppression - added in order to prevent ionization. Commonly used at high concentration of another more easily ionized element. The elements used normally are the alkaline elements (potassium or cesium).

Method of Standard Addition
The major element chemical matrix of the standards and samples are matched. Matching is accomplished best by the method of standard additions, where a small spike of the standard is added to a split of the sample solution.

LEUCH, Kathryn K
LLORENTE, Cindy C
NATIVIDAD, James Thomas G

Greetings from Baffin Bay

Greetings from Baffin Bay

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chem proverb #1

If you are not part of the solution,
you are part of the precipitate.

-http://chem.chem.rochester.edu/nvdcgi/proverbs.cgi

magnesium diboride

In January 2001, Akimitsu’s research group in Japan revealed that MgB2 turns out to be superconducting at 39 Kelvin, making it one of the highest known transition temperatures (Tc) of any superconductor. Hundreds of papers have been produced in the first rush to examine this material, but researchers using different techniques have reported many different, often unusual, and sometimes conflicting properties. Magnesium diboride is an intermetallic compound that loses its resistance at 32 K in nanocrystalline form while 39 K in the wire-form. Relatively high temperature superconductors are celebrated nowadays for they no longer require, at least minimize, the use of cryogenics to maintain their superconducting state. The compound also has a pronounced difference in the resistivity of about 10 μΩ cm at low temperature and of about 0.5 μΩ cm at above Tc.

The structure of MgB2 plays a very important role in its conducting property. It is a simple hexagonal AlB2-type of structure which is typical of borides.

The structure is consisting of layers of boron that is separated by closed-packed layers. Located at the center of the hexagons that are formed by boron atom are the magnesium atoms. These magnesium atoms donate electron density to the boron layers.

Preparation of MgB2 powder (Canfield, et al., 2008). Stoichiometric amounts of powdered B and Mg at were reacted in 950◦C for approximately an hour to form MgB2 powder. The vapor pressure of Mg is approximately 200 Torr at 950◦C. Given this, it is assumed that MgB2 forms via a process of diffusion
of Mg vapor into the boron grains.

Preparation of MgB2 wires (Canfield, et al., 2008). One hundred micrometer diameter of boron fiber10 and M were sealed in a tantalum tube to produce MgB2 wire. The said tube was sealed in quartz and was then placed for approximately an hour into a 950◦C box furnace. The reaction container was removed from the furnace and was allowed to quench to room temperature.

Preparation of Nanocrystalline MgB2 by Ball Milling Method (Lorenz, et al., 2006). Nanocrystalline MgB2 is synthesized by high-energy ball milling of Mg and B. Milling is conducted in Ar atmosphere using jar and balls made from tungsten carbide (WC). 20 h of milling time was done and the Mg and B were partially reacted through cold alloying to form MgB2. The reaction was completed by hot uniaxial pressing at 973 K/640 MPa for 10 min. This treatment resulted in a complete conversion of Mg and B to MgB2 with only minor traces of impurity phases left.

References:

Brown, T. L., et al (1994). Chemistry: The Central Science. Englewood Cliffs, New Jersey: A Simon & Schuster Company
Canfield P. C., and S L Bud’ko. (2002). Magnesium diboride: one year on. Physics World. pp 29-34.
Canfield, P and Crabtree G. (2003). Magnesium boride: better late than never. Physics today. pp. 34-40
Canfield P. C., et al., (2008). Superconductivity in Dense MgB2 Wires. Iowa: Ames Laboratory, U.S. Department of Energy and Department of Physics and Astronomy Iowa State University. Pp 1-4.
Lee, J.D. (1992). Conscise Inorganic Chemistry (4th ed.).Singapore: Fong and Sons Printers Pte Ltd.
Lorenz, B. Et al., (2006). Superconducting properties of nanocrystallineMgB2. Supercond. Sci. Technol. 19 (2006) 912–915
Physicsweb, Nature and LANL. (2003, May 1). Magnesium diboride Thoery confirmed. Retrieved September 13, 2008, from http://www.superconductors.org/news.htm.