For 2 of the 3 cases below, choose an analytical method or methods that would al

Question

For 2 of the 3 cases below, choose an analytical method or methods that would allow you to perform the analysis described. Give SPECIFIC DETAILS on experimental parameters (including justification). You may also comment on sample preparation. (a) Coal-fired power plants are notorious for releasing mercury (in its elemental form Hg (l)) because North American coal deposits tend to have large mercury concentrations. Describe a method to identify and quantify (i.e. show that it's mercury and how much there is) mercury emissions from a coal plant smoke stack. (b) Robots have been exploring the surface of Mars since the Soviet Union's Mars 3 probe in 1971. Currently, the American Opportunity is operating on the Martian surface and the Mars Science Laboratory is scheduled to land in August 2012. One of the primary purposes of all these rovers is to look for water. Describe an instrument that might work in a Martian rover: it would have to be rugged, lightweight, and able to detect water. (c) For recent archaeological samples (e.g. a few hundred years old),^14 C dating is unreliable. A more accurate technique is to determine the L-to-D isomerisation of DNA. Basically, all living DNA is in the L-form. After organism death, it gradually isomerises to racemic (50/50 L/D ratio) DNA over time. Assuming the isomerisation rate is known, how would you determine the age of some human remains found frozen on a mountain?

Explanation / Answer

Solution:

a) Mercury (l) in elemental form can be easily identified as mercury reacts with powdered sulfur to form a stable mercuric sulfide (black in color). Further the soluble form of mercury, Hg22+, Hg2+ can be identified using group analysis for the respective cations (eg. Hg22+, Hg2+), which form the insoluble chlorides. The dilute hydrochloric acid is used as the group reagent. The most important cations of 1st group are Ag+, Hg22+, and Pb2+. The analysis of group is done by adding dilute HCl to the aqueous solution of the mercury (Hg22+) sample. This results in a white precipitate. NH4OH is added to the white precipitate. The color of the white precipitate turns black, indicating the presence of Hg22+.

On allowing hydrogen sulfide gas to pass through the aqueous solution of the mercury sample (Hg2+) in presence of dilute HCl, if black precipitates occurs and the precipitate is insoluble in dilute nitric acid and soluble in aqua regia. Then this confirms the presence of Hg2+.

Activated carbon injection technique is used to remove the mercury from the flue gas duct in coal-fired powered plants at a temperature of 250 – 350 degree Fahrenheit. Mercury gets adsorbed on to the surface of activated carbon, which can be collected in the electrostatic precipitator where mercury (l) is separated from the activated carbon. The separated mercury then can be quantified using electronic balance.

c) The L and D enantiomers (L/D:50/50) of the DNA molecule can rotate the plane polarized light in the different direction, i.e. in left direction (Levorotatory, L) and right direction (Dextrorotatory, D) respectively. The intensity of the rotated plane polarized light is directly proportional to the concentration of the isomer (either L or D). The extract of the archeological sample after collection can be subjected to the plane polarized light using Polarimeter instrument. The concentration of the remaining L-isomer in the extracted sample can be directly evaluated by measuring the intensity of the plane polarized light by the L-isomer.

Once the reference concentration of the L-isomer in the living sample is known then the concentration of the L-isomer converted to D-isomer can be easily calculated by subtracting the evaluated L-isomer in the sample to the reference L-isomer. Now if the rate of conversion of L-isomer to D-isomer is known, then one can find the time taken by the by the L-isomer to convert to D-isomer.

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