Short answer and calculation questions. Answer 4 of the following 6. (a) Uranium

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Short answer and calculation questions. Answer 4 of the following 6. (a) Uranium-thorium dating is an alternative to^14C dating, often used to date the age of coral fossils. It requires the separation of U from Th. Determine if this can be done quantitatively using coulommetry by calculating the potential (voltage) ranges over which U^3+ and Th^4+ reduce. Assume naturally occurring U and Th react via the reactions below. Assume a possible concentration range of 1 times 10^-2 to 1 times 10^-7 M. U^3+(aq) + 3e^- rightarrow U(s) E degree = -l.66V Th^4+ (aq) + 4e^- rightarrow Th(s) E degree =-1.83 V (b) The absorbance information of two pure dyes are given. A commercial food product contains both dyes. If the absorbance is 0.6S3 at 430 nm and 0.182 at 600 nm through a 1 cm cuvette, what is the concentration of each dye in the food? (c) Explain the difference between a hard source and a soft source for a mass spectrometer, and give an example of each. (d) This chromatogram was obtained in an HPLC experiment using the parameters shown. (e) Explain the peak shape (example is shown at right) of the cyclic voltammetric output. Specifically, explain what is happening (in terms of species in solution) at the 4 points labelled A. B. C. and D. f) Briefly describe these two GC detectors: flame ionization (FID) and electron capture (ECD). Be sure to include advantages and/or limitations of the detectors.

Explanation / Answer

b) for both of dyes lambda max is 430 nm.

using lambert-beer law (A=xi cl) , concentration of dye A is 8.1 x 10-5 and of dye B it is 8,4 x 10-4 M

c) Hard sources leave molecule in excited energy states which relax via bond cleavage. Give “daughter ion” fragments at lower m/z. examples are electron impact ionization method. Soft sources minimize fragmentation. Resulting spectra has fewer peaks. example is chemical ionization, fast atom bombardment.

e) the given cyclic voltamogram represents reversible reaction occuring at electrode surface.

At a potential well positive of E0´ , only nonfaradaic currents flow for awhile (point D). When the potential reaches the vicinity of E0´ , the reduction begins and current starts to flow .As the potential continues to grow more negative, the surface concentration of the reactant must drop, hence the flux to the surface and the current increase (point A). As the potential moves past , the surface concentration drops to near zero and mass transfer of reactant to the surface reaches a maximum rate (point B). Then it declines as the depletion effect sets in (Point C).

f) FID is the Most common detector for GC. In an FID, effluent from the column is directed into a small air-hydrogen flame. Most carbon atoms (except C=O) produce radicals (CHO+) in the flame: CH + O CHO+ + e- • Electrons are used to neutralize the CHO+ atoms and the ions are collected at an electrode to create a current to be measured. This current is proportional to the number of molecules present.

Advantages: 1. universal detector for organics 2. does not respond to common inorganic compounds 3. mobile phase impurities not detected 4. carrier gases not detected 5. limit of detection: FID is 1000x better than TCD 6. linear and dynamic range better than TCD

Disadvantage: destructive detector

ECD is Radioactive decay-based detector . It is selective for compounds containing electronegative atoms, such as halogens, peroxides, quinones, and nitro groups.The sample effluent from a column is passed over a radioactive emitter, usually 63Ni. An electron from the emitter causes ionization of the carrier gas (often N2) and the production of a burst of electrons.

Advantages: useful for environmental testing detection of chlorinated pesticides or herbicides; polynuclear aromatic carcinogens, organometallic compounds. selective for halogen- (I, Br, Cl, F), nitro-, and sulfur-containing compounds. detects polynuclear aromatic compounds, anhydrides and conjugated carbonyl compounds Disadvantages: could be affected by the flow rate

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