1. Briefly explain (a) Why atomic absorption lines are very narrow? (b) Why atom
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Question
1. Briefly explain (a) Why atomic absorption lines are very narrow? (b) Why atomic emission is more sensitive to flame instability than atomic absorption? (c) Why fluorescence measurements have the capability of greater sensitivity than absorption measurements? (d) What causes blue shift and red shift in spectra? (e) The differences between ionization interference and spectral interference. (26 points) 2. A solution contains 1.0 mg of KMnO4/L. When measured in a 1.00 cm cell at 525 nm, the transmittance (T) was 0.300. When measured under similar conditions at 500 nm, T was 0.350. (a) Determine the absorbance (A) at each wavelength. (b) Determine the molar absorptivity at each wavelength (c) What would T be if the cell length were in each case 2.00 cm? (d) Calculate the absorptivity (if concentration is in mg/L) for the solution at each wavelength (19 points) 3. (a) Draw a picture of the following cell. P(ejc 300 102M)or2 (4.00-102M) Sn2 2.00-102M)sn4 (200-10-4M P) (b) Calculate the cell voltage. (c) Is the cell as written galvanic or electrolytic?Explanation / Answer
Answer:
A) In the atomic absorption spectroscopy, when the white light is incident on a gas,then some of its wavelengths (photons) bump electrons from lower energy state E1 to higher energy state E2. Those the photons (wavelengths) whose energy is exactly equal to E2?E1 are absorbed by the electrons and these wavelengths that one sees the dark lines in the spectrum. The lines are narrow because theoretically only one wavelength of light corresponds to the energy difference E2?E1, thereby predicting absorption lines to have zero width.
B) Flames in atomic emission are more sensitive to flame instability because optimum excitation conditions vary widely from element to element. High temperatures are needed for excitation of some elements and low temperatures for others. The region of flame that gives rise to optimum line intensities varies from element to element. Flame is rarely use in atomic emission because atomization is more complete when using a plasma due to the production of high temperatures. Also the plasma helps reduce the ionization interference effects. In flame absorption, after the sample is nebulized by a flow of gaseous oxidant, mixed with a gaseous fuel and carried into the flame it is then atomized. Then some of the atoms in the gas ionize to form cations and electrons. In flame emission the sample is introduce with argon, carries the sample into the flame. The flame is suppose to atomize the sample, while the flow of gas takes the ions and electrons to be detected.
C) Reasons for greater sensitivity of fluorescence measurements than absorption measurements are:
• Sensitivity: The sensitivity of fluorescence detection is approximately 1,000 times greater than absorption spectrophotometric methods. This leads to greater limits of detection, while potentially using less sample material. This is important especially when working with precious or limited-quantity materials.
• Specificity: Only molecules that fluoresce are detected by this method, resulting in greater specificity compared with UV/Vis absorption.
• Wide concentration range: Fluorimetry generally can detect more than three to six log orders of concentration without sample dilution or modification of the sample.
• Accurate results: The sensitivity and specificity of fluorescence measurement leads to potentially more precise and accurate readings.
D) The fluorescence spectrum may depend on the presence of quencher for a variety of reasons but in general this requires the presence of more than one emitting species. Even in the case of a single fluorophore in the presence of the excited-state solvent relaxation one may observe the blue shift of the fluorescence band upon addition of a dynamic quencher which shortens the excited-state lifetime and therefore gives more weight to the emission from only partially relaxed excited-states (which are higher in energy than totally solvent-relaxed states).
There can be many causes: red shift due to shift in the excited state potential compared to the ground state, shifts due to solvent relaxation, excited state reactions such as excited state proton transfer.
E) Ionization interference
Ionization interference is more common in hot flames. The dissociation process does not stop at formation of ground state atoms. Excess energy of the flame can lead to excitation of ground state atoms to ionic state by loss of electrons thereby resulting in depletion of ground state atoms. In cooler flames such interference is encountered with easily ionized elements such as alkali metals and alkaline earths.
Spectral Interferences
Spectral interferences are caused by presence of another atomic absorption line or a molecular absorbance band close to the spectral line of element of interest. Most common spectral interferences are due to molecular emissions from oxides of other elements in the sample.
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