Spectroscopy Problem Set February 22, 2018

Transcription

1 Spectroscopy Problem Set February, In the diagram above which of the following represent vibrational relaxations? 1. Which of the following represent an absorbance? 3. Which of following represents an emission? 3 4. Which of the following is an intersystem crossing? 4 5. Which of the following is a forbidden electronic state transition? 5 S S 1 S 0 S 1 S 1 S 0 T 1 S 0 None of the above

2 6. Draw a Jablonski diagram and clearly label the following: 6 Vibrational Relaxation Absorption Fluorescence Phosphorescence Excitation 7. Which of spectra is most likely Na vapor? 7 8. Which is most likely biphenyl in hexane? 8 9. Which is most likely benzene in hexane? Which is most likely benzene vapor? 10

3 11. Which of the following is a form of white noise? 11 a) shot noise b) flicker noise c) 60 Hz d) W lamp e) rap & C+W music 1. Which of the following is a form of environmental noise? 1 a) shot noise b) flicker noise c) 60 Hz d) W lamp e) thermal noise I II 13. Which of the compounds above are expected to have greater fluorescent signal and why? 13 a) compound I because its longer lived triplet state b) compound II because of collisional deactivation c) compound I because of great structural rigidity d) compound II because of great structural rigidity e) compound I because of collisional deactivation 14. Beer s law is obeyed if A < The wavelength of fluorescence is than the corresponding excitation What are bands observed for molecular spectra and lines for atomic spectra? 16

4 F = fluorescence; P = phosphorescence; E = excitation 17. In the diagram above the three labeled regions represent: Why is the phosphorescence lifetime longer than the one for fluorescence? What are flicker, 60 Hz, and shot noises, how does appear in a power density vs. frequency spectrum Why do methods based on fluorescence have generally a lower limit of detection than those based on absorbance? 0 1. Why is spectrofluorometry potentially more sensitive than spectrophotometry? 1. A Beer s law measurement was made at 355 nm on a 9.00 ml sample with analyte X. Its absorbance A, was found to be A spike of 1.0 ml of 1.00 mm compound X was made on that sample and A was found to be What is the concentration of X in the sample? 3. What is a disadvantage of using fluorescence as opposed to absorption? 3 4. The UV-vis absorbance analysis of Fe 3+ was conducted by complexation with 1,10 phenanthroline. The maximum absorption was found to be 510 nm. A 56. mg solution of Fe(NO 3) 3(aq) (MW ) was treated with 1,10 phenanthroline and diluted to 1.00 L. The absorbance of the complex was found to be An unknown solution was treated with 1,10 phenanthroline and its absorbance was found to be What is the concentration of iron in the unknown? 4

5 5. The diagram above is a representation of 5 a) UV-vis absorbance spectrophotometer b) Scanning IR c) FT-IR d) Fluorometer e) monochrometer 6. What are the advantages of a dual beam spectrophotometer over a single beam instrument? 6 7. Why is aqueous state analysis by IR difficult? Why does Raman excel here? 7 8. Raman Active stretches are a result of changes in 8 9. Spectral absorbance in the near-ir region are a result of Draw the major features of the Michelson interferometer. Label the moveable and the stationary mirrors of this device. Also label the source and the detector. Illustrate the optical paths with arrows indicating the direction of the photon flux. Why is there usually a laser in this device? 30

6 31. The block diagram above is a representation of 31 a) FT-IR b) photodiode array c) CCD d) scanning IR e) potentiostat 3. The diagram above is a representation of a 3 a) FT-IR b) CCD spectrometer c) PDA spectrometer d) Scanning IR e) Scanning UV-vis spectrophotometer

7 33. What is the difference between atomic emission and atomic fluorescence? What is Doppler broadening in AA spectroscopy? What are the refractory oxides? Name two examples. Why are they are a problem in AA spectroscopy? How does the graphite furnace AA spectrometer achieve a lower limit of detection than the flame AA one? Describe two reasons as to how ICP-AE achieve a much lower detection limit than flame AE Analysis by flame AA spectroscopy was conducted on an archeological sample consisting of pottery shard for cadmium. The.3451 g shard sample was digested by addition of -ml of 40% HF and -ml of 65% HNO 3. This sample was then diluted to 5.00 ml with doubly distilled water. A standard addition analysis was conducted with the treated sample. Aliquots of 1-mL of the treated sample were added to 5 10-mL volumetric flask. The following volumes of 00 ppm Cd(NO 3) standard solution were added followed by dilution to the 10-mL mark. The flame AA signal for measured for each and summarized below: 38 vol spike (ml) signal

8 Part a) What is the concentration of Cd in the pottery shard (in ppm)? Part b) What is the uncertainty for that answer in Part a? 39. An unknown element X was analyzed by AA spectrometry. The unknown was mixed with a µg/ml standard solution of X. The results are reported below. 39 Volume of unknown (ml) Volume of standard (ml) Total volume after dilution (ml) Absorbance at 566 nm What is the concentration of that unknown? Answers D 6

9 S IC VR IS S 1 VR T 1 F EC P EC S 0 VR VR Excitation (Absorbance) Fluorescence (10-7 s) Phosphorescence (10-4 s) 7 A 8 D 9 C 10 B 11 A 1 C 13 D Longer 16 The electronic states of molecules are coupled to vibrational ones. Atoms do not have vibrational states E, -F, 3-P 18 The relaxation route for phosphorescence goes through a spin forbidden triplet to singlet transition, whereas the one for fluorescence goes through a singlet to singlet transition.

10 19 Flicker Is low frequency noise whose origins are not clearly understood. Shot Noise - Arises from the statistical flucuations across electrical junctions, e.g N-P juction of a transistor. It occurs at all frequencies. 60 Hz Is a form of environmental noise that comes from AC wiring. 1/f Noise Environmental Noise Watts/ Johnson (thermal) and 0, DC f, 0 The intensity of the fluorescence signal is directly proportional to the power of the incident radiation source (P 0). I = kp 0c The signal in optical absorbance is due to a ratio of the emergent beam power relative to the incident one. A = - log (P/P 0) Increasing beam power will increase signal in fluorescence unlike absorption. Fluorescence is a scattering technique. The signal is measured outside of the axis of incident beam and therefore without the background of that beam. This background is inherent to absorption techniques. 1 the analytical signal, F is proportional to P 0, whereas absorbance, A is proportional to the ratio P 0/P 0.86 mm 0.00/0.50 = x/{(1/10) (9/10)x} X = 0.86 mm 3 There could be many, I must read and consider your answer. One obvious problem is that fewer molecules fluoresce when compared to the absorption phenomenon e-3 (mol/ g)(1/1 L) = 3.1e-4M A = ebc, eb = 0.447/3.1e-4 = = 1433(c) c = 0.361/1433 =.5e- 4M 5 C

11 6 The light beam from the source is split into sample beam and reference beam by the mechanical chopper. The reference beam monitors the lamp energy whereas the sample beam reflects sample absorption. The observed absorbance measurement is the ratio of the sample and reference beams which are recombined before moving to the monochromator. This arrangement compensates the effects due to drift in lamp intensity, electronic and mechanical fluctuations which affect both the sample and reference beams equally. Modern improvements in optics permit high level of automation and offer the same or even better level of detection as compared to earlier single beam systems. Instability factors due to lamp drift, stray light, voltage fluctuations do not affect the measurement in real-time. Little or no lamp warm up time is required. This not only improves throughput of results but also conserves lamp life 7 Water has a very strong IR absorption, and does not have a Raman active stretch 8 Polarization 9 Δν > 1 30 Please see your notes 31 B 3 B 33 AE is based on the relaxation of atomic electrons that are promoted by flame temperature. AF based on the relaxation of atomic electrons promoted by an external radiation source 34 Doppler Broadening chaotic motion in the flame itself will cause some atomic species to move away or closer to the detector. This causes line broadening as the AA and AE lines now assume a band of frequencies as opposed to a single frequency. Red shifted P 0 movement P Blue shifted movement towards detector Detector

12 35 The refractory oxides are the translucent heat-stable forms of the metal/metalloid oxides that cause light scattering within the flame. This is an non-absorption route for the decrease in the power of the emergent beam, and thus adds to the background. Examples: Al O 3, SiO, B O 3, SnO 36 It is based on the GFAA creating a nearly instantaneous plume of concentrated analyte as opposed to the flame AA which requires a constant feed of sample solution into the flame. 37 Higher temperatures with the plasma increase the population of excited state atoms. (see the discussion on the Boltzmann distribution) Also higher temperatures within the plasma are better able to break up the refractory oxides. 38 Problem 11 - Exam y = E-03x E-01 R = E-01 AA signal signal Linear (signal) Conc spike in Sample Find the x-int: 0 = 5.895e-3 (x) x = 6.56 ppm 6.56 ppm (10 ml/1 ml) = 56.6 ppm in the 5-mL treated sample solution. (56.6 g Cd / 1e6 g solution) * 5 g solution = g Cd ( g Cd /.3451 g shard sample) * 1e6 = 735 ppm Cd in pottery shard

13 Part B) Part b) You can use the spreadsheet I distributed to you earlier: vol spike (ml) conc spike in sample (ppm) signal x^ difference y- y(line) d^ E E E E E-06 Equation A: s sum E-05 xi y x int n( x int) ( x int) ( xi ) m s D n is the number of data points, m is the slope, D is as follows (5-5): D x i n xi x i s y is the standard deviation in the y-axis. It calculated as (5-7) where d is the difference between the least squares fitted line and the data point. s y d n i Now for D xi n xi xi Plug into A 3 s x int.3e e 3.00e4 3 5 ( 6.56) ( 6.56) So the x-int with uncertainty is x = 6.56 ppm ± 0.64 in relative uncertainty its x = 6.56 ppm ±.4%

14 The final answer is 735 ±.4% ppm Cd in pottery shard 39 A = ebc 0.11 = (10.0/100.0) ebc x 0.49 = (10.0/100.0) ebc x + (5.00/100.0) ebc s With c s = ppm c x = 145 ppm