CHM 424 EXAM 4 CRIB - COVER PAGE FALL

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1 CHM 44 EXAM 4 CRIB - COVER PAGE FALL 007 There are six numbered pages with five questions. Answer the questions on the exam. Exams done in ink are eligible for regrade, those done in pencil will not be regraded. 1 coulomb = charges 1 amp = 1 coulomb per 1 second 1 volt = 1 joule per 1 coulomb ε 0 = C J -1 m -1 h = J s k = J K -1 c = 3 8 m s -1 1 amu = kg k = cm -1 K -1 m p = amu 1 ev = J ν vib (H ) = 4,400 cm -1 φ f σa F = qvb σ f = P= ε0χe P= Nqx 4π I s (λ) = I L σ s lnωf(λ)e(λ)d(λ) ( νl νvib) ( νe + νl) = Δ IR ( νe νl) N 1 N 0 exp( E kt) 1 k ν = μ π μ = E k mv = = qv s mm 1 m + m 1 kt L = Pσ 0.61λf ρ= = 1. λ f # r z1m1 = M + z1mp jm ( m ) z = M = z ( m m ) p ( m m1) p πr m π t = = v q B 1 q B f = = t m π m r B = q V s E qe = r E r = k qe x y x y Φ = Φ = π ( U V cos( f t) ) xy, 0 0 r0 r0 k Φ F = ma = zq x x Φ m = zq t x zqt Φ x = m x t d m d = = v z Ve s

2 CHM 44 EXAM 4 CRIB FALL 007 Name Score / pts. 1. Provide the following definitions or facts at 6 pts each. a. What is the physical origin of Rayleigh scatter? Which scatters most a pure gas, pure liquid, or pure crystal? The variation in the number of polarizable electrons within a volume given by the wavelength cubed. A gas shows the greatest number density variation, thus the largest scatter. b. In an FTIR the sample is placed between the interferometer and the detector. Where in the optical path is the sample placed in an FT Raman instrument? The sample replaces the IR source. Note that the laser is not the Raman source, it simply makes the sample emit. c. Which molecular fragmentation reaction is more likely? (1) ABCD + 6 AB + CD + (1) since even-number electron fragments are more stable () ABCD + 6 AB + CD + d. Name the four ionization methods covered in the notes, Section 0. Ion Sources. electron impact ionization, chemical ionization, matrix assisted laser desorption, and electrospray (collision induced dissociation will be accepted though it was covered in 0.1) e. On what basis does a (1) magnetic sector, () electric sector, and (3) time-of-flight analyzer separate charges? (1) momentum, () kinetic energy, (3) velocity Exam 4 Crib, 007, page 1

3 30 pts.. Perform the following calculations at pts each. a. A compound has a fluorescence spectrum with three vibronic peaks at 500.0, 540.5, and 588. nm. If this compound is excited with a laser at 63.8 nm, at what wavelength will this vibration appear? nm = 0,000 cm nm = 18,501 cm -1 or a vibrational frequency of 1,499 cm nm = 15,803 cm -1-1,499 14,304 cm -1 = nm will be the wavelength of the Raman peak b. If a mass spectrometer can be built with a source-to-detector length of 1 mm, what is the highest pressure (torr) that will be absent intermolecular collisions. Use σ = m, room temperature, and 1 Pascal = torr L = = 6.5 Pascal = 4.85 torr c. A 0 amu singly-charged ion is accelerated with a 1,000 V potential. What velocity will the ion have when it leaves the source? 16 E = qv = 0 ev = 1.6 J v 16 E 3. 9 = = = m v = 4.4 ms Exam 4 Crib, 007, page

4 30 pts. 3. Answer the following two homework questions for 15 pts each. a. My group used a frequency doubled dye laser to observe Raman spectra with ultraviolet excitation. With the particular laser we used, only % of the light at 600 nm was converted to 300 nm. If you don't know much about Raman excitation, it would seem that the signal with 300 nm excitation would be % that obtained at 600 nm (some people actually told me it was a stupid experiment when I proposed it). Is this true? With this particular laser what signal strength would you get at 300 nm compared to 600 nm? Going from 600 nm to 300 nm would predict a 16 increase in signal strength according to the 1/ 4 relationship. (600/300)4 = 4 = 16. The drop in intensity by % would mean that excitation at 300 nm would be 1.6 that obtained at 600 nm. b. The ion-accelerating voltage in a particular quadrupole mass spectrometer is 5.00 V. How long will it take a singly charged benzene ion to travel the length of the rod assembly, a distance of 15.0 cm? An acceleration of 5 V will give the ion 5 ev of energy. Using 1 ev = J, the ion energy will be 8-19 J. The mass of benzene is 78 amu. Using 1 amu = kg, the ion mass will be kg. The velocity can now be computed v = v= = 3,5ms The time for the ion to travel 15 cm (0.15 m) is 0.15/35 = 4 μs Exam 4 Crib, 007, page 3

5 30 pts. 4. Consider the Raman spectrometer drawn in block diagram form at the right. This is a single-beam instrument much like a fluorometer. As such it has to be calibrated for at least three types of signal variation: (1) source intensity changes from sample to sample, () emission optical component efficiencies, and (3) changes in the laser wavelength. Discuss how each of these signal variations will affect the spectrum and how one might correct for them. laser beam is vertical to match the monochromator slits sample lens lens holographic Rayleigh rejection filter grating CCD array detector slit mirror mirror (1) Source fluctuations will cause the entire spectrum to change intensity and prevent quantitation. Can be taken into account by using an internal standard that has a constant concentration and a spectrum not interfering with the sample, e.g. carbon tetrachloride with unchlorinated organic compounds. The sample intensity is normalized by the standard intensity. () Optical component efficiencies will affect the intensity of individual Raman bands with respect to each other. As an example, the efficiency with a shift of 3,500 cm -1 might be less than a shift closer to the laser wavelength. This means that relative heights can't be used as an aid in identification or differentiation of structurally related compounds. The wavelength response can be calibrated by using an emission standard such as those used in the fluorescence experiment. Within the last year NIST has started to supply such standards for Raman. (3) Changes in laser wavelength will cause the entire spectrum to change intensity because of the 1/λ 4 relationship. The Albrecht-Hutley equation given on the front page can be used in conjunction with an internal standard to make this adjustment. The internal standard must have a known ν e. The standard is normalized to unity at one wavelength. For all other wavelengths the Albrecht-Hutley equation is used to compute the expected intensity of the standard. The standard is normalized to this value. The sample spectrum is then normalized by the same value. Exam 4 Crib, 007, page 4

6 30 pts. 5. The following questions pertain to the DESI paper by Cooks, et al., given out in class. a. Briefly describe the experiment. b. What is the primary advantage of the DESI experiment in comparison to standard mass spectrometric instruments? c. What highly unusual type of surfaces (for normal mass spectrometry) can be examined by DESI? Exam 4 Crib, 007, page 5

7 d. Describe how chemical reagents can be used as an aid in identification? e. How quantitative is the technique? f. What living tissue example was given? Exam 4 Crib, 007, page 6