4. How can fragmentation be useful in identifying compounds? Permits identification of branching not observed in soft ionization.

Transcription

1 Homework 9: Chapters Assigned 12 April; Due 17 April 2006; Quiz on 19 April 2006 Chap. 20 (Molecular Mass Spectroscopy) Chap. 21 (Surface Analysis) 1. What are the types of ion sources in molecular mass spectroscopy? Hard source o Ion in excited state often with direct excitation of analyte molecule with excitation source. Fragments are often produced with this method. Soft source o Little fragmentation with this method. Analyte ionization mainly from secondary effects. The analyte molecule has minor fragmentation.

2 2. List 3 ion sources used in molecular mass spectroscopy 3. Describe Matrix Assisted Laser Desorption/Ionization a. Soft Ionization b. Sample dissolved in solution containing UV-absorber and solvent c. Solution evaporated and precipitate formed d. Pulsed laser used to excite precipitate e. Molecular ion desorbed from surface of precipitate In MALDI a laser is used to excite an organic matrix that absorbs in the UV. The solution evaporates and a precipitate is formed. The precipitate is excited by a pulse laser, liberating a molecular ion. 4. How can fragmentation be useful in identifying compounds? Permits identification of branching not observed in soft ionization. 5. How can element isotopics effect the mass spectra of molecules? The peak ratios will reflect the isotopes of the elements in the molecule. For example, those molecules containing Cl will reflect the different Cl isotopes 35,37 Cl. Other isotopic ratios are listed below.

3 6. List the ionization methods for the spectra below 7. List the ionization methods for the spectra below electron impact field ionization desorption 8. Describe electrospray ionization A solution is pumped through a needle. The needle is at kv potential compared to surrounding electrode so the droplets become charged. As the solvent evaporates

4 the droplet shrinks and the charge density increases. Eventually the charge migrates to the molecule. 9. Describe the chemical transformations that may be observed at a surface? At a surface an interaction between solid-solution, solid-gas, or a vacuum-solid occur at a surface. An example of a typical surface transformation is oxidation. The bulk solid differs from the surface with respect to chemical species. The depth of oxidation or surface variation can be evaluated. 10. What are some common characteristics of surface techniques? List some of the techniques. Surfaces are probed with an incident beam of photons, electrons, or other suitable particles. A secondary beam is detected and measured. The incident and secondary particles do not have to be the same. If the incident beam goes beyond the surface the secondary beam must originate with the surface. 11. What is the difference in probing a surface with electrons versus photons? Electrons have shorter path and will interact with the surface to a greater degree than photons. The secondary particles from a primary photon beam need to have a short pathlength. 12. Describe X-ray photoelectron spectroscopy A surface is excited by an x-ray (hv) and an emitted electron is measured. The momentum p of the outgoing electron is determined from the kinetic energy used p=(2me kin ) 0.5. The bonding energy of the electron is determined from the difference between the energy of the incoming x-ray and the kinetic energy of ejected electron and work function of the spectrometer (w). * E b =γ-e k -w

5 13. What are the 3 different x-ray transitions commonly seen in Auger Electron Spectroscopy? Describe the transitions. What type of elements are favored by Auger Electron emission? The transitions involve electron removal, transition to the removed state, and ejection of electron. For the figure below, the transition is KLL. Other transitions include LMM and MNN. Auger is favored by lighter elements.