Chem 155 Quiz 3 Review Topics: Quiz 3 outline

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1 Quiz 3 outline 1. Atomic absorption spectrometry a. Principles of FAAS where selectivity and sensitivity arise b. Spectrometer design c. Atomization processes d. Sensitvitiy and atomization: protecting agents, releasing agents, radiation buffers e. Bandwidth considerations and calibration curve linearity 2. Atomic emission spectrometry a. ICP principles i. Ar plasma ii. Inductive coupling iii. torch design b. Ar-plasmas versus flames as atomizers c. Bandwidth considerations and noise d. Stability considerations, matrix effects and internal standards e. Monochromators, simultaneous multielement detection, Echelle designs and CCD array detection 3. UV-Vis absorbance spectrometry a. Beer s law b. Molecular cross sections c. Non-Beer s law behavior 4. UV-Vis spectroscopy of Molecules a. Pi, sigma, pistar and sigmastar molecular orbitals b. Broad classifications a. * b. n * c. n *, * c. Solvatochromism a. Increase in solvent polarity blue shift implies n * character b. Increase in solvent polarity blue shift implies * character c. Conjugation usually: i. Increases extinction of and ii. red-shifts n *, * type transitions d. Aromatic substitution with (-) charge density (e.g. O - ) can destabilize pi system and red shift * e. Aromatic substitution with (+) charge density (e.g. -NH 3 + ) can stabilize pi system and blue shift * d. Aqueous transiton metal ions a. Vis transitions are d-d based and weak b. Ligands like SCN- and o-phenanthroline can make charge transfer complexes with large and therefore low DL (C MIN ). e. Photometric titration a. uses C T and V T to determine concentration at inflection of A vs V T b. does not use A, therefore does not i. require strict Beer s law behavior ii. is insensitive to matrix effects that could change.

2 Path of analyte through plasma or flame Chem 155 Quiz 3 Review Topics: 1. Fill in the empty boxes consistent with the diagram. SEE NOTES In boxes 1 to 6 inddicate in what form the analyte is likely to exist: 6 5 Emission In boxes 7 and 8 below put the equation that relates concentration and light power corresponding to emission (7) and absorption (8): 7 HCL h 4 Absorption Nebulize Liquds Solids Gases

3 2. Compare Ar-plasma and air-c 2 H 2 flames in the following categories: Quality circle correct Electron density Ar-plasma has higher lower equal e - density than air-c 2 H 2 flame. Temperature Ar-plasma is higher lower equal temperature than air-c 2 H 2 flame. Chemical Ar-plasma has higher lower equal air-c 2 H 2 flame. reactivity reactivity than Formation of Ar-plasma is higher lower equal air-c 2 H 2 flame. oxides likelihood than Atomization efficiency Ar-plasma is superior inferior equal to air-c 2 H 2 flame. 3. Compare inductively coupled plasma atomic emission spectrometry (ICP-AES) to flame atomic absorption spectroscopy (FAAS) in the following categories: Quality circle correct choice Cost to buy / ICP-AES is higher, lower, FAAS operate equal cost than Simultaneous ICP-AES is more, less, FAAS Multielement detection equally competent than Dynamic Range ICP-AES has larger, smaller, FAAS equal dynamic range than Detection Limit ICP-AES has higher, lower, equal detection limits than FAAS 4. Because of the mulit-element capability and atomization / temperature sensitivity of emission intensity, the following analytical method has proved highly successful in ICP-AES: a. Standard addidions c. D2 background correction b. Internal standards d. nonlinear calibration analysis 5. Formation of oxides or molecular species in flame/plasma can be suppressed by: a. EDTA or other complexing agents b. Addition of KCl to the matrix c. Addition of oxyanions such as sulfate or phosphate. d. Standard additions methods. 6. Ionization of analyte atoms in flame/plasma can be suppressed by: a. EDTA or other complexing agents b. Addition of KCl to the matrix c. Addition of oxyanions such as sulfate or phosphate. d. Internal standards methods.

4 7. Fill in the following table assuming Beer s law is obeyed: Absorbance P Po %T C / M A log %T A %T 100 %T A %T 100 P P0 P %T 100 P0 8. Based on this, comment on the relative reliability (i.e. accuracy or precision) of absorbance measurements at A=1 and A=3. 9. Associate each of the following spectra with the following solvent: a. Gas phase b. Water c. Cyclohexane d. Perfluorooctane e. Dioxane

5 10. What is the function of the reagent blank in the metals experiment? To evaluate analyte levels in the Blank. 11. What is the function of the spike recovery analysis in the metals experiment? To validate the sample preparation step. 12. Circle the correct answer. In comparison to double-beam scanning spectrophotometers, diode array spectrophotometers are: Quality Spectrometer Type Diode Array Double Beam more accurate absorbance more accurate wavelength much faster more complex more expensive 13. Solvatochromism: correlate the following: Spectral Shift on transfer from hexane to alcohol Transition type Predominant effect seen in spectrum is: Shift nonbonding (n) * orbital direction orbital red blue stabilize destabilize stabilize destabilize n* * 14. Aromatic substitution: Spectral Shift on 254 nm * transition of benzene upon substitution with: Substituent Shift direction Effect on orbital Effect on * orbital OH O - NH 2 NH 3 + red blue stabilize destabilize stabilize destabilize

6 15. Assuming that the absorption noise in your spectrometer, σ A = 0.001, what is your detection limit for Fe +2 aquo ( MAX =1.0 M -1 cm -1 ) and FeSCN +2 ( MAX =5000 M -1 cm -1 ) in the nm range? a. C MIN =3s b /m b. s b = A c. m = slope of calibration curve d. slope of calibration curve is (M -1 cm -1 ) C M 3s b m M 1 cm cmm M 1 cm cmm 16. Photometric titration of Fe+3 with SCN- solution to make FeSCN would give what titration curve? 17. Why such a photometric titration be more accurate than relying on Beer s law to make the concentration estimate? Select all that apply: a. Matrix effects are unimportant in photometric titration. b. Photometric titration uses an internal standard. c. Beer s law is always obeyed in photometric titrations. d. Beer s law need not be obeyed in photometric titrations. e. Photometric titration relies on titrant volume and titrant concentration which can be measured accurately. f. Photometric titration relies on absolute absorbance which can be measured exactly. g. Photometric titration does not require sample preparation.

very high temperature for excitation not necessary generally no plasma/arc/spark AAS

very high temperature for excitation not necessary generally no plasma/arc/spark AAS Atomic Absorption Spectrometry (Chapter 9) AAS intrinsically more sensitive than AES similar atomization techniques to AES addition of radiation source high temperature for atomization necessary flame