Chem 155 Midterm Exam Page 1 of 10 Spring 2011 Terrill

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1 Chem 155 Midterm Exam Page 1 of 10 Spring 011 Terrill ame Signature Experimental design: 1. Selenium (Se) is toxic and teratogenic above about 100 μg / day. Yet vitamin tablets often list 100 μg of selenium (Se) on the label as it is an essential nutrient. Regulating levels of Se in vitamin tablets is thus important. The following pertain to Se analysis by ICP, an atomic spectroscopy method similar to atomic absorption. i. ICP atomic emission spectroscopy has a linear dynamic range of about 100 to 5000 ppb (ng/ml) Se. ii. ICP requires about 10 ml of sample iii. You have 1000 ppm Se as a primary standard solution. iv. Assume that you can pipet no less than 100 μl and use no volume larger than 100 ml to minimize waste in the following: b. If you are able dissolve a vitamin tablet that purports to contain 10 μg of Se in acid and then dilute it to 5 ml, what will the approximate concentration of the [Se] be in the solution? 0.4 ppm g 5 10 L 1000 mg g = 0.4 mg L _400 ppb c. Give the concentrations of calibration standards that would be appropriate for the analysis of such a sample. (if you don t know the answer to the above question assume 1000 ppb) 100, 500, 1000 ppb d. Suggest a method to prepare the calibration standards from the primary stock standard solution provided. Observe the 100 μl pipetting and 100 ml maximum volume for dilution limitations. Preliminary dilution(s): Final Dilutions: Target (final) calibration standard conc. / ppb Conc. Of soln. pipetted / ppb 1000 mg L Volume pipetted / ml Final volume / ml o further dilution needed 0.1 ml 100 ml = 1 mg L

2 Chem 155 Midterm Exam Page of 10. Express in your own words the meaning of a 95% confidence interval. The interval within which you may guess the true mean of a population to be with the confidence that there is only one chance in 0 that you are wrong.. The following set of measurements were made for determining the caffeine content of a diet soda (mg per can): = The expected value is exactly 45.0 mg per can. Is there evidence at the 95% confidence level that the soda sample tested differs from the labeled amount of caffeine? In your answer, assume that the method is valid. ( ) + ( ) + ( ) =.085 Since 1.7<4., there is no evidence that exp value differs from 45 mg = % confidence interval freedom CI ts x 1 x μ + t calc s 4. What is the probability that the true caffeine content of the can is less than the lower boundary of the 95% confidence interval?.5%

3 Chem 155 Midterm Exam Page of When is the method of standard additions helpful? When matrix effects, i.e. dependences of instrumental sensitivity on sample composition, are significant. 6. What is an internal standard and when is it helpful? An internal standard is a chemical, which is clearly instrumentally resolved from analyte and has no interferant of its own, which is added in constant concentration to samples and standards to provide an internal signal reference intended to compensate for the effects of small changes, i.e. drift, in instrumental parameters affecting overall instrument sensitivity. 7. What is the function of a reagent blank? A reagent blank is a sample comprised of all chemicals used in sample preparation which serves to identify interferences arising from sample preparation steps. 8. What is the purpose of a spike recovery analysis? A spike recovery analysis serves to quantitatively validate the sample preparation steps indicating loss or gain of analyte or interferant during sample preparation.

4 Chem 155 Midterm Exam Page 4 of Draw an energy state diagram for the Kr, F and the KrF* excimer as pertains to it s function in the excimer laser. a. Identify the atoms or molecules in all four states. b. Identify the two states involved in the stimulated emission process. c. Draw an arrow indicating the transition that produces laser radiation. Kr * + F KrF * KrF States in lasting transition. Lasing transition. Kr + F 10. Explain why this 4state configuration may have a lower pump energy threshold than a state laser. The lower state in a state laser is the ground state, which by definition is nearly always the predominant one in terms of population. Since the lower state produces absorption loss in a laser, it must be depopulated by means of pumping to achieve light amplification. This is difficult. In a 4 state laser, the lower state may be unstable and therefore easily depopulated. 11. Fill in the blanks: ame of EM regime: Wavelength Predominant Excitation Xray 0.1 to 10 nm Core e ame of Spectroscopy 4 Ultraviolet nm Valence e Uv or uvvis Visible nm Valence e Vis or uvvis Infrared.540 μm Molec vibration Radiowave 0.11 meter uclear spin in mag field IR or FTIR or vibrational MR

5 Chem 155 Midterm Exam Page 5 of If a CzernyTurner monochromator has the following specifications: Holographicallyruled diffraction grating with 158 grooves per mm. 50 mm focal length Grating position such that the diffracted angle is 45 degrees Operation in first order A slit width of 0.5 mm a. What is the reciprocal linear dispersion of this monochromator (use appropriate units)? 1mm 10 6 d cos 45 π nm 180 d := F := 50 mm D := D = 1.8 nm 158 mm 1F mm b. What is the effective bandwidth of this monochromator (use appropriate units)? w := 0.5 mm Δλ eff := wd Δλ eff = 0.89nm 1. Give the vocabulary word or short phrase that corresponds to the following definitions: (4 pts.) a. The ratio of the sensitivity of an selectivity instrument to an analyte to that of an interferant. b. The slope of a graph of instrument Sensitivity response versus analyte concentration. 4 c. The minimum concentration of analyte for which an accurate determination of concentration can be made. d. The largest concentration for which a calibrationcurve remains linear. e. The range of concentrations (or signal strengths) between the LOQ and the LOL. f. The minimum detectable concentration of analyte. g. Closeness of mean of population to the true or theoretical value. h. Closeness of a distribution of measurements to each other. Limit of quantitation Limit of linearity Linear dynamic range Limit of detection Accuracy Precision

6 Chem 155 Midterm Exam Page 6 of Sketch the approximate form of the Φ (chisquared, sum of the squared residuals) versus the parameter a in the model y=a+b*x. This pertains to the fitting of an x,y data set to a linear model. Assume data have a good fit to a line. Indicate at what point along this curve the bvalue is optimal? Φ parabolic b 15. Draw a Jablonski diagram for an organic molecule and identify: a. the ground electronic state, S0 b. the first singlet excited state S1 c. the vibrational levels within the singlet excited state d. the triplet excited state T1 e. transitions corresponding to absorption of a. IR radiation b. UV radiation f. Fluorescent emission g. Phosphorescent emission S1 VIB LEVELS T1 UV ABS FLUOR PHOS IR ABS S0

7 Chem 155 Midterm Exam Page 7 of Indicate true answer(s) below. An internal standard is: a. Compensates for random changes in measurement sensitivity. b. Compensates for matrix effects. c. Prevents errors due to injection volume variation in chromatography. d. Prevents changes in detector sensitivity. e. Is always used when sample and standard are in different matrices. 17. Indicate true answer(s) below. A matrix effect is: a. A chemically induced change in measurement sensitivity. b. An interferant chemical that gives a signal at the analyte wavelength. c. A thermally induced change in measurement sensitivity. d. Due to changes in detector sensitivity. e. Due to differences between samples and standards. 18. Elaborate the acronym for PMT and CCD detector: P HOTO M ULITPLIER T UBE C HARGE C COUPLED D DEVICE 19. Draw on the diagram below to indicate the sequence of events that leads to light detection in a CCD pixel. Fill in the boxes with explanatory text. 10V 10V 10V Al contact SiO nsi e are repelled from Al contact. hν strikes depletion region, electron and hole are created. h+ accumulate at Al contact

8 Chem 155 Midterm Exam Page 8 of 10 Statistics: Mean : μ lim x Population Standard Deviation: σ x lim x μ ( ) Average: x avg x Sample Standard Deviation: s x x x ( avg ) 1 Bias or absolute systematic error = x avg μ Relative standard deviation = s x avg Confidence limits: μ x±ts n μ x±zσ n

9 Chem 155 Midterm Exam Page 9 of 10 Multiplication/division rule: Propagation of Error: Addition/subtraction rule: If: a b±σ b + c±σ c d±σ d If: a c±σ c b±σ b d±σ d Then: Signals: σ a σ b + σ c + σ d Then: σ a a σ b b + σ c c + σ d d S S b + mc General transducer response S min S b + σ Blank Minimum detectable signal Standard Additions: C x C min bc s mv x σ Blank m C x Beer's Law: A ε b Detection Limit ( ) V s_0 C s V x V x = sample vol C x = sample conc. V s = spike vol C s = spike conc. V T = total vol V S_0 = xintercept C log P P o C k P P o Beer's Law Absorption Boltzmann Partition Function: i o ΔE g i kt e g o Emission ( ) k jk 1 Boltzmann Constant Electromagnetic (EM) Radiation: Constants: h js Plank Constant c ms 1 Speed of light in vacuum k jk 1 Boltzmann Constant a mol Avogadro's umber J To convert ev to Joules multiply by: ev Equations: E λ ν h ν c Photon Energy Velocity of light in vacuum λ max T 10 6 nm K Wien Displacement Law for Blackbodies

10 Chem 155 Midterm Exam Page 10 of 10 Interactions of EM Radiation with Matter: c medum E e Diffraction and Monochromators: ( ) n λ d ( sin( r) + sin() i ) D 1 Effective bandwidth = w D 1 R f n λ λ Δλ F d ( ) ( ) ( ) I R n n 1 I o n + n 1 h ν c n medium n 1 sin θ 1 n sin θ φ d sin θ Δλ Δy n ( ) d cos() r nf Refractive index Snell's Law of Refraction Fresnel Formula for ormal Incidence Reflection Photoelectric effect Grating Equation normal incidenc Grating Equation Reciprocal Linear Dispersion Resolution Speed Michelson interferometry: n λ a ( b) δ Principle equation f Δν v m λ 1 δ Interferometer output frequency (Hz) Interferometer resolution (cm 1 )