27-1 (a) Resonance fluorescence is observed when excited atoms emit radiation of the same

Transcription

1 Chapter (a) Resonance fluorescence is observed when excited atoms emit radiation of the same wavelength as that used to excite them. (b) Vibrational relaxation occurs when excited species collide with molecules, such as the solvent, and in doing so lose energy without emission of electromagnetic radiation. The energy of the excited species is decreased by an amount equal to the quantity of vibrational energy transferred. (c) Internal conversion is the nonradiative relaxation of a molecule from a low energy vibrational level of an excited electronic state to a high energy vibrational level of a lower electronic state. (d) Fluorescence is a photoluminescence process in which atoms or molecules are excited by absorption of electromagnetic radiation and then relax to the ground state, giving up their excess energy as photons. (e) The Stokes shift is the difference in wavelength between the radiation used to excite fluorescence and the wavelength of the emitted radiation. (f) The quantum yield of fluorescence is the ratio of the number of fluorescing molecules to the total number of excited molecules. (g) Self-quenching occurs when the fluorescent radiation from an excited analyte molecule is absorbed by an unexcited analyte molecule. This process results in a decrease in fluorescence intensity For spectrofluorometry, the analytical signal, F, is given by F = 2.3K εbcp 0. The magnitude of F, and thus sensitivity, can be enhanced by increasing the source intensity, P 0, or the transducer sensitivity. For spectrophotometry, the analytical A is given by A =

2 P / P 0. Increasing P 0 or the detector s response to P 0 is accompanied by a corresponding increase in P. Thus, he ratio does not change nor does the analytical signal. Consequently, no improvement in sensitivity accompanies such changes (a) Fluorescein because of its greater structural rigidity due to the bridging O groups. (b) o,o -Dihdroxyazobenzene because the N=N group provides rigidity that is absent in the NH NH group Compounds that fluoresce have structures that slow the rate of nonradiative relaxation to the point where there is time for fluorescence to occur. Compounds that do not fluoresce have structures that permit rapid relaxation by nonradiative processes Organic compounds containing aromatic rings often exhibit fluorescence. Rigid molecules or multiple ring systems tend to have large quantum yields of fluorescence while flexible molecules generally have lower quantum yields Excitation of fluorescence usually involves transfer of an electron to a high vibrational state of an upper electronic state. Relaxation to a lower vibrational state of this electronic state goes on much more rapidly than fluorescence relaxation. Fluorescence almost always occurs from the lowest vibrational level of the excited electronic state to various vibrational levels of the ground electronic state. Such transitions involve less energy than the excitation energy. Therefore, the emitted radiation is longer in wavelength than the excitation wavelength See Figure A filter fluorometer usually consists of a light source, a filter for selecting the excitation wavelength, a sample container, an emission filter and a detector.

3 A spectrofluorometer uses monochromators instead of filters for excitation and emission. There are also hybrid instruments that use an excitation filter and an emission monochromator. In corrected spectrofluorometers, there is also a reference detector for monitoring and correcting for fluctuations in the light sources intensity. Emission is usually detected at right angles to the incident radiation to maximize the fluorescence signal Most fluorescence instruments are double beam to compensate for fluctuations in the analytical signal due to variations in source intensity Because fluorometers don t disperse the emitted radiation the instrument can detect the entire spectrum of fluorescence emission (as long as a suitable filter is used to remove scattered light from the excitation source). Thus, a fluorometer can provide lower limits of detection than a spectrofluorometer. In addition, fluorometers are substantially less expensive and more rugged than spectrofluorometer, making them particularly well suited for routine quantitation and remote analysis applications.

4 A B C D E F G H I Determination of NADH 2 Part (a) 3 Concentration in M Fluorescence unknown Part (b) 14 Regression equation 15 Slope Intercept 4.E Concentration of unknown Parts (c), (d), (e), and (f) Spreadsheet Documentation 19 Error Analysis B15=SLOPE(B4:B11,A4:A11) 20 s r (standard error in y) B16=INTERCEPT(B4:B11,A4:A11) 21 N 8 B17=(B12-B16)/B15 22 S xx 0.42 B20=STEYX(B4:B11,A4:A11) 23 s m 0.27 B21=COUNT(B4:B11) 24 y bar (average fluoresence) B22=B21*VARP(A4:A11) 25 M for part (e) 1 B23=SQRT(B20^2/B22) 26 M for part (f) 3 B24 =AVERAGE(B4:B11) 27 Standard deviation in c for part (e) B25= Replicates part (e) (entry) 28 RSD in c for part (e) B26=Replicates part (f) 29 Standard deviation in c for part (f) B27 =B20/B15*SQRT(1/B25+1/B21+((B12-B24)^2)/((B15^2)*B22)) 30 RSD in c for part (f) B28=B27/B17 31 B29=B20/B15*SQRT(1/B26+1/B21+((B12-B24)^2)/((B15^2)*B22)) 32 B30=B29/B17

5 27-11 In the first printing of the text, volumes of the standard solution were presented in both the text and the table. Using only the data in the table, we find A B C D E F G H 1 V (Zn 2+ ) c S, ppm 1.10E Part (a) Volume Standard Fluorometer 5 Solution, ml Reading Part (b) 12 Slope Intercept Part (c) 15 s r SD m SD b LINEST Values 18 Part (d) m b 19 c (Zn 2+ ), ppm SD m SD b 20 Part (e) R s r 21 SD c (Zn 2+ ), ppm F DOF 22 SS regr SS resid Spreadsheet Documentation 25 B12 = SLOPE(B6:B9,A6:A9) 26 B13 = INTERCEPT(B6:B9,A6:A9) 27 E18:F22 = LINEST(B6:B9,A6:A9,TRUE,TRUE) 28 B15 = F20 29 B16 = E19 30 B17 = F19 31 B19 = B13*B2/(B12*B1) 32 B21 = B19*SQRT((B16/B12)^2+(B17/B13)^2) c Q = 100 ppm 288 / 180 = 160 ppm 100 ml 1mg quinine 1g solution 160 ppm 500 ml = 533 mg quinine 3 15 ml 1 10 g solution 1mL

6 A c V (540)(50 ppm)(10.0 ml) s s c Q = A A V 540 (20.0 ml) 2 1 Q = 225 ppm 1mg quinine 1g solution 225 ppm 1000 ml g solution 1mL = 225 mg quinine g Q g Tablet 6 10 = ppm