Experiment FLU: Excitation and emission spectra, determination of quinine in tonic water

Transcription

1 Experiment FLU: Excitation and emission spectra, determination of quinine in tonic water Learning Goals: to learn to use a fluorometer to learn to normalize results obtained on different scales to improve volumetric technique to calculate dilution factors and results correctly Flu-1 Pre-lab Assignment: 1. In your notebook, prepare a flowchart for the experiment 2. Write pre-lab calculations for standard curve preparation and sample dilution for this experiment in your notebook. 3. On looseleaf: 1. Use diagrams to describe the phenomenon of fluorescence. How is fluorescence different from absorbance spectroscopy? Discuss instrumentation, the theory of fluorescence/absorbance, advantages and disadvantages of both methods with reference to atomic energy levels. 2. What food products contain quinine? 3. If a sample shows an intensity reading of 30 on the 10% scale, what reading would it show on the 30% scale? The 100% scale? The 3% scale? 4. Why is it important to close the fluorometer shutter before opening the sample compartment lid? 5. Calculate the volume of sulfuric acid needed to prepare 2 L of 0.05 M H2SO 4. The Handbook of Chemistry and Physics lists the molarity of concentrated acids Calculate the mass of Quinine Sulfate (C20H24N2O 2) 2H2SO42H2O), MW = g required to prepare 1 L of a 0.1 g/l solution of Quinine (C20H24N2O 2), MW = g. Introduction The small number of compounds which fluoresce both limits the scope of the method and provides its greatest advantage-specificity. If a fluorometric procedure can be devised for an analysis, it is often the most simple and fastest method, and as such it is common to find fluorometric procedures used in clinical labs, radiochemical labs, and pollution control labs. The most common type of instrument is the filter fluorometer, which relies on the specificity of the compounds being analysed for its usefulness. In addition to analytical information, a large amount of information can be obtained about the electronic properties of fluorescing compounds by using double monochromator fluorescence spectrometers. The Aminco SPF 125 is used for this experiment. Fluorescence spectroscopy is extremely sensitive, usually several orders of magnitude more sensitive than UV/Vis. Where multiple methods exist for a particular determination, fluorescence is usually the most sensitive. Other advantages of fluorescence are selectivity and an extremely large linear dynamic range. Most aromatic compounds fluoresce, often with very high quantum efficiencies. Although

2 Flu-2 aliphatic and cyclic carbonyl compounds, as well as highly-conjugated compounds also fluoresce, it is the aromatics which are the most important, partly because most molecules of biological interest are aromatic or contain multiple aromatic rings, and thus exhibit high quantum efficiencies for fluorescence. If the compound being analyzed for natural fluorescence, as in the case of quinine, very little work-up is required for the analysis. In other cases, such as many main-group cations, the addition of organic chelating groups provides a fluorescent species in solution. This is the case for the determination of calcium, for example. Until the advent of atomic absorption methods, such fluorometric methods were very common. Theoretical considerations show that fluorescent output varies linearly with concentration only at low concentrations. This results in a number of design criteria for fluorometers. Detection Limits Detection limits for quinine are in the order of 0.05 g/ml. Applications Figure Flu-1: Optics of Fluorometer Quinine or its derivatives are used in many different products. The web site has an informative summary of the history and applications of quinine, with links to medical pages. Also, try using Google to search for quinine using the advanced search; look for quinine, but exclude malaria. The average threshold concentration of the standards used to test taste perception in humans are : sucrose (sweetness) 0.7%; sodium chloride (saltiness) 0.2%; hydrochloric acid (sourness) 0.007%; quinine sulfate (bitterness) %. Instrumentation The SPF-125 Spectrophotofluorometer is composed of three units; the main spectrometer, a lamp power supply (on a shelf to the right) and a small starter unit. The lamp power supply provides power to the lamp (duh). The lamp also requires a separate starter unit, which rests on the benchtop beside the instrument (you don t have to do anything with it, it just sits there). Refer to figures 1 through 5. The instrument can separately scan the excitation and emission wavelengths from 200 to 800 nanometers or observe the fluorescence at fixed wavelengths in that region. Note the geometry of the beam of radiation from the source, and how it differs from the straight-

3 through geometry used for UV/Vis. The reason for reading the fluorescence intensity at right angles is that scattering is a minimum at 90 ; it is a maximum at 180. Flu-3 Figure Flu-2b: % full scale Figure Flu-2a: Relative intensity scales on fluorometer. Figure Flu-2a shows the fluorometer meter display; figure Flu-2b shows the % relative full scale switch, with settings (from right side, reading counterclockwise) of 0.1, 0.3, 1, 3, 10, 30 and 100. When using the 0.1, 1, 10 or 100% full scale settings, read the top scale (from 0-100). The maximum reading of the meter is the number of the full scale setting; for example, if you are using the 0.1 scale, the maximum reading is 0.1. You would divide all of your readings by 1000 to get the absolute intensity (why?). When using the 0.3, 3 or 30 settings, use the bottom scale (from 0 to 33.3). When calculating absolute intensity for the 3 scales, use 30 as the maximum reading - not 33.

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5 Flu-5 Table 1: Listing of Spectrometer Components 1. RELATIVE Indicates intensity of light detected by the photomultiplier tube. The INTENSITY meter RELATIVE INTENSITY scales are read in conjunction with the PERCENT FULL SCALE multiplier switch position. 2. SENSITIVITY control Increase measuring circuit sensitivity from times when rotated from completely (calibrate position) counterclockwise to completely clockwise. 3. SCAN controls The OFF position disconnects the scanning motor and the DC voltage. The SWEEP position connects the proportional DC voltage to the SWEEP jacks for application to the X-axis of a recorder. The drive position activates the scanning motor for either the excitation or the emission monochromator. The scan rate is 2.5 nm per second. The EM position on the inner switch selects the emission monochromator for scanning when the SCAN control is set to DRIVE. The EX position on the inner switch selects the excitation monochromator for scanning when the SCAN control is set to DRIVE. 4. POWER control Controls application of power to SPF-125. ON position places instrument in a standby condition. HV position applies high voltage to the PM tube and enables the instrument to make measurements. 5. PERCENT FULL SCALE switch 6. Lamp Cooling Fan switch 7. BLANK ADJUST RANGE switch and FINE control 8. EXCITATION WAVELENGTH The 100 through.1 positions select the calibrated sensitivity of the microphotometer's measuring circuit. This switch selects the meter scale read and the scaling factor used. Controls application of power to the lamp blower and to rear panel convenience outlet, thus applying power to the Xenon Lamp Power Supply. This switch selects the coarse range (i.e., low, medium or high) for nullification of PM tube dark current and the fluorescence/scatter generated by a blank. The FINE control is a ten-turn potentiometer used for fine nullification of these currents. These controls apply an internally generated current that is subtracted from the PM tube output signal. Enables selection and visual observation of excitation monochromator wavelength setting. Number's are in nm. 9. Excitation slit Adjusts width of excitation monochromator entrance and exit slits in the following increments: 0.1 mm (1.5 nm bandpass), 0.2 mm (2.2 nm), 0.5 mm (5.5 nm), 1 mm (11nm), 2 mm (22 nm), and 4mm (44 nm). 10 Lamp vertical Knurled knob is used to adjust vertical alignment of Xenon lamp adjustment control 11 PM shutter When up, shutter is closed, thus preventing light radiation from reaching photomultiplier tube. When pushed down, shutter is opened, thus enabling the passage of light to the PMT. 12 Emission slit width control 13 Emission Wavelength control Adjusts width of emission monochromator entrance and exit slits in the following increments: 0.1 mm (1.5 nm bandpass), 0.2 mm (2.2 nm), 0.5 mm (5.5 nm), 1 mm (11nm), 2 mm (22 nm), and 4mm (44 nm). Enables selection and visual observation of emission monochromator wavelength setting. Number's are in nm.

6 Flu-6 Figure Flu-4: Lamp Power Supply Figure Flu-5: Lamp Starter Unit Experimental procedure Solutions Preparation Dilute Sulfuric Acid, 0.05M: Dilute the appropriate volume of concentrated sulfuric acid (Caution - add acid to a large quantity of water) to 2 liters; there are 2 L volumetric flasks in the cupboard next to the sink in ELL 336 that can be used). Standard quinine solution: Prepare 1 L of a 0.1 g/l solution of Quinine solution in 0.05 M H2SO 4; verify the compound and the purity used to prepare the standard and adjust the mass as required. Quinine sulfate photo decomposes. Stock solutions should be made up fresh daily or stored in brown bottles in a cool place. If stored in brown bottles in the fridge, quinine sulfate is stable for several months. Prepare a series of standards containing between 0.1 mg/l and 0.5 mg/l Quinine in 0.05 M H SO solution. 2 4 Dilute 1.00 ml of unknown tonic water to ml with 0.05 M H2SO 4 in a volumetric flask. Prepare duplicate samples, a method spike and a sample spike for this experiment.

7 Flu-7 Instrumental Procedure Do not forget to close the PM shutter when changing samples. The fluorescence of quinine is constant in the concentration range from to 0.1M H SO. 2 4 Rinse the cell several times with each new solution between samples. Before lighting the lamp check the following: Fluorometer shutter is closed. If the fluorometer shutter is left open when the sample compartment is also open, a surge of light (from the room lights) will hit the photomultiplier tube (PMT) and swamp it. It takes the PMT about 15 minutes to recover after this happens, and causes it to age prematurely. Fluorometer scan switch is off. Fluorometer power switch is off. Exhaust vent damper is open. (Door marked Vent on the front right corner of the fluorometer). Analysis Turn on the lamp power supply (to the right of the fluorometer, on a shelf) and the lamp cooling fan switch (top right side of fluorometer). Press the "ignite" button on the lamp power supply momentarily, until the red light over the power switch lights up. Do not hold the button depressed for more than 3 seconds at a time. Turn the fluorometer power switch on (top left side of fluorometer). Allow the instrument to warm up for 15 minutes before switching to HV. This applies a warmup current to the detector (photomultiplier tube, or PMT), to help reduce the shock of the HV power application. Start with the following settings on the instrument: percent full scale 10 blank adjustment low slit widths both set at 2 emission wavelength 450 nm excitation wavelength 350 nm Note that the quartz fluorescence cuvette will be marked with a letter "Q" in one corner; always insert the cuvette the same way each time. With the shutter closed, insert the cuvette containing the blank and set the reading to zero. Use the most sensitive scale (e.g., 0.1) and leave the blank adjust control on low. Set the blank reading to zero by using the small knob in the center of the blank adjust control. Open the shutter and rezero. Close the shutter and remove the blank. Record the excitation and emission spectra of quinine as follows. With the shutter closed, insert the cuvette containing the 0.5 mg/l standard, replace the cover and then open the shutter. Adjust the % Full Scale until the reading is less than the maximum of the dial. Manually adjust the 2 monochromators to obtain the maximum intensity. If the reading goes off scale (is pinned at the far right side) then increase the %Full Scale and

8 Flu-8 continue adjusting the two monochomators. Then, turn on the chart recorder. Make sure the clip is on the end of the chart paper. The recorder will start running automatically once the instrument begins scanning. Mark the starting/ending positions and the starting/ending wavelengths on the spectrum. Record the EXcitation spectrum: leave the emission wavelength at the wavelength giving maximum intensity, and set the excitation wavelength to the minimum value on the monochromator dial. Set the inner scan knob to EX. The scan is started by turning the outer scan knob to Drive ; simultaneously start the chart recorder. When finished, turn the outer scan knob back to Off Record the EMission spectrum: set the excitation wavelength to the wavelength giving maximum intensity and then set the emission wavelength to the minimum value on the monochromator dial. Set the inner scan knob to EM, and scan as above. When you have completed the two spectra, set the monochromators to give the maximum intensity for the sample; then adjust the sensitivity control so that the maximum reading of the high standard is between 90 and 100. These wavelengths will be used for the remainder of the experiment. Close the shutter and remove the sample. Next, insert the blank again. Close the sample compartment, open the shutter and adjust the blank to zero on the most sensitive scale (i.e., 0.1%) using the fine blank adjust inner knob (leave the outer Range knob set to Low ). 1 Now, begin using the GLP order as you measure the relative intensities of the emission of the blank, standards and unknown. Adjust the percent-full-scale control to get the highest reading for each solution, without it going off scale. Make sure that you record both the percent-full-scale and the displayed intensity from the correct meter scale. When reading the sensitivity on the meter: use the display scale that corresponds with the percent-full-scale switch setting. The top scale (0-100) is used if the percent full scale is on any multiple of 10. The middle scale (0-33) is used if the percent-full-scale switch is on any multiple of 3. Supposing on the 10 scale the intensity of a sample is about 15 %; in this case you should switch to the next most sensitive scale, which is the 3 scale. To read the intensity using the 3 scale or multiples thereof use the middle scale, the intensity should still be about 15 % but you may see that now it is actually 15.5 or 16.0 % instead of just 15 %. Note that for the 3 scale the maximum reading possible is 33 1/3 %, compared with 100 % on the 10 scale. Set the %full scale to get the maximum reading, without having the needle read off scale. If the sample does not read within the range of the standard curve, you will need to reevaluate your dilution calculations; prepare a different dilution of the sample and run it again. 1 see GLP appendix.

9 Flu-9 Instrument Shutdown Shut down the instrument as follows: Switch off the lamp power supply. Switch off the fluorometer power. Allow the blower to cool the lamp for 10 minutes before switching off the fluorometer lamp fan. Close the fluorometer shutter after switching off the fan power. Treatment of Data/Reporting A full report is required for this experiment. 1. Convert all the relative intensity readings to absolute intensity. 2. Hand in the excitation spectrum and the emission spectrum. From the spectra, measure and calculate max. Compare the max/min of the spectra; do you see any correlations or similarities? Explain. 3. Plot an analytical calibration curve. 4. Determine the concentration of quinine (mg/l) in the original unknown. Also determine the concentration of quinine in the QC sample. Calculate and report your spike recoveries. 5. Calculate and report the results from the quality control sample. Record your results in the QC Data binder.