A CHEM - 1. Discuss whether IR spectrometry is a...

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Unformatted text preview: 1. Discuss whether IR spectrometry is a quantitative or qualitative technique (or both, and if both, when is this the case?) IR spectrometry is both a quantitative and qualitative technique, due to the fact thatIR spectrometry follows Beer ’s law and is rich in spectral information. Absorption ofIR light follows Beer ’s law, as do all forms of spectroscopy no matter the frequency of the transition, and thus by preparing standard solutions oflcnown concentration and plotting their obtained absorbance values at a given wavelength {or multiple wavelengths), one may determine the concentration of an unknown by use of these data and a least-squares fit of the data. In addition, IR spectrometry is qualitative due to the wealth oflR transitions associated with many molecules, particularly organic molecules. This abundance of IR transitions for a given molecule effectively malces its spectrum unique, and as a result, readily identifiable using computer algorithms. T his latter issue is of importance to analysis of mixtures by IR spectrometiy/fspectroscopy. {+1 for trying, +3 for saying both, +2 for saying follows Beer-Lambert Law, +2 for stating calibration plots can be used, +2 for qualitative due to wealth of bonds that are active} 2. From the list below, circle the molecule(s) that are infrared active. Then, explain why this is so, and then identify in words What groups are active and Where they would absorb infrared light. 0 \ / D/ \D H—<‘3—(’3—(,3—H H\ /C:C\ C H H H H H/ \H All of the molecules are infiared active, because for a molecule to be infrared active, it must possess at least one bond with a change in dipole moment upon vibration, and all of the molecules listed above meet that criterion. The C—H stretches would be found at about 2800-3000 cm], and the olefin (alkene) C=C transition at about 1600 cm}, and the 0-D band at about 2300-2100 em"T (based on ratio of reduced masses of water and heavy water). {+3 for saying all active — if they pick one then they get 1 pt and 2 pts if 2 picked, +3 for change in dipole moment upon vibration, +1 each for identification of C-H, C=C, and 0-D and then 2 pt each for each frequency} 3. Why is it that detectors in FT—IR instruments typically have fast response times (respond quickly to changes in IR radiation striking them)? Give the name of a given type of [R detector. Due to the fact that the interferometer output to the detector is fast, the detector must respond quickly to changes in IR light intensity. {+2 for trying, +3 for saying that the interferometer output is fast (scans fast, etc.), +3 for providing a type of detector such as DTGS, pyroelectric, semiconductor, MCT, etc.} 4. A student performed a series of FT-IR experiments wherein the student measured the signal (S) of a given IR band at 3100 cm'1 and the noise (N) of the signal, as a function of the number of scans (n) averaged (number of spectra added together). The plot below was obtained (data in black diamonds, best fitted curve is the solid line). The best fit line to the data yielded an equation = K5105”, where K is a constant of no consequence here. In two short sentences, discuss why it is that these results were obtained. 16GDG 14cm] 0 12cm // 100w / BEBE EGUU AGDG ZGDU U 64 128 192 255 320 384 448 512 Number of Spectra Co—Added The observed increase in signal—to-noise ratio is as expected for as the number of scans n is increased, the S/N should increase as a power of the square root of n the number of scans. Indeed, we see that the exponent of the number of scans is 0. 502, which is close to the expected value of 0. 5. {+2 for trying, +3 for saying that the S/N ratio should increase with increased number of scans, and +2 for stating that the relationship should follow an n to the one-half power dependency.} 5. An analyst at Gulf States Research Institute (GSRI) was provided with a sample of liquid n- hexane, CH3(CH2)4CH3, from the Exxon-Mobil refinery on Scenic Highway. The analyst prepared the sample for IR evaluation by placing a drop of the neat liquid on a brand new NaCl plate, and covering it with another new salt plate. The IR spectrum below was obtained. First, in the right-hand white space of the spectrum, identify as many of the the functional groups present in the sample as you can and provide approximate energy (frequency) positions for them. Second, please give a one- to-two-sentence response to whether or not the spectrum is what you think it should be and your rationale as to why that is the case. Functional Group Frequency 1 oo _ R H k, _ °/n 7 s '1' {AM _ “ I l T 80— t l t r ' I .r " a l‘ 4 {Il J n ' t s Goi l\ t i m ‘ f’ I . -. t 40— ” t - . . a \ Fingerprint n 20; region 7 not ° . germane to this 3 7 question 4000 3500 3000 2500 2000 1 500 1 000 500 Wavenu mbers It is clear that the presence of the 0-H stretch at about 3300 wavenumbers indicates that the hexane sample is contaminated with water or some alcohol, most lilcely from point of collection at Exxon- lufobil, during its transport to GSRl, or by the analyst. {+3 for identifying 0-H stretch, +3 for C—H stretch, +2 for stating sample became contaminated with water or alcohol, +2 for giving some scenario under which it occurred} 6. Discuss the nature ofionization interference and its impact on quantification in flame AAS. Do not give any discussion on how to deal with this common interference. Ionization interference is quite common and occurs when one is trying to measure the absorbance of an atomic species that becomes easily ionized at increasingly high temperatures, but these temperatures are not high enough to afiect other zero-valent elements in the flame (’ they are not ionized at this temperature). As a result, the absorbance signal for the easily ionized analyte will be lower than expected, and its apparent concentration will be lower than it should be (this is because the ion form of the analyte does not absorb light at the same wavelength as the zero-valent analyte, e.g. Na(0) vs Na(l)). {+4 for stating that zero—valent species becomes ionized due to high temp flame, +3 for stating that the apparent concentration of the zero—valent species is lower than expected, +2 for saying that the wavelength of ion is different than zero—valent species} 7. Jack Bauer of the former Counter Terrorism Unit (CTU — Los Angeles) has extracted a piece of tissue from a suspect who died suddenly. Jack surveyed the area where the suspected terrorist was found and noted the presence of metal ion solutions used in the preparation of Pb(lI)-based batteries. However, the terrorist was suspected of planning an attack using an organophosphorous nerve agent. Jack has little time and very little tissue (less than 100 mg) with which to work, and fortunately. Chloe O’Brien, his CTU partner, has access to two different atomic absorption spectrometers (flame and furnace) that she uses routinely to test for metals contamination in her baby’s food (she is well- trained in their use). Thus, Jack decides to perform Pb analysis of the lUO-mg sample ofhuman tissue in order to see if the terrorist was merely exposed to high levels of Pb(ll) and died. or if he was exposed to the nerve agent. Please identify what type of AAS method he would use= and then discuss why this is the case, in a very concise manner. Jack would choose furnace atomic absorption spectrometry (GFAAS), because the method is amenable to direct evaluation ofsmall amounts ofsolid analyte. In addition, because the amount of lead in the .sample is probably low, he will need a method with a very low detection limit (LOB). He would choose this because the efiiciency of the GFAAS method is higher, which in turn allows him to use smaller amounts of sample with low concentrations. {+2 for trying, +4 for choosing GFAAS, +2 for stating that it can handle solid samples, +2 for stating that it is more efficient than a flame AAS method} 8. As a laboratory technician. you are presented with an aqueous sample that was collected from floodwaters resulting from Hurricane Ike in Galveston, TX. As you can imagine, there will be many elements and compounds present in the solution. You are to quantitati\-'ely analyze for Pb. Zn and Cu in the aqueous sample using flame AAS. You construct a calibration curve using various concentrations of these three elements dissolved in pure water. However, you find that the FAAS response of the standards in pure water is quite different than in the flood waters. What is the issue here and what method would you use to best determine the concentration of the analytes in the presence of the other elements and compounds? Do not discuss the steps of this method! H Chemical or Matrix interferences arise due to chemical reactions of ionic analytes in the aqueous sample with other ligating type species, the products of which are often times not volatile or only somewhat volatile. In the latter case of limited volatility, the analyte response (absorbance) is suppressed, that is, the sensitivity of the response is much less than theoreticallypredicted when using pure analyte in pure solvent. Thus, if a calibration curve is constructed using pure analyte and pure water, say, then the environment of the standard concentration analytes will not be at all lilce that of the sample, and large deviations in the true concentration of the sample analyte will occur. T he best approach is to use the method of standard addition to solve this problem. {+5 for stating chemical/matrix interference is the problem, +2 for saying that the response is suppressed, +5 for using standard addition method} 9. In the following sentences. please fill in the blanks or circle one of the italicized words-"phrases to make the statements as factually correct as is possible. A. In general. a hollow cathode lamp (HCL) functions by formation of very narrow “lines” of light as the result ofa sputtering excitation of atomic species. B. Atomic absorption spectra exhibit very narrow/broad absorption bands due to the fact that atoms possess/do nof possess vibrational modes. C. Furnace atomic absorption spectrometry requires more/less training of the user because the equipment is less/more straightfon-vard to use. D. A is used in AAS to remove any extraneous spectral lines emitted from the hollow cathode lamp. A. Fluorescent, B. narrow and possess, C. more, less and D. monochromofor. {+3 for each in A and D, +2 for each in B and C.} ...
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A CHEM - 1. Discuss whether IR spectrometry is a...

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