The identity of a white powder inside a plastic bag and the material used to make the plastic bag were determined using infrared (IR) spectroscopy. It is possible to identify unknown compounds using IR spectroscopy because every compound absorbs infrared light at a different frequency. The identity of the unknown was identified as 4-Bromobenzoic acid, based on information obtained from the spectrum of samples of the powder. The material used to make the bag was found to be polyethylene based on IR spectrum analysis. The identity of the unknown powder and material of the bag were confirmed by comparing the spectra obtained in the experiment to known spectra of the predicted compounds.
Infrared (IR) spectroscopy is the analysis of how infrared light affects vibration of atoms within a molecule. IR spectroscopy can be used to determine the functional groups present in a compound because different functional groups absorb different amounts of infrared radiation. Atoms in molecules are always vibrating at temperatures above absolute zero. When infrared radiation is passed through a sample, molecules in the sample will absorb energy if the frequency of a specific vibration in the molecule matches the frequency of directed radiation.
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The infrared spectrum of a sample is obtained using a Fourier transform infrared (FTIR) spectrometer. Signals from a sample (interferogram) are first collected by a FTIR spectrometer using an interferometer. An interferometer produces a signal that contains all infrared frequencies encoded into it. The FTIR spectrometer then performs a Fourier transform and displays the infrared spectrum of a sample. A Fourier transform represents a relationship between a signal in time domain and its representation in frequency domain.4
IR spectroscopy is used widely in forensics to identify samples in the solid, liquid or gaseous phase and confirm their identity. The purpose of this experiment was to familiarize students with the IR instrumentation and its application to a forensic investigation. The identity of a white powder was identified by using the FTIR spectrometer for qualitative and quantitative analysis.
A small plastic bag with a white powder (unknown # 11) was selected to be identified. 10 mg of the white powder and 1 g of potassium bromide (KBr) were placed inside a mortar and grinded into a fine powder. A small amount of the mixture was placed KBr press to compress the grinded powder into a pellet. A sample of the unknown sample was placed into the press to be compressed into a separate pellet.
Using a Perkin Elmer 1600 FTIR spectrometer, a background scan was conducted without any sample inside the spectrometer. A pellet was placed in between salt plates and two drops of oil were added. The plates containing the pellet were then placed into the sample holder and scanned using the Perkin Elmer FTIR. This procedure was conducted for both pellets.
A small piece of the plastic bag was cut, stretched and taped into the IR sample holder. The material used to make the plastic bag was identified using the FTIR spectrometer. A new background scan was conducted, followed by the scan of the plastic bag.
The spectra for background scans (Figure 1A and 2A-Appendix) displayed numerous narrow peaks that were very close in proximity to be classified and labeled.
4-Bromobenzoic acid and caffeine were the two possibilities for the identity of unknown # 11. The spectrum for unknown # 11 + KBr (Figure 1B-Appendix) displayed a high number of narrow peaks between 400-500. Two isolated peaks appeared around the mid-region of the spectrum at 1376.63 and 1459.21. These peaks were assumed to be C=C groups present on aromatic ring using the table for IR absorptions for representative functional groups the functional groups. Two additional broad peaks appeared at 2854.72, 2924.58 and were identified as the -OH group of a carboxylic acid.
The spectrum for the unknown # 11 (Figure 1C-Appendix) displayed an extremely large number of high intensity peaks between 400-500. Peaks that appeared at 1376.34 and 1458.49 were also believed to be the C=C group of an aromatic ring. A single strong peak occurred at 2924.23 and was identified as a -OH group like the previous spectrum.
Polyethylene and polystyrene were the two possibilities for the identity of the material of the bag containing the sample. The spectrum for the plastic bag (Figure2-B) displayed several very distinguishable peaks. Two very sharp and close peaks occurred at 720.05 and 729.78. These peaks were predicted to be the C-H bend bonds of an alkene. The next main peaks to display high intensity occurred at 1302.54, 1377.45 and1471.07. These peaks were predicted to be caused by the C-C bonds of present in a conjugated alkene. The last peak occurred at 2898.00 and displayed a large area in comparison to the others. This peak was predicted to be caused by the C-H stretch bonds present in an alkene.
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The identity of unknown # 11 and the bag in which it was found, were determined by analyzing their infrared spectra. The infrared spectra for the compounds were obtained using a FTIR spectrometer. The information obtained from the spectra of the unknown sample was used to predict that identity of unknown # 11 was 4-Bromobenzoic acid. To confirm the identity of the unknown powder, a sample of pure 4-Bromobenzoic acid was analyzed using the same FTIR spectrometer. The IR spectrum for 4-Bromobenzoic acid (Figure 1D-Appendix) displayed the same number of main peaks than unknown # 11 and unknown # 11 + KBr. All the IR spectra found in Figure 1 were superimposed to display that all the spectra share the same similarities and therefore are the same compound. (Figure 3-Appendix)
It was determined that the bag was composed of polyethylene. Information obtained from the spectra of the bag and comparisons to a known polyethylene IR spectrum (Figure 4-Appendix) were used for the identification of the polymer.
The Perkin-Elmer 1600 Fourier transform infrared spectrometer was used to obtain all the spectra in this experiment. The program used to operate the spectrometer was activated by clicking the "Spectrum" icon on the desktop. Administrative settings were entered once the program loaded. The background scan must be conducted before analyzing any sample. The background scan consists of activating the instrument to scan the sample holder, without any sample in position. The background scan is used to measure the absorbance of air and water molecules in the laboratory and subtract them from the sample. The background scan is conducted by clicking "Instrument" in the main menu and selecting "Scan Background".
Samples containing solids (unknown # 11) can be grinded into a fine powder and mixed with potassium bromide (KBr is used because it is transparent in IR). The samples can then be placed into a press to be compressed into a pellet form. The pellet in this experiment was formed by applying pressure to the press by placing two wrenches at each end of the press and moving them in opposite directions. A drop or two of oil are added to the pellet and it is then placed in between two salt plates in the sample holder. Some solid samples can be analyzed without adding the KBr, and other solid samples don't need to be in between salt plates in the sample holder as long as they don't move while the scan is on progress. Liquids and gases have separate cells used to contain the sample while it is being analyzed. The materials of all the cells used to hold samples in the FTIR spectrometer are transparent to the infrared scan.
The FTIR spectrometer consists of a source that produces infrared radiation over a series of frequencies. The infrared radiation is travels to an interferometer which splits the infrared beam into two. One mirror reflects off a flat mirror which stays in the same position. The second beam is reflected off to a mirror that is moving a few millimeters at a time. Both beams reconnect at the beam splitter and interfere with one another to form an interferogram. The infrared beam travels to the the sample holder by mirrors located throughout the instrument to a detector which designed to measure the interferogram signal. A Fourier transformation is made by the computer to convert the interferogram to a IR spectrum of the sample. Figure # 5 (Appendix) illustrates the path the infrared radiation travels and how the IR spectrum is found.
Infrared spectroscopy can be used to positively identify an unknown compound because every compound will provide a different IR spectrum. Bonds in every compound will absorb different amounts of infrared radiation because every compound is made up of different combinations and arrangements of atoms.
Using IR spectrometry, the identity of unknown # 11 was found to be 4-Bromobenzoic acid. In the spectrum for the unknown sample, a broad peak that occurred at 2924.23 was matched to the peak in the unknown sample + KBr spectrum that occurred at 2954.58. It is probable that these peaks were produced by the -OH bond in a carboxylic acid. The presence of isolated peaks at 1458.71 and 1376.34 in the unknown sample spectrum were relatively close to the peaks found at 1459.21 and 1376.63 in the spectrum for the unknown sample + KBr. It is possible that these isolated peaks were caused by the C=C bonds present within an aromatic ring. Using a table for the IR absorptions of different functional groups the presence of an -OH bond (3400-2400) and C=C bonds (~1475) in aromatic rings was confirmed. The only compound available that contained the -OH group of a carboxylic acid and an aromatic ring was 4-Bromobenzoic acid. A sample of 4-Bromobenzoic was analyzed using the FTIR spectrometer. The spectrum displayed the -OH peak at 2923.96 and C=C peaks at 1458.49 and 1376.70. The resemblance of these peaks to the peaks displayed on the spectra of the unknown confirmed the unknown's identity to be 4-Bromobenzoic acid.
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Using similar data interpretation of the IR spectrum of the sample bag, the identity of the material used to make the bag was found to be polyethylene. In the spectrum of the bag, two very tall and narrow peaks occurred at 720.05 and 729.78. The peaks were predicted to be caused by the C-H bending groups of an alkene. The next set of isolated and high intensity peaks occurred at 1302.54, 1377.45 and 1471.07. These peaks were predicted to be caused by the C-C bonds of an alkene. The last peak occurred at 2898.00 and was predicted to be caused by the C-H stretch bonds present in an alkene. Using a table for the IR absorptions of different functional groups confirmed the presence of an alkene with C-H bend (675-1000 and stretch groups (3100-3000, as well as conjugated C-C groups (1680-1640).
The infrared spectrum of polystyrene was obtained from an IR card spectrum from the University of Arizona. The spectrum of polyethylene displayed peaks at 2916.50 and 2855.34 which correspond to the C-H stretch bonds present in an alkene. Two additional peaks at 1492.23 and 1466.02 can be attributed to the conjugated C-C bonds in the alkene. The last set of narrow peaks occurred at 749.51 and 723.30. These peaks were caused by the C-H bend bonds of an alkene. The resemblance between the intensity of the peaks found in the IR card spectrum for polyethylene and the spectrum of the bag obtained in the experiment indicates that the bag in which the unknown was contained was made of polyethylene.