Lab Report 14 (Exp 15) PDF

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Lab Report 14 (Exp 15)...

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Winter 2017/18 CH 228- Afternoon Class

Experiment #15: Forensic Analysis of Ink Using Thin-Layer Chromatography

Prepared for: Emma Tran Prepared by: David McNeely

February 27, 2018

Introduction Chromatography is a technique used to separate any given sample into its constituent components in order to further analyze the sample or possible comparison against other samples for a potential conclusion of its identification. There are many different types of chromatography that can be used (usually dependent on the type of sample being analyzed), but thin-layer chromatography was the method of choice for this laboratory experiment. Thin-layer chromatography (or TLC) involves making a highly-concentrated solution containing the sample to be analyzed. Then, this solution is blotted onto a chromatography plate and partially submerged into a solvent. The solvent moves up the plate through capillary action and takes with it the individual components of the sample solution to different positions inversely related to their polarity (the most polar components will move the shortest distance up the plate). This process can then be performed on a sample pool of other solutions in order to determine the most likely math for the original sample solution. Two questions that were discussed before the conduction of this lab experiment were “how does solvent polarity relate to ink separation?” and “how does paper chromatography allow us to identify the pen that was used on the original sample?”. Due to the volatility of the solvents used in this lab experiment, it is important to handle them carefully in order to avoid potential injuries or waste of experiment materials. This lab experiment can be conducted by first testing a number of different solvents with a sample pen in order to determine which one will likely give the most easily-interpretable results. This is done by performing TLC tests with the same ink but with each different solvent and then identifying the solvent that creates the best separation of constituent components on the chromatography plate. Once the best solvent has been identified, this solvent should be used in test the suspicious sample provided from the threatening letter and compared to the pool of sample pens also provided. A comparison between the results of all the TLC tests should then be performed; the sample pen that most closely resembles the TLC result from the original sample is likely to be the same pen used to write the suspicious letter. Experimental Data and Observations Pen Used: Pen #1 TLC Test Observations Using Different Solvents: Solvent #1: no visible separation of ink components Solvent #2: very slight grey-tinted separation just above original ink blot Solvent #3: yellow-to-pink gradient visible near original ink blot, slight purple/blue streak near top of plate Solvent #4: slight yellow-to-pink gradient just above original ink blot

Solvent #5: blue-to-yellow-to-orange-to-pink-to-purple gradient observed ranging from just above original ink blot all the way to the solvent front on the plate Solvent #6: murky green-to-blue-to-yellow-to-pink-to-purple gradient observed ranging from just above original ink blot all the way to the solvent front on the plate Solvent #7: murky green-to-purple-to-pink gradient observed ranging from just above original ink blot all the way to the solvent front on the plate; no clear distinction between color separation; colors blended together Observations of Final TLC Plate Under UV Light: No new ink spots observed on TLC plate after viewing under UV light

Ink Sample

R_f (Yellow)

R_f (Pink) R_f (Green)

R_f (Blue)

1

0.93

0.8

0.76

0.7

2

0.93

0.8

0.76

0.6

3

0.83

0.73

0.7

0.63

Unknown

0.8

0.73

0.7

0.63

Table 1: R_f Values Calculated from Thin-layer Chromatography Test Plate Calculations and Graphs R_f Values Calculation (Table 1) Ex. Unknown Ink Sample ¿ ˙ (cm) =Rf Solvent Front Distance(cm) 2.5 cm =0.8=Rf (¿) 3 cm

Claims and Evidence The more polar the solvent, the more the constituent components of the sample being tested tend to separate. The most polar components will move the shortest distance, and the least polar components will move the farthest distance. However, higher polarity does not always equate to better interpretability of results. A determination must always be done before conducting a TLC for a new sample in order to determine which solvent/adsorbent combinations yields the best results. Paper chromatography allows us to likely determine the identity of the pen used on the suspicious note by comparing TLC tests from the note and a pool of sample pens (one of which was used to write the note). After the TLC tests have been performed, one is able to measure and calculate Rf values for each sample and compare the results to the TLC test from the suspicious note. The result which most closely resembles and matches the suspicious note sample is likely to be from the same pen that was used to write the note. After reviewing the data in Table 1, it can be deduced that the identity of the unknown ink sample is likely pen #3. The Rf values for the unknown ink sample most closely match those from pen #3. Analysis and Discussion After comparing my results with those of other students, it seems as though the results from my experiment are likely correct. Due to the lack of very clear ink dot separation in my experiment, some rounding and estimation was required when measuring and calculating the Rf values; however, the degree of error was low enough to still ensure a positive identification of the unknown ink sample. Possible sources of error that could inaccurately skew the results of this experiment include improper solvent choice and insufficient ink sample extraction. Firstly, if one of the less effective solvents were to have been chosen to be used in this experiment, it could have resulted in insufficient ink separation, which would have then led to an inability to compare the TLC tests from the suspicious note and sample pen inks. If the components of each ink sample were not sufficiently separated, proper identification of the suspicious note ink sample may not have been properly concluded due to the inability to accurately analyze the positions of the separated ink dots. Similarly, if an insufficient amount of ink were to have been extracted from the written sample of any of the sample inks, it would then be possible for the TLC test results to be inconclusive. If not enough ink were extracted, it could have resulted in unclear and not-easilyvisible ink component dots or gradients, therefore preventing an accurate identification of the ink from the suspicious note. Both of these potential sources of error can be classified as determinate as the error source can be found and accounted for; if the sources of error were indeterminate, it would not be possible to consistently and accurately account for the error as it would be random. Unique Discussion Topics The solvent from jar #3 (50% ethyl acetate/25% ethanol/25% water) was determined to be the best solvent for producing the most easily identifiable and interpretable results from the

TLC tests. This solvent was chosen because it produced the most drastic and clear separation between the constituent components of the ink sample tested. In general, it seems as though the more polar solvents tend to produce the better separation of sample components compared to less polar solvents. In this case, however, a mixture of medium to highly polar solvents produced and best separation of ink sample components. It would have been possible for this experiment to result in a clearer separation of ink sample components, but this would come at the expense of environmental sustainability and general adherence to Green Chemistry principles. A balance between environmental conscientiousness and sufficient experimental outcomes has been reached with the use of the ethyl acetate/ethanol/water mixture solvent. A “greener” solvent could have been used, but this would have likely resulted in an inability to accurately and efficiently interpret the results at the end of the experiment. Based on the analysis of the final TLC test plate results and comparison of Rf values from each sample test result, it seems as though pen #3 was used to write the suspicious note. Potential sources of error that may weaken the analysis of these ink samples include insufficient ink sample extraction and retrieving the TLC plates from the solvent before they have had enough time to sufficiently separate. If not enough ink were to have been extracted before blotting the TLC plates, it is likely that the ink separation would not be clear or visible enough to result in a probable identification of the ink sample. If the TLC plates were to have been removed from the solvent before they were given enough time to fully separate, it is likely that the results would be entirely inconclusive due to the utter lack of ink sample separation. Different pigments from the ink samples end up at different locations on the TLC plates after the experiment is conducted mainly due to the difference in polarity between the solvent used and the components of each ink sample. When a solvent of relatively high polarity is used, ink components of lower polarity will travel farther up the TLC plate than those or higher polarity (which will remain closer to the original ink blot on the plate). As long as an efficient solvent is used in the conduction of the chromatography, it seems as though the environmental conditions in which the experiment is conducted are largely irrelevant as long as they are kept consistent throughout the duration of and between each TLC conduction.

Reflection Topics Extend If this experiment were to be extended, it would be interesting to analyze mixed ink samples of different colors and determine which colors were mixed together from a sample pool of multiple colors. Also, it would be interesting to compare similar colors from different manufacturers to determine which general color maintains the highest degree of uniqueness in terms of constituent components. CH222 Topics A lecture topic from CH222 that can be related to this lab experiment is that of polarity. The greater the difference in polarity between two different substances, the greater attraction that

exists between them. In this case, the less polar components of the ink samples move farther up the TLC plates than do the more polar components. This is due to the high polarity of the solvent used in this experiment. Since the difference in polarity is greater between the highly polar solvent and the slightly polar ink components than between the highly polar solvent and the highly polar ink components, the slightly polar ink components are attracted to the solvent more and move farther up the TLC plate. Applications The major use of the techniques used in this lab experiment is that of forensic analysis. For example, if someone were to write a threating letter to the President or another person of power, it could be useful to be able to determine likely identities of the pen used to write said letter by performing a chromatography experiment and comparing the results from the ink used in the letter to those of the ink from pens owned by potential suspects. Related Reading According to Science of Chromatography, the distance that the components of a test sample will move is entirely dependent on the differences in polarity between the components and the solvent used in the experiment; the more polar components will stick to the stationary phase more than the less polar components (“Adsorption Chromatography”). This finding is in agreement with the conclusion described in paragraph three under the “Unique Discussion Topics” subheading discussed earlier. Green Chemistry The most relevant principle of Green Chemistry applied to this lab experiment is that of environmental damage mitigation. While more easily-interpretable data could have been found using different solvents, a happy medium between sufficient experimental results and environmental sustainability has been achieved through the use of the ethyl acetate/ethanol/water mixture solvent solution.

Works Cited “Adsorption Chromatography.” Science of Chromatography, chromatographyscience.blogspot.com/2012/08/adsorptionchromatography.html#.WpVQTGbMwmI....