A CLUE from Chromatography:

Transcription

1 A CLUE from Chromatography: Professor Plum was murdered in his new SMC science building office! Unfortunately, the killer was clever enough to eliminate most physical evidence of how the crime was committed. Your boss, Inspector Clouseau of Santa Monica Homicide has called in your team of forensic chemists to examine a blood stain found on the carpet under Professor Plum's desk for possible clues. It is hoped that chemical residues found in the stain will positively identify the murder weapon and thus help determine whom the murderer is List of Suspects Compiled by Santa Monica Homicide: Colonel Mustard A war veteran, explorer, and close personal friend of Professor Plum's. Col. Mustard is suspected of jealousy following rumors that Professor Plum and his new office mate Miss. Scarlet (a former lover of Col. Mustard's) were having an affair. An old rope with dark red stains that might be blood was found in the basement of Col. Mustard's Westside mansion. Mercury residue from an old-fashioned bilge pump used on container ships was found on the rope. Miss Scarlet Professor Plum's office mate. Miss Scarlet, a biology teacher, was recently moved into Professor Plum's office. Other instructors heard her complain on numerous occasions about Prof. Plum's many books, his messy desk, his old clothes, and how many students he invited to office hours. It is suspected that Scarlet secretly detested Plum and wanted the entire office for herself. A revolver with special cobalt-tipped bullets was found in Miss. Scarlet's purse. When asked about it she responded, "a woman needs a little something to protect herself with around this city."

2 Mr. Green Mr. Green has been a local plumber for the past 15 years. When he attended SMC he wanted to be a dentist, but failed to get into UCLA dental school because of a low grade in Prof. Plum's chemistry class. Mr. Green has complained to the school about Prof. Plum's class on numerous occasions, but in all cases has failed to have the grade changed. A bent copper pipe (plumbers no longer use lead pipes) with Mr. Green's fingerprints on it was found in his workshop on 25th street. A dart-board with a picture of Plum pasted to the bulls-eye was also found with numerous perforations. Mrs. White Mrs. White a former English housekeeper is now head of the custodial staff at the SMC science building. She was heard to complain on numerous occasions about the state of Plum's office and his, "intolerably messy collection of old books and papers." Miss white would like nothing better than to see all of Plum's old stuff thrown out and his office thoroughly cleaned. She also detests his new office mate Miss Scarlet and suspects her of smoking in the building. A heavy silver candlestick with a large dent and crack in one side was found in her broom closet. Miss White claims it was a gift from an old Priest she once visited in France. Mrs. Peacock Mrs. Peacock is the head librarian at SMC. She has long begrudged the fact that Plum checked out more books than any other faculty member at SMC. When informed of the murder she was quoted as saying that it was about time his books were returned to their, "proper resting place." Some of the staff noted that Mrs. Peacock (a widow) often sat with Prof. Plum at lunch and that she was extremely jealous of him sharing his office with the new younger biology instructor. An old rusty iron knife was found in her desk. Mrs. Peacock claims it was only used as a letter opener.

3 Chromatography Inspector Clouseau takes your team to the crime lab where he expects you to use chromatography to examine the material extracted from the carpet and find the culprit! The crime lab director explains: Separation techniques are of vital importance to chemists. Some of them are of ancient origin, such as distillation and fractional crystallization. Many others have been developed during modern times. Chromatography is a method of separating various components of a mixture that takes advantage of the differing attraction molecules have for various substances. Figure 1 below is a sample illustration showing two soluble substances mixed together and placed on the bottom of a piece of absorbent paper (the media) which is then dipped in solvent. In this example the spirals are more attracted to the paper than the triangles are. Thus, as the solvent rises (the shaded area), the triangles move up the paper more rapidly than the spirals and eventually the two are separated. All chromatography is based on this principle of differing attraction of various species for the media and the solvent Figure 1 A good analogy for this process is a shopping mall. Suppose two people enter a mall filled with clothing shops. One person is very fashion conscious and enjoys trying on new outfits; the other only has only a passing interest in such activities. If both people enter the mall at the same time and move through it separately at their own rate, the one who is not really interested in clothing may examine several outfits, but will probably emerge from the mall within the hour. The fashion conscious person will be "attracted" to each store, and may take several hours to finish shopping. The clothing stores in the mall act in the same way as the absorbent paper in the example above. The strength of attraction for the media (in this case the shops) determines the retention time (how long is spent in the mall), and permits separation of two species with different attractions to the media (one person exits before the other).

4 The use of paper chromatography was first described in 1906 as a method for separating plant pigments. The word "chromatography" derives from the Greek word "khroma," meaning color; although the technique works equally well with uncolored compounds. All chromatography techniques use the same principle: the stationary phase and the mobile phase attract the various substances in the mixture to a different degree (as illustrated in the two examples above). In the first described use, a mixture of plant pigments was spotted on a piece of paper (the stationary phase), and water creeping up the paper by capillary action (the mobile phase) separated the pigments. Those pigments more attracted to the paper did not move far from where they were placed. Those pigments more attracted to water rose closely behind the moving front of the water. Those pigments attracted to both rose part-way. Other methods of chromatography include liquid and gas chromatography. Liquid chromatography employs a column (for example a large burette) that is filled with a porous medium. A mixture of two or more species in solution is introduced at the top and separates as it flows down the column. Sometime the column is a thin pressurized metal tube such as in HPLC (high performance liquid chromatography). Many organic chemists and pharmacists use such techniques to separate or purify samples. Gas chromatography works in much the same way as HPLC, but the sample is gaseous and the media is generally coated on the walls of a long thin capillary tube. A gas chromatograph (or GC) can be interfaced to a variety of detectors such as flame ionization or a mass spectrometer to yield even more detailed information about the nature of a sample. For example, GC can be employed by scientists doing environmental testing of ground water and soils to determine what compounds are present in these samples. Today, both paper and thin-layer chromatography (or "TLC" for short, a method similar to paper chromatography except that the fixed medium is a thin silica-based compound affixed to a plastic backing) are used as quick methods of identifying the number (and in some cases nature or identity) of components in a chemical mixture. These techniques are routinely employed by forensic scientists to determine the chemical make-up of samples and mixtures, and can be used to solve crimes such as whether questionable historic documents are frauds (by comparing ink and paper samples to those of known authentic documents), or in our case to find out what tell-tale substances were left behind on a bloody carpet! In order to positively identify what is in the stain extract, we need to use samples of each of the compounds that might be present and see how they behave on the paper. We can then compare our extract to each of these known samples in order to positively identify the substance(s) present in the carpet stain.

5 Our test to determine if two samples are identical will be to compare their numeric R f factors. The R f factor is a measure of the distance each substance (or ion) moved, D i, compared to the total distance that the solvent moved, D s : R f = D i / D s Note that the larger the difference between the different chemical's R f values (i.e. the more the spots are separated), the better the overall resolution of the chromatogram. D i D s D i is the distance travelled by the ion. D s is the distance travelled by the solvent. Figure 2 Your team will need to determine the R f factor for each of the known cations and to compare these with the results from the carpet sample in order to make a positive identification. Because not all of the cations make a visibly colored spot, a staining reagent will be added after the separation is complete to produce colored products. This will better help us to identify each cation.

6 Experimental Procedure: Your team will be provided with a series of vials containing 0.1 M solutions of each of the following five samples to be tested: AgNO 3 Co(NO 3 ) 2 Cu(NO 3 ) 2 Fe(NO 3 ) 3 Hg(NO 3 ) 2 You will also be provided with a sixth sample that is a mixture of each of these species which will be your "combined known." Finally, your team will be provided with a seventh sample labeled "unknown" which will contain the extract removed from the blood stain on Professor Plum's carpet. The media we will use is a sheet of 19cm x 11cm filter paper. Along the 19cm edge, draw a pencil line (not ink) about 1 cm from the edge. Then make seven evenly spaced marks along this line and label them: Ag +, Co 2+, Cu 2+, Fe 3+, Hg 2+, Know, and Unknown. Scotch tape 19 cm Tape 11 cm Ag + Co 2+ Cu 2+ Fe 3+ Hg 2+ Know Unk 1 cm Figure 3 Gloves have been provided for this experiment (latex gloves are provided, nitrile gloves are available at the stockroom for persons with latex allergies). You should wear gloves throughout the remainder of this experiment. Dispose of your gloves in the waste chemicals jar along with any other waste at the end of the experiment. Using the capillary tube provided with each of the samples, apply each of the knowns and the unknown to the paper, forming spots that are roughly 3-8mm in diameter. Be careful not to mix the capillary tubes from one sample with another or the samples will become contaminated during this process. Allow the spots to air-dry and then reapply each of the samples. Repeat this procedure 3-4 times. Be sure each of the spots is completely dry before reapplying the solution or the spot size will grow to larger than 8mm and you will need to start over.

7 Put about 15 ml of eluting solution into a clean dry 600 ml beaker and cover with a watch glass. This solution is a mixture of hydrochloric acid, ethanol, and butanol. Check to make sure that the spots on the filter paper are all dry. Place a 4-5 cm length of Scotch tape along the upper end of the left edge of the filter paper as shown in Figure 3, so that about half of the tape is on the paper. Form the paper into a cylinder by attaching the tape to the other edge, in such a way that the edges are parallel but do not overlap. When you are finished, the pencil line at the bottom of the cylinder should for a circle and the two edges of the paper should have at least a 1-2 mm gap between them. Stand the cylinder up on the lab bench to check that such is the case and readjust the tape if necessary. Do not tape the lower edges of the paper together. Place the cylinder in the eluting solution in the 600 ml beaker, with the sample spots down near the liquid surface. The paper must not touch the walls of the beaker and the spots should be above the surface level of the solution. Cover the beaker with the watch glass. The solvent will gradually rise by capillary action up the filter paper, carrying along the cations at different rates. Do not move the beaker once the paper is inserted splashing of the solution may ruin the results of this experiment. While the experiment is proceeding, you can test the effect of the staining reagent on the different cations. Put an 8mm spot of each of the 5 cation solutions on a clean strip of filter paper, labeling each spot. Dry the spots as before. Some of them will have a little color; record these colors in your notebook. Wearing gloves, dip the strip(s) briefly into the staining reagent located in one of the fume hoods in the lab room. This reagent forms colored precipitates or reaction products with many cations, including all of those used in this experiment. Remove the strip and place on a paper towel and dry under a heat lamp. Record the colors obtained with each of the cations. Considering that each spot contains less than 50 micrograms of cation, the tests are quite definitive. When the eluting solution has risen to within 2 cm of the top of the filter paper remove the cylinder from the beaker, take off the tape, and immediately draw a pencil line at the solvent front (the highest point the solvent reached). Note: It will take about 1-2 hours for the solution to reach this point. Be sure to keep an eye on the paper during this time, because the experiment will not be valid if the solvent goes all the way to the top of the paper. After removing and marking the paper, dry it completely by laying it on a paper towel and placing the paper under a heat lamp. Be careful to keep the paper a safe distance away from the lamp to avoid burning the paper. Note any cations that you can see and mark their location using your pencil. Then, wearing gloves, dip the paper as before into the tray containing the staining reagent. You should be able to see all of the cations in the known and unknown solutions. Dry your chromatogram on a paper towel under one of the heat lamps.

8 Measure the distance from the straight line where you applied the spots to the solvent front and record this as D s. Then, measure the distance from the pencil line to the center of the spot made by each of the cations; this is distance D i. Calculate the R f factor for each of the known cations and compare these to the results from your unknown solution. Can you tell who the murderer is? Was it one person or a conspiracy? What does the evidence suggest? Pour all waste into the properly marked containers in the hood. Your crime lab report Your report to inspector Clouseau is due at the end of the lab period. It should be written in your lab notebook and should include: 1. A detailed analysis of the color of each of the compounds before and after spraying. 2. The distance each compound and the solvent front moved. 3. The calculated R f factors for each of the spots in the knowns and unknown. 4. Your chromatogram to be submitted as evidence. 5. A short one paragraph written summary of your team's analysis of the unknown sample from the crime lab listing which ion(s) are present and which suspect(s) the evidence points to as being involved in the murderer.