Thin Layer Chromatography

Transcription

1 Thin Layer hromatography Introduction: Sandra, a child of six, was racked with chills, vomiting and in convulsions when her mother found her. Sandra had been at a friend's house, had gotten into the medicine chest and had swallowed a large number of `headache pills'. The symptoms Sandra was experiencing is not uncommon for those suffering from a drug overdose. Fortunately for Sandra, her parents were able to determine from the open empty bottle the drug the child had taken and, with this information, the doctor was able to take proper corrective action in time to prevent permanent damage. For a doctor to take proper action in cases of drug overdoses, it is of vital importance the doctor know precisely what drug the individual ingested. ne method to rapidly determine drug identification is through chromatography of a portion of the victim's blood. In the experiment you will be performing today, the brand name of an unknown analgesic will be determined by identifying the components in it by thin layer chromatography. RMATGRAPY: TE GEERAL PRIIPLE There are many forms of chromatography used to separate mixtures. All forms involve three things: 1. a stationary (or non-mobile) phase 2. a mobile (or moving) phase 3. an analyte (the compound or mixture being analyzed) hromatography separates the components in a mixture so they can be separately identified. Separation is achieved by placing a portion of the mixture on the stationary phase and allowing the mobile phase to pass through. Depending on the properties of the compounds in the mixture, some will be more attracted to the stationary phase and not move (or move very slowly) others will be more attracted to the mobile phase and will move very quickly. TI LAYER RMATGRAPY In thin layer chromatography, or TL, a small portion of dissolved material is applied as a small dot near the bottom edge of a plastic or glass plate coated with a thin layer of adsorbent. (The adsorbent constitutes the stationary phase). The plate is placed in a closed container containing a small amount of solvent. apillary action pulls the solvent (the mobile phase) slowly up the plate like water being soaked up by a sponge. Actually, most compounds will be partially attracted to both the coating and the solvent. ompounds more attracted to the The compounds in the sample on the TL plate (the analyte) can do two things as the solvent moves up the plate: (1) If a compound is attracted to the coating it sticks and does not move up the plate: TL Plate Sample Dot Solvent in a Beaker (2) If a compound is not attracted to the coating it will not stick and it does move up the plate:

2 coating move up the plate slowly, while those more attracted to the solvent travel more quickly and separation is achieved. What features cause some compounds to prefer the stationary phase to the mobile phase? A major factor is polarity of the bonds. The coating is composed of aluminum oxide (Al 2 3 ). The aluminum - oxygen bonds are very polar. The solvent is usually a nonpolar or very moderately polar organic solvent, in this experiment ethyl acetate is used (formula ). In general, the more polar bonds a compound has the more attracted it is to the very polar aluminum oxide and the more slowly it moves up the plate. The structures of the compounds you will be analyzing are shown below. The bonds possessing the important polarity are those bonds between the very electronegative atom and or and, and between and, or and. It is primarily these bonds that determine the overall polarity of these compounds. Remember that it is the difference in electronegativity of the two atoms in the bond that makes a bond polar. With this in mind, carbon-carbon bonds are not polar at all, while carbonhydrogen bonds are only very slightly polar so both of these types can be ignored. 3 3 AFFEIE Analgesics 3 SALIYLAMIDE 3 AETYLSALIYLI AID AETAMIPE The best known and most widely used analgesic is acetylsalicylic acid (known as aspirin). ther commonly used analgesics are those shown previously. affeine is not an analgesic, but is sometimes added for its stimulating effects. affeine increases the body's sensitivity to carbon dioxide causing an increase in both the rate and depth of respiration. The analgesics given above work to inhibit pain by preventing transmission of pain impulses from the hypothalamus gland (located in the base of the brain) to the cerebral cortex. These analgesics also reduce fever by the following modes of action: 1. causing blood vessels to dilate (enlarge) so more blood can be cooled when it reaches the skin; 2. stimulating diaphoresis (perspiration) so the skin cools blood even more efficiently; 3. by inhibiting the production of certain prostaglandins which normally function to increase body temperature. When viruses or bacteria invade the body, prostaglandins are activated to IREASE body temperature. Because the invading organisms are more susceptible to high temperatures than your own uninfected cells, this increase in temperature serves as one of the body's methods of ridding itself of invading organisms. Thus, while taking an analgesic drug serves to reduce symptoms associated with infectious disease, it does TIG to attack the invading organisms and may, in fact, be counterproductive! 3

3 Aspirin, and to some extent salicylamide, also exhibit an anti-inflammatory action that is especially useful to people suffering from arthritis. Aspirin is able to reduce swelling and relieve the pain. Some analgesics tablets include other compounds for therapeutic effects such as antihistamines, decongestants and sedatives. In addition to the active ingredients, the tablets must also contain substances such as starch that act as binders to hold the tablets together. Experimental In this experiment you will obtain as an unknown, a well-known analgesic tablet. The object is to identify the drug by comparison with several known compounds. The tablet will be one of those listed below. Drug (Brand ame) Ingredients AAI EXEDRI VAQUIS TYLEL acetylsalicylic acid, caffeine acetylsalicylic acid, caffeine, acetaminophen, salicylamide acetylsalicylic acid, caffeine, acetaminophen acetaminophen Procedure 1. apillaries for applying samples can be made by heating and turning a glass tube until the glass softens and then pulling it in opposite directions. The instructor will demonstrate this for you. Be careful, hot glass looks just like cool glass. Break off six short pieces of the small capillary you just made. 2. btain from the stockroom a TL plate. Don't touch the surface of the plate, or finger prints will appear on the plate. Lay the TL plate on a page in your note book. With a pencil mark on the page where the corners of the plate are. Also make a mark on the page about 1 cm up from the bottom of the plate. Below the plate make five evenly spaced marks on the page and number them btain from the stockroom your unknown analgesic in a small test tube. A small amount of solvent will be added, then crush the tablet. Don't worry if it doesn't all dissolve. Using a clean capillary tube for each sample, apply one dot of the following in the order named on the TL plate (use the lines as a guide): 1. acetaminophen 2. acetylsalicylic acid 3. unknown 4. caffeine 5. salicylamide 4. When all five samples have been applied, place the TL plate spotted end down in a developing jar and screw on the lid. Don't disturb the jar while the TL plate is developing or the samples may be ruined. About fifteen minutes are needed for the solvent in the jar to be pulled up the plate. Keep you eye on the TL plate, and when the solvent is about 1 cm from the top, pull out the plate and quickly mark the solvent line on top of the plate. The solvent will evaporate very easily so don't be slow. Allow the solvent to evaporate in the fume hood.

4 5. What do you see? Probably nothing. You need to visualize the spots on the plate. Shine an ultraviolet light on the plate and circle any spots you see with a pencil. Record the colors and any other observations you can make about the spots. Analysis of the Plate In TL, the compounds are compared by their RETETI FATRS (or R f 's). These are determined by measuring the distance each sample moved and dividing by the distance the solvent moved: R f = d sample / d solvent 1. Start at the origin (the place where the sample was applied) and measure the distance to the ETER of a spot. This is the sample distance. 2. ow measure the distance from the origin to the line where the solvent reached (called the solvent front). This is the solvent distance. 3. Divide the sample distance by the solvent distance, this is the R f for that spot. 4. alculate the R f for each spot on your TL plate. 5. Using the R f values for each spot and the observations you made when you visualized the plate, determine which of the known standards are in your unknown analgesic. 6. Using the listed ingredients given above, identify the brand name of the analgesic you analyzed. hemical Waste Disposal onsult the instructor for waste disposal instructions. The glass capillaries are sharps and cannot go in the trash (it s not unknown for a custodian to be injured from a sharp poking through a garbage bag. hemistry sharps pose an additional poison hazard.

5 PreLab ame: 1. (onceptual, not required to answer this question in your lab notebook) The TL system can be optimized by adjusting the polarity of the solvent. To do this, Ethyl acetate (moderately polar) can be mixed with hexane (very non-polar). a. If a sample is polar, will it dissolve better in pure ethyl acetate, or a mixture of ethyl acetate & hexane? b. Will a polar sample move farther up the TL plate in pure ethyl acetate, or a mixture of ethyl acetate & hexane? c. n the empty TL plate, sketch where you might expect the 3 dots to show up. ote that #2, #3 are slightly lower than # The following data was made up to resemble the TL experiment. ompute the retention factor for each peak in the known samples (#1, 2, 4, 5). The dotted line is the solvent distance. (a) ompute the retention factor for each peak in the known samples (#1, 2, 4, 5). The dotted lines are where the solvent got to. Assume the samples all started at 0.3 cm. Read the ruler to cm. It may help to write the ruler readings down on the TL plate diagram. Sample name Distance (cm) R f 0. solvent 1. caffeine 2. acetominophen 3. Unknown 4. acetylsalicylic acid salicylamide (b) What chemical(s) are present in the unknown? ote: You have to subtract the starting point from each distance (solvent & sample). For example, if sample 2 is 3.4 cm from the bottom of the TL plate, it s distance is =3.1 cm.

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7 Report AME and Drawer # Lab Section: Afternoon Evening Follow the directions below and answer the questions as directed. Your report will consist of the attached pages. Remove these pages and answer the questions. Turn in your report with the TL plate stapled to the upper left corner. Reproduce your TL plate on the outline below. Label the samples with their names. Report the R f for each. Indicate what was seen for each spot using both visualization techniques. 1 Sample name Visualization R f List the ingredients in your unknown: 2. What is the brand name of your unknown?

8 3. R f values have no units, explain why. Show how you calculated the R f value for each component in your unknown. 4. An aspirin tablet contains 300 mg of aspirin. An arthritic patient requires 0.90 grams of aspirin daily. ow many tablets should you give the patient each day? Show unit analysis. 5. A patient requires 1500 mg of sodium bicarbonate (a 3 ). ow many 2.5 grain tablets of sodium bicarbonate should be administered to the patient? ( 60 mg equals 1 grain ) Finally, write a conclusion for this experiment.