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Analytical Chemistry 7 - Problems A7 and A10: Chromatography 1. Gas-liquid chromatography (GLC) is a powerful analytical technique. Outline the principles of this technique, from the injection of the sample. - sample is heated to vaporize it but sample must not decompose at high temperatures - mobile phase = inert gas like nitrogen, helium or argon - stationary phase liquid long chain alkane adsorbed on solid support; - different sample components carried through apparatus in stream of carrier gas - each component has a different attraction for the hydrocarbon, depending on polarity and molecular mass - components move through at different rates - have different retention times - more volatile components move through faster and have shorter retention times - components converted to ions by flame ionizer as they leave the column - charged ions can be converted to electrical signal by the detector 2. Paper chromatography may be used to separate a mixture of sugars. (a) State the stationary phase and an example of a mobile phase used in paper chromatography. Stationary phase: water in the paper Mobile phase: water or other solvent or solvent mixture (2) (b) The identity of two sugars in a mixture can be determined by measuring their Rf values, after staining. (i) Describe how an Rf value can be calculated. (1) Rf = distance travelled by substance distance travelled by solvent front (ii) (c) Calculate the Rf value for sugar 2 in the chromatogram at the right. Explain how the Rf value of sugar 2 could be used to identify it. - compare Rf of unknown to Rf of known samples - when paper chromatography is carried out under the same conditions (same mobile and stationary phases, and same temperature), different substances have unique Rf values 3. A mixture of two alcohols was analysed using high-performance liquid chromatography, HPLC, and produced chromatogram 1 below. a) In the space provided in chromatogram 2, sketch the chromatogram if the column of the chromatograph was less tightly packed and all other variables were kept constant. Chromatogram 2 should be: two broader peaks as chromatogram 1 for each peak; two peaks of same area as chromatogram 1 for each peak; two peaks of the same area as chromatogram 1 are closer to each other; AND eluted earlier b) Suggest, with a reason, a difference between alcohol 1 and alcohol 2 that would account for the different retention times. Alcohol 1 is less soluble in the stationary phase (or more soluble in the mobile phase) than alcohol 2 so it moves through the column faster. 4. (a) Identify a possible mobile phase and stationary phase for HPLC and gasliquid chromatography, GLC. Chromatographic technique Stationary phase Mobile phase HPLC silica (silicon dioxide) with adsorbed long chain hydrocarbon hexane or water or alcohol or other liquid solvent GLC long chain alkane on column inert N2, He or Ar gas (4) (b) Deduce which technique, HPLC or GLC, can be used to analyse the urine sample of an athlete for the anabolic steroid, tetrahydrogestrinone, THG. (1) GLC - commonly used technique in drug testing, and steroids do not decompose at high temperatures SEE the clarification notes in the Assessment Statements: 10.2 Deduce which chromatographic technique is most appropriate for separating the components in a particular mixture. Aim 8: HPLC can identify compounds that are temperature-sensitive. Uses include: analysis of oil; alcoholic beverages; antioxidants, sugars and vitamins in foods; pharmaceuticals; polymers; biochemical and biotechnology research; and quality control of insecticides and herbicides. GLC can identify compounds that can vaporize without decomposing. Uses include: analysis of urine samples from athletes for drugs, underground mine gases and blood alcohol levels. NOW YOU KNOW! (c) Outline how the technique selected in part (b) would be carried out to confirm the presence of the steroid THG in the urine sample. - measure retention times for THG and sample - if THG present in sample then retention times will be the same under the same conditions (2) 5. An analgesic tablet contains 400 mg of aspirin and 80 mg of caffeine. The molecular formula of aspirin is C9H8O4 and that of caffeine is C9H10N4O2. (a) State and explain which method, gas-liquid chromatography (GLC) or high performance liquid chromatography (HPLC), would be best for the separation and mass determination of aspirin and caffeine in the tablet. (2) - HPLC is the better technique - component would decompose (because of the higher temperature) in GLC (b) State and explain which of the two components would have the shorter retention time. (2) - aspirin because it has a lower molecular mass (~180 vs 206 for caffeine) and it will move through the HPLC column faster (c) Sketch a chromatograph for the separation of the aspirin and caffeine in the analgesic tablet. aspirin lower molecular mass (180 vs 206 for caffeine) so shorter retention time AND present in greater amount (800 mg vs 40 mg for caffeine) so a higher peak and greater area under the peak caffeine higher molecular mass (206 vs 180 for aspirin( so longer retention time AND pesent in lower amounts (40 mg vs 800 mg for aspirin) so a lower pear and smaller area under the peak