School of Chemistry UNIVERSITY OF KWAZULU-NATAL, WESTVILLE CAMPUS JUNE 2009 EXAMINATION CHEM340: INSTRUMENTAL ANALYSIS DURATION: 3 HOURS TOTAL MARKS: 100 Internal Examiners: Professor A Kindness Dr T Msagati External Examiner: Professor N Torto Rhodes University NOTE: This paper consists of 14 pages including a Periodic table and Data sheet. Please check that you have them all. Answer all the questions. Each section to be answered in a separate Answer Book. Students are requested, in their own interest, to write legibly.
Question 1 School of Chemistry, University of KwaZulu-Natal, Westville Campus Page 2 SECTION A The following questions relate to the analysis of the drug 2-(4-isobutylphenyl) propanoic acid, ibuprofen, by various methods of analysis. a) According to the following paper: Quantitative analysis of ibuprofen in pharmaceutical formulations through FTIR spectroscopy by S. R. Matkovic et al., it is possible to quantitatively measure the drug using both FTIR and UV spectroscopy. They state that the equation of the lines (y = m x + c) for the analysis is Method Calibration range (m/v %) c m R 2 IR spectroscopy 0.000 0.609 0.001 168.4 0.999 UV spectroscopy 0.000 0.200 0.003 10.61 0.990 y is the peak area of the infrared band at 1721 cm -1 and Absorbance at 273 nm for UV. x is the concentration of ibuprofen expressed as mass (g) per 100 ml (m/v %) i) Define sensitivity for an analytical method. (1) ii) Which method is most sensitive? Briefly explain your choice. (1) iii) Which method produces the largest linear range? Explain why this is a good attribute in an analytical method. (2) iv) A tablet weighing 1.042 g is crushed and finely ground. A 250.0 mg portion is removed and dissolved in 25 ml of chloroform. The solution is filtered and the resultant solution was measured by FTIR using a liquid cell. The area of the peak was found to be 64.7 cm -1. What is the mass of ibuprofen in a tablet; express your results as mg ibuprofen per tablet. (The tablet contains the active ingredient and inert fillers.) (3)
School of Chemistry, University of KwaZulu-Natal, Westville Campus Page 3 b) Ibuprofen can also be analysed by reversed phase HPLC using valerophenone as an internal standard. Valerophenone Ibuprofen The following data was obtained when a 5 μl standard solution was injected into an HPLC instrument. 10.0 mg/ml valerophenone peak area = 877.3 and for 12.0 mg/ml ibuprofen = 849.7. A tablet was dissolved in 50.0 ml solvent (same solvent as the mobile phase) and internal standard was added such that the concentration of the internal standard was 10.0 mg/ml. The peak area measure for the valerophenone was 860.1 whilst the ibuprofen peak had an area of 416.5. i) List four attributes that an internal standard should have when being used in HPLC analysis. (2) ii) What was the concentration of the ibuprofen in the dissolved sample? (3) iii) What was the mass of ibuprofen in the tablet? (1) iv) Afterwards it was noted that the injection volume for the sample had been changed to 4 μl instead of the 5 μl that was used for the standard. What problems, if any, did this lead to in the analysis of the sample? Briefly explain your answer. (2) [15]
Question 2 School of Chemistry, University of KwaZulu-Natal, Westville Campus Page 4 The analysis of iron in a vitamin tablet was carried out as follows. The vitamin tablet was dissolved in water and diluted to 100.0 ml in a volumetric flask. Five aliquots each 5.00 ml in volume was removed by pipette and placed in 5 clean 100.0 ml volumetric flasks. To each volumetric flask a series of standard amounts of iron standard solution (50 μg/ml) was added. A reducing agent was added to the solutions to reduce the iron to the +2 state. The solution was then reacted with o-phenanthroline to form a coloured complex that was measured by visible spectrophotometry @ 508 nm. N N 3 + Fe 2+ N N Fe 2+ o-phenanthroline 3 The following table summaries the analytical procedure. Flask Volume sample added /ml Volume of Fe standard added /ml Final volume /ml Absorbance @508 nm 1 5 0 100 0.201 2 5 1.0 100 0.298 3 5 2.5 100 0.429 4 5 5.0 100 0.732 5 5 10.0 100 1.215
School of Chemistry, University of KwaZulu-Natal, Westville Campus Page 5 The following calibration graph was obtained. 1.4 Absorbance @ 508 nm 1.2 1 0.8 0.6 0.4 0.2 0 y = 0.2029x + 0.2097 R 2 = 0.9988 0 1 2 3 4 5 [Fe] (added) μg/ml a) Why was the o-phenanthroline added? (1) b) What material were the cuvettes made from? Explain why this material was used. (2) c) What was the iron content of the vitamin tablet? (4) d) What is the name of this type of calibration method and when is it used? (1) e) Which will be the most sensitive method of spectrophotometric analysis for Fe: dithizone (ε = 4.16 10 3 cm -1 mol -1 L), o-phenanthroline (ε = 1.06 10 4 cm -1 mol -1 L) or thiocyanate (ε = 7.90 10 3 cm -1 mol -1 L)? Briefly explain your reasoning. (2) [10]
Question 3 School of Chemistry, University of KwaZulu-Natal, Westville Campus Page 6 a) There are several methods that are used for sample atomization in atomic spectrometry. One of these methods involves hydride generation. i) Give two examples of elements that may suitably be determined by this method? (2) ii) What is the source of hydride that is normally used? (1) iii) Briefly describe what happens in the process of atomization by hydride generation. (2) b) Describe the type of atomization technique that is most suitable for the determination of mercury by atomic spectroscopy. (2) c) What characteristics of inductively coupled plasmas make them suitable for atomic emission spectrometry? (2) d) From instrumental point of view, give the main differences between inductively coupled plasms-optical emission spectroscopy (ICP-OES) and atomic absorption spectrometry (AAS). (2) [11]
Question 4 School of Chemistry, University of KwaZulu-Natal, Westville Campus Page 7 SECTION B a) A water sample was analyzed for the presence of barium (Ba) using atomic absorption spectrometry (AAS). Preliminary tests on the sample indicated that the water sample contained Ca. The atomic absorption measurement for Ba was determined using atomic absorption spectrometry at wavelength of 553.6 nm. However, the AAS results indicated that Ba was below the detection limit. When the same sample was determined using other methods such as stripping polarography a signal above the detection limit for Ba was obtained. i) What could have been the problem in the analysis of Ba using the AAS method? (1) ii) What is the solution to the problem? (1) (b) In the determination of sodium and vanadium using AAS, the analyst encountered some unusual observations. He found that the signal for sodium was markedly decreased by the presence of hydrochloric acid. For vanadium, there was a significant enhancement of absorption when aluminium or titanium was present especially in fuel-rich flames of the AAS. (i) What caused the decrease of signal for sodium? Briefly explain. (2) (ii) What caused enhancement of signal for vanadium? Briefly explain. (2) (c) Sample preparation methods as well as the choice of proper solvents are very crucial in infrared spectroscopy. (i) Give the advantages and disadvantages of using potassium bromide methods as sampling method for solids in infrared spectroscopy. (2) (ii) In most cases it has been difficult to measure the infrared spectrum of biological materials in water due to the spectrum of water obscuring the information. Mention two strategies that may help minimize the problem. (2) [12]
Question 5 (a) School of Chemistry, University of KwaZulu-Natal, Westville Campus Page 8 In each of the following types of chromatographic methods, indicate the types of compounds that can be separated. (i) Gas chromatography (2) (ii) Ion exchange chromatography (2) (iii) Gel permeation chromatography (or size exclusion chromatography) (2) (b) Describe the fundamental difference between: (i) Adsorption and partition chromatography (2) (ii) Gel-filration and gel-permeation (2) (c) Discuss factors that influence resolution in chromatography and how they can be manipulated. (4) (d) In a normal phase partition column, a solute was found to have a retention time of 29.1 minutes and an unretained sample had a retention time of 1.05 minutes when the mobile phase was 50% by volume chloroform and 50% n- hexane. Calculate capacity/retention factor (k ) for the solute. (2) (e) Optimization of parameters such as the retention factor ( k ) and selectivity factor (α,) is harder in HPLC than in GC. Explain. (2)
School of Chemistry, University of KwaZulu-Natal, Westville Campus Page 9 (f) The following plots show the effect of particle size on plate height for a packed GC column. The numbers to the right hand of each curve are particle diameters. (i) Briefly explain the trend observed. (2) (ii) Explain how film thickness of the stationary phase particles affects retention and column resolution. (2)
School of Chemistry, University of KwaZulu-Natal, Westville Campus Page 10 (g) The following results were obtained from a chromatographic column. The aim was to get complete resolution of all six components of the mixture at the shortest possible elution time. Conditions of the experiment such as the ratio and flow rate of the mobile phases were varied to arrive at this objective. From the results shown below, the experiment was not a success. How would you achieve complete resolution of all the components of this mixture at a reasonable elution time? (2) (h) Stationary phases in GC and HPLC are normally either bonded and/or crosslinked. Define the two terms (bonded phases and cross-linked phases). (2) (i) What is the purpose of using bonded phases or cross-linked stationary phases in column stationary phases? (2) (j) Distinguish between the different types of support used in capillary (open tubular) GC columns, giving the advantages and disadvantages of each. (3) [31]
Question 6 School of Chemistry, University of KwaZulu-Natal, Westville Campus Page 11 (a) What is the source of a junction potential in a glass/calomel electrode system? (2) (b) The following cell was found to have a potential of 0.124 V: Ag gcl (sat d) Cu 2+ (3.25 x 10-3 M) membrane electrode for Cu 2+. When the solution of known copper activity was replaced with an unknown solution, the potential was found to be 0.086V. What was the pcu of this unknown solution? (Neglect the junction potential). (2) (c) How does the information obtained by a direct potentiometric measurement of ph differ from that obtained from a potentiometric acid/base titration? (3) (d) Use the figure below which shows the response of a liquid-membrane electrode to variations in the concentrations and activity of calcium ion to explain why the electrode response is related to analyte activity rather than analyte concentration. (2) (i) What does the value 29.58 mv (in the figure above) indicate? (2) (ii) Account for the non linearity of the curve for potential (E) versus concentration. (2)
School of Chemistry, University of KwaZulu-Natal, Westville Campus Page 12 (e) Distinguish between potentiometry and voltammetry. (2) (f) Why is it necessary to buffer solutions in organic voltammetry? (2) (g) A compound Q (structure below) undergoes a reversible reduction (to form HQ) at a voltammetric working electrode, the reaction is given below. Assume that the diffusion coefficient for compound Q and HQ are approximately the same. Calculate the approximate half-wave potential (versus SCE) for the reduction of HQ at an RDE (rotating disc electrode) from a solution buffered to a ph (i) of 7.0 and then a ph of 5. (4) O OH + 2H + + 2e - E 0 = 0.599 V O OH [21]
School of Chemistry, University of KwaZulu-Natal, Westville Campus Page 13
School of Chemistry, University of KwaZulu-Natal, Westville Campus Page 14