You are advised to spend an equal amount of time on each question.

Transcription

1 UNIVERSITY OF EAST ANGLIA School of Chemistry Main Series UG Examination INSTRUMENTAL ANALYTICAL CHEMISTRY CHE-2F52 Time allowed: 2 hours Answer THREE questions. You are advised to spend an equal amount of time on each question. All questions carry an equal number of marks. Answer EACH question in a SEPARATE answer book. The breakdown of marks within each question is indicated by the percentage figures in brackets on the right. The following is provided: Graph paper Do not take this question paper out of the examinations room. Notes are not permitted in this examination. Do not turn over until you are told to do so by the Invigilator. (CHE-2F52) Module co-ordinator: Dr Andrew Mayes (CHE) Copyright of the University of East Anglia Version 1

2 2 1. Answer ALL parts (a) to (f). Most underivatised amino acids have no absorbance in the UV-visible range, but they can be reacted with a reagent called AQC to make fluorescent derivatives. A high performance liquid chromatography (HPLC) system was set up to run the following analytical method to separate the fluorescent amino acid derivatives. Mobile phase A consisted of 140 mm sodium acetate, 17 mm triethylamine and 3 mm EDTA adjusted to ph 5.05 using phosphoric acid. Mobile phase B was 60% acetonitrile in H2O (v/v). Separation was carried out on a Waters Nova-Pak 4 µm C18 column, mm, thermostatted to 37 C and operated at a flow rate of 1 ml min 1. The gradient conditions (all segments linear) were as shown in the table below: Time 0 min 0.5 min 12 min 17 min 30 min Gradient 100% A 98% A 93% A 87% A 68% A The gradient was then immediately raised to 100 % B for 5 minutes. At the end of the separation the column was re-equilibrated for 10 minutes at 100 % A, prior to injection of the next sample. The chromatogram for this separation, applied to an AQC-derivatised amino acid standard mixture, is shown below: question 1 continued / (CHE-2F52) Version 1

3 3 /question 1 continues (a) Draw a block diagram showing the HPLC instrument modules required to carry out this separation and the order in which they would be connected. [15%] (b) Is this a normal phase or a reversed phase separation? State which information in the method allows you to infer this. [10%] (c) Explain how a mobile phase gradient is produced in an HPLC system. [15%] (d) Draw a graph of the mobile phase gradient profile throughout the separation. [15%] (e) What kind of detector would be used to detect the analyte peaks in this method? Draw a labelled diagram to show the key components of such a detector. [35%] (f) A rather complex gradient was used for this separation. How would the appearance of the chromatogram change if a simple linear gradient, starting at 100% A and ending with 100% B after 30 minutes, was applied instead? [10%] (CHE-2F52) TURN OVER Version 1

4 4 2. Answer ALL parts (a) to (c). (a) Explain the different levels of analytical methodology: technique, method, procedure, protocol with reference to lead in water. [24%] (b) Concern has been raised about the leakage of heavy metals, in particular chromium, into a listed aquifer at an old industrial site and because of this the Environmental Agency has been brought in to carry out an intrusive site investigation. With reference to this case, state and illustrate the steps involved in solving analytical problems. [40%] (c) As part of a study looking at heavy metal contamination of water the concentration of hexavalent chromium, CrO4 2 was determined by first derivative UVvisible spectrophotometry after reacting with diphenylcarbazide. The hexavalent chromium compound with diphenylcarbazide was detected at nm while Erioglaucine A, added as an internal standard, was determined at nm. The set of standard solutions of hexavalent chromium and internal standard were prepared by pipetting various volumes of a standard containing 10.5 µg/l chromium into a 10 ml volumetric flask, adding a constant amount of Erioglaucine A and making up to mark. A pre-concentration procedure was then carried out followed by the UV-visible measurements. The intensities at the two wavelengths in the visible spectra are shown in the table below. Sample Volume of standard / ml Intensity at nm Intensity at nm ml of the unknown was taken, the internal standard added and the solution made to 10 ml. The same pre-concentration procedure was used as for the standards. If the following intensities, I (591.30) = and I (653.50) = were seen for the unknown, what is the concentration of hexavalent chromium in the water sample? [36%] (CHE-2F52) Version 1

5 5 3. Answer ALL parts (a) to (c). The redox chemistry of A at ph 7 is described below. A + e A E o = V A + 2e A 3 E o = 0.24 V (a) Describe a protocol to prepare 3 ml of a M solution of A in 0.10 M phosphate at ph 7 from 1.0 g of A and 0.05 L of 0.10 M phosphate at ph 7. State any assumptions that are made in the protocol and take the molecular mass of A to be g mol 1. [30%] (b) Cyclic voltammetry at 20 mv s 1 of an unstirred M solution of A containing 0.10 M phosphate at ph 7 was performed between +0.3 and 0.7 V. The voltammetry was initiated at +0.3 V and produced a Nernstian response. (i) Illustrate the excitation profile of the experiment with a plot of electrode potential versus time. [15%] (ii) Illustrate the cyclic voltammogram that was obtained and indicate how the output defines the value of E o and electron stoichiometry for each couple. [35%] (iii) Explain how the diffusion coefficient of A 3 could be obtained by cyclic voltammetry. [10%] (c) The experiment in (b) was repeated in a different laboratory. Their cyclic voltammogram indicated the following description for the redox chemistry at A. A + e A E o = V A + 2e A 3 E o = 0.19 V What is the most likely explanation for their results? [10%] (CHE-2F52) TURN OVER Version 1

6 6 4. Answer ALL parts (a) to (c). A typical UV-visible spectrometer contains two radiation sources: a deuterium lamp and a tungsten filament lamp. (a) (i) Sketch and describe the components of a deuterium lamp and explain how light is produced. [20%] (ii) Over what wavelength range is the deuterium lamp useful? What prevents its use at other wavelengths? [10%] (iii) What is the advantage of using D2 in place of H2? [10%] (b) (i) Explain how light is produced by a tungsten filament lamp and how the colour temperature is related to the wavelengths of the useful output. [20%] (ii) What is the purpose of adding halogen to a tungsten lamp? [15%] (c) In Atomic Absorption Spectroscopy, a flame can be used to atomise the sample. (i) Make a sketch of the cross section of a typical flame labelling the regions. What processes occur in each region? [15%] (ii) The most commonly used combination of fuel and oxidant is acetylene and air. In what circumstances would the air be replaced by nitrous oxide and why? [10%] END OF PAPER (CHE-2F52) Version 1