CHE2035: Advances in Analytical Instrumentation

Question 1

A team of analysts has been charged with the task of determining the concentration of parabens and other preservatives in shampoos and cosmetics. For this purpose they decide to analyse the extracted samples by high pressure liquid chromatography (HPLC) using a C18 column (7.5 cm x 3 mm I.D., 2.7 µm particles) and water: methanol; (55:45) as mobile phase under isocratic conditions, at a flow of 0.6 mL/min using an UV-vis detector at 250 nm. Figure 1 below shows the molecular structure of the 7 target analytes.

(a) The laboratory has several liquid chromatography instruments (HPLC) equipped with UV-vis (UV-vis), refractive index (RI) and mass spectrometry (MS). Indicate the pros and cons of each of the available detectors and justify the selection of the analysts. [10 marks]

(b) If the dead volume of the C18 column is vM= 0.3 mL and the flow of the mobile phase is constant at 0.6 mL/min, using the information in the chromatogram presented in Figure 1 calculate the following:

(i) The capacity factors (k’) for the analytes 3 (methylparaben) and 4 (ethylparaben). [4 marks]

(ii) The selective factor (α) between for the analytes 3 (methylparaben) and 4 (ethylparaben). [2 marks]

(iii) The resolution (R) between peaks 3 (methylparaben) and 4 (ethylparaben), and between peaks 6 (butylparaben) and 7 (benzylparaben) [4 marks]

(c) The chromatogram in Figure 1 was obtained using a C18 column and water: methanol; (55:45) as mobile phase under isocratic conditions. Suggest how the analysts can optimise the elution conditions to reduce the analysis time and improve the resolution of peaks 6 (butylparaben) and 7 (benzylparaben), using the same instrumentation. Indicate in your answer the pro- and cons- of your suggestion(s). [10 marks]

(d) Explain the impact on the chromatographic efficiency of the following column parameters, indicating what would be the effect on the height of theoretical plates of the column:

(i) Reduction of the particle size of the C18 stationary phase.

(ii) Increase of the flow of the stationary phase. [10 marks]

(e) In order to determine the concentration of the parabens depicted in Figure 1 in two shampoo samples (M1 and M2), 0.9803 g M1 and 0.9679 g of M2 were weighted. The samples were dissolved by stirring them in water (final volume 20 mL) for 15 min. Then 3.00 mL of each solution were passed through a C18 SPE cartridge (solid phase extraction) at 10 mL/min flow to remove interferences. The analytes were eluted with 5 mL of an organic solvent. Then the eluate was evaporated and solubilized in 0.100 mL and a 20-µL aliquot was injected into the liquid chromatograph.

The calibration equation for propylparaben was: Peak Area = 27786×[Propylparaben] + 52, where the units for [Propylparaben] were in µg/mL. The linear range was found to be 0.03-10 µg/mL.

Calculate the content of propylparaben in both shampoos (expressed as µg/g propylparaben) considering that M1 and M2 yielded peak areas of 438738 and 109204, respectively. [10 marks]

Question 2

(a) Discuss the processes of Quality Assurance and Quality Control in an analytical laboratory and why they are necessary. Keep your answer to one page. [25 marks]

(b) Clozapine is one of the newer types of antipsychotic drugs used in the treatment of schizophrenia and other psychotic disorders. A technology company has approached you to help with the development of a miniature electrochemical sensor that will allow real-time monitoring of clozapine levels in microliter volumes of serum samples. Describe your solution taking the following into account:

(i) Describe what method of electrochemical analysis you will use and what advantage this would give compared to other methods.

(ii) Describe what type of electrodes you would use in your test?

(iii) What is measured during the test and give other important considerations for the electrochemical measurement.

(iv) What gives rise to the signal that you are seeing? (v) Is it important to functionalise the electrode and what are the advantages and disadvantages to your approach?

(vi) What important sensor characteristics do you want, and which ones do you want to avoid with your test? [25 marks]

Question 3

(a) What is the principle of Atomic Absorption Spectroscopy (AAS)? (Maximum of 40 words). [3 marks]

(b) In terms of applications of AAS –

(i) When should Atomic Absorption Spectroscopy (AAS) be used? (Maximum 40 words) [3 marks] (ii) Name some applications and purposes of AAS? (Maximum of 30 words per application, [1/2 mark per application to a maximum of 3 marks]) [5 marks]

(c) List the advantages and disadvantages of Electrothermal – Atomic Absorption Spectroscopy. (Maximum of 150 words). [8 marks]

(d) A Hydride-Generation Atomic Absorption Spectroscopy (HG-AAS) method was developed for the analysis of total Hg in liquid matrices of mercury-rich plants and mine tailings samples. The HG system used a flowrate of 280 to 320 mL/min for nitrogen and 15 mL/min for NaBH4. Under these conditions, linear calibration curves were obtained over the range of 75 to 1000 ng/mL of Hg

(i) Plot the calibration curve [3 marks]

(ii) Comment on the accuracy and precision of this method [3 marks]

(iii) Calculate the LOD and LOQ of this method [3 marks] (iv) Explain why hydride generation method was used [5 marks]

(e) A flow system was developed for the determination of total mercury concentration in fish samples by Cold Vapor Atomic Absorption Spectrophotometry (CV-AAS), based on the Multicommuted Flow Injection Analysis (MCFIA) approach. The system uses independently controlled solenoid valves for the introduction of reagents and samples. When not injected, solutions were recirculating to the reservoir bottles, in this way reducing the waste produced by the analytical system and the sample consumption. Results were compared to those obtained by the reference flow injection procedure. Accuracy was also assessed by recovery studies using a certified reference material as well as spiked samples; recovery percentages in the range of 90.7% to 99.8% were found.

(i) Explain the principle of CV-AAS to measure Hg. (Maximum of 60 words) [5 marks]

(ii) The new method using the multicommuted flow system was compared to a classic system of cold vapor AAS mercury determination using FIAS 100. Comment on the new method compare to the more conventional one. (Maximum of 40 words)[5 marks]