c) The tar product produced in a coke oven is a very complex mixture of volatile

Question

c) The tar product produced in a coke oven is a very complex mixture of volatile polynuclear organic molecules. A particular cause for concern is the possible presence of very small amounts of carcinogenic benzo(a)pyrene (C20H12) shown below. i) Suggest an analytical method with the highest sensitivity for identifying the presence of benzo(a)pyrene present in the tar and give reasons for the selection of your method [6 marks] i) Discuss the ways in which you might identify the species using the selected technique and the spectral features you might examine in the identification. [7 marks)

Explanation / Answer

I. A simple and precious analysis of benzo pyrene in coal tar by dissolving it in benzene. Benzopyrene in the solution is directly separated by a one-dimensional dual band thin-layer chromatography. The Benzopyrene spot on the thin-layer will be scraped off into a small centrifugal-tube. After adding 5 ml of dimethyl sulfoxide (DMSO) into the tube, ultra sonic extraction to be carried out for 10 minutes in order to dissolve Benzopyrene completely. The extracted DMSO solution then centrifuge for 5 minutes. Benzopyrene in the supernanant will be identified by comparison of its fluorescence and excitation spectra with those of standard Benzopyrene solution and to be determined spectrofluorometrically by means of narrow base line method.

This analytical method has the following advantages over the conventional methods :

(i) Spotting process of solution in the present chromatography is easy and not time-consuming and made a quantitative determination possible, because large volume of sample solution can be applied to easily.

(ii) The extraction procedure of Benzopyrene is simplified by the use of DMSO as extracting solvent because of its high dissolving capacity

(iii) An accurate determination of Benzopyrene is easily obtainable because narrow base line method can eliminate the interference of the other polynucler aromatic hydrocarbons mixed in Benzopyrene which was scraped off together with a part of its neighbouring spots in order to recover Benzopyrene .

II. Detailed technique: This method consists of the following procedures; i) extraction of polynuclear hydrocarbons in coal tar by liquid-liquid partitions, ii) separation of polynuclear hydrocarbons into each component by two-dimensional dual band thin-layer chromatography, and iii) spectrofluorometrical analysis of each component. Benzopyrene in coal tar were extracted by a series of liquid-liquid partitions of cyclohexane containing coaltar-dimethyl sulfoxide(DMSO), (DMSO +20 vol% HCl; 1: 1, v/v)-cyclohexane, cyclohexane-water, cyclohexane-5% NaOH, and cyclohexane-water. The residual water in the final cyclohexane solution was removed by adding small amounts of sodium sulfate. anhydride. The cyclohexane solution was then evaporated to dryness in a reduced pressure and at low temperature with the help of a rotary evaporator. The residue was dissolved in a small volume of benzene. A part of the benzene solution was applied onto the aluminum oxide G layer of the thin-layer chromatoplate which had been composed of two adsorbent layers of aluminum oxide G and 26% acetylated cellulose. The first development was carried out on aluminum oxide G layer with n-hexane-ethyl ether (19: 1, v/v) in a chamber kept at about 20% in relative humidity. The second development was carried out on the acetylated cellulose layer with methyl alcohol-ethyl ether-water (4:4: 1, v/v). The polynuclear hydrocarbons thus separated were detected on the acetylated cellulose layer as small fluorescent spots under ultraviolet rays (365 and 253 m). Each spot on the thin-layer was scraped off into a small centrifuge-tube, and benzopyrene in it was extracted with benzene by centrifugation. Benzopyrene in the benzene extract was analysed by spectrofluorometry.

Related Questions

Navigate

• Browse (All)
• Browse C
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.