Analytical techniques and protocol adopted

3.1.2.1 SEM

The morphology of the soot was investigated using an FEI Nova NanoSEM 30 Series with Digital Micrograph Software at the Electron Microscope Unit (EMU) at the University of Cape Town (UCT).

Dry diesel soot requires minimal sample preparation prior to observation in the SEM. For each experiment, a sample stub was coated with double coated carbon tape. Carbon tape was used to negate charge build up in the soot samples as soot is a nonconductive material. Soot was then directly applied onto the carbon tape and the sample stub was inserted into the SEM. The microstructure of the samples was observed in scanning mode as well as with

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back-scattered electron detection. The acquisition parameters used to obtain each micrograph are displayed on the each respective micrograph in the results section.

3.1.2.2 TEM

Transmission electron microscopy was employed to study soot microstructure in greater detail. The instrument used for this analysis was a FEI Tecnai G2 20 TEM, equipped with a lanthanum hexaboride (LaB6) filament and a Gatan Tridiem image filter (GIF), housed at the EMU at UCT. The protocol highlighting the sample preparation undertaken by this study for all soot samples for TEM analysis is listed below.

Sample preparation protocol:

Note: Steps 1-3 applied only to test bed engine soot while steps 4-6 applied to both the commuter bus and test bed engine soot.

1. A 25 mm diameter TIGF filter paper containing ± 4mg of soot sample was placed in a 100m beaker containing 20m ethanol (Kimix, Cape Town, South Africa).

2. The filter paper with adsorbed soot was ultrasonicated for 5min or until the soot de- adsorbed from the filter paper and dispersed into the ethanol solvent.

3. The stripped filter paper was removed from solution.

4. Using a dropper, a few drops of the soot and ethanol mixture from the beaker were transferred onto a 3mm carbon-coated copper grid.

5. The 3mm grid containing the sample was held under a UV light (250W xenon lamp source) until the ethanol completely evaporated.

6. The dry copper grid was then inserted into the TEM grid holder. TEM experimental parameters

The TEM was operated at 200keV to obtain high resolution bright field images. The signal- to-noise ratio, and hence the contrast, in the images was enhanced by employing a 20eV energy slit (using the GIF) to filter out elastically scattered electrons. All images were captured using a Gatan Ultrascan 1000 digital camera with a CCD chip comprising of 1024 x 1024 array of pixels of width 15 m.

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3.1.2.3 Inductively coupled plasma – mass spectrometry (ICP-MS with ICP-AES)

Analysis were performed by an external service provider – the Central Analytical Facility at the University of Stellenbosch. ICP-MS was conducted on soot to determine the quantities of trace elements while ICP-AES was measured the major elements. The instruments used for ICP-MS and ICP-AES were an Agilent 7700 ICP-MS and a Thermo iCap 6300 ICP-AES respectively. Analytical conditions were optimized for each instrument, with an instrument calibration done using NIST traceable standards. Accuracy of measurement was verified using a separate standard at the beginning of the analysis.

In each analysis a 20 mg sample of soot was microwave digested in HNO3 and then analysed by ICP-AES and ICP-MS.

3.1.2.4 Soxhlet extraction

The SOF of the commuter bus soot was obtained via Soxhlet extraction. Sample size limitations for the test bed engine soot meant that testing the effect that the solvent has on the quantity and type of SOF during extraction was exclusively investigated using commuter bus soot. Six solvents were chosen to test the influence of the extraction solvent on the quantity of SOF. The solvents that were used are highlighted in Table 3.3 while the procedure that was adopted for the extraction of commuter bus soot is indicated thereafter.

Table 3.3: Solvents used for soot extraction

Dichloromethane Toluene Hexane Acetone Methanol Cyclohexane

All solvents were supplied by Kimix (Cape Town, South Africa). During Soxhlet extraction, samples are typically placed inside a thimble. Initial work was done using cellulose thimbles but these were replaced with glass ones. The motivation for this may be found in Appendix D.

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Extraction protocol:

1. A glass fibre extraction thimble, extractor and flask were cleaned with acetone to remove any laden particles.

2. The thimble, extractor and flask were dried in an oven at 50°C for 15min.

3. The extraction thimble was weighed using a Mettler Toledo AT 20 microbalance ( 2 g).

4. Approximately 100mg of commuter bus soot was added to the thimble and the combination was reweighed.

5. The thimble, containing the soot sample, was inserted into the extraction cavity of the Soxhlet and 30m solvent was added to the flask.

6. Soxhlet extraction was continued under reflux for 24 h.

7. On completion of the extraction process the thimble with sample was removed from the extraction cavity and dried in a fumehood for 1 h. Thereafter they were dried in an oven at 50°C for 48 h.

8. The dried thimble with sample was then weighed. 9. SOF was calculated as follows

100 s m s m t m s m t m (%) SOF i f

where m = mass, t = thimble, s = sample, i = prior extraction and f = after extraction. 3.1.2.5 Chemical speciation of the SOF

Chemical speciation of the SOF of the soot was conducted. The extraction process adopted was the same as in section 3.1.2.4, except steps 7-9 were omitted. The solvents used were the same as those in Table 3.3. The protocol undertaken in determining the chemical speciation of the SOF from extraction commuter bus soot is highlighted below.