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UCL Home  /  Geography  /  Resources  /  Laboratory  /  Laboratory Methods  /  Lake Sediment Analysis  /  Inorganic Fly Ash

Inorganic Fly Ash

Inorganic Ash Spheres from lake sediments

(improved version of Rose, N.L. (1990))

1. 0.1 - 0.2g of dried lake sediment is accurately weighed into a 12ml glass test-tube and 2ml of 30% H2O2 is added to each sample. This is left covered overnight at room temperature.

2. An additional 5ml of 30% H2O2 is added then added to each sample. This is heated in a water-bath at 80 - 90°C for 3 hours.

3. Cool, top-up each tube with distilled water and centrifuge for 5 minutes at 1500 r.p.m. Pipette off the supernate.

4. To each tube add 5ml 0.3M NaOH and heat in a water-bath at 80 - 90°C for 3 hours. This stage removes biogenic silica (e.g. diatoms, chrysophyte cysts etc.).

5. Cool, top-up each tube with distilled water and centrifuge for 5 minutes at 1500 r.p.m. Pipette off the supernate.

6. To each tube add 5ml 3M HCl and heat in a water-bath at 80 - 90°C for 1 hour. This stage removes the less robust carbonates and bicarbonates in the sediment.

7. Cool, top-up each tube with distilled water and centrifuge for 5 minutes at 1500 r.p.m. Pipette off the supernate.

8. Repeat the washing and centrifuging procedure.

9. Label and weigh a small sample vial with air-tight lid for each sample. (This is the mass of the empty vial 'VE')

10. Transfer the sample residue to these bottles and re-weigh. (This is the mass of the vial + sample 'VS')

11. Place a cover slip for each sample onto a metal settling tray. Pipette a drop or two of each sample onto the respective coverslip. Heat gently (lowest hotplate setting) to evaporate the water.

12. Re-weigh the sample bottles. (This is the weight of the vial after subsampling 'VSUB')

13. When the water has evaporated, mount the cover-slips onto standard microscope slides using an optical mountant such as 'Naphrax' or similar.

Counting

IASs should be counted using a light microscope at x400 magnification. All the IASs on the cover-slip should be counted (number 'N') and care should be taken to ensure the full area of the dried sample droplet(s) is covered.

The concentration of IASs in the sediment sample is thus:

IAS concentration = 100N/E * M

where M is the mass of sediment and E is the percentage of the final suspension evaporated onto the coverslip:

E = 100*(Vs - VSUB)/ (VS - VE)

The IAS concentration is in units of 'number of particles per gram dry mass of sediment' (or gDM-1)

Note: It is important to remember that these are quantitative techniques and so care should be taken to ensure that particles have settled out properly before pipetting off supernatant liquid and when transferring sample residues from one sample vessel to another. Also it should be remembered that fly-ash particles are deposited from the air and are present in high concentrations in many urban areas. Therefore, all containers must be kept covered unless they are in the fume cupboard with the extractor on. Special care regarding the possibilities of cross-contamination between samples must be taken when undertaking preparations from lakes in 'clean' areas.