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UCL Home  /  Geography  /  Resources  /  Laboratory  /  Laboratory Methods  /  Lake Sediment Analysis  /  Carbonaceous Particles from lake sediments

Carbonaceous Particles from lake sediments

Carbonaceous Particles - Procedure

A. Afternoon before booked time

1. Accurately weigh 0.1-0.2g dried sediment from selected samples into labelled 12ml polypropylene tubes on a 4-figure balance. (All weighings should be done on such a balance and recorded). (This is the mass of the sediment 'M')

2. To each tube, add 1.5ml of conc. nitric acid. Cover and leave in a safe place overnight. This allows for any reactive organic material to be removed prior to heating and saves loss of any sample by excessive reaction when the sample is heated later.

 

B. Day 1

1. To each tube, add another 1.5ml of conc. nitric acid. Place in the waterbath. Heat at 80°C for 2 hours (not including heating up time).

2. Remove tubes from water bath and top-up with distilled water. Centrifuge at 1500 rpm for 5 minutes.

3. Pipette off the supernatant nitric acid taking care not to disturb the sample pellet at the bottom of the tube. Removed acid must be neutralised before it can be discarded. Please follow all laboratory protocols for the neutralisation and removal of chemical waste.

4. Add 3ml hydrofluoric acid to each tube and return to the water-bath. Heat at 80°C for 2 hours (not including heating up time).

5. Remove from the waterbath, top-up with distilled water and leave, covered, overnight in the fume cupboard.

 

 

C. Day 2

1. Centrifuge at 1500 rpm for 5 minutes. Pipette off the supernatant liquid taking care not to disturb the sample pellet at the bottom of the tube. This discarded supernate must be neutralised as above.

2. Add 3ml 6M HCl to each tube and heat in a water-bath at 80°C for 2 hours (not including heating up time).

3. Remove from the waterbath. Top-up the tube with distilled water and centrifuge at 1500 rpm for 5 minutes.

4. Pipette off the HCl taking care not to disturb the sample pellet at the bottom of the tube (again neutralising the acid this before discarding)

5. Wash the sample with distilled water. Centrifuge at 1500 rpm for 5 minutes. Pipette off the liquid taking care not to disturb the sample pellet at the bottom of the tube.

6. Repeat this washing and centrifuging once more.

7. Label and weigh a small sample vial (c. 10ml is sufficient) and an air-tight lid for each sample. (This is the mass of the empty vial 'VE')

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

9. 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.

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

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

 

Counting

SCPs should be counted using a light microscope at x400 magnification. All the SCPs 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 SCPs in the sediment sample is thus:

SCP concentration = 100N/E * M

where E is the percentage of the final suspension evaporated onto the coverslip:

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

The SCP concentration is in units of 'numbers 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.