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Magnetic Susceptibility Protocol (V. 1-1)

Magnetic Susceptibility is a good and potentially non-destructive method of analysis which can give an initial look inside the core this can be measured in two ways. Either discrete samples can be placed in pots, weighed and the magnetic susceptibility per unit weight measured, or readings can be taken along a core measuring the fluctuations in magnetic susceptibility for a constant volume.

Equipment

  • Bartington magnetic susceptibility metre.
  • Either, pot (discrete sample) coil or a (core) reader attachment.
  • Plastic pots and lids (if using discrete samples).
  • Recording sheet appropriate to method.
  • Booked bench space to use the equipment in!

Preparation of Discrete Samples

  1. Sediment samples are put into 10 cm3 plastic pots so that the pot is full to the top, and weighed. An average weight of empty pots is taken to subtract from the full pot weight. The pots are labelled with permanent marker pen.
  2. Samples are put in a drying oven overnight at no higher than 105oC – take care not to melt the plastic pots!
  3. The samples are then removed from the oven, the lids fastened on and the dry weights recorded.
  4. The magnetic susceptibility meter is set up and zeroed, it may take some time for the meter to stabilise so it is worth setting up the machine half an hour before you start taking readings. Any large metal objects such as metal trays, watches or belt buckles should be kept away from the meter.

Analysis of Discrete Samples

The readings are taken as follows:

  1. The machine beeps at regular intervals (15.01 sec.). At the first beep the sample hole is empty and the value noted (A).
  2. The sample is then lowered into the hole and the second beep value ignored as this covers the time whilst the pot was being placed into position.
  3. The third and fourth beep values are recorded (B & C) the sample is then removed.
  4. The fifth beep is ignored and the sixth beep value recorded (D). The values recorded measure a background magnetic field before and after the sample value.
  5. The next sample is then measured in the same way.
  6. When a routine is established there is usually enough time to move samples so that blank value D can become blank value A for the subsequent sample. Taking these two readings allows compensation to be made for natural drift in the earth’s magnetic field.
  7. As a final reading an empty pot is measured.
  • Top tip – now empty the sample into a sediment tube and use for particle size analysis.

Calculation

The magnetic susceptibility of a sample is calculated in the following way:

(B+C)/2 – (A+D)/2

dry weight of sample

  1. This gives the magnetic susceptibility of the sample in SI units /g dry weight.
  2. Then the magnetic susceptibility value for the empty pot is subtracted to allow for the magnetic properties of the sample pot, to give a final value.