STUDY OF GUAM CLUSTER OF NEURODEGENERATIVE DISEASE.
Mark Purdey . April 2004
Magnetic susceptibility testing of soils;
Soil was drawn from the regions where the Chamorro populations harvested their food supplies using identical methods employed whilst soil sampling for metal analyses. Due care was taken to avoid collecting samples from areas where any polluting activities had been practiced that were out of character for the region and could potentially influence magnetic susceptibility readings – eg. fires or the dumping of metal materials.
After drying the soil on trays, the samples were taken to a wooden bench in a building that was relatively free of any electromagnetic influences ( eg. noise, bright sunlight, electrical cables/apparatus ) and metal surfaces/objects that could influence the operations of the magnetic susceptibility meter.
Each soil sample was ground by pestle and mortar into a fine grained, homogenous powdery medium to remove air pockets. Twenty five grams were then drawn from each sample, placed into a plastic vial which was then tamped down to achieve high density packing. The vial was lowered into the chamber slot which centres the sample into the field of the sample coil of the PCSM magnetic susceptibility meter.
The portable PCSM field meter, from Pike
Agri-lab supplies, employs two matched coil chambers with a
differential amplifier high gain circuit that responds to slight
variations in impedance when magnetic dipoles are switched within
one of the coils. The frequency of excitation is 4.3 KHz. This
meter is designed on a different principle to the standard design
of most standard magnetic susceptibility meters, such as the
Bartington.
The PCSM employs two chambers which are filled with oxygen that
is the most paramagnetic of all gases ( uCGS 3449). In this
respect, any sample of soil or rock added to the sample chamber
will unbalance the impedance match (XL*XC = 0), so that the
unbalanced side will read the mismatch as a figure converted on
the meter to a uCGS reading
Prior to commencement of Magnetic susceptibility readings, the vial containing the reference sample of rock dust - of a ‘known’ magnetic susceptibility uCGS reading measured on the Bartington MS2 meter - was lowered into the sample chamber and the reading that was displayed on the meter was balanced until it matched its ‘known’ reading. This procedure was repeated after the recording of every 10th sample had been run through the meter to check if any untoward external/ internal influences had emerged and effected the overall operation of the meter.