A method of and system for determining the total magnetic content of a fluid sample introduced within at least one coil. A sample withdrawal event time is determined and, at the sample withdrawal event time, the response of the at least one coil is analyzed in two zones, a first zone before the fluid sample withdrawal event time and a second zone after the fluid sample withdrawal event time. The total magnetic content of the fluid is calculated based on the difference in the response of the at least one coil in the first and second zones at the fluid sample withdrawal event time.