about 400 parts per trillion ppt by mass the concentration of cesium in other naturally occurring contexts is not expected to vary greatly from this level in one embodiment then a generous estimate of the maximum cesium concentrations likely to be encountered in nature is 4 000 ppt or 4 parts per billion ppb by mass assuming this hypothetical maximum allows concentration of a hypothetical minimum concentration that would always provide at least a ten fold factor between the resultant invaded concentration in the core and the noninvaded concentration in the formation namely 40 ppb therefore for example to obtain 40 ppb in the core with a 2 invasion one would need a concentration 50 u00d7 higher in the drilling fluid specifically 2 ppm as the threshold level to obtain a 1 resolution of core invasion one would need a concentration 100 u00d7 higher in the drilling fluid specifically 4 ppm as the threshold level because there is the possibility of cesium concentration dilution it is safer to spike the drilling fluid to a larger concentration than necessary therefore by spiking the drilling fluid to ten times the threshold level one ensures in all practical cases that dilution will never hinder the resolution for the determination of drilling fluid contamination in the core the.