proppant or onto the materials to be blended with the proppant so that each proppant stage or each fracturing job treatment can be identified by a unique tracer material suitable metals for use as the tracer material are generally commercially available from sigma aldrich inc as well as from mallinckrodt baker inc it is understood however that field grade materials may also be used as suitable tracer materials for tagging onto proppant material or materials to be blended with proppant material samples of flowback proppant collected from the field may be analyzed according to a process known as the inductively coupled plasma icp method to determine from which proppant stage and which production zone the proppant has been produced according to the icp method an aqueous sample is nebulized within an icp spectrophotometer and the resulting aerosol is transported to an argon plasma torch located within the icp spectrophotometer the icp spectrophotomer measures the intensities of element specific atomic emissions produced when the solution components enter the high temperature plasma an on board computer within the icp spectrophotomer accesses a standard calibration curve to translate the measured intensities into elemental concentrations icp spectrophotometers for use according to the icp method are generally commercially available from the thermo arl business unit of thermo electron corporation agilent technologies and several other companies depending upon the model and the manufacturer the degree of sensitivity of currently commercially available icp spectrometers can generally detect levels as low as 1 to 5 parts per million for most of the metals listed above it is understood that depending on the materials used as tagging agents other spectroscopic techniques well known to those skilled in the art including atomic absorption spectroscopy.