Calibration by standard addition Technique To a 100mL aliquot of the unknown ( which is between land 5 ppm ) , add 0.2 mL of the 100 - ug / mL stock solution of Cu in a clean , dry beaker . This will increase the Cu concentration of the unknown by 2 ppm . Repeat with 10.0 mL aliquots of the unknown and the addition of 2,4,6,8 mL of the Cu stock solution , respectively , to increase the unknown samples by 4 , 6 , 8 , and 10 ppm . Zero the instrument with distilled water and aspirate the unknown solutions with the adde copper successively , aspirating water between each sample .