When measuring mercury at sub pg/g level, there is a strict quality control procedure you need to comply.
You need to prepare at least 5 calibration standards, with the lowest standard being 0.5 pg/g or 1 pg/g depending on you are in America or Europe.
The linearity of the calibration curve needs to be in a accepted range
The intercept of the calibration curve needs to be less than half of the slope
The recovery of the lowest standard need to be within certain range
An ‘on going standard’ needs to be used for controlling the drift
An ‘independent standard’ needs to be prepared by another person or from independent source to ensure the accuracy of the calibration standards
The usage of reference material is highly recommended
Random sample needs to be spiked in duplicate, the spike recoveries of these two
sample need to be within certain range