General Spectroscopy - Trace metal impurity data

I have some questions about trace impurities reported on the certificate of analysis for 1000 ug/mL and 10 000 ug/mL Al solutions. The impurity values are reported with 4 to 6 significant figures for most analytes – how do you get such accurate values? Are the less than (<) values IDL, MDL or RL – how to assign the same or different impurity values for 1000ug/mL and 10000ug/mL Al solutions, respectively, that are directly reported from analytical results (with/without correction), scientific judgment or computer generated? Calibration standards and their concentration used for impurity analysis? How long to complete impurity analysis and report of each product.