Hi, colleagues. There are some questions need you help.Please give me some information. 1,Is there any difference between working standard and secondary standard? 2,"Analytical procedures used to characterize a reference standard should not rely solely on comparison testing to previously designated reference standard." Taken from FDA guidance.So,what analytical procedures used to characterize a reference standard are acceptable? 3,How do I prepare the SOPs of reference standard characterization? Thank you advanced.