
 Posts: 24
 Joined: Thu Apr 28, 2016 11:45 pm
I am curious how to determine the calculation by external cal and standard addition method. I get quite different result.
For external cal I prep on 1ml dmf and get calibration curve as 20,50, 100 and 200ug/ml hexane STD. Then I add 2g sample (or lesser weight) plus 1ml dmf. Mean the concentration obtain need to multiply 1ml and decide sample weight.
For STD addition method, I prep series of 20,50,100 and 200ug/ml hexane STD. Then each vial I put 2g sample then add 1ml of series hexane concentration. For STD addition, I will plot y=my+c. Then assuming y equal zero to calculate the concentration of the sample.
Question is this two method can not get tally result.
Then I now abit confuse on STD addition do I still need devide my sample weight? Theoritically no need because each vials have 2g sample, but I not able to cancel of the unit to obtain ug/g. Another way is I convert become ug (mean let say stock is 5000ug/ml, I take 0.1ml and top up to 5ml, equal 100ug/ml. From this 100ug/ml, I take 1ml to dissolve either STD addition or external cal, mean my cal point will be 100ug. At this stage if I decide my sample weight will be ug/g. However this three method non of it tally.
Can anyone help to advise on this?
Thanks