Rules on weighing your standard

[jayroseville58]

Are there any guidelines pertaining to proper weighing? Let say the method says weigh 80.0 mg can I set a rule that the weight I have should fall between 98 to 102 percent? Thanks.

[tristanewalters]

What environment is your lab in?

I am in the pharmaceutical environment and our methods will usually say "accurately weigh about." In this instance, the USP has guidelines. "Accurately" means using an analytical or microbalance, i.e. weighing with a balance that will give you appropriate accuracy. "About", per the USP, means within +/- 10% of the stated value.

In other instances, the method will say something like "Accurately weigh 80.0mg +/- 5.0 mg."

The window that you mentioned is pretty little. If you can make a rule, I would recommend using +/- 10% or a range of +/- so many mg. This makes it easier for whoever uses it to know "okay, the standard I weighed is within the stated range" without having to do any calculations.

Also, the above is assuming you are correcting for the weight of the standard in your final calculations. If you are not correcting for the weight of your standard (you should be), you need to weigh 80.0 mg.

[danko]

Make sure the amount you’re weighing will result in a signal (peak) which is in the linear range of your method. That range is hopefully documented during the method validation.

Best Regards

[Fernando]

Hi all

Also watch out for the minimum weight of your analitycal balance, see <41> in USP 37 for more details.

Best regards.

Fernando

[Peter Apps]

The balance manufacturers have very good guidance on their web sites. In particular the importance of how many figures the balance reads to and its impact on how many significant figures you have in your weighing result is often underestimated.

Peter