Chromatography Forum

Hello guys, I am working as a QC chemist, sometimes do a bit development work. I was wondering has any of you feel stressed out when you need to weigh out material to make standard? I think I enjoy most part of the work except the balance part, which makes me feel anxious.

Any idea about how to wight out 1~3 mg accurate?

It all depends on the quality of your balance. Using a "normal" analytical balance, e.g. Mettler Toledo XSR205DU you should generally avoid weights below 20 mg. Check it with reference weights daily and have it checked on a regular basis by a certified service provider.
If you really need to weigh 1 mg, you need a micro balance which might be more expensive than ordering larger amounts of your standards for many years.

I always found it easier to weigh out 10mg-100mg into a larger volume of solvent to make a standard, or into smaller volumes of solvent and then do a secondary dilution. Without at least a 5 place balance and probably having it in a glovebox I wouldn't try for 1mg.

If you weigh out 11mg instead of 10mg, you just account for that in a secondary dilution to bring it to the proper concentration instead of trying for exactly 10mg.

Accurately weighing 1 mg needs a microbalance - a balance that reads to 1 microgram. A microblance needs to be on a proper weighing table in a draught=proof, thermostatted room. Better to weigh bigger quantities and dilute them more.

Peter

If you do some propagation of error calculations and use standard deviations that reflect the nominal error in a volumetric flask and for the 4-place analytical balance, you will find that what was indicated above is true. 0.0100 g in a 25 mL volumetric flask will give you about 5% error in the final calculated concentration of the standard.

If you change the mass to 0.025 g, keeping the same standard deviations for the flask and the balance, the error drops to 2% (within the error of a good analytical measurement).

1 mg on a 4-place balance is about 50% error (I'm taking the standard deviation for the mass measured as +/- 0.0005 g).

As you might expect, the error in the actual dilution volume has much less of an effect on the error of the final concentration than does the mass of analyte.