Recently, I had the opportunity to work with some really high level testing protocol that required equipment that could test to a level on 1 Micron. That's incredibly small. For example, a human hair is about 40 microns wide. So imagine measuring to only 1 micron. We used a Radwag MYA 2/2Y. Here's a link to the manufacturer information:
Here's a link to Nevada Weighing where you can buy this unit in a "package" that provides you all the accessories necessary to insure accuracy - the antivibration table, the anti-static ionizer, the environmental module, the software, etc. - best prices anywhere:
Radwag also have an excellent (and economical ) Semi Micro balance that will weigh as low as 0.01mg (0.00001g). Incredible. Here's the link to the manufacturer technical information:
And here's the link to Nevada Weighing for the best prices on these semi-micro balances:
Incredible stuff for such accuracy at such extreme levels. You can find these items on ebay, or webstore, etc.. Economical yet accurate. Incredible.