how to convert "units"of reagent in weight ?

Alex Brands abbrands at
Wed Jun 6 11:15:00 EST 2001

On 6 Jun 2001, M D wrote:

> Excuse me for this very trivial question, but I have a doubt on my method to
> calculate a concentration using international units (IU). For instance, in a
> paper from Davidson and Cronin on NAP culture media, the composition of
> polymyxin B solution is 0,03 % w/v. But this reagent is sold in vials
> containing 30,000 units per vial. What is the exact way to convert these
> units ?

Does the label not say how many mg are in the vial?

The Sigma catalog sells a 5mg vial of polymyxin B.  They also sell a
1,000,000 unit vial that is listed as "minimum 6,000 USP units per mg"
The exact units per mg are probably printed on the label, and the purity
may also be on the label.

The lack of vital information in materials & methods sections is a pet
peeve of mine.  In one paper I read, the authors thought it necessary to
write that they got their Whatman paper from Whatman, but neglected to
write where they got their enzymes!

The other bugger is when they write "as previously described in Smith,
1995".  In that paper, they write "as previously described in Smith 1992",
then in the '92 paper, "as previously described in Smith 1989" and so on
down the line.


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