[gordybird]

I am not sure if this is the right place to post this question but it seams like it.
Anyway I am adding insulin to media that I grow cells in and in the literature 5ug/ml is commonly quoted however many papers quote the concentration as 20nM.  I was wanting to work out if 5ug/ml is actually just 20nM. However I am a bit unsure of the calculations to do this.

My working was:
5ug/ml of insulin would give 5mg (or 5000ug) in 1 litre (i.e. 1000*5).
I then worked out no of moles = 5mg/5733.49g (formula mass of insulin) which gives 8.72x10-7moles.
Then I dived by the volume which I assume is 1 as I worked out the total mass in 1 litre? (8.72x10-7/1) therefore giving 8.72x10-7M or 872nM.

This to me seams inccorrect as this is massively above the 20nM quoted in many papers and I think it seams unlikely there would be this big a difference.

So to get to the point finally is my working correct? any help would be massively appreciated.