[Immunology] Converting pmol/L to pg/mL?

[david eyles]

Ian:
you wrote:
moles = (mass in grams)/MW.
pmol = pg/MW, so pg = pmol x MW.
MW is about 21,000 daltons (unglycosylated), so 1 pmol/L = 21,000 pg/L =
21 pg/mL.
So, multiply the number of pmol/L by 21 and you have pg/mL.


but, 
if pg = pmol x MW, 
Why is this not:   pg = pmol x 21000

pg = 21000pmol
pg/L = 21,000pmol/L = 21pmol/ml

David