Since it is fully evaporated, isn't it as simple as
1,000 µg/mL x 0.050 mL = 50 µg
in the vial? What are the remaining details of your method? Are you pulling all of those vaporized molecules through your ITEX device and trapping them on a sorbent? I'm not an authority on the ITEX device/technology but it looks like a variation on the theme of thermal desorption.
I'd say your calibration curve is likely going to be some plot of mass of analyte correlated to instrument response. Once you know the mass of your analyte in the unknowns, you'll calculate the concentration in the sample from the amount of sample you vaporized in the first place.