ok, finally the curves look awful :)

Please write which set do these STD's concentration belong to

There seems to be some systematic error - intsrcept (that's why I ask all the time about blank runs)

There also seems to be some errors that make your curve not linear - this can be weighting/diluting etc.

First thing to do is to find out the source of your systematic error, eliminate it, and than proceed

Also I see something with your instrument - it gives quite different injections from time to time