Q:

What happened to Sheldon's assistant, Alex?

In some episodes of Season 6, Sheldon had a young assistant named Alex Jensen. However, she did not appear in Season 7 nor 8. Is there any mention or explanation as to why she did not appear in later episodes?
Did she leave her job, or was she fired by Sheldon? On the other hand, does she still work for him, but we don't see her anymore?
I am looking for facts, not theories, I know that it is likely that she left because of the unmet love for Leonard or the sexual harassment committed by Sheldon, but is this mentioned anywhere in the series or by the creators? Will she reappear in a later episode?

According to Wiki:

Alex's presence was to force Penny to realize that other smart and
  pretty women are also interested in Leonard.

But there is no mention of what happened with her after season 6.

A:

It wasn't mentioned and probably won't be because it's not relevant to the narrative of the show.  She was introduced, served her purpose and was never heard from again (though one could infer that after the 'sexual harassment' episode that she up and quit).  As for will she return, probably not.  She was a plot device, it played out, and now she is no longer needed

