First off, did you wash your hands?
What did you do during that half hour, before masturbation?
I don't know if i'm allowed to quote other websites... (if i'm not allowed, someone let me know, and I will delete this post right away!!!)
i got this from plannedparenthood.Com:
"how long can sperm survive outside of the human body?
sperm in the cervical mucus of a woman's vagina can survive for up to seven days. Without the cervical mucus to both nourish and protect the sperm, the sperm will die within one-half to four hours. Anyone who has touched semen should wash and dry or at least wipe off before touching the vulva or vagina. "
so i'm guessing from that, that the sperm can live up to four hours.
I looked around a bit more on other less reliable websites that i'm not going to quote, and from that, I have gathered that when semen dries, the sperm dies. Was there still moist semen (pre-cum) on your hands?
The precum should have been way dried up 30 minutes later. And im sure you would have wiped it off or touched something else right? So it couldnt have survived all that traveling to get in you. Even though precum contains sperm, I dont think you have a high chance of getting pregnant.