Removing Vinyl floor tile adhesive (TAR) from ceramic floor tilling

Hi All
We are planning on tiling on kitchen floor.
So far using a hot air gun and scraper we have removed the hard vinyl tiles and also most of the black adhesive (TAR) that they are held on with.
We are left with a think film on the black tar on the concrete floor. Can I just tile on top of this or do I have to remove this film?
If I have to remove it, what is the best way?
I have searched through the google archives but I cant find a definate answer.
Cheers all
Ben
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