Natural ALA Vs Regular ALA

I'm Curious, does anyone know of any research that indicates which is better for chelation? I've learned there are 2 types of ALA, R-Alpha Lipoic Acid, which is the natural form in the body and S-Alpha Lipoic Acid which is an artificial analog.

Most supplements have both. Some supplements are advertising they only have R-Alpha Lipoic Acid.

Any info would be appreciated. I've been chelating with regular ALA from Source Naturals, which probably has both and it seems to work. But I don't like to use anything artifical, so I was thinking of switching to pure R-Alpha Lipoic Acid.

cheers,
Mike