You can't compare the two directly. A "mL" is a milliliter, and is a measure of volume (it is one thousandth of a liter). A "ppm" is a part-per-million, and is a concentration. A ppm is used in exactly the same way as a percent (%) is used. A percent is really part-per-hundred. A ppm is a much lower concentration, but is used in the same way.
Without knowing more information about the problem, there is NO WAY to go from milliliters to ppm. You must have more information to go from volume to concentration.
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