ppb - parts-per-billion. Refer to ug/L or ug/Kg or ng/g or ng/mL or pg/uL.
ppm - parts-per-million. Refer to mg/L or mg/Kg or mg/mL or ng/uL.
µg/L - micrograms-per-liter A unit of measure equivalent to ppb.
mg/kg - milligrams-per-kilogram A unit of measure equivalent to ppm.
mg/L - milligrams-per-liter A unit of measure used with liquid samples. 1 mg/L is equivalent to 1 ppm.