If you have 1 million parts of something 10 parts would be 10 parts per million. If you had 2 million parts of something 20 parts would be 10 parts per million.

parts per million = ppm

There are one million milligrams in one liter. 4.8 milligrams is 4.8 parts per million (parts per million) There are one million milligrams in one liter. 4.8 milligrams is 4.8 parts per million (parts per million)

Nickel is an element. Therefore, in any nickel item it is 1 billion parts per 1 billion or 1 million parts per million.