One part per billion concentration notation is which of the following notations?

• A. 1.0 x 10^-11
• B. 1.0 x 10^-7
• C. 1.0 x 10
• D. 1.0 x 10^-9

Answer: (D)

Parts per billion expresses a very small concentration as a ratio of one part in a billion parts. In scientific notation, that is 1 divided by 1,000,000,000, which is 1.0 × 10^-9. So this notation specifically represents one part per billion. The other options correspond to different magnitudes: 1.0 × 10^-7 equals 100 parts per billion, 1.0 × 10^1 equals 10 (a much larger quantity), and 1.0 × 10^-11 equals 0.01 parts per billion, far smaller than a single ppb. Therefore, 1.0 × 10^-9 is the correct way to denote one part per billion.