PPB to µg/kg Converter
Common Conversions
| ppb | µg/kg |
|---|---|
| 0.1 | 0.1 |
| 1 | 1 |
| 5 | 5 |
| 10 | 10 |
| 50 | 50 |
| 100 | 100 |
| 500 | 500 |
| 1000 | 1000 |
| 5000 | 5000 |
| 10000 | 10000 |
| 100000 | 100000 |
| 1000000 | 1000000 |
Why this conversion matters in chemistry
Soil residential risk assessment crosses this identity. A 50 ppb arsenic residential soil RBSL on an EPA risk-assessment output is 50 µg/kg on the analytical-laboratory ICP-MS data — the same concentration in different notations. The identity is the usual type cast at the boundary between regulatory threshold documentation and instrumental measurement output. The same equality holds for any trace-contaminant mass ratio calculation in soil, sediment, or biological-tissue matrices.
Formula
Where the factor comes from
This one resolves to a factor of one, and the prefix algebra shows why without appeal to any measurement. The SI prefixes are exact by definition: micro at 10⁻⁶ over kilo at 10³ puts a microgram per kilogram at one part in 10⁹. A mass-basis ppb is that same fraction, by definition. The gram cancels, no molar mass appears, and no density is ever consulted. What the identity does require is that the ppb was built on mass to begin with. The three letters carry no denominator of their own, and at trace levels the mole reading is common: an ambient air figure in ppb is a mole fraction of gas, with no µg/kg equivalent at all until the analyte is named and the air is weighed.
Precision and significant figures
Nothing is multiplied, so nothing about the number's quality changes — 4 ppb is 4 µg/kg, and 4.0 µg/kg claims a figure the relabel did not supply. The real constraints sit in the sample. At ppb in a solid, subsampling variance usually dominates: a 0.5 g aliquot taken from a kilogram of sediment or tissue is a narrow window on a heterogeneous object, and two of them can land a long way apart with the instrument behaving perfectly. Blanks matter here too, since a few nanograms of carryover is a measurable fraction of a ppb-level result. Two significant figures is generally the most a trace solid supports.
Worked Examples
The conversion anchor — same ratio in different prefix combinations.
100 µg/kg — about a typical mid-range trace-contaminant level.
1 mg/kg = 1 ppm — the bridge step between trace and bulk regimes.
Sub-ppb — about the limit of routine ICP-MS quantitation.
Common mistakes
µg/kg read as µg/g
µg/g is ppm; µg/kg is ppb. One character separates results a thousandfold apart, and both turn up on the same report whenever major and trace analytes are tabulated together. Check the header rather than trusting the magnitude to look wrong — an unusual sample is exactly the one where a plausible-looking number is off by a thousand.
Applied to a gas-phase ppb
Ambient and stack ppb figures are mole fractions of gas, not mass fractions. The one-to-one identity holds on a mass basis only, so relabeling an air ppb as µg/kg yields a number with no physical referent. Find out what the ppb was divided by before treating the two notations as interchangeable, since a bare ppb declares nothing about its denominator.
Wet weight and lipid-normalized results mixed
Tissue burdens for bioaccumulative organics get reported wet weight, dry weight, or normalized to lipid content. At 5% lipid the lipid-normalized figure is twenty times the wet-weight one for the same fish. Renaming the unit carries the original basis through untouched and settles nothing between them, so put both reports on one basis before their numbers mean anything together.