Basics of Environmental Sampling and Analysis - Fundamentals of Environmental Sampling and Analysis - page 12

contaminants that have low concentrations. Concentration units also vary with the
types of environmental media (air, liquid, or solid) as described below.

Chemicals in Liquid Samples

For chemicals in liquid samples (water, blood, or urine), the mass/volume (m/v) unit
is the most common. Depending on the numerical value, the concentration is
expressed as mg/L,

g/L, or ng/L. For freshwater or liquids with density equal to
1.0 g/mL, the following units are equivalent. The units will not be equivalent if sea
water, and denser or lighter liquids are concerned.

m

1mg = L ¼ 1 ppm 1 m g = L ¼ 1 ppb 1 ng = L ¼ 1 ppt ð2 : 1Þ

Chemicals in Solid Samples

For chemicals in solid samples (soil, sediment, sludge, or biological tissue), the
concentration unit is mass/mass (m/m) rather than mass/volume. Units such as mg/L
or g/L should not be used to express contaminant concentration in solid samples.

The following two sets of units are equivalent:

m

1mg = kg ¼ 1 ppm 1 m g = kg ¼ 1ppb 1ng = kg ¼ 1 ppt ð2 : 2Þ

In reporting such mass/mass units in solid samples, it should specify whether the
mass is on a wet basis or on a dry basis. A dry basis is commonly adopted for
comparison purposes when such samples have a large variation in moisture contents.
A subsample should always be collected for the determination of the moisture
content in addition to concentration measurement. Use the following to convert from
wet basis to dry basis:

mg = kg on dry basis ¼ mg = kg on wet basis =ð1% moistureÞ ð2 : 3Þ

Chemicals in Gaseous Samples
For chemicals in air, both sets of mass/volume (mg/m 3 , g/m 3 , and ng/m 3 ) and

volume/volume (ppm v ,ppb v , ppt v ) are used, but they are not equivalent:

m

m 3 m 3 m 3 1mg = 6¼ 1 ppm 1 m g = 6¼ 1 ppb 1 ng = 6¼ 1 ppt

To convert between these two sets of units at standard temperature and pressure
(25 C, 1 atm), the following formula can be used.

m 3 m 3 mg = ¼ ppmMW = 24 : 5 ppm ¼ðmg = Þð24 : 5 = MWÞ ð2 : 4Þ

Note that at other temperature and pressure conditions, the conversion factor (24.5)
will be slightly different.