Environmental Engineering Reference
In-Depth Information
through the container walls, which is a common problem with plastic containers. Loss
might also be through septa in the tops of bottles. To assure that these types of losses do
not take place, both positive and negative control samples can be prepared in the field at
the time of sampling. If transportation and storage are of particular concern or are
particularly challenging, positive and negative standards or controls can be processed
through transportation and storage before actual samples (called transport controls). In
this way it is possible to determine if there are critical steps that need to be changed or
time constraints that need to be applied to ensure sample validity.
Sometimes samples will come under rigorous conditions during transport and storage.
Rigorous conditions are most often changes in pressure and temperature. Samples taken
at high altitude or in deep water and analyzed at low altitude or on the water's surface
will undergo significant pressure changes. Temperature changes cause pressure changes
in the sample container. If the sample contains volatile components or volatile organic
compounds (VOCs), component vapor pressures will change with temperature, and
appreciable pressure can build up when the sample is obtained under cold conditions and
is subsequently warmed to room temperature. Warm samples will lose components when
they are opened and the pressure is released. Likewise, a sample obtained at warm
temperatures and stored at cold temperatures can have considerable vacuum buildup in
the container. Opening the container while it is cold can result in contamination by
material moving into the container. Desert temperatures well over 40°C and arctic
temperatures well below 0°C can be encountered while sampling. Samples taken under
either of these extremes will undergo significant pressure changes when brought to room
temperature.
The third characteristic of a container is that it must not allow contamination of its
surroundings during transportation. This means that the outside of the container is clean
and that as discussed above, no part of the sample should be able to exit the container. A
common loss would be via breakage, which can be controlled by having sufficient and
proper packing material in the shipping container.
8.3. CONTROLS
To make sure that no changes occur during transport, storage controls are used. Common
types of controls are listed in Table 8.2. One type would simply be a known; that is, a
sample with a known concentration of the component of interest is transported and stored
with the test samples. Any change, loss, or gain would indicate a potential problem with
the transport
TABLE 8.2 Types of Sampling Controls
Name
Characteristics
Known samples A sample of known component concentration of interest is included with
samples collected in the field. Inclusion must be done in the field.
 
Search WWH ::




Custom Search