Traps, Mistakes, and Errors - Field Sampling: Principles and Practices in Environmental Analysis - page 286

to be present. The analytical laboratory can change analytical and extraction procedures
to take into account different retention times or other confusing interactions with
analytical methods so that an accurate measurement of the analyte can be made.
These types of method adjustments cannot be made without the laboratory having a
complete history of the field as well as the nature and levels of contamination present.
However, in some cases, well-meaning but sometimes not well thought out actions of the
persons submitting a sample will result in interferences. A good example of this is a field
that was used as a wood treatment facility and was heavily contaminated with both
organic and inorganic contaminants. Soil cleanup was being done by arc furnace, which
reduced the soil to ash. The ash was analyzed for such compounds as benzene,
naphthalene, polyaromatic hydrocarbons (PAHs), and pyrenes to ascertain if they were
within regulatory limits. Purge and trap was used for benzene, the other components were
analyzed by extraction, and initially none of these compounds was found in the ash.
After 2 months and hundreds of samples benzene started to be observed in samples.
However, its recovery was poor and the observed concentrations inconsistent, varying
from sub-ppb to 50-150 ppb levels. Little change happened during the following 2 weeks
of checking and further testing except that the recovery levels got worse and more erratic.
During this time the client asserted that no change had been made in its sampling
procedures and so the problem had to be in the laboratory procedures. After another 2
weeks the client admitted that it had started adding lime to the samples before
incineration.
It was determined that the poor recoveries and inconsistent analytical results were
attributable to this lime addition. During incineration lime was reduced and when mixed
with the ash produced cement. Thus when water was added to the samples during
extraction the contents were solidifying and this interfered with the purge and trap
process. This explained the erratic results, but not the occurrence of benzene in the
sample because it should have been destroyed during incineration. At this point the client
again said that nothing had changed in its procedures that would account for the
appearance of benzene.
It turned out that the client had begun using water to cool the ash when it exited the
furnace. The cooling water was being taken from an open pit at the field, and was
contaminated with various chemicals leached from the contaminated field's soil and into
the water in the pit. Benzene was the most mobile of these chemicals and so was the first
to show up during analysis. Water in the pit was found to contain benzene in excess of
several hundred ppb. The good intention of cooling the waste ash to speed the process
resulted in several hundred tons of ash and soil being passed through the furnace, again at
considerable expense.
This illustrates the incompatibility of procedures being used at the sampling field with
those being used in the analytical laboratory. Note here that a detailed description in a
project notebook of procedures being carried out in the field would have greatly
facilitated finding the source of the problem and correcting it and would have saved
considerable time and money.