safe) conditions. Laboratory safety procedures that call for proper ventilation and
“trial” doses of these pre-treatment chemicals are warranted in case of potential
reactions.
If the sample contains heavy levels of
inorganic particulate, serial dilutions must
be run. Sources of inorganic particulate
include carbon combustion product from
post-fire sites, diatomaceous interferences
from ponds and wells, clay, dendritic or
other residual precipitates. Often, the addition of traditional matrix modifiers (for
example, AgNO3) will aid in precipitating
out unwanted metal particulate. A centrifuge can be used to concentrate select
components of a sample, but this method
may require diligent investigation techniques that warrant study of all resulting
phases. Diminished analytical sensitivities
are, of course, the drawback to having to
run a series of dilutions. Many times, this
cannot be avoided in order to produce a
“readable” final filtration, which is then
prepared for transmission electron microscopy (TEM) analysis.
Other creative approaches may warrant using sieves of various sizes to
separate solids and then capturing the
liquid phase(s) for further preparation.
The solid fractions can be studied and
measured for moisture content, and,
through gravimetric reduction, possible
mechanical, thermal, and chemical separation techniques, prepared and analyzed
as sub-samples for asbestos content by
both polarized light microscopy (PLM)
and TEM. Off-the-shelf methods can then
be employed for these fractions. ASTM
D22.07 is currently working on asbestos
in soil methods that may further standardize each field professional and each
laboratory’s approach to these unusual
samples.
Laboratories often ultrasonicate liquid
samples in order to “free” any asbestos
mineral from binding particles. Careful
documentation with in-house standards is
necessary to ensure that the energy going
in to the sample can be calibrated and
controlled. An unintended false positive-rich environment might be “created”
when bundles or aggregates of asbestos
mineral become disassociated and separated during aggressive sonication. Likewise, loss of analyte may “create” the
unintended consequence of false nega-tive-rich samples due to poor practices of
filtration, dilution, and sample transfer.
Overall, laboratories must document their
practices to achieve maximum sensitivity
and recovery of analyte in standards. This
can be a time-consuming and expensive
practice, but the value added to a client’s
final analytical data can lead to assurances of quality results.
Analysis can proceed by TEM using
USEPA 100.1 or 100.2 protocols. Additionally, modified or proprietary analytical regimens may prove more effective if
all asbestos minerals are sized at high
magnifications and mineral specific grav-ity/density ratios are utilized to calculate
percent by weight for solids and numbers
of structures per volume of liquid.
Analysis
Many labs, including our laboratory, avoid
the seemingly easy final step of drop
mount preparations, which lead to possible
systematic error and potential loss of analyte, in favor of a final filter transfer to
grid step. Final filter preparations are prepared using standard techniques involving
collapsing the filters chemically and etching them in a high-temperature plasma,
coating them in a high-vacuum carbon
evaporator, and transferring portions of
the carbon replica to indexed grids on a
solvent wick.
Now What?
How would the industrial hygienist interpret a final result for just one sample of
total asbestos at 10MFL (million fibers per
liter), asbestos >10µm at 0.1MFL, and the
solid fraction at 0.01 percent asbestos by
weight?
If this were a drinking water sample,
the EPA threshold of 7MFL for structures
>10µm would not have been eclipsed, so
the water could be disposed of through
municipal treatment systems. But this fictitious sample does not closely resemble
