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ERDC/CRREL TR-02-1
results of the TNT screening, which often reflect the sum of TNT and TNB con-
centrations, correlate well with results obtained in the laboratory with Method
8330.
RDX On-Site Determination
On-site analysis for RDX is similar to the TNT colorimetric method. The
acetone extract is passed through an anion exchange resin to remove any nitrate
and nitrites (this step may be avoided when the site is not suspected of containing
detectable levels of these ions). Zinc and acetic acid are then added to the extract;
this converts the RDX to nitrous acid. Note that the same reaction will occur with
HMX, NG, or PETN because they are all degraded to nitrous acid using this
treatment. The test can therefore be used to estimate if any one of these four
explosives is present, or their sum. The extract is then filtered and placed in a vial
with a Hach Nitriver 3 powder pillow. If the extract develops a pinkish color, it
contains at least one of the analytes. Similarly for the TNT detection procedure,
quantitative analysis of RDX, HMX, PETN, or NG can be obtained from absorb-
ance measurements. The maximum absorbance of the colored reaction end prod-
uct is at 507 nm, so the reading should be done at that wavelength. The results of
the RDX on-site analysis, which often reflects the sum of RDX and HMX, also
correlate well with results obtained in the laboratory.
Advantages and Limitations of Colorimetric Methods
The colorimetric field methods have several advantages. They are rapid (35
min or less per soil sample), use only inexpensive solvents, are very simple to
apply, and have shown a strong correlation with results obtained by Method
8330. These methods have a low incidence of false negative responses and low
detection limits for most analytes (Table H2).
The main limitation of the spectrophotometric colorimetric method for TNT
is that the procedure is subject to positive interference from humic materials
(often a yellow hue), particularly if the requirement to visually detect only a
reddish hue in the extract after base addition isn't followed. Compared to the
immunoassay field screening method, the spectrophotometric colorimetric
method requires more in-field manipulations. However, the spectrophotometric
colorimetric methods produce more precise results and have a larger analytical
range (0200 ppm) as compared to the immunoassay field screening methods. In
addition, the reagents used for the colorimetric methods have a much longer shelf
life and are far less sensitive to temperature. Lastly, because of the larger sample
size (even larger than 20-g samples could be handled, if desirable) for soils,
heterogeneity, especially when dealing with a moist material, is not as large a
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