10
ERDC/CRREL TN-05-2
Table 1. Detectability results.
Faint color
Medium color
Brightest color
Explosive
Tenfold dilution
Hundredfold dilution
Thousandfold dilution
TNT
20 to 100 ppm
100 to 500 ppm
500 to 1000 ppm
RDX
30 to 100 ppm
100 to 500 ppm
500 to 1000 ppm
HMX
--
100 to 500 ppm
500 to 1000 ppm
NG
50 to 100 ppm
100 to 750 ppm
750 to 1000 ppm
The results indicate that there is a dramatic difference in the detectability
between the differing types of paper. The copy paper provided the worst
detectability and it is believed that chemicals introduced in the manufacturing
process may be adversely affecting the reaction. This paper is relatively smooth
and glossy in comparison to the filter papers and therefore may have an effect on
the absorption of the sample, yielding poor results. Experience has shown that on
a few occasions false negatives were reported from field samples blotted on copy
paper, when in fact the concentration of RDX and/or TNT was significantly high
enough that it should have yielded some color indication. Retesting with either
Expray test paper, cellulose, or glass fiber paper produced an indication consis-
tent with the concentration. Copy paper was thus eliminated from the study.
The glass fiber paper had the best detectability, giving a more intense indica-
tion relative to the other papers. This enhanced detectability may be due to the
paper being very absorptive, not allowing the aliquot of standard to spread
laterally, thereby concentrating the sample in a smaller area. However, this paper
is very easily torn, and so does not allow for writing upon in order to identify the
sample. This was a significant drawback and hence this paper was eliminated
from the study. The Expray test paper at the time of study was available only in
the testing coupons (size 6 4.4 cm), and because of this extremely small size it
was eliminated from the study. However, 8.5- 11-inch sheets, which are desir-
able for large-scale blotting, are now available.
The remaining cellulose fiber papers all performed similarly. The cellulose
papers do have some ash content, and the Whatman's has resin with an unknown
quantity of nitrogen. It is felt these additives were not a detriment to the study
and the cellulose filter papers performed as well as the Expray test paper.
Whatman's #1 is familiar and easily obtainable, therefore all further testing was
completed using this paper.
As mentioned earlier, false negatives have been observed, and this we attri-
bute to the testing paper used in the procedure. However, we cannot discount the
possibility of other interferences resulting in poor results, such as the soil matrix
and/or organics masking the colorimetric results. Some false positives have been
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