CalibrationStandards
Analytical standards of the 14 explosives-related analytes listed in Method 8330, as well as nitroglycerine and
PETN, were purchased either as a mixed stock standard (each analyte at 1.00 mg/mL) or individually from Ac-
cuStandard, Inc. (NewHaven, CT). These stock standards were specially prepared using acetone as the solvent. The
preparation and handlingofmixed-analyteworking standardswas reported elsewhere (Hewitt and Jenkins 1999).
SamplePreparation
To screen surfaces of range scrap, either the entire piece was submersed in acetone or the exposed surface was
wiped (rubbed) with an acetone-moistened cotton swab held with metal tweezers (Hewitt 2001). To estimate the
surface concentration, the surface area of the piece submersed or swabbed should bemeasured. Moreover, the swab
should be air-dried prior to extractionwith 5mL (or more) of acetone. For qualitative information, an area estimate
is not necessary, and if a cotton swab is used, it can be placed directly into the barrel of a 5-mL disposable plastic
syringe, followed by1mLof acetone.
Soil samples were prepared by extracting 1.0 to 40 g of field-moist soil with a one-to-five-fold greater volume of
acetone (i.e., 1:1 to 1:5). Extractions were performed in either glass or plastic bottles bymanually shaking the soil /
solvent slurry several times for 30 seconds over a 30-minute period.
Following extraction, an aliquot of the acetone was passed through a 25-mmMillex FH (0.45-µm) filter that was
attached, via a Luer-Lok
TM
fitting, to a disposable 3-mL plastic syringe. The filtered extract was directly transferred
to a 2-mL amber deactivated glass vial.
Colorimetric screeningwas performed by transferring a 5-
µ
L aliquot of solvent extract to a test sheet. Several (6 to
12) sample extracts canbe screened simultaneouslybypre-marking the test paper and carefullyplacing each aliquot.
After allowing the acetone to evaporate, the surface of the test sheet was sprayed per kit instructions. If color ap-
pears following application of the first aerosol, then polynitroaromatics (e.g. TNT, TNB, DNT, picric acid, tetryl,
etc.) were likely present. Some of the colors that may appear upon the application of this first aerosol are blue, red,
or orange. A bluish color appears when 2,4-DNT or 2,6-DNT is the dominant compound, a reddish-brown color
appears for TNT andTNB, and an orange color for tetryl and picric acid. After application of spray from the second
aerosol can, the formation of a pink color indicated the presence of nitramines or nitrate esters (e.g., RDX, HMX,
NG, PETN, NC, NQ, and/or tetryl). Application of the first two aerosol cans allowed for the sequential detection of
both polynitroaromatic and nitramines. If there was no color development, then the sample was sprayed with the
third aerosol can. The development of a pink color after applying the third aerosol indicated the presence of an inor-
ganic nitrate (ammonium, potassium, sodium, barium, strontiumnitrate, or blackpowder).
Visual and InstrumentCalibration
A visual scale for the colorimetric screening test was prepared by spraying (see
above) 5-
µ
L aliquots of 10-, 100-, and 1000-mg/L standards of TNT andRDX after
they had been placed on test sheets. All six aliquots were placed on the same sheet;
however, the TNT standards had to be covered when applying the second aerosol.
This screeningmethod candetect the presence of 0.05
µ
gof explosive analytewhen
concentrated in a discrete location on awhite surface. In general, the color intensity
changes from a very light shade for 0.05
µ
g to a distinct light color for 0.5
µ
g and to
a dark color for 5
µ
g.
For GC-TID, a five-point calibration curve is recommended for each analyte of
concern. This number of standards allows non-linearmodels to be usedwhen neces-
sary. A non-linear model (quadratic through the origin) should be chosen when the
linear regression through the origin fails to establish a correlation coefficient (r) of
greater than 0.990. Calibration checks should be made after every five samples by
randomly running one of the four highest working standards. When the calibration
model fails to establish a concentration within +20% of the expected value for a
standard, re-calibration should be performed. The concentrations of the working
standards ranged anywhere from 0.01 to 50
µ
g/L, depending on the analysis objec-
tives. Table 1 shows MDLs obtained for spiked Ottawa sand. Typical chroma-
tograms are available elsewhere (Hewitt et al., 2001).
Table 1.Methoddetection
limits (MDLs) based on
matrix (Ottawa sand)
spike samples.
Compound MDL (mg/kg)
NG
0.10
1,3-DNB
0.012
2,6-DNT
0.0054
2,4-DNT
0.0016
TNB
0.0024
TNT
0.0016
RDX
0.0094
4AmDNT
0.010
2AmDNT
0.0068
Tetryl
0.0017
HMX
0.027
Column: DB-1, 15m, 0.5-
µ
m film.
