Colorimetric methods

Analysis time

Matrix interferences

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and nitrocellulose turn pink. An orange color in-

from other nitroaromatics because cleanup steps

dicates that both TNT and RDX are present. An-

(solid phase extraction and liquidliquid transfer)

other advantage of the broad response of some

are used.

colorimetric methods is they may be used to de-

When several polynitroaromatic analytes are

tect compounds other than the primary target

present in soil, the EnSys and CRREL colorimetric

analyte. For example, the colorimetric RDX meth-

field results sum the analytes that respond to that

ods may be used to screen for HMX when RDX

test. For example, if a soil sample (as analyzed by

levels are relatively low, and for NQ, NC, NG, and

Method 8330) contains TNT, TNB, RDX, HMX, and

PETN in the absence of RDX and HMX. The

tetryl, the concentration estimate from the CRREL

USACE and ENVIROL colorimetric procedures

and EnSys RISc colorimetric methods for TNT

are more specific to TNT than the CRREL and

would sum contributions from TNT, TNB, and

EnSys RISc colorimetric methods, but have not

tetryl, and the RDX test kit would sum contribu-

been as thoroughly evaluated. If a secondary tar-

tions from RDX and HMX. Because response fac-

get analyte is present at only low concentrations

tors are not identical for each compound, the re-

in a sample, the effect on the analytical result is

sulting concentrations will not quantitatively sum

minimal. If the objective is to determine the con-

the analytes, but will provide an estimate that is

centration of TNT or RDX when relatively high

adequate in light of the substantial spatial hetero-

levels of other nitroaromatics and nitramines are

geneity always encountered for these analytes in

present, immunoassay or the USACE or ENVIROL

soil.

methods may be appropriate.

Extremes of temperature, pH, and soil water

Immunoassay methods

content can interfere with on-site analytical meth-

For TNT kits, the primary target analyte is TNT,

ods. According to the California Military Environ-

and the secondary target analytes are other

mental Coordination Committee, the following

nitroaromatics, such as TNB, DNTs, Am-DNTs,

physical conditions are generally not recom-

and tetryl. For the RDX kit, the primary target

mended for both colorimetric and immunoassay

analyte is RDX, and there is cross-reactivity with

methods: temperatures outside the 4 to 32C range,

HMX (3%). If the primary target analyte is the

pH levels less than 3 or greater than 11, and water

only compound present in soil, the immunoassay

content greater than 30% (CMECC 1996). Specific

methods measure the concentration of that com-

product literature should be consulted for more

pound.

information.

If multiple analytes are present in soil, the im-

munoassay kits measure the primary target

Colorimetric methods

analyte plus some percentage of the cross-reactive

For TNT methods, the primary target analyte

secondary target analytes. The response of immu-

is TNT, and the secondary target analytes are other

noassay kits to the secondary target analytes is not

nitroaromatics, such as TNB, DNB, 2,4-DNT, 2,6-

equivalent to that of the primary target analyte.

DNT, and tetryl. For RDX methods, the primary

Also, the response does not remain constant

target analyte is RDX, and the secondary target

throughout the concentration range of the kits. In

analytes are nitramines (HMX and NQ), and ni-

addition, different immunoassay kits have differ-

trate esters (NC, NG, and PETN). If the primary

ent cross-reactivities to secondary target analytes

target analyte is the only compound present in soil,

based on the antibodies used to develop each

the colorimetric methods measure the concentra-

method. Cross-reactivities for immunoassay kits

tion of that compound. If multiple analytes are

are usually reported at the 50% response level

(IC50), typically the midpoint of the concentration

present in soil, the CRREL and EnSys field meth-

range of the kits. Table 5 shows the reported cross-

ods measure the primary target analyte plus the

reactivities at IC50 for the immunoassay kits. A

secondary target analytes: nitroaromatics for the

complete cross-reactivity curve for the entire con-

TNT test kit, and nitramines plus nitrate esters for

centration range should be obtained from the

the RDX test kits. Also, the response of the CRREL

manufacturers for the immunoassay kits being

and EnSys colorimetric methods to the secondary

considered. Where multiple analytes exist in soil

target analytes is similar to that of the primary tar-

samples, immunoassay results may not directly

get analyte, and remain constant throughout the

compare with EPA Method 8330 results. For ex-

concentration range of the methods, although the

ample, an immunoassay kit may have cross-reac-

observed colors may be different. The ENVIROL

tivities of 23% for TNB and 35% for tetryl for the

method is much less susceptible to interference

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