Sae-Im Nam August 1997

On-Site Analysis

of Explosives in Soil

Evaluation of Thin-Layer Chromatography

for Confirmation of Analyte Identity

SPECIAL REPORT 97-21
Abstract: Two colorimetric-based methods are commonly used for on-site analysis of explosives in soil.

For the TNT method, acetone soil extracts are reacted

with base to produce reddish-colored Janowsky ions.

For RDX, acetone extracts are acidified and reacted with

zinc to reduce RDX to nitrous acid, and

the nitrous acid is determined by reacting the resulting solution with a Griess reagent. The TNT

method is subject to interference from the presence of other polynitroaromatic compounds such as

TNB, tetryl and the isomers of DNT. Likewise, the

RDX method is interfered with by the presence of

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other nitramines such as HMX and tetryl, and organonitrate esters such as NG, PETN, and NC. This

study investigates the use of thin-layer chromatography (TLC) as a simple on-site method to confirm the

identity of analytes detected using colorimetric on-site

methods. Separations using both laboratory-grade and

locally available solvents were developed. The major limitation of this method is detection capability, which

was estimated to be about 0.1 μg of analyte. This corresponds to a concentration of 17 μg/g when using 30 μL

of spotting volume, or 500 μg/g when using 1 μL of

spotting volume.
Special Report 97-21

Sae-Im Nam August 1997

Prepared for

U.S. ARMY ENVIRONMENTAL CENTER

SFIM-AEC-ET-CR-97030

Approved for public release; distribution is unlimited.

On-Site Analysis

of Explosives in Soil

Evaluation of Thin-Layer Chromatography

for Confirmation of Analyte Identity

US Army Corps

of Engineers®

Cold Regions Research &

Engineering Laboratory
PREFACE

This report was prepared by Dr. Sae-Im Nam, Research Chemist, Geological Sciences Division, Research and Engineering Directorate, U.S. Army Cold Regions

Research and Engineering Laboratory. Funding was provided by the U.S. Army

Environmental Center (AEC), Aberdeen Proving Ground, Maryland, Martin H.

Stutz, Project Monitor.

The author gratefully acknowledges Martin H. Stutz, AEC, and Marianne E. Walsh,

CRREL, for their technical review of this manuscript. In addition the author acknowledges Martin H. Stutz for his suggestion to evaluate this approach and his

useful comments throughout the study. Daniel C. Leggett (CRREL) and Thomas F.

Jenkins (CRREL) are also acknowledged for useful consultation. Jane G. Mason,

CRREL, is acknowledged for her assistance in the preparation of this report.

This publication reflects the personal views of the author and does not suggest or

reflect the policy, practices, programs, or doctrine of the U.S. Army or Government

of the United States. The contents of this report are not to be used for advertising or

promotional purposes. Citation of brand names does not constitute an official

endorsement or approval of the use of such commercial products.

ii
CONTENTS

Page

Preface .......................................................................................................................... ii

Introduction ................................................................................................................. 1

Brief history of TLC ............................................................................................ 2

Objective .............................................................................................................. 3

Materials and methods .............................................................................................. 3

Instrumentation and equipment ...................................................................... 3

Chemicals and reagents ..................................................................................... 3

Preparation of visualizing agents ..................................................................... 3

Preparation of standards and sample extracts ............................................... 4

General thin-layer chromatography procedures ........................................... 4

Results and discussion ............................................................................................... 5

Separation of nitroaromatics using laboratory-grade and

commercial-grade solvents ........................................................................... 5

Separation of nitramines and nitrate esters using laboratory-grade

and commercial-grade solvents .................................................................... 6

Evaluation of TLC plates ................................................................................... 6

Evaluation of HPTLC plates ............................................................................. 7

Evaluation of visualizing agents ...................................................................... 7

Estimation of detection capability .................................................................... 10

Testing of soil samples collected from the field ............................................. 10

Recommendations for specific separations and visualizing reagents ........ 12

Conclusions ................................................................................................................. 12

Literature cited ............................................................................................................ 12

Abstract ........................................................................................................................ 15

TABLES

Table

1. Separation of nitroaromatics with laboratory-grade solvents ......................... 5

2. Development times ................................................................................................ 6

3. Separation of nitroaromatic compounds with commercial-brand

paint thinners and laboratory-grade isopropanol ..................................... 6

4. Separation of nitramines and nitrate esters with laboratory-grade

solvents............................................................................................................. 7

5. Separation of nitramines and nitrate esters with commercialbrand solvents ................................................................................................. 7

6. Comparison of HPTLC plates and TLC plates ................................................... 8

7. Visualization of nitroaromatics, nitramines, and nitrate esters ....................... 9

8. Lowest level of visualization ................................................................................ 11

iii
On-Site Analysis of Explosives in Soil

Evaluation of Thin-Layer Chromatography

for Confirmation of Analyte Identity

SAE-IM NAM

INTRODUCTION

Environmental concerns over explosives contamination in soil have resulted in the determination of the extent of this contamination at numerous Department of Defense installations.

Laboratory analytical methods were developed to

enable the determination of the most commonly

found components of explosives such as 2,4,6-trinitrotoluene (TNT), hexahydro-1,3,5-trinitro-1,3,5-

triazine (RDX), and related impurities and environmental transformation products in the soil

matrix (Jenkins et al. 1989, U.S. EPA 1995). On-site

methods for TNT and RDX, the most commonly

encountered contaminants (Walsh et al. 1993),

were also developed to provide a more expedient

means of rapidly characterizing these sites prior

to extensive laboratory analyses (Jenkins and

Walsh 1992, Teaney and Hudak 1994). Overall, the

use of on-site methods has been successful in providing rapid site characterization at explosivescontaminated sites.

Two of the most commonly used on-site methods for determining the presence of TNT and RDX

in soil are based on research conducted at the U.S.

Army Cold Regions Research and Engineering

Laboratory (CRREL). These methods are based on

the production of colored products when acetone

soil extracts are reacted with the appropriate reagents. In the field screening methods by Jenkins

(1990) and Walsh and Jenkins (1991), TNT and

RDX, respectively, are converted to color-specific

compounds that are quantified spectrophotometrically. Kits containing the associated reagents and

supplies are commercially available from EnSys

Corporation (now Strategic Diagnostics, Inc.,

Newark, Delaware). In the TNT method, acetone

soil extracts are reacted with strong base as shown

in eq 1 to produce reddish-colored Janowsky anions when TNT is present. Reddish-colored anions

are also produced, however, when 1,3,5-

trinitrobenzene (TNB) or N-methyl-N-2,4,6-

tetranitrobenzenamine (tetryl) is present, and a

bluish-colored anion is produced when 2,4-

dinitrotoluene (2,4-DNT) is present (Jenkins and

Walsh 1991). Thus a positive response on the TNT

test does not unequivocally prove that TNT is

present since several other polynitroaromatics can

give a similar response.

For the RDX test, soil extracts are first acidified

with acetic acid and reacted with zinc to reduce

any RDX present to nitrous acid, and the resulting solution is reacted with a Griess reagent to

produce a reddish-colored azo dye (eq 2). Reddishcolored azo dyes are also produced if other

nitramines (such as octahydro-1,3,5,7-tetranitro1,3,5,7-tetrazocine [HMX] or tetryl) or organonitrate esters (such as nitroglycerin [NG],

pentaerythritol tetranitrate [PETN], or nitrocellulose [NC]) are present. In addition, nitrate and nitrite ion, if not removed using an anion exchange

column prior to reaction with zinc, will also respond. The ion exchanger is specified in the

CRREL-developed method, but is not recommended for routine use by EnSys.

For both of these methods, the intensity of the

CH3 — C — CH3

O

CH2 — C — CH3

O

+ B

CH3

NO2

NO2

O2N

CH3

NO2

NO2

O2N

CH2 — C — CH3

O

CH2 — C — CH3

O

+

Equation 1.
color is directly proportional to the concentration

of the analyte of interest, and concentrations are

determined by measuring absorbance at 540 nm

for TNT and at 507 nm for RDX. Method detection limits for TNT and RDX in soil samples using

these methods are 1.1 μg/g and 1.4 μg/g, respectively.

Often the capability of the TNT test to detect

other polynitroaromatics can be quite useful. For

example, in a recent study in Sparks, Nevada, areas of contamination with 2,4-DNT were detected

using this test (Jenkins et al. 1996). Likewise the

capability of the RDX test to determine HMX concentrations was recently demonstrated at an active anti-tank range at Valcartier, Quebec (Jenkins

et al. [in press]). As stated previously, these two

methods are capable of responding to these TNTand RDX-related nitroaromatics and nitramines in

a like manner. This results in the inability to unequivocally identify which specific compounds are

present in many cases without more in-depth laboratory analyses. It is important to be able to discriminate among the various compounds that respond to these tests since cleanup levels for the

various explosives can be set at somewhat different concentrations. Therefore it would be quite

useful if a simple, inexpensive, on-site method

were available to qualitatively determine which

of the potentially detectable analytes are giving

rise to the colored reaction products from either

the TNT or RDX tests.

Thin-layer chromatography (TLC), also known

as planar chromatography, is an inexpensive,

readily fieldable chromatographic method for

separating compounds. As with other chromatographic methods, TLC separations utilize a stationary phase and a mobile phase. The stationary

phase is a solid material coated on a glass or plastic plate and the mobile phase is a solvent(s) of

variable polarity. Separation is achieved as

analytes partition between these phases based on

polarity differences as the solvent rises by capillary action through the coated solid. The major advantages of TLC as an on-site method, relative to

other chromatographic methods, include the ability to rapidly process a number of samples simultaneously, low capital cost of equipment and supplies, and minimal power requirements.

Brief history of TLC

The earliest development of TLC is attributed

to two Russian scientists, Ismailov and Shraiber,

in 1938, when they were able to separate certain

medicinal compounds on unbound alumina

spread on glass sheets (Coker et al. 1993). Their

technique was termed “drop chromatography”

largely due to the fact that drops of solvent were

applied to the plate containing the sample. Then,

around 1949, Meinhard and Hall enhanced the

method by using binder material to adhere the alumina to the glass plate (Sherma and Fried 1996).

However, it was not until 1951 when Kirchner and

his colleagues from the U.S. Department of Agriculture further enhanced the method to resemble

what is now known as thin-layer chromatography.

Kirchner utilized sorbent materials in conjunction

with binders on the plates and developed the

plates in an ascending fashion, which is commonly

used in modern TLC. Then in 1958, Stahl and his

colleagues popularized the technique by providing laboratory manuals, which standardized procedures, materials, and nomenclature associated

with thin-layer chromatography. By the mid 1960s,

instruments such as densitometers were being utilized to quantitatively measure the results of TLC.

For the past 30 years continued improvements

have been made in the field of thin-layer chromatography, including fully automated instruments

that will spot samples, develop plates, analyze the

results, and provide quantitative measurements.

Classical TLC was used as early as the mid-1960s

to determine components of explosives (DiCarlo

et al. 1964, Yasuda 1964, Hoffsommer and

McCullough 1968) and more recently, automated

multiple development high-performance thinlayer chromatography (AMD-HPTLC) has been

used to identify explosives or components of explosives in abiotic matrices such as soil and water

Equation 2.

+ Zn

Acetic Acid

3 HNO2

RDX Franchimont Reaction (1897)

NO2

N

N N

O2N NO2

R

HNO2 +

+

R

NH2 N2

Griess Reaction (1864)

+

+

R

N2 NR'2

Azo Dye

N N NR'2

R

2
and in a biotic medium such as urine (Yucang et

al. 1991, Steuckart et al. 1994). Classical or conventional TLC served to provide more of a qualitative determination of explosives, whereas modern TLC or HPTLC provides both qualitative and

quantitative results. The advantage of modern

TLC is sensitivity, providing very low limits of

detection. However, the initial capital cost of the

necessary instrumentation can be high, and usage

of highly sensitive equipment is not ideal for field

situations unless the equipment is housed in a

mobile laboratory. The conventional TLC methods are not as sensitive, but because of very simple

equipment requirements, they can be used in the

field without significant modifications, at reasonable cost.

Objective

The objective of this work is to evaluate the use

of conventional TLC as an adjunct to current onsite colorimetric methods for TNT, RDX, and related compounds. The most popular colorimetric

methods often cannot readily distinguish among

the various polynitroaromatic compounds that

respond to the TNT test, or the various nitramines

and nitrate esters that respond to the RDX test.

This work evaluates the ability of TLC to separate

TNT and the other common polynitroaromatic

compounds, and RDX and the other commonly

encountered nitramines and nitrate esters in a costeffective and timely manner.

MATERIALS AND METHODS

Instrumentation and equipment

A basic thin-layer chromatography starter kit

was purchased from Alltech Associates, Inc.

(Deerfield, Illinois). The starter kit included the

following: a 20- × 20-cm TLC tank with glass lid,

20- × 20-cm tank liners, microcap (microcapillary)

dispensers (for sample spotting), disposable spray

box, spotting template, reagent spray unit with

glass jar, and 20- × 20-cm Adsorbosil Plus 1 TLC

plates. Additional glass-backed plates consisting

of EM silica gel 60 F254 (20 × 20 cm, 250 μm), EM

silica gel 60 F254 with preconcentration zone (20

× 20 cm, 250 μm), EM silica gel 60 with

preconcentration zone (20 × 20 cm, 250 μm), EM

HPTLC silica gel 60 F254 with preconcentrated and

prechanneled zones (10 × 10 cm, 200 μm), and

Adsorbosil HPTLC with phosphor and

preconcentrated zone (10 × 10 cm, 150 μm) were

purchased from EM Science, Gibbstown, New Jersey. A multiband (254 and 366 nm) UV lamp with

a viewing box was obtained from UVP, Inc., in San

Gabriel, California.

Chemicals and reagents

Analytical standards for TNT, TNB, tetryl, 2,4-

DNT, 4-A-DNT, 2-A-DNT, RDX, HMX, NG, and

PETN were prepared from Standard Analytical

Reference Materials (SARM) obtained from the

U.S. Army Environmental Center, Aberdeen Proving Ground, Maryland.

Visualizing chemicals and reagents consisting

of p-(diethylamino)benzaldehyde (DEAB), p-

(dimethylamino)benzaldehyde (DMAB), titanium

(III) chloride (TiCl3), ethylenediamine (EDA), potassium permanganate (KMnO4), sodium

periodate (NaIO4), diphenylamine (DPA), n-(1-

naphthyl)ethylenediamine dihydrochloride and p-

(dimethylamino) cinnamaldehyde (DMACA), acetic acid, and zinc were purchased from Aldrich

Chemical Co., Milwaukee, Wisconsin. TNT EnSys

developer reagent was obtained from EnSys, Inc.,

Research Triangle Park, North Carolina. Potassium

hydroxide, hydrochloric acid, hexane, acetonitrile,

and sulfanilic acid were purchased from Baker Inc.,

Phillipsburg, New Jersey. Sodium hydroxide, sodium nitrite, petroleum ether, stoddard solvent,

and ethanol were purchased from Fisher

Scientifics, Fair Lawn, New Jersey. Butyl alcohol

and dimethylsulfoxide (DMSO) were obtained

from Mallinckrodt, St. Louis, Missouri. Xylene was

furnished by MCB Reagent, Cincinnati, Ohio, and

sulfanilamide was from Eastman Kodak Co.,

Rochester, New York. Acetone and chloroform

were from EM Science, Gibbstown, New Jersey,

and Burdick & Jackson, Muskegon, Michigan, respectively. Finally, the Hach NitriVer3 powder pillows were from Hach Co., Loveland, Colorado.

Commercial-grade solvents were purchased

from local paint and hardware stores. These solvents consisted of Sunnyspec paint thinner, Sterling Lynsol, Sterling Thin-X (red), Sterling VM &

P naphtha, Savogram deglosser, Parks VM & P

naphtha, Recordsol paint thinner, PPG Industries

Duracryl, Sterling acetone, Ace paint thinner,

Woolworth-brand 70% isopropanol, Sterling solvent alcohol, and 3M general purpose adhesive

cleaner. Mobil gasoline with octane 93 was purchased from a local gas station.

Preparation of visualizing agents

TiCl3 reagent

In a 50-mL volumetric flask, 5 mL of aqueous

TiCl3 solution (10% TiCl3 in 20–23% HCl) is combined with 5 mL of 2 N HCl and the solution is

3
brought to volume with ethanol (Yinon and Zitrin

1981). The solution should be thoroughly mixed

and made fresh prior to use.

EDA : DMSO (1 : 1)

A 25-mL portion of EDA is mixed with 25 mL of

DMSO. It forms a clear, colorless solution

(Hoffsommer and McCullough 1968).

DEAB reagent (0.25% DEAB in 0.25 N HCl

in ethanol)

A 250-mg portion of DEAB is mixed with 100

mL of 0.25 N HCl in ethanol until dissolved

(Yasuda 1964). The resulting solution is yelloworange in color and may be stored in the dark at

4°C for up to three weeks.

DMAB reagent

A 1-g portion of DMAB is mixed with 3 mL of

HCl, 30 mL of ethanol, and 18 mL of butanol. The

resulting solution is light yellow in color and may

be stored in the dark at 4°C for up to three weeks

(Jork et al. 1994b).

Bratton-Marshall reagent

A 0.1-g portion of N-1-naphthylethylenediamine•2 HCl is mixed with 5 mL of 2 N HCl

and 95 mL of butanol (Yinon and Zitrin 1981). The

solution is stirred until dissolved. The resulting

solution is light gray in color.

DMACA reagent (0.25% DMACA in 0.25 N HCl

in ethanol)

A 250-mg portion of DMACA is mixed with 100

mL of 0.25 N HCl in ethanol until dissolved (Jork

et al. 1994b). The resulting solution is yelloworange in color and may be stored in the dark at

4°C for up to three weeks.

NaIO4 and KMnO4 reagent (0.1% NaIO4 and 0.5%

KMnO4 in 4% NaOH)

A 0.1-g portion of NaIO4 is combined with 0.5 g

of KMnO4, and 100 mL of 4% NaOH is added and

mixed until dissolved (Carlson and Thompson

1986).

1% DPA

A 500-mg portion of DPA is dissolved in 50 ml

of ethanol (Bagnato and Grasso 1986).

NaNO2

A 0.5-g portion of sodium nitrite is dissolved in

10 mL of water and the solution is brought to 50

mL with an ethanol and HCl mixture (41.5 mL of

ethanol and 8.5 mL of HCl) (Jork et al. 1994b).

5% KOH

A 5-g portion of KOH is dissolved in 100 mL of

ethanol (Jork et al. 1994a).

Hach NitriVer3 solution

Two packets of prepackaged Hach NitriVer3

powder pillows are dissolved in 40 mL of water

(Walsh and Jenkins 1991).

Griess reagents

Peak’s Griess reagent (Peak 1980). Solution A: A

0.1-g portion of N-1-naphthylenediamine dichloride is dissolved in 100 mL of water.

Solution B: A 0.5-g portion of sulfanilic acid is

dissolved in 14 mL of acetic acid and brought to

100 mL with water. Equal amounts of solutions A

and B are added together immediately before use.

Individual solutions may be stored in the dark at

4°C for up to three weeks.

Higg’s Griess reagent (Higgs and Hayes 1982). Solution A: A 20-mg portion of N-1-naphthylethylenediamine dichloride is dissolved in 100 mL

of 0.1 N HCl.

Solution B: A 200-mg portion of sulfanilic acid

is dissolved in 100 mL of 2 N HCl. Equal volumes

of both solutions are mixed together immediately

before use.

Preparation of standards and sample extracts

Stock standard solutions of TNT (1000 mg/L),

TNB (725 mg/L), tetryl (505 mg/L), 2,4-DNT (1000

mg/L), 2-A-DNT (1000 mg/L), 4-A-DNT (1000

mg/L), RDX (1000 mg/L), HMX (1000 mg/L),

PETN (2000 mg/L), and NG (2000 mg/L) were

prepared by dissolving the dry material in either

HPLC-grade acetone or acetonitrile. The stock solutions were further diluted when smaller concentrations were needed. Soil samples used in this

study were previously obtained from Defence

Research Establishment Valcartier (DREV) Tank

Firing Range in Quebec, Canada; Hawthorne

Ammunition Plant in Nevada; Savanna Army

Depot in Illinois; and Umatilla Army Depot in

Oregon. Soil extracts were prepared as described

by Jenkins (1990) and Walsh and Jenkins (1991).

General thin-layer chromatography procedures

The developing tank was prepared by the addition of the mobile phase (200 mL) and equilibrated

for approximately 30 to 40 minutes or until the

tank liner had been saturated. The TLC plates were

prepared for sample spotting by designating the

4
line of origin (about 2–3 cm above the bottom of

the plate) and the solvent front line (10 cm from

the line of origin). Using capillary micropipettes,

samples were spotted along the line of origin approximately 1 cm apart. Spotting volumes ranged

from 0.5 to 30 μL, but in most cases, samples were

spotted 1 μL at a time. The plates were then placed

in the developing tank (containing the freshly prepared mobile phase) and developed in an ascending manner until the mobile phase had reached

the solvent front line. The plates were removed

from the tank and either air dried or dried with

hot air from a heat gun prior to observation. The

fluorescence-containing plates were observed under the UV lamp (set at 254 nm) and/or sprayed

with visualizing agents. Nonfluorescence-containing plates were sprayed with visualizing agents.

The position of the resulting nonfluorescing spots

(observed under the UV light) or colored spots was

marked and the retention factor (Rf) values were

determined by dividing the distance traveled by

the compound by the distance traveled by the solvent front. When analyzing soil samples, analytes

were identified by comparing the Rf values to the

Rf values of the standards that were spotted on

the same plate.

RESULTS AND DISCUSSION

Separation of nitroaromatics using laboratorygrade and commercial-grade solvents

Numerous mobile phase systems were tested to

determine the best solvent or combination of solvents that would result in a distinguishable separation of nitroaromatic compounds such as TNT,

TNB, DNT, tetryl, and the isomers of amino-DNTs.

The evaluated mobile phase systems using laboratory-grade solvents included hexane : chloroform (4 : 1), chloroform, petroleum ether : acetone

(3 : 1) and (2 : 1), petroleum ether : isopropanol (4

: 1), xylene, and Stoddard solution : isopropanol

(2 : 1) and (1 : 1). These solvents were chosen

according to their eluting strength. The combination of four parts petroleum ether with one part

isopropanol resulted in the best distinguishable separation of nitroaromatic compounds compared to the other evaluated mobile phase systems (Table 1). The solvent system of hexane : chloroform (4 : 1) resulted in no movement of the

compounds. Chloroform, by itself, resulted in

identifiable separation of TNT, TNB, DNT, and

tetryl, but the Rf values of the aminoDNTs were nearly identical. The combination

solvents of petroleum ether and acetone at various ratios also failed to separate the compounds

efficiently. Xylene, by itself, and Stoddard solution

(a clear petroleum distillate) with isopropanol

were fairly effective in separating the

nitroaromatic compounds, but their development times were considerably longer compared

to the other laboratory-grade solvent systems

(Table 2).

Commercial-grade paint thinners and alcohols,

which are readily available in any local hardware

or paint store, were also evaluated for their ability

to separate nitroaromatic compounds. However,

because of the high water content in commercially

available alcohols (i.e., Woolworth-brand 70% isopropanol and Sterling solvent alcohol), all mobile

Table 1. Separation of nitroaromatics with laboratory-grade solvents.

Solvent TNT TNB 2,4-DNT Tetryl 2-A-DNT 4-A-DNT

systems Rf ± S.D. Rf ± S.D. Rf ± S.D. Rf ± S.D. Rf ± S.D. Rf ± S.D.

1 0.52 0.36 0.48 0.20 0.16 0.14

2 0.42 0.40 0.40 0.27 0.22 0.23

3 0.47 0.46 0.45 0.33 0.30 0.30

4 0.64 ± 0.02 0.58 ± 0.02 0.52 ± 0.02 0.37 ± 0.02 0.23 ± 0.02 0.27 ± 0.02

5 0.75 0.71 0.63 0.62 0.55 0.55

6 0.74 0.72 0.65 0.67 0.62 0.62

7 0.54 0.50 0.47 0.28 0.1

Solvent system key:

1—Chloroform

2—Petroleum ether : acetone (3 : 1)

3—Petroleum ether : acetone (2 : 1)

4—Petroleum ether : isopropanol (4 : 1), n = 3

5—Stoddard solution : isopropanol (2 : 1)

6—Stoddard solution : isopropanol (1 : 1)

7—Xylene

5
phases prepared with these alcohols resulted in

very little or no movement of the nitroaromatic

compounds. However, when commercially available paint thinners were mixed with laboratorygrade isopropanol (4 : 1), compounds such as TNT,

DNT, TNB, and tetryl could be distinguished from

each other (Table 3). Commercial brands such as

Recorder paint thinner, Sterling VM & P naphtha,

Sterling Thin-X paint thinner, and Ace paint thinner were among the tested brands that have resulted in good-to-fair separation of TNT, DNT,

TNB, and tetryl. However, in most cases, the development time took longer than when the mobile phase was composed solely of laboratorygrade solvents (Table 2).

Separation of nitramines and nitrate esters

using laboratory-grade and commercial-grade

solvents

The solvent system of petroleum ether and acetone (1 : 1) was found to be very effective in separating the nitramines, such as RDX and HMX, and

to produce a fair separation of the nitrate esters,

PETN and NG (Table 4). RDX, PETN, and NG were

also effectively separated with petroleum ether :

isopropanol (4 : 1), but HMX failed to move from

the line of origin.

Mobile-phase solvents consisting of commercialbrand acetone and some paint thinners (1 : 1) were

also effective in producing a good separation of

RDX and HMX (Table 5). However, the separation

of PETN and NG was not as clear, thus resulting

in very similar Rf values.

Evaluation of TLC plates

TLC plates used in this study were all glass

plates precoated with silica gel. The differing features of these plates included different commercial brands, fluorescence vs. nonfluorescence, and

preconcentration zone vs. no preconcentration

zone. The preconcentration area, located on the

bottom of the plate, is made up of inert material

that is meant to absorb sample solvent. The two

brands of TLC plates, EM and Adsorbosil, did not

show any significant differences regarding separation of analytes. When using the same solvent

system and visualizing procedures, both plates

were nearly identical in results with slight color

Table 2. Development times.

Solvent system Time (min.)

Stoddard solution : isopropanol (2 : 1) and (1 : 1) 50

Chloroform 10

Petroleum ether : acetone (3 : 1) and (2 : 1) 16

Petroleum ether : isopropanol (4 : 1) 17

Xylene 25

3M adhesive cleaner : isopropanol (4 : 1) 24

Duracryl : isopropanol (4 : 1) 26

Deglosser : isopropanol (4 : 1) 44

Recorder paint thinner : isopropanol (4 : 1) 46

Parks VM & P naphtha : isopropanol (4 : 1) 30

Sterling VM & P naphtha : isopropanol (4 : 1) 23

Sterling Thin-X paint thinner : isopropanol (4 : 1) 50

Ace paint thinner : isopropanol (4 : 1) 45

Table 3. Separation of nitroaromatic compounds with commercial-brand paint thinners and

laboratory-grade isopropanol (4 : 1).

Solvent TNT TNB 2,4-DNT Tetryl 2-A-DNT 4-A-DNT

systems Rf ± S.D. Rf ± S.D. Rf ± S.D. Rf ± S.D. Rf ± S.D. Rf ± S.D.

1 0.78 ± 0.04 0.76 ± 0.04 0.65 0.68 ± 0.05 0.54 ± 0.01 0.56 ± 0.01

2 0.88 ± 0.02 0.88 ± 0.01 0.86 ± 0.01 0.85 ± 0 0.83 ± 0.01 0.84 ± 0.01

3 0.74 ± 0.01 0.69 ± 0.02 0.71 0.65 ± 0.04 0.59 ± 0.04 0.61 ± 0.04

4 0.70 ± 0.07 0.67 ± 0.05 0.61 ± 0.04 0.52 ± 0.04 0.39 ± 0.01 0.43 ± 0.04

5 0.72 ± 0.02 0.68 ± 0 0.63 ± 0.01 0.57 ± 0.02 0.40 ± 0.01 0.43 ± 0

6 0.65 ± 0.00 0.60 ± 0 0.55 ± 0 0.48 0.35 0.40

7 0.74 ± 0.01 0.68 ± 0 0.64 ± 0.01 0.53 ± 0 0.42 ± 0.02 0.44 ± 0

8 0.79 ± 0.01 0.76 ± 0.02 0.66 ± 0.01 0.62 ± 0.02 0.45 ± 0.01 0.49 ± 0.02

Solvent system key:

1—3M adhesive cleaner : isopropanol, n = 2

2—Duracryl : isopropanol, n = 2

3—Deglosser : isopropanol, n = 2

4—Recorder paint thinner : isopropanol, n = 2

5—Parks VM & P naphtha : isopropanol, n = 2

6—Sterling VM & P naphtha : isopropanol, n = 2

7—Sterling Thin-X paint thinner : isopropanol, n = 2

8—Ace paint thinner : isopropanol, n = 3

6
intensity variations. The fluorescent and

nonfluorescent plates were found to produce identical separations. However, the plates having

preconcentration zones did give better analyte

resolution, compared to plates having no

preconcentration zones, when the spotting volume

exceeded 5 μL. This result was in agreement with

Rabel and Palmer (1992) and Hauck and Mack

(1990), in which they report enhanced resolution,

reproducibility, and recovery of analytes spotted

on preconcentration zones.

Evaluation of HPTLC plates

According to Fenimore and Davis (1981), when

high-performance thin layer chromatography

(HPTLC) plates are used in conjunction with modern scanning equipment, the limit of detection can

be similar to those obtained by high-performance

liquid chromatography (HPLC). HPTLC plates,

like conventional TLC plates, are usually coated

with various binders to hold sorbent material together. However, the dimensions of HPTLC plates

are approximately half the size of conventional

plates. The particle sizes of the sorbent material

are much smaller and the size distribution of these

particles is much tighter. HPTLC plates are also

thinner and the surface is more uniform than conventional plates. These differences often can result in use of smaller sample volume, smaller solvent volume for the mobile phase, shorter solvent

migration distance, and greater sensitivity for the

detection of separated compounds.

HPTLC plates were evaluated here to determine

if the separation and resolution of compounds

were better relative to standard TLC plates when

using conventional techniques. Two different

brands of HPTLC plates were evaluated (EM and

Adsorbosil). Both brands had preconcentration

zones, with the EM plates also having channeled

zones while the Adsorbosil did not. The compounds were spotted along the preconcentration

zone using microcapillary dispensers. Because of

the size of the plates (10 × 10 cm) and thinner thickness (150–200 μm), the developing time was usually between 10 and 15 minutes, half the development time of standard TLC plates. The HPTLC

plates also required less mobile phase volume

compared to standard TLC plates. However, when

the compounds were visualized with UV light or

with visualizing agents, the separation and resolution of compounds, including nitroaromatics,

nitramines, and nitrate esters, were similar to the

standard TLC plates (Table 6). The two brands of

HPTLC plates behaved similarly and results

showed no difference in separation and resolution

of compounds between plates having channeled

zones and no channeled zones.

Evaluation of visualizing agents

Numerous visualizing agents as well as UV light

were evaluated for their effectiveness in visualizing components of explosives. In most cases, the

evaluated visualizing agents were chosen in accordance with the literature. The summary of the

Table 4. Separation of nitramines and nitrate esters with laboratory-grade solvents.

RDX HMX PETN NG

Solvent systems n Rf ± S.D. Rf ± S.D. Rf ± S.D. Rf ± S.D.

Petroleum ether : acetone (1 : 1) 3 0.72 ± 0.01 0.62 ± 0.01 0.91 ± 0.03 0.88 ± 0.02

Petroleum ether : isopropanol (4 : 1) 3 0.30 ± 0.02 No movement 0.84 ± 0.03 0.79 ± 0.02

Table 5. Separation of nitramines and nitrate esters with commercial-brand solvents.

RDX HMX PETN NG

Solvent systems Rf ± S.D. Rf ± S.D. Rf ± S.D. Rf ± S.D.

3M adhesive cleaner : Sterling acetone (1 : 1)* 0.58 ± 0.04 0.53 ± 0.04 0.80 ± 0.05 0.78

Parks VM & P naphtha: Sterling acetone (1 : 1)* 0.54 ± 0.01 0.45 ± 0.03 0.73 ± 0.05 0.69 ± 0.06

Sterling VM & P naphtha: Sterling acetone (1 : 1)* 0.53 ± 0.01 0.42 ± 0.02 0.65 ± 0.06 0.63 ± 0.04

Sterling Thin-X : Sterling acetone (1 : 1)** 0.53 ± 0.03 0.43 ± 0.08 0.59 ± 0.03 0.60 ± 0.03

Ace paint thinner : Sterling acetone (1 : 1)* 0.48 ± 0.07 0.36 ± 0.09 0.61 ± 0.04 0.59 ± 0.05

*n = 2

**n = 3

7
results is shown in Table 7. In most cases, the evaluated visualizing agents worked well in visualizing nitroaromatic compounds, but were fairly limited in visualizing nitramines and nitrate esters.

UV light and EnSys TNT developer

The simplest method for visualizing nitroaromatics and nitramines was viewing the developed

plate under shortwave (254 nm) UV light (Glover

and Hoffsommer 1973, McCormick et al. 1978,

Malotky and Downes 1983, and Zou et al. 1994).

We found the fluorescence-containing plates to

have bright green backgrounds with light-to-dark

spots representing nitroaromatic and nitramine

compounds. Following the UV viewing, nitroaromatics could be further distinguished by placing

approximately 1 μL of the EnSys TNT developer

on the dark spots. The EnSys TNT developer reacts with the nitroaromatic compounds to form

Meisenheimer complexes and results in the following color formations: purple for TNT, orange

for tetryl, light yellow for 4-A-DNT, orange for

TNB, light yellow for 2-A-DNT, and light green

for 2,4-DNT. The intensity of the color varied depending on the concentration of the compound.

TiCl3 and DEAB, TiCl3 and DMAB, and TiCl3 and

DMACA

These three visualizing agents worked well in

visualizing nitroaromatic compounds. They are

grouped together due to similar reaction principle

(Yasuda 1964, 1970, Yinon and Zitrin 1981, and Jork

et al. 1994b). The developed plates are initially

sprayed with the TiCl3 reagent, which reduces the

nitroaromatic compounds to amines. When the

plate is dried, it is sprayed with either DEAB,

DMAB, or DMACA, which react with the amines

to form Schiff bases. Compounds sprayed with

DEAB resulted in yellow-colored products,

DMAB-sprayed compounds were orange-yellow

in color, and compounds sprayed with DMACA

resulted in purple colors. The color developments

were immediate after the application of DEAB,

DMAB, or DMACA and were best viewed when

the plate was still wet.

EDA : DMSO

The solution of EDA : DMSO (1 : 1) reacts with

nitroaromatic compounds to form Meisenheimer

complexes. The results we obtained were similar

to Hoffsommer and McCullough (1968). Upon

spraying the plate with EDA : DMSO solution, the

resulting colors were purple for TNT, orange for

tetryl, red-orange for TNB, faint yellow for 2-ADNT, and light green for 2,4-DNT (while the plate

was still wet).

Alkaline solution (5% KOH, 1 N NaOH, 0.1 N NaOH)

and Griess reagent, 5% KOH and Bratton-Marshall

reagent, and 5% KOH and Hach NitriVer3

These visualizing agents behave similarly in that

their reactions all follow a similar principle. The

alkaline solution is initially used to reduce the

compounds to form nitrite ions. These ions are

then converted to azo compounds in the acidic

medium and coupled to naphthalene derivatives

(which are found in the Griess reagent, the BrattonMarshall reagent, and the Hach NitriVer3) to form

the characteristic colors. Slight variations in the

formulation of the Griess reagent resulted in different color formation (Table 7). According to the

literature, nitrate esters and nitramines reacting

Table 6. Comparison of HPTLC plates and TLC plates.

Petroleum ether : isopropanol Petroleum ether : acetone

(4 : 1) (1 : 1)

HPTLC TLC HPTLC TLC

Compounds Rf* ± S.D. Rf* ± S.D. Rf* ± S.D. Rf* ± S.D.

TNT 0.83 ± 0.03 0.64 ± 0.02

TNB 0.79 ± 0.02 0.58 ± 0.02

2,4-DNT 0.73 ± 0.03 0.52 ± 0.02

Tetryl 0.68 ± 0.01 0.37 ± 0.02

2-A-DNT 0.57 ± 0.00 0.23 ± 0.02

4-A-DNT 0.62 ± 0.02 0.27 ± 0.02

RDX 0.32 ± 0.02 0.30 ± 0.02 0.65 ± 0.05 0.72 ± 0.01

HMX No movement No movement 0.57 ± 0.06 0.62 ± 0.01

PETN 0.82 ± 0.02 0.84 ± 0.03 0.82 ± 0.04 0.91 ± 0.03

NG 0.75 ± 0.02 0.79 ± 0.02 0.79 ± 0.04 0.88 ± 0.02

*n = 3

8
Table 7. Visualization of nitroaromatics, nitramines, and nitrate esters.

Visualizing agent Visualized compounds*

Ensys TNT developer • TNT- purple • TNB- orange

• Tetryl- orange • DNT- light green after acetone

TiCl3 and DMAB • TNT- orange yellow • TNB- orange yellow

• Tetryl- light brown • 2-A-DNT- light orange yellow

• 4-A-DNT- golden yellow • DNT- light orange yellow

TiCl3 and DEAB • TNT- bright yellow • TNB- orange-yellow

• Tetryl- light yellow • 2-A-DNT- yellow

• 4-A-DNT- yellow • DNT- yellow

TiCl3 and DMACA • TNT- pinkish purple • TNB- purple

• Tetryl- purple • 2-A-DNT- purple

• 4-A-DNT- purple • DNT- light purple

TiCl3 , NaNO2, and Bratton-Marshall • TNT- purple • TNB- purplish pink

• Tetryl- light pink • 2-A-DNT- purple

• 4-A-DNT- purple • DNT- purple

EDA : DMSO (1:1) • TNT- purple • 2-A-DNT- faint yellow

• Tetryl- orange • DNT- faint light green when the plate is wet

• TNB- red-orange

5% KOH and Hach NitriVer3 • TNT- peachy orange • TNB- light pink

• Tetryl- light yellow • 2-A-DNT- light yellow

5% KOH and modified Griess reagent • TNT- light purple • TNB- light purple

• Tetryl- light purple

5% KOH and Bratton-Marshall reagent • TNT- faint light purple • TNB- faint light purple

• Tetryl- faint light purple

0.1 NaOH and Peak’s Griess reagent • Tetryl- very faint yellow • NG- pink

• 2-A-DNT- faint yellow

1 NaOH and Higg’s Griess reagent • RDX- light reddish brown • 2-A-DNT-very light yellow

• TNT- reddish brown • NG- purplish pink

• Tetryl- light yellow

5% KOH and Higg’s Griess reagent • TNT- peach • 2-A-DNT- light yellow

• Tetryl- yellow • NG- pink

• TNB- faint pink

5% KOH and Hach NitriVer3 • TNT- peachy-brown • TNB- light pink

• Tetryl- light yellow • 2-A-DNT- light yellow

NaIO4

/KMnO4 spray • TNT- yellow • 2-A-DNT- light yellow

• Tetryl- light yellow • DNT- light yellow

• TNB- light yellow

5% EDA in ethanol • TNT- light brown • TNB- light orange

• Tetryl- peachy brown • 2-A-DNT- light, light yellow

Griess reagent and UV exposure for 30 • PETN- lime green • RDX- light blue

min. followed by heat drying • NG- lime green • HMX- light blue

1% or 5% DPA and UV exposure for 15 min. • PETN- olive green • RDX- light purple

• NG- olive green

*Spotted 1 μg of compound.

9
with Griess reagents yielded colors that characterize these compounds, but in most cases we

found nitroaromatic compounds to be more visible with the exception of 2,4-DNT and 4-A-DNT.

Griess reagent formulation by Peak and Higgs resulted in a pinkish color for NG. The Hach

NitriVer3, when used in conjunction with acidified zinc, detects RDX from soil and groundwater

extracts (Walsh and Jenkins 1991, Jenkins et al.

1994), but Hach NitriVer3 used in conjunction with

5% KOH did not yield any color formation for nitrate esters (PETN and NG) nor for nitramines

(RDX and HMX). The absence of color(s) may have

been due to the omission of an acidic environment.

When using the visualizing agents in this category

the best viewing of colors was after the plates had

completely dried.

NaIO4/KMnO4

According to Carlson and Thompson (1986),

sodium metaperiodate (0.1%) and potassium permanganate (0.5%) in a 4% sodium hydroxide solution yields yellow spots for NG on a dry plate

and green spots for PETN on a wet plate, all

against a violet background. DiCarlo et al. (1964)

also observed similar results for PETN when

sprayed with a solution containing 4 : 1 : 1 of sodium metaperiodate (2%) : potassium permanganate (1%) : sodium bicarbonate (2%). However, our

results yielded no color formation for nitrate esters. Some nitroaromatics developed yellow spots

after the plate had been dried in a 100°C oven for

five minutes (Table 7). The failure to observe the

nitrate esters may partly be due to spotting of

lower levels of NG and PETN (1 to 2 μg) compared

to the literature values of 2 to 25 μg.

1% or 5% DPA and UV exposure

DiCarlo et al. (1964), Parker et al. (1975), and

Bagnato and Grasso (1986) all report color visualization of PETN with either 1% or 5% DPA followed by UV light exposure. Parker et al. (1975)

also included NG and RDX while Bagnato and

Grasso (1986) were able to also observe HMX. The

results we obtained were similar. PETN and NG

appeared green–gray and RDX was light pinkish–

purple.

Griess reagent and UV exposure

Spraying with Griess reagent followed by UV

light exposure for approximately 30 minutes resulted in pink-colored spots for RDX, HMX, PETN,

and NG. However, when the plates were dried in

a 110oC oven for 20 minutes, RDX and HMX

yielded light blue spots while PETN and NG had

lime green spots.

Estimation of detection capability

A preliminary estimation of the minimum detectable level for each explosive was determined

by spotting different volumes (ranging from 0.5

to 30 μL) of each standard solution. The concentration of the various standard solutions ranged

from 5 to 2000 mg/L. Depending on the analyte

of interest, UV viewing and visualizing agents

were employed to visualize (or detect) the intended compound. Table 8 lists the lowest detectable quantity that could be visualized for each explosive. All analytes were visible at 1-μg quantity

either by UV viewing or by spraying with a visualizing agent. In samples containing high concentration of analyte (>1000 mg/L), a spotting volume of 1 μL was sufficient to allow visualization

of the analyte, but in samples containing lower

levels (<100 mg/L), the spotting volumes could

be as high as 30 μL (on a preconcentration zone)

without causing streaking or too much spreading

of the analyte. TNT and TNB were the only

analytes that were consistently visualized at 0.1-

μg quantity, which was equivalent to a spotting

volume of 10 μL of standard solution of 10 mg/L

or 1 μL of 100 mg/L. The remaining nitroaromatics

were also visible at 0.1-μg quantity when sprayed

with TiCl3 and DMACA; however, more tests are

needed to fully assess the reliability of detection

at this level. RDX was also visualized at 0.1-μg

quantity under UV viewing, but the observations

were more consistent with HPTLC plates than

with conventional plates. While the HPTLC plates

did not play a role in significantly enhancing the

separation of analytes, with regard to detection

capabilities, the properties of HPTLC plates

seemed to enhance the detectability of nitramines

and nitrate esters.

Testing of soil samples collected from the field

Soil samples collected from the field (ranging

from ammunition plants to firing ranges) were

analyzed using the conventional TLC methods to

determine or confirm the accuracy of its separation procedures. The concentration of the analytes

from these soil samples was previously determined by standard HPLC methods. In most cases,

soil samples were extracted with acetone (1 to 5

ratio) and the filtered extracts were spotted on TLC

plates. Soil from Umatilla Army Depot, which contained 716 μg of TNT per gram of soil, was extracted and spotted (10 × 1 μL). This yielded a vis10
ible spot under UV light that corresponded to the

same Rf value as the spot from a standard solution of TNT. A soil sample from Hawthorne Ammunition Plant containing an array of explosives,

including HMX (2.4 mg/g), RDX (8.1 mg/g), TNB

(0.088 mg/g), DNB (0.002 mg/g), TNT (13.9 mg/

g), and 2,4-DNT (0.007 mg/g), was extracted.

When 1 μL of the extract was spotted and developed, two spots that corresponded to TNT and

RDX were visible under UV light. Soil collected

from DREV had tested positive for RDX using the

RDX field screening kit (Walsh and Jenkins 1991),

but the explosive most commonly used in that area

was HMX. When 10 μL of the DREV soil extract

was spotted and developed using the solvent system of petroleum ether : acetone (1 : 1), a single

pink spot appeared after spraying the plate with

Griess reagent and exposing the plate under UV

light for 30 minutes. When the Rf value for the

spot from the DREV sample was compared to the

Rf values of the standard RDX, HMX, PETN, and

NG, it corresponded to HMX, and the identity was

confirmed by standard HPLC methods. In most

cases, soil samples collected from the field contained a high concentration of TNT with very low

levels of other nitroaromatics. When soil extracts

from these samples were spotted, because of the

high TNT concentration, other nitroaromatics

could not clearly be identified. To determine the

effectiveness of the TLC method in separating

nitroaromatic compounds in extracts from field

samples, soil from Savanna Army Depot, which

Table 8. Lowest level of visualization.

Lowest level Visualizing

Compound (μg) agent Frequency

Nitroaromatics: TNT 0.1 UV exposure 4/4

TiCl3 and DMACA 8/8

EDA : DMSO (1 : 1) 1/1

HPTLC*/UV exposure 4/4

HPTLC*/TiCl3 and DMACA 1/1

HPTLC*/EDA : DMSO (1 : 1) 3/3

Tetryl 0.01 TiCl3 and DMACA 1/1

0.05 EDA : DMSO (1 : 1) 1/1

HPTLC*/TiCl3 and DMACA 1/1

HPTLC*/EDA : DMSO (1 : 1) 2/2

TNB 0.1 UV exposure 4/4

TiCl3 and DMACA 8/8

HPTLC*/UV exposure 3/3

HPTLC*/EDA : DMSO (1 : 1) 2/2

4-A-DNT 0.1 UV exposure 1/1

TiCl3 and DMACA 1/1

2-A-DNT 0.1 UV exposure 3/3

TiCl3 and DMACA 1/1

2,4-DNT 0.1 UV exposure 3/4

TiCl3 and DMACA 6/6

HPTLC*/TiCl3 and DMACA 1/1

Nitramines: RDX 0.1 UV exposure 4/7

Griess and UV exposure 1/1

HPTLC*/UV exposure 8/8

HPTLC*/5% DPA and UV exposure 1/5

HMX 0.1 HPTLC*/UV exposure 3/3

0.2 Griess and UV exposure 2/2

Nitrate esters: PETN 0.2 HPTLC*/1% DPA and UV exposure 1/1

0.4 Griess and UV exposure 2/2

NG 0.2 HPTLC*/1% DPA and UV exposure 1/1

0.4 Griess and UV exposure 2/2

*High-performance thin-layer chromatography plates.

11
contained similar levels of TNT (14 μg/g) and TNB

(9.4 μg/g), was used. Soil samples were extracted

in acetone as described above, but to maximize

the extract concentration, the soil-to-solvent ratio

was increased to 1 : 2 (1 gm of soil to 2 mL of

acetone). A volume of 20 μL (which was equivalent to 0.14 μg of TNT and 0.094 μg of TNB) was

spotted and developed in the solvent system of

Sterling VM & P naphtha and laboratory-grade

isopropanol (4 : 1). Plates were sprayed with TiCl3,

NaNO2, and Bratton-Marshall reagent (Jork et al.

1994b). Two spots, light purple in color, were identified as TNT and TNB. These results seemed to

indicate that TLC could be used to separate and

distinguish different explosives components in

actual field samples when appropriate solvent system and visualizing procedures are utilized.

Recommendations for specific separations and

visualizing reagents

The solvent system of petroleum ether and isopropanol (4 : 1) is recommended to separate various species of nitroaromatic compounds, including TNT, TNB, and DNT. The most sensitive

visualizing agents tested for nitroaromatics are

TiCl3 followed by DMACA spray. For the separation of nitramines such as RDX and HMX, and

nitrate esters PETN and NG, the solvent system

of petroleum ether and acetone (1 : 1) is recommended with visualization with Griess reagent

followed by UV exposure. Optimal separation

occurs with laboratory-grade solvents; however,

in cases where laboratory-grade solvents are not

readily available, commercially available solvents

such as paint thinner and acetone may be substituted. If sensitivity is not an issue, commercially

available solvents will be more cost-effective and

more readily available in the field.

CONCLUSIONS

For the purposes of this evaluation, the conventional TLC approach was used rather than the

modern TLC techniques for the following reasons:

1) the conventional TLC techniques involve less

equipment, and so are less expensive and are field

portable, and 2) the purpose of this report was to

evaluate a method that can be used in conjunction with on-site colorimetric methods. The results

indicate TLC methods could indeed be used to

separate various components of explosives such

as TNT, TNB, DNT, RDX, HMX, PETN, and NG

from soil samples. Using appropriate solvent systems such as petroleum ether and isopropanol (4 :

1) and petroleum ether and acetone (1 : 1),

nitroaromatic, nitramine, and nitrate ester compounds could be effectively separated. In most

cases, commercial-brand solvents (which are

readily available in hardware stores) were also

effective in giving good-to-fair separation of components of explosives. However, as mentioned at

the onset of this report, the detection capability of

conventional TLC methods is poor, and this remains the major limitation of this method. The conventional TLC method evaluated in this report is

capable of detecting 0.1 μg of TNT or RDX with

either UV, TiCl3 spray followed by DMACA, or

Griess reagent followed by UV exposure. This is

equivalent to spotting a volume of 1 μL of sample

extract containing 100 μg/mL of TNT or RDX, and

if the sample extract was prepared using the soilto-solvent ratio used in the on-site colorimetric

methods (20 g of soil to 100 mL of acetone), the

concentration of TNT or RDX in soil would

correspond to approximately 500 μg/g. This is

about 500 times above the minimum detection

limit for TNT (1.1 μg/g) and RDX (1.4 μg/g) colorimetric on-site tests. Even if the maximum spotting volume of 30 μL is used, the detection capability remains at about 17 μg/g if the

soil-to-solvent ratio is maintained at 20 g and 100

mL. If a larger soil-to-solvent ratio is used to obtain an extract for TLC analysis, the detection capability could be further improved. More experiments aimed at optimizing detection capability by

either concentrating sample extracts and/or utilizing higher soil-to-solvent ratios are needed to

fully assess the practical limit of detection for the

TLC method.

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August 1997

On-Site Analysis of Explosives in Soil: Evaluation of Thin-Layer

Chromatography for Confirmation of Analyte Identity

Sae-Im Nam

U.S. Army Cold Regions Research and Engineering Laboratory

72 Lyme Road

Hanover, New Hampshire 03755-1290

Special Report 97-21

Approved for public release; distribution is unlimited.

Available from NTIS, Springfield, Virginia 22161

SFIM-AEC-ET-CR-97030

Two colorimetric-based methods are commonly used for on-site analysis of explosives in soil. For the TNT method,

acetone soil extracts are reacted with base to produce reddish-colored Janowsky ions. For RDX, acetone extracts are

acidified and reacted with zinc to reduce RDX to nitrous acid, and the nitrous acid is determined by reacting the

resulting solution with a Griess reagent. The TNT method is subject to interference from the presence of other

polynitroaromatic compounds such as TNB, tetryl, and the isomers of DNT. Likewise, the RDX method is interfered with by the presence of other nitramines such as HMX and tetryl, and organonitrate esters such as NG, PETN,

and NC. This study investigates the use of thin-layer chromatography (TLC) as a simple on-site method to confirm

the identity of analytes detected using colorimetric on-site methods. Separations using both laboratory-grade and

locally available solvents were developed. The major limitation of this method is detection capability, which was

estimated to be about 0.1 μg of analyte. This corresponds to a concentration of 17 μg/g when using 30 μL of spotting volume, or 500 μg/g when using 1 μL of spotting volume.

U.S. Army Environmental Center

Aberdeen Proving Ground, Maryland 21010-5401

21

UNCLASSIFIED UNCLASSIFIED UNCLASSIFIED UL

Colorimetric methods Planar chromatography TNT

Explosives RDX Thin-layer

On-site analysis Soil chromatography