separating compounds. As with other chromato-
NO2
graphic methods, TLC separations utilize a station-
N
Acetic Acid
ary phase and a mobile phase. The stationary
+
Zn
3 HNO2
N
N
phase is a solid material coated on a glass or plas-
O2N
NO2
tic plate and the mobile phase is a solvent(s) of
RDX
Franchimont Reaction (1897)
variable polarity. Separation is achieved as
analytes partition between these phases based on
+
polarity differences as the solvent rises by capil-
NH2
N2
lary action through the coated solid. The major ad-
+
HNO2
vantages of TLC as an on-site method, relative to
other chromatographic methods, include the abil-
R
R
+
ity to rapidly process a number of samples simul-
N2
NR'2
taneously, low capital cost of equipment and sup-
+
plies, and minimal power requirements.
NR'2
N
N
R
R
Brief history of TLC
The earliest development of TLC is attributed
Azo Dye
Griess Reaction (1864)
to two Russian scientists, Ismailov and Shraiber,
Equation 2.
in 1938, when they were able to separate certain
medicinal compounds on unbound alumina
color is directly proportional to the concentration
spread on glass sheets (Coker et al. 1993). Their
of the analyte of interest, and concentrations are
technique was termed "drop chromatography"
determined by measuring absorbance at 540 nm
largely due to the fact that drops of solvent were
for TNT and at 507 nm for RDX. Method detec-
applied to the plate containing the sample. Then,
tion limits for TNT and RDX in soil samples using
around 1949, Meinhard and Hall enhanced the
these methods are 1.1 g/g and 1.4 g/g, respec-
method by using binder material to adhere the alu-
tively.
mina to the glass plate (Sherma and Fried 1996).
Often the capability of the TNT test to detect
However, it was not until 1951 when Kirchner and
his colleagues from the U.S. Department of Agri-
other polynitroaromatics can be quite useful. For
culture further enhanced the method to resemble
example, in a recent study in Sparks, Nevada, ar-
what is now known as thin-layer chromatography.
eas of contamination with 2,4-DNT were detected
using this test (Jenkins et al. 1996). Likewise the
Kirchner utilized sorbent materials in conjunction
capability of the RDX test to determine HMX con-
with binders on the plates and developed the
centrations was recently demonstrated at an ac-
plates in an ascending fashion, which is commonly
tive anti-tank range at Valcartier, Quebec (Jenkins
used in modern TLC. Then in 1958, Stahl and his
et al. [in press]). As stated previously, these two
colleagues popularized the technique by provid-
methods are capable of responding to these TNT-
ing laboratory manuals, which standardized pro-
and RDX-related nitroaromatics and nitramines in
cedures, materials, and nomenclature associated
a like manner. This results in the inability to un-
with thin-layer chromatography. By the mid 1960s,
instruments such as densitometers were being uti-
equivocally identify which specific compounds are
lized to quantitatively measure the results of TLC.
present in many cases without more in-depth labo-
For the past 30 years continued improvements
ratory analyses. It is important to be able to dis-
criminate among the various compounds that re-
have been made in the field of thin-layer chroma-
spond to these tests since cleanup levels for the
tography, including fully automated instruments
various explosives can be set at somewhat differ-
that will spot samples, develop plates, analyze the
ent concentrations. Therefore it would be quite
results, and provide quantitative measurements.
Classical TLC was used as early as the mid-1960s
useful if a simple, inexpensive, on-site method
to determine components of explosives (DiCarlo
were available to qualitatively determine which
et al. 1964, Yasuda 1964, Hoffsommer and
of the potentially detectable analytes are giving
McCullough 1968) and more recently, automated
rise to the colored reaction products from either
multiple development high-performance thin-
the TNT or RDX tests.
Thin-layer chromatography (TLC), also known
layer chromatography (AMD-HPTLC) has been
used to identify explosives or components of ex-
as planar chromatography, is an inexpensive,
plosives in abiotic matrices such as soil and water
readily fieldable chromatographic method for
2
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