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.