substance. R f values in TLC are the ratio of the individual elution distance and the
maximal elution distance as given by the solvents stretch (see Fig. 3.20).
Generally, the quality of separation in TLC depends on the optimal matching of
polarities of both substance and solvent. For TLC various solvents as well as solvent
mixtures are used. An overview on suitable solvents acting as eluents with various
polarity, the so-called eluotropic series lists common solvents according to their
polarity or elution power. This eluotropic series is nearly the same as given in Fig. 3.3.
The technical realization of TLC covers various methods as illustrated in Fig. 3.19
differing only in the way of eluent flow and shape of the thin layer. Also, 2D-TLC
has been used by preparing one fractionation in one direction and repeating the
procedure with another solvent and with turning the sheet by 90
.
By far more common is the usage of second order chromatography for fractionation. This is usually performed as column chromatography consisting of a (glass)
column filled with silica gel, a facility to fill the eluent on the top allowing to
percolate through the silica gel, and the possibility to collect the eluent leaving the
column at the bottom. As well, the selection of the polarity of the eluent is the
dominant parameter determining the quality of the separation. For complicated
separation problems, e.g. isolation of an analyte from various chemically very
similar substances (as purification step), one solvent or solvent mixture is used
with a very fine collection of fractions. Coarse fractionations of a wider range of
polarity, e.g. for separating roughly the broad spectrum of compounds in raw
extracts, are carried out by changing the eluent composition and to collect the
fractions according to the different eluents.
In organic-geochemical analyses of fossil materials, a coarse separation of crude
extracts or oils into three fractions is used. The individual fractions represent mainly
different compound classes, the aliphatic and aromatic hydrocarbons as well as the
functionalized compounds, also called NSO-compounds. For environmental analyses a broader fractionation can be used as illustrated in Fig. 3.21. But as well, the
separation of individual compounds or substance classes is aimed at.
start ing line
end line of
solvent
max. distance
elut ion distance
elut ion distance
R f = max. distance
elut ion distance
R f = 0.8
R f = 0.3
Fig. 3.20 How to calculate
R f -values in TLC
36
3 Sample Treatment
maximal elution distance as given by the solvents stretch (see Fig. 3.20).
Generally, the quality of separation in TLC depends on the optimal matching of
polarities of both substance and solvent. For TLC various solvents as well as solvent
mixtures are used. An overview on suitable solvents acting as eluents with various
polarity, the so-called eluotropic series lists common solvents according to their
polarity or elution power. This eluotropic series is nearly the same as given in Fig. 3.3.
The technical realization of TLC covers various methods as illustrated in Fig. 3.19
differing only in the way of eluent flow and shape of the thin layer. Also, 2D-TLC
has been used by preparing one fractionation in one direction and repeating the
procedure with another solvent and with turning the sheet by 90
.
By far more common is the usage of second order chromatography for fractionation. This is usually performed as column chromatography consisting of a (glass)
column filled with silica gel, a facility to fill the eluent on the top allowing to
percolate through the silica gel, and the possibility to collect the eluent leaving the
column at the bottom. As well, the selection of the polarity of the eluent is the
dominant parameter determining the quality of the separation. For complicated
separation problems, e.g. isolation of an analyte from various chemically very
similar substances (as purification step), one solvent or solvent mixture is used
with a very fine collection of fractions. Coarse fractionations of a wider range of
polarity, e.g. for separating roughly the broad spectrum of compounds in raw
extracts, are carried out by changing the eluent composition and to collect the
fractions according to the different eluents.
In organic-geochemical analyses of fossil materials, a coarse separation of crude
extracts or oils into three fractions is used. The individual fractions represent mainly
different compound classes, the aliphatic and aromatic hydrocarbons as well as the
functionalized compounds, also called NSO-compounds. For environmental analyses a broader fractionation can be used as illustrated in Fig. 3.21. But as well, the
separation of individual compounds or substance classes is aimed at.
start ing line
end line of
solvent
max. distance
elut ion distance
elut ion distance
R f = max. distance
elut ion distance
R f = 0.8
R f = 0.3
Fig. 3.20 How to calculate
R f -values in TLC
36
3 Sample Treatment