Band
The term band originated from the first chromatographic separation which was carried out by Tswett in the late 1890s. Tswett separated some plant pigments employing a liquid-solid phase system in a tubular column and the separated pigments formed different colored stripes across the diameter of the tube forming a strong contrast with the white adsorbent; these stripes were given the name bands. The individual pigments were recovered by extruding the adsorbent from the tube and slicing each band away from its neighbor with a knife. Each band was then extracted with a solvent. The term band persisted for some years and when elution peaks were obtained from the early gas chromatographs they were also sometimes termed solute bands. Today, the term band has largely fallen into disuse and is rarely used in modern column chromatography. The terminology, however, is still used thin layer chromatography and in electrophoresis.
Author: RPW Scott
Book:Principles and Practice of Chromatography
Section:Principles TLC Sample-Application
the solutes are
slowed down by the more retentive layer and are thus focused as a sharp band at
the front of the plate. As development proceeds, the solutes separate in the
normal high retentive layer in the usual manner. This procedure has other
advantages. If the sample is contaminated with salts or biological polymers,
these will be trapped in the concentration zone and, thus, will not pass onto
the separation region of the plate and effect the quality of the separation.
Band applicators
operate differently and are usually fully automated. The sample is atomized in
a stream of air or nitrogen depending on the nature of the sample and its
tendency to oxidation. A diagram of the type of atomizer used in band
application is shown in figure 35.
Figure 34
TLC Plate with Sample Concentrating Zone
Principles TLC Sample-Application
Author: RPW Scott
Book:Dispersion in Chromatography Columns
Section:Dispersion Summation-of-Variances
The width of
the band of an eluted solute relative to its proximity to its nearest neighbor
determines whether two solutes are resolved or not. The ultimate band width as
sensed by the detector is the result of a number of individual dispersion
processes taking place in the chromatographic system, some of which take place
in the column itself and some in the sample valve, connecting tubes and
detector (see
Extra Column Dispersion
). In order to determine the ultimate dispersion of the
solute band it is necessary to be able to calculate the final peak variance.
This is achieved by taking into account all the individual dispersion processes
that take place in a chromatographic system. It is not possible to sum the band
widths (standard deviation or (s))
resulting from each individual dispersion process to obtain the final band
width, but it is possible to sum all the respective variances. However,
the summation of all the variances resulting from each process is only possible
if each
Dispersion Summation-of-Variances
Author: RPW Scott
Book:Liquid Chromatography Detectors
Section:HPLC-Detectors Dispersion Connecting-Tubes
parabolic velocity profile that
occurs under conditions of Newtonian flow, (i.e. when the velocity is
significantly below that which produces turbulence). Under condition of
Newtonian flow, the distribution of fluid velocity across the tube adopts a
parabolic profile as shown in figure 1. The velocity at the walls is virtually
zero and that at the center a maximum. This situation is depicted
diagramatically in Figure 1.
Figure 1.
The Parabolic Velocity Profile of a Solute Band Passing Through a Tube
Due to the
relatively high velocity at the center of the tube and the very low velocity at
the walls, the center of the band of solute passing down the tube will move
ahead of that situated at the walls. The resulting effect of band dispersion is
depicted in figure 2
HPLC-Detectors Dispersion Connecting-Tubes
Author: RPW Scott
Book:Gas Chromatography - Tandem Techniques
Section:GC-Tandem GC-Spectroscopic-Systems UV-Absorption-Characteristics
nbsp;
The different excitation frequencies can be used in various types of analytical applications some of which are broadly summarized in table 1.
The bands given in table 1, for the most part, comprise all of the analytically useful bands in the ultra violet-visible region. The energy associated with a specific transition, theoretically, should produce a single sharp absorption band.
Table 1 Application Areas for Absorption at Different UV Wavelengths
Adsorption Band- Substance Analytical Application
180-250 nm bands of most aromatic hydrocarbons Excellent for trace analysis and characterization
160-180 nm bands of single olefins Good for trace analysis with an appropriate spectrophotometer
250 nm bands of benzene Reasonable for trace analysis good for characterization
260-290 nm bands of saturated aldehydes and ketones Poor for
GC-Tandem GC-Spectroscopic-Systems UV-Absorption-Characteristics
Author: RPW Scott
Book:Preparative Chromatography
Section:Preparative Mass-Overload
with the benzene. In fact, as a result
of solute–solute interaction the benzene is, in effect, partially eluting itself.
The effect of
the mass overload of benzene on the other solutes is also clearly demonstrated.
The presence of the high concentration of benzene in the mobile phase increases
the elution rate of both the naphthalene and the anthracene. Its also seen,
however, that the effect of the high concentration of the benzene on the closer
eluting peak naphthalene is to produce band dispersion, whereas the anthracene
band does not suffer significant dispersion and the retention of both the front
and the rear of the anthracene peak appear to be linearly reduced with sample
mass. The chromatograms shown in figure
7 also show that the anthracene peak maintains its symmetry throughout all
sample sizes. The impact of the high concentration of benzene on the elution of
the other solutes only occurs while the solutes are still in contact with one
another at the beginning of
Preparative Mass-Overload
Author: RPW Scott
Book:Dispersion in Chromatography Columns
Section:Dispersion Alternative-Axes
Thus, assuming
there are (n) non-interacting, random dispersive processes occurring in the
chromatographic system, then any process (p) acting alone will produce a
Gaussian curve having a variance ,
Hence,
where,
()
is the variance of the solute band as sensed by the detector.
The above
equation is the algebraic enunciation of the principle of the summation of
variances and is fundamentally important. If the individual dispersion
processes that are taking place in a column can be identified, and an
expression for the variance arising from each dispersion process evaluated,
then the variance of the final band can be calculated from the sum of all the
individual variances. This is how the Rate Theory provides an equation for the
final
Dispersion Alternative-Axes