Thin-Layer Chromatography

Physical Methods of uetection i.i ueieciwn oj Aosoromg auostances
Note: Gratings should never be "polished" with the fingers or breathed on. This
is also true of coated or "bloomed" gratings which have magnesium or lithium
fluoride evaporated onto them.
Samples of spectrometers with grating monochromators
• TLC scanner II, CAMAG (Fig. 10)
• CD-60 densitometer, DESAGA (Fig. 11)
• CS 9000 Flying-spot scanner, SHIMADZU (Fig. 12)
• FTR-20 scanner, SIGMA/BIOCHEM (Fig. 13)
Light Sources
Lamps to be employed in photometry should
• produce radiation that is as constant as possible both in origin and intensity and
• be as good approximation as possible to a point source in order to facilitate the
production of parallel beams [40].
A distinction must be made between continuous sources (hydrogen or deuterium
lamps, incandescent tungsten lamps, high pressure xenon lamps) and spectral line
sources (mercury lamps), which deliver spectrally purer light in the region of their
emission lines.
A continuous source has to be employed to record absorption spectra. Fluorescence
is usually excited with mercury vapor lamps; in the region of their major
bands they radiate more powerfully than do xenon lamps (Fig. 14).
High pressure
mercury vapor lamp
Xenon lamp
-A,
Fig. 14: Radiation characteristics of a high pressure Hg lamp (OSRAM HBO 100; continuous
line) and of a xenon lamp (PEK 75; broken line) [4]. The intensity /is represented logarithmically
in relative units.
Continuous sources: The sources of choice for measurements in the ultraviolet
spectral region are hydrogen or deuterium lamps [1]. When the gas pressure is 30
to 60 x 10" 3 Pa they yield a continuous emission spectrum. The maxima of their
radiation emission occur at different wavelengths (H2: k = 280 nm; D2: k =
220 nm). This means that the deuterium lamp is superior for measurements in the
lower UV region (Fig. 15).
Fig. 15: Relative intensity distribution of the radiation produced by a hydrogen and a
deuterium lamp.
Because of the high rate of diffusion of hydrogen the energy consumption resulting
from thermal conduction is very large in the short-wave UV region and the
radiation yield is relatively low. Deuterium diffuses more slowly, its thermal conductivity
is lower and the radiation yield is ca. 30% higher than is the case for a
hydrogen lamp.
The continuum produced by both of these lamps is accompanied by emission lines
in the visible spectral region at k = 486.12 nm (H2) and k = 485.99 nm (D2); these
can be employed for adjustment and calibration of the wavelength scale.
Hydrogen lamps are equipped with a rectangular slit for adjusting and centering
the gas discharge; this ensures that the radiation intensity is particularly high in
this region and the position of the radiation source is stable. But long-term drift
cannot be excluded completely [1].
Tungsten incandescent lamps are primarily employed in the visible region (A =
320...700 nm). They consist of an evacuated glass bulb [11] containing a thin,
coiled tungsten wire which is heated to incandescence. Since tungsten melts at
3655 K the usual operating temperature is 2400 to 3450 K. The higher the temperature
the higher is the vapor pressure of tungsten. The metal vapor is deposited on
the relatively cool glass bulb so that the "transparency" of the glass is reduced,
thus, reducing the operating life which is reported to be ca. 1000 hours at 2400 K.
In order to reduce the rate of evaporation krypton or argon are often employed
as protective gases, which means that 70 to 90% of the electrical energy is converted
H2
500