Lamps radiation characteristics

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.

Sources emitting radiation characteristic of element of interest (hollow-cathode lamp). Flame or electrically heated furnace or carbon rod. Monochromator, photomultiplier, recorder. 

The main radiation source for AAS is the hollow-cathode lamp (HCL). The HCL (Fig. 27.3) emits radiation characteristic of a particular element. The choice of HCL for AAS is simple. For example, if you are analysing for lead, you will need a lead-coated HCL. It is normal to pre-warm the HCL for about 10 min prior to use. This can be done either by using a separate pre-... 

Figi 14 Radiation characteristics of a high pressure Hg lamp (Osram HBO 100 continuous line) and of a xenon lamp (PEK 7S broken line) [4]. The intensity / is represented logarithmi-cally in relative units.------------------------- -----------------------------------------------... 

This is practically similar to that of emission flame photometry. An important point of difference is the need to have a radiation source. It is practically impossible to isolate a particular resonance wavelength from a continuous source by using a prism or a diffraction grating or both simultaneously. This problem was solved with the development of hollow-cathode discharge lamps. Such lamps produce monochromatic radiation characteristic of the element analyzed. In these lamps the cathode is a hollow tube which is lined by the element in question. The lamp will thus emit monoehromatic radiation characteristic of the emission spectrum of the element Involved. Such lamps have now become commercially available for a long range of elements. In less sophisticated instruments, a continuous discharge lamp with double monochromators is used. 

A further example of the static method of vapour pressure measurement which after teething troubles is finding increased favour is absorption spectroscopy. The method consists of heating a specimen in a closed vessel of known dimensions, usually a silica cell, maintained at a uniform temperature. A characteristic spectral source of high intensity is provided, for example, by the incorporation of the element of interest into a hollow cathode lamp. Radiation from the lamp is passed through the closed... 

Essentially the same spectrometer as is used in atomic absorption spectroscopy can also be used to record atomic emission data, simply by omitting the hollow cathode lamp as the source of the radiation. The excited atoms in the flame will then radiate, rather than absorb, and the intensity of the emission is measured via the monochromator and the photomultiplier detector. At the temperature achieved in the flame, however, very few of the atoms are in the excited state ( 10% for Cs, 0.1% for Ca), so the sample atoms are not normally sufficiently excited to give adequate emission intensity, except for the alkali metals (which are often equally well determined by emission as by absorption). Nevertheless, it can be useful in cases where elements are required for which no lamp is available, although some elements exhibit virtually no emission characteristics at these temperatures. 

Spectroscopy is the study of the absorption and emission of radiation by matter. The most easily appreciated aspect of the absorption of radiation is the colour shown by substances that absorb radiation from the visible region of the spectrum. If radiation is absorbed from the red region of the spectrum, the transmitted or unabsorbed radiation will be from the blue region and the substance will show a blue colour. Similarly substances that emit radiation show a particular colour if the radiation is in the visible region of the spectrum. Sodium lamps, for instance, owe their characteristic orange-yellow light to the specific emission of sodium atoms at a wavelength of 589 nm. 

It is already a fact that lasers are replacing conventional lamps in a great variety of spectroscopic applications. The origin of this substitution lies in their superior performance over incoherent light in many experimental situations. Many spectroscopic experiments have been improved, and moreover new techniques have been developed due to the particular advantages provided by lasers. The characteristics of laser radiation on their own constitute real advantages and justify their widespread use in many applications. 

For recording of the emission spectrum, the emitted radiation is focussed on the slit of a monochromator and intensities measured attach wavelength. Since sensitivities of photocells or photomultipliers are wavelength dependent, a standardization of the detector-monochromator combination is necessary for obtaining true emission spectrum This can be done by using a standard lamp of known colour temperature whose emission characteristics is obtained from Planck s radiation law. The correction term is applied to the instrumental readings at each wavelength. Very often substances whose emission spectra have been accurately determined in the units of relative quanta per unit wavenumber intervals are... 

Many technical applications require high radiant power, which cannot be furnished by an operationally reasonable number of lamps having otherwise optimal emission characteristics. Examples of this situation are mostly found in applications oTthe 254-nm fine, where a number of low-pressure mercury lamps may be replaced by one medium-pressure mercury arc. This substitution represents a compromise where spectral selectivity and energy wasting (VIS and IR radiation) is traded against a compact production unit which is less expensive (number of reactors, quartz, safety requirements) and easier to operate (number of reactors, space, and overview). 

Such radiation models have been in permanent development over the last 30 years, and the published results may be classified in two main categories incidence models which may be characterized by mathematical models assuming the existence of a given radiant energy distribution in the vicinity of the reactor, and emission models in which lamp characteristics, reaction, and flow processes are taken into account. 

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