Radiant deuterium lamp

If a continuum source is needed for absorption spectroscopy, this can be provided by discharge lamps fdled to higher densities, such that pressures can exceed 100 bar at operational temperatures. The result is a broad continuum emission with superimposed line spectra, as shown for several lamps in Fig. 14. In commercial spectrometers the deuterium lamp is commonly used for the UV region below 350 nm while the tungsten-halogen lamp is convenient for the 350 to 900 nm range. The latter is an example of a thermal source whose radiant excitance per unit wavelength closely approximates that predicted by the Planck formula for a blackbody radiator " ... 

A tungsten light source does not supply sufficient radiant energy for measurements below 320 nm. In the UV region of the spectrum, a low-pressure mercury-vapor lamp that emits a discontinuous or line spectrum is useful for calibration purposes but is not very practical for absorbance measurements, because it can be used only at certain wavelengths. Hydrogen and deuterium lamps provide sources of continuous spectra in the UV region with some sharp emission... 

As primary sources, continuous sources such as a tungsten halogenide or a deuterium lamp can be used. They have the advantage that multielement determinations are possible. However, because of the low radiant densities saturation is not obtained and the power of detection is not fully exploited. With line sources such as hollow cathode sources and electrodeless discharge lamps much higher radiances can be obtained. Even ICPs into which a concentrated solution is introduced can be used as a primary source, through which multielement determinations become possible. 

FIGURE 13-11 (a) A deuterium lamp of Ihe typo used in spectrophotometers and (b) its spectrum. The plot is of irradiance / , (proportional to radiant power) versus wavelength. Note that the maximum intensity occurs at 225 nm. Typically, instruments switch from deuterium to tungsten at -350 nm. 

As shown by Equation 15-6, the magnitude of the output signal in luminescence measurements, and thus the sensitivity, is directly proportional to the source radiant power l i. For this reason, more intense sources are u.scd in luminescence methods than the tungsten or deuterium lamps used in absorption measurements. 

The design of a typical double-beam instrument that allows for relative background correction is schematically shown in Fig. 6 and consists of a radiant source, such as a hydrogen-deuterium lamp for ultraviolet and a tungsten lamp for visible radiations a filter, prism, or grating monochromator for wavelength selection sample and reference cells and a photocathode, usually associated with a photomultiplier for detection. 

The spectrometer can be equipped for quasi-simultaneous measurement of the line and background absorption [159]. Radiation from a second, continuous source, such as a deuterium arc lamp, is rapidly switched with the radiation of the primary source and fed through the ab.sorption volume, by a semitransparent mirror. The radiant flux of the continuous source is not decreased significantly by atomic absorption, owing to the low spectral resolution of the monochromator however, it will be weakened by broad-band absorption of molecules, or stray radiation. By this means, nonelement-specific background absorption can... 

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