Hydrogen continuum lamp

When determining those elements which absorb in the UV portion of the spectrum, corrections for nonatomic absorption are almost essential. This is especially true for solutions of high solids content, such as those which result from fusion techniques. This correction is generally made by using a hydrogen continuum lamp. Some of the newer instruments have provisions for automatic and continuous background correction. A field installable kit is available to retrofit the model 1200 now used. 

Measure the background absorbance at 234.9 nm for the sample with the hydrogen continuum lamp. 

In addition to the value of 1.0 for the CO quantum yield obtained by Warneck near 150 nm. Quick and Cvetanovic (66) identified 0( D) as a primary product of the photolysis at the emission wavelengths of a bromine lamp, 157.5 and 163 nm. It was previously (67) established that insertion of 0( D) into neopentane leads to a 65 percent production of neopentanol. Thus the yield of neopentanol in combination with the effect of various additive gases serves as a qualitative test for the presence of 0( D). The quantum yield of neopentanol can then be used to measure the quantum yield for 0( D). These authors find the 0(1d) quantum yield to be near unity at these two wavelengths. This result is in conflict with that of Inn and Heimerl (50), who measured a quantum yield of 0.5 for CO production in the interval from 150 to 167 nm. These latter measurements were made by Fourth Positive fluorescence with a hydrogen continuum serving as the photolytic source. They have... 

High pressure xenon lamps are also employed in some TLC scanners (e.g. the scanner of Schoeffel and that of Farrand). They produce higher intensity radiation than do hydrogen or tungsten lamps. The maximum intensity of the radiation emitted lies between k = 500 and 700 nm. In addition to the continuum there are also weak emission lines below k = 495 nm (Fig. 14). The intensity of the radiation drops appreciably below k = 300 nm and the emission zone, which is stable for higher wavelengths, begins to move [43]. 

Background correction is carried out with a continuum source, e.g. a hydrogen hollow-cathode lamp or a deuterium-arc lamp. 

Continuum spectrum Radiation consisting of a band of wavelengths as opposed to discrete lines. Incandescent solids provide continuum output (blackbody radiation) in the visible and infrared regions deuterium and hydrogen lamps yield continuum spectra in the ultraviolet region. 

Deuterium and Hydrogen Lamps. A continuum spectrum in the ultraviolet region is produced by electrical excitation of deuterium or hydrogen at low pressure. Ihe mechanism by which a continuum spectrum is produced involves initial formation of an excited molecular species followed by disscKiation of the excited molecule to give two atomic species plus an ultraviolet photon. The reactions for deuterium are... 

Both deuteriuin and hydrogen lamps produce outputs in the r.ingc of 160- KOO nm. In the ultraviolet region (191) 400 nm), a continuum spectrum exists as... 

Quartz windows must be employed in deuterium and hydrogen lamps because glass absorbs strongly at wavelengths less than about 3s()nm. Although the deuterium lamp continuum spectrum extends to wavelengths as short as IbOnm.the useful lower limit is about 190 nm because of absorption by the quartz windows. 

As we have already mentioned, atomic absorption lines are very narrow (about 0.002 nm). They are so narrow that if we were to use a continuous source of radiation, such as a hydrogen or deuterium lamp, it would be very difficult to detect any absorption of the incident radiation at all. Absorption of a narrow band from a continuum is illustrated in Fig. 6.4, which shows the absorption of energy from a deuterium lamp by zinc atoms absorbing at 213.9 nm. The width of the zinc absorption line is exaggerated for illustration purposes. The wavelength scale for the deuterium lamp in Fig. 6.4 is 50 nm wide, and is controlled by the monochromator bandpass. If the absorption line of Zn were 0.002 nm wide, its width would be 0.002 x 1/50= 1/25,000 of the scale shown. Such a narrow line would be detectable only under extremely high resolution (i.e., very narrow bandpass), which is not encountered in commercial AAS equipment. 

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