A low pressure of argon in the lamp insures that the line width from the hollow cathode lamp is less than the line width of the absorbing species. Collisions of excited state sputtered atoms with argon atoms will lead to broadening of the output of the hollow cathode lamp and potentially lead to the same problem described above with the use of a continuum source. Note that some of the elements have complex spectra. Set the monochromator EXACTLY to the wavelength peak approaching from the low wavelength side. Ensure that the hollow cathode lamp is operating correctly at the recommended lamp current. The pressure of the argon is low to minimize collisions of argon atoms with sputtered atoms. path before attempting to align the hollow cathode lamp and adjust the monochromator. With this understanding we can ask why the hollow cathode lamp has a low pressure of argon filler gas. Therefore, hollow cathode lamps, which emit intense narrow lines of radiation specific to the element being analyzed, are needed for atomic absorption measurements. Reducing the slit width on a continuum source to a level that would provide a narrow enough line to respond to atomic absorption would reduce the power so that it would not be much above the noise. Here, a sealed quartz tube is used, which is also filled with a noble gas and which additionally contains traces of the desired metaloptionally. An Alternative: The EDL An alternative to the HCL is the electrodeless discharge lamp, EDL for short. The problem with reducing the slit width is that it reduces the number of photons or source power reaching the sample. The schematic structure of such a hollow cathode lamp can be seen in Fig. Several wavelengths are available for tin determination by AAS using both hollow cathode lamps and electrodeless discharge lamps. The hydrides are heated in the quartz tube to around 1000C where they are atomized. What is the problem with reducing the slit width of the monochromator to get a narrower line? With GC separation, AAS is typically used with a quartz furnace following hydride generation. Atomic absorption line superimposed onto the overall output of a continuum source/monochromator system. Flame microsampling is accomplished using a micropipet to place 50–250 μL of sample in a Teflon funnel connected to the nebulizer, or by dipping the nebulizer tubing into the sample for a short time.\( \newcommand\). The hollow cathode is coated with the element you wish to measure. An illustration of a hollow cathode lamp is shown in Figure 6.3A. Atomic absorption spectrophotometers require a separate source lamp, called a hollow cathode lamp, for each individual element that you wish to measure. For example, continuously aspirating a sample that has a high concentration of dissolved solids-sea water, for example, comes to mind-may build-up a solid de- posit on the burner head that obstructs the flame and that lowers the absorbance. One of these concerns the radiation source. Continuous aspiration is sample intensive, typically requiring from 2–5 mL of sample.įlame microsampling allows us to introduce a discrete sample of fixed volume, and is useful if we have a limited amount of sample or when the sample’s matrix is incompatible with the flame atomizer. The most common means for introducing a sample into a flame atomizer is a continuous aspiration in which the sample flows through the burner while we monitor absorbance. How do Hollow Cathode Lamps Work Construction Hollow cathode lamps are essentially two electrodes enclosed in a low-pressure fill-gas. The relative size of each zone depends on many factors, including the choice of fuel and oxidant, and their relative proportions. The light produced by the lamp is emitted from excited atoms of the same element that is to be determined, therefore the radiation energy corresponds directly to the. Profile of typical flame using a slot burner. The free atoms are then exposed to light, typically produced by a hollow-cathode lamp, and undergo electronic transitions from the ground state to excited electronic states.
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