Single element hollow cathodes

The degraded materials should not leave any regulated heavy metals in the compost soil after degradation. The compost soil from each of the degraded samples was tested for lead and cadmium for all of the materials. The acceptable limit is 30 mg/kg for lead and 0.3 mg/kg for cadmium. The compost soil for each sample was put into solution and the regulated heavy metal in the compost soil was measured with Fisher-brand (2013) hollow-cathode single-element 2-inch-diameter lamps with elements for lead and cadmium. 

EDL or hollow cathode lamps are used to determine Na, K, Mg, and Ca. Single-element lamps are preferred, but multielement lamps may be used. EDLs are more intense than hollow cathode lamps, and are preferred for K and Na. When performing analyses in emission mode, no lamps are needed. 

With this technique, problems may arise with interference, such as background absorption—the nonspecific attenuation of radiation at the analyte wavelength caused by matrix components. To compensate for background absorption, correction techniques such as a continuous light source (D2-lamp) or the Zeeman or Smith-Hieftje method should be used. Enhanced matrix removal due to matrix modification may reduce background absorption. Nonspectral interference occurs when components of the sample matrix alter the vaporization behavior of the particles that contain the analyte. To compensate for this kind of interference, the method of standard addition can be used. Enhanced matrix removal by matrix modification or the use of a L vov platform can also reduce nonspectral interferences. Hollow cathode lamps are used for As, Cu, Cr, Ni, Pb, and Zn single-element lamps are preferred, but multielement lamps may be used if no spectral interference occurs. 

As explained in Chapter 1, section 7, unless a very high resolution monochromator, e.g. an echelle monochromator, is used to isolate a very narrow (< ca. 0.005 nm) band of light from a continuum spectrum prior to absorbance measurement, the sensitivity will be very poor.1,2 Although there are occasional reports of analysis by flame AAS using continuum sources such as xenon arc lamps, these are invariably from research laboratories. The vast majority of reported applications use single element line sources, and more than 99% of these applications use hollow cathode lamps. 

Hollow-cathode lamps for about 70 elements are available from commercial sources. For certain elements, high-intensity lamps are available. These provide an intensity that is about an order of magnitude higher than that of normal lamps. Some hollow-cathode lamps are fitted with a cathode containing more than one element such lamps provide spectral lines for the determination of several species. The development of the hollow-cathode lamp is widely regarded as the single most important event in the evolution of atomic absorption spectroscopy. 

Hollow-cathode lamp A source used in atomic absorption spectroscopy that emits sharp lines for a single element or sometimes for several elements. 

Figure 13.7 Typical model of a hollow cathode lamp. The cathode is a hoUow cylinder whose central axis corresponds to that of the optical axis of the lamp. The fill gas (normally neon) is always chosen so that the spectral output of the cathode is free from interference. Right, in the hox, pictorial representation of atoms of the cathode being excited by the impact of neon ions (Ne+). Hollow Cathode lamps are available as either single element or multi-element depending on the application. This particularity of AAS renders it impractical to perform qualitative work.

As in FES, the light source of the spectrometer is nothing other than the sample of which all the atoms have been excited. Qualitative analyses of unknowns can therefore be made. This marks an important difference from atomic absorption in which measurements can only be made for elements for which the instrument has been customized (choice of the hollow cathode lamp). Erom a single run, a multielement analysis can be obtained in strict contrast to AAS. However only elements for which a calibration has been carefully undertaken can really be measured. 

A variety of hollow-cathode lamps is available commercially, The cathodes of some consist of a mixture of several metals such lamps permit the determination of more than a single element. 

In theory, no monochromator or lilter should be necessary for alomic fluorescence measurements when an LDI. or hollow-cathode lamp serves as the excitation source becau.se the emitted radiation is, in principle, that of a single element and will thus excite onlv atoms of that element. A nondispersive system then could be... 

The advent of the resonance detector has greatly simplified the potential problem of multi-element analysis. It is in principle possible to line up a number of resonance detectors, and, combining this with a multi-element hollow cathode lamp and a single burner, analyze a sample for a number of elements simultaneously. 

The analyses were performed on a Jarrell-Ash Model No. 32-360 multipass atomic absorption spectrophotometer equipped with a Beckman total consumption three burner set operated on a hydrogen-air mixture, and Westinghouse single element hollow cathode lamps. 

Another approach is to introduce two or more hollow cathodes, each of a single element, into one envelope. This technique also is subject to the problem of selective volatilization but at a lower rate than the single hollow cathode design. 

The intensity of emission from multiple-element hollow cathodes is generally lower than single-element tubes, although several successful combinations are possible if proper choice of elements is made, taking into account volatilization rates and noninterfering emission spectra. For example, a multiple-element combination of magnesium, calcium, and aluminum is available, as is a combination of silver, lead, and zinc. 

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