Hollow cathode tubes

Following the work of Lundegardh in the twenties, emission flame spectroscopy became established as an analytical tool in almost every branch of science. Although hollow cathode tubes were first studied by Paschen (P2) in 1916, and although atomic absorption spectroscopy had found occasional application, notably in the mercury vapor detector W20), it remained for Walsh (W2) in Australia in 1955 to recognize the essential advantages inherent in absorption over emission methods and revive general interest in this technique. Shortly thereafter but apparently independently, Alkemade and Milatz (A2, A3) in Holland devised instruments and applied atomic absorption spectroscopy in their laboratory. Walsh and his co-workers have since contributed a remarkable volume of work on instrumentation and application, and patents are held by Walsh on his method in Australia, Europe, and America. 

Fig. 1. Spectral scan of the emission of calcium hollow cathode tube (Ransley Glass, Melbourne, Australia). The emission spectrum is dominated by the strong resonance line of calcium at 4227 A (reproduced from reference (Z3) by permission from the editor of Clinical Chemistry).

Performance of Calcium Hollow Cathode Tube at Various Lamp Currents ... 

While the first hollow cathode tubes were constructed in such a way that they could be repeatedly flushed with the purified noble gas, the inconvenience connected with such equipment led to the development of permanently sealed tubes. In order to insure a reasonable lifetime of such tubes, they have to be of a certain minimum volume. One of the reasons for the lifetime limits is leakage of air into the tube, but more important seems to be the loss of the filler gas which is slowly absorbed by the metal and the glass surface. Since the lamp operates by the sputtering off of the cathode lining, gradual loss of the latter leads to eventual deterioration of the lamp. Lamps for metals that sputter abundantly, like the alkali metals, or zinc and cadmium, have short lifetimes, mostly well below a hundred hours. 

The principle of the hollow cathode tube, production of a vapor of atoms by cathodic sputtering, has been employed by Gatehouse and Walsh (Gl) for sample vaporization. The sample is introduced into a vacuum chamber and is made the cathode which produces a cloud of activated atoms. The light of a separate hollow cathode tube is passed through this vapor and absorption is measured in a spectrophotometer. 

When solutions of high salt content (2% or higher) are aspirated, the salt particles formed from the aerosol are of sufiBcient size to pass through the flame without disintegrating (W15). These particles are capable of scattering the light from the hollow cathode tube, which will show up in the measurement as erroneously high absorption. 

Willis (Wll), using a potassium hollow cathode tube instead of the commonly employed discharge lamp, determined potassium in blood serum. At the 1 50 dilution no interference was encountered from calcium, magnesium, and phosphate at serum levels, but sodium gave a small enhancement. The sodium interference was controlled by the addition in excess of sodium chloride or of the disodium salt of EDTA to samples and standards alike. 

Zinc in atomic absorption spectroscopy is remarkably free from interferences as contrasted to the difiiculties encountered in polarography or with colorimetric methods (M4). Gidley and Jones (G4, G5) studied the influence of 27 elements and the only effect seen was a depression with silicon. The absorption enhancement encountered by these authors with haloid acids could be traced back to the attack of the brass burner by the samples and to the use of a brass hollow cathode tube as zinc line source. Methods for the determination of zinc in various metals and alloys are described by these authors. 

High-intensity sources other than hollow cathode tubes also have been investigated. Most promising are the microwave-excited electrodeless discharge lamps. These lamps produce very high-intensity, sharp line spectra but suffer stability problems and generally have short lifetimes compared to hollow cathode tubes. 

With the radiation from a hollow cathode tube passing four times through an atomic beam of rhodium, Kuhn and Woodgate [14] have been able to observe in absorption hfs doublets of separation 0.023 cm in the lines 3396, 3434, and 3692 A, all transitions to the ground level Fg/2, and concluded that ° Rh has a nuclear spin of 1 = 1/2. 

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