Vapor discharge lamps

Vapor discharge lamps produce emission by passing an electric current through a vapor composed at least partly of the element of interest. Such lamps are produced by Osram in Germany and Philips in Holland. Osram lamps exist for the elements mercury, thallium, zinc, cadmium, and the alkalis. 

Until quite recently, vapor discharge lamps were recommended for the determination of sodium and potassium, despite the fact that they are relatively troublesome to use. However, newly available hollow cathode lamps are as bright as the discharge lamps at both the primary... 

The schematic layout of a luminescence spectrometer for recording of fluorescence, phosphorescence and excitation spectra is shown in Fig. 20. As excitation source a mercury vapor discharge lamp or, as a continuous source, a pulsed xenon flashlamp is used. Excitation spectra are obtained by setting monochromator b2 at the maximum of the luminescence spectrum and varying by means of the monochromator bi the exciting wavenumber continuously over the whole absorption spectrum of the sample. The luminescence spectriun is corrected by diverting a portion of the incident light on a beamspHtter d to a quantiun counter... 

Fluorescent lamps generate light through a low-pressure mercury vapor discharge that has strong emission tines in the UV, namely at A = 254 nm and around 366 nm. The fluorescent layer is excited by the UV radiation and emits in the visible part of the spectrum. While remains of the 254 nm tine are efficiently rejected by the glass tube, some fraction of the 366 nm radiation can be measured in the emission spectrum of the lamp. 

The in-line source depicted in Figure 8.7 was designed by Bruins et al. to be mounted on a PE-Sciex triple quadrupole and it was derived from the standard heated nebulizer of their APCI source. The corona needle is replaced by a discharge lamp. Nitrogen is used as the nebulizing and the lamp gas, while air is used as the auxiliary gas. A dopant improves the efficiency of ionization and it is supplied through the auxiliary gas line and vaporized together with the solvent in the heated nebulizer. 

Gaseous Discharge Lamps. Gaseous discharge lamps consist of an electrically operated source of radiant energy characterized by the emission of radiation from a stream of ionized gas carrying current between electrodes in the lamp (See Fig G10). Lamps in common use include fluorescent, mercury-vapor and neon lamps. In general,... 

Immediately after ignition, mercury is still liquid and the discharge takes place in the fill gas (argon). At this point the voltage is low and the current is limited essentially by the short-circuit current delivered by the power supply. As the temperature within the lamp increases, mercury vaporizes. The lamp impedance increases, and this causes the lamp voltage to increase and the current to decrease. After about 1 min, the bum-in period is finished and the lamp reaches stationary conditions. 

Sources of ultraviolet radiation include (a) tungsten-filament incandescent lamps (b) tungsten-iodine cycle lamps with quartz envelopes (c) mercury-vapor lamps and (d) the zinc discharge lamp. Odier types are available, but enjoy only limited application. The hydrogen or deuterium lamps are used in the laboratory, but are delicate and costly for process uses. 

Even a technique of higher detection power as ET-AAS may require some sort of previous analyte enrichment for difficult elements. In the determination of As and Se in mineral waters described by Hudnik and Gomiscek [23], coprecipitation of both elements on hydrated Fe(III) oxide was employed to improve LoDs, otherwise impaired by matrix effects. A graphite tube furnace was the atomization cell, with the atomic vapor sampled with element electrodeless discharge lamps. After treatment of the sample with Fe(III) solution at the appropriate pH, the oxide precipitate was filtered and dissolved and the solution volume reduced to 5 mL of 0.2 M H2SO4. Ten-microliter volume aliquots of sample and standard solutions were injected into the furnace. Reported LoDs were 0.2 and 0.5 p,g l-1 for As and Se, respectively. 

The APPI source is one of the last arrivals of atmospheric pressure sources [80,81]. The principle is to use photons to ionize gas-phase molecules. The scheme of an APPI source is shown in Figure 1.34. The sample in solution is vaporized by a heated nebulizer similar to the one used in APCI. After vaporization, the analyte interacts with photons emitted by a discharge lamp. These photons induce a series of gas-phase reactions that lead to the ionization of the sample molecules. The APPI source is thus a modified APCI source. The main difference is the use of a discharge lamp emitting photons rather than the corona discharge needle emitting electrons. Several APPI sources have been developed since 2005 and are commercially available. The interest in the photoionization is that it has the potential to ionize compounds that are not ionizable by APCI and ESI, and in particular, compounds that are non-polar. 

Another approach to the production of UV photons includes the development of electrode-less discharge lamps driven by microwave excitation (e.g. Fassler et al., 2001, Ametepe et al., 1999, He et al., 1998). This type of lamp is shown in Fig. 4-17. In this case, the excitation of mercury vapor within the discharge gap is achieved by coupling in the energy with a water-cooled high-frequency spool. This concept may be a very convenient tool for microwave photochemistry experiments by simultaneous combination of microwave and VUV/UV irradiation of aqueous systems (c.f Klan et al., 2001, 1999). 

Nl. Nelson, L. S., and Kuebler, N. A., Vaporization of elements for atomic absorption spectroscopy with capacitor discharge lamps. Spectrochim. Acta 19, 781-784 (1983). 

Cold vapor mercury detection limits were determined with a FIAS(ji )-100 or FIAS-400 flow-injection system with amalgamation accessory. The detection limit without an amalgamation accessory is 0.2/ig/L with a hollow cathode lamp, 0.05 /ig/L with a System 2 electrodeless discharge lamp. (The Fig detection limit with the dedicated FIMS(ji )-100 or FIMS-400 mercury analyzers is <0.010/ig/L without an amalgamation accessory and <0.001 /ig/L with an amalgamation accessory.) Flydride detection limits shown were determined using an MFlS-10 Mercury/Flydride system. 

---