High-Pressure Discharge Vapor Lamps

These lamps show moderately good color rendering properties, the very broad spectrum being one of their main utilities. For instance, high-pressure Xe lamps or deuterium lamps are commonly used in optical spectroscopy techniques. 

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Mercury-xenon lamp (Arc) An intense source of ultraviolet, visible, and near infrared radiation produced by an electrical discharge in a mixture of mercury vapor and xenon under high pressure. 

While some of the mid-IR sources emit light below 400 cm the intensity drops off. A more useful source for the far-IR region is the high pressure mercury discharge lamp. This lamp is constructed of a quartz bulb containing elemental Hg, a small amount of inert gas, and two electrodes. When current passes through the lamp, mercury is vaporized, excited. 

Gaseous discharge lamps which contain internal electrodes also can serve as sources for atomic absorption. They are variously called arc lamps, spectral lamps, vapor lamps, and by the name of the manufacturer, such as Osram lamps and Philips lamps. Gaseous discharge lamps contain an inert gas at low pressure and a metal or metal salt. They are especially suited to metals of relatively high vapor pressure, such as the alkali metals and some other metals such as mercury, cadmium, and lead. 

The setup for atmospheric pressure photoionization (APPI) (Bos et ah, 2006 Hanold et ah, 2004 Raffaelli and Saba, 2003 Robb et ah, 2000) is very similar to that of APCI (Fig. 8.7). Only the corona discharge is replaced by a gas discharge lamp (krypton,10.0 eV) that generates ultraviolet (UV) photons in vacuum. The liquid phase is also vaporized by a pneumatic nebulizer and different geometries are used. Most analytes have ionization potentials below 10 eV, while high-pressure liquid chromatography (HPLC) solvents have higher ionization potentials (water 12.6 eV, methanol 10.8 eV, and acetonitrile 12.2 eV). 

Nearly all the practical phosphors for three-band fluorescent lamps are either oxides or oxyacid salts (collectively called oxides ), which can be efificiently excited by 254-nm ultraviolet light generated by low-pressure mercmy vapor discharge. Those phosphors include simple oxide, aluminate, phosphate and borate, etc. Simple oxide combined with aluminate phosphors are the most widely phosphors used in three-band fluorescent lamps mainly due to their high emission efiiciency as well as physical and chemical stability. 

The Na D emission can be efficiently generated by an electrical discharge through sodium vapor and so such lamps are used for high-power illumination. But some people find the intense yellow color uncomfortable. The solution use a high-pressure lamp so as to shift the emission toward the red, as we discuss below. 

Mercury is directly below cadmium in the periodic table, but has a considerably more varied and interesting chemistry than cadmium or zinc. Elemental mercury is the only metal that is a liquid at room temperature, and its relatively high vapor pressure contributes to its toxicological hazard. Mercury metal is used in electric discharge tubes (mercury lamps), gauges, pressure-sensing devices, vacuum pumps, valves, and seals. It was formerly widely used as a cathode in the chlor-alkali process for the manufacture of NaOH and Cl2, a process that has been largely discontinued, in part because of the mercury pollution that resulted from it. 

Osram lamps for mercury cannot be used in atomic absorption, because they contain vapor at so high a pressure that the emission line is almost completely self-absorbed, and the sensitivity is very small. It is also generally agreed that hollow-cathode lamps for thallium, zinc, and cadmium are superior to the discharge lamps. 

Probably the most commonly used instruments for cation impurity analysis of silicates are flame atomic absorption spectrophotometers and ion selective electrodes. In most cases, separation of silica is required to reduce interferences. The sample may also have to be diluted to bring the analyte concentration within the linear operating range. For cations, the atomic absorption spectrophotometer is more versatile than ion specific electrodes. If the analyst is concerned with the presence of heavy metals, then accessories such as a hydride system for the elements that form high vapor pressure compounds, e.g., Sb, and a mercury vapor cold trap are useful. If a large number of elements are to be determined, a substantial investment in hollow cathode and electrode discharge lamps must be made. Several gas mixtures will also be required. 

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