Metal filament atomizers

Schematic diagram showing the development of a dipolar field and ionization on the surface of a metal filament, (a) As a neutral atom or molecule approaches the surface of the metal, the negative electrons and positive nuclei of the neutral and metal attract each other, causing dipoles to be set up in each, (b) When the neutral particle reaches the surface, it is attracted there by the dipolar field with an energy Q,. (c) If the values of 1 and <() are opposite, an electron can leave the neutral completely and produce an ion on the surface, and the heat of adsorption becomes Q,. Similarly, an ion alighting on the surface can produce a neutral, depending on the values of I and <(), On a hot filament the relative numbers of ions and neutrals that desorb are given by Equation 7.1,which includes the difference, I - <(), and the temperature, T,...

In a number of cases, the temperature of the filament and thermodynamic parameters allow one to calculate [9] the flux intensity of free atoms produced in dissociation of molecules. Specifically, in the case of dissociation of hydrogen, oxygen, and nitrogen molecules on hot metal filaments under pressures of molecular gases higher than lO" Torr, the flux intensity of atoms A originating from A2 molecules is given by... 

Mass discrimination with distillation effects. Let us assume that the isotope composition of an element is being measured by thermal ionization. This method consists in ionizing the sample atoms by evaporation on a metal filament. Statistical thermodynamics (e.g., Denbigh, 1968) tells us that, while vapor pressure is a function... 

Nebulization is inefficient and therefore not appropriate for very small liquid samples. Introducing samples into the plasma in liquid form reduces the potential sensitivity because the analyte flux is limited by the amount of solvent that the plasma will tolerate. To circumvent these problems a variety of thermal and electrothermal vaporization devices have been investigated. Two basic approaches are in use. The first involves indirect vaporization of the sample in an electrothermal vaporizer, e.g. a carbon rod or tube furnace or heated metal filament as commonly used in atomic absorption spectrometry [7-9], The second involves inserting the sample into the base of the... 

Thermal vacuum evaporation. This method is used for evaporation and the subsequent deposition of various metals. Rather volatile metals such as Ag, Au, Cu, and Pd can be evaporated from heated containers. Evaporation of less volatile metals, in particular, Ti or Mo, occurs by electrical heating of metal filaments or bands [32]. In certain conditions chemical active gases, such as oxygen, sulfur vapors, and others, introduced in evaporation zone react with metal atoms giving semiconductor compounds (for example, oxides, sulfides). 

From the late 1960s onwards, a number of research groups around the world began to investigate alternatives to pneumatic nebulization for sample introduction, in an attempt to overcome transport efficiency limitations. The most successful approaches were those which involved heating small, discrete liquid samples, and sometimes even solid samples, directly on a metal filament, boat, or cup which could be positioned reproducibly into a flame. However, since the temperature of the metal would be lower than that of the flame itself, the techniques were confined to the determination of relatively easily atomized elements such as arsenic, bismuth, cadmium, copper, mercury, lead, selenium, silver, tellurium, thallium, and zinc. 

Thermal ionization is based on the production of atomic or molecular ions at the hot surface of a metal filament [95,96]. In this ionization source, the sample is deposited on a metal filament (W, Pt or Re) and an electric current is used to heat the metal to a high temperature. The ions are formed by electron transfer from the atom to the filament for positive species or from the filament to the atom for negative species. The analysed sample can be fixed to the filament by depositing drops of the sample solution on the filament surface followed by evaporation of the solvent to complete dryness, or by using electrodeposition methods. 

In the case of hot-filament CVD, refractory metal filaments (e.g., W, Ta, Re, etc.) are electrically heated to very high temperatures (between 2000 and 2700°C) to produce the necessary amount of atomic hydrogen that is necessary for the reasons mentioned above for the synthesis of diamond. Except for combustion flame CVD, hot-filament CVD is considered the simplest of all of the methods and also the most inexpensive. Plasma-jet and laser-assisted CVD methods rely on a plasma torch or laser to attain the very high temperatures that are needed to... 

Argon is used in incandescent light bulbs to permit the filament to be heated to a higher temperature, and thus to produce a whiter light, than would be practical in a vacuum. The argon decreases the rate at which the metallic filament evaporates, by keeping vaporized metal atoms from diffusing away from the filament. 

To find out if electrons present a diffraction pattern as light waves do, physicists Clinton Davisson and Lester Germer at the Bell Laboratories in New Jersey in 1927 were able to "shoot" electrons through a nickel crystal, whose structure is arranged by nature to have atomic planes just the right distance apart. They heated a metal filament to eject electrons that then traveled through the crystal to land in an electron detector. 

This technique uses an electrothermally heated graphite furnace, a carbon rod, or metal filament to atomize the sample. The sample is placed in the form of a solution or a solid in the atomizer. 

In thermal-ionization mass spectrometry (TIMS), samples consist of fg-ng quantities of chemically separated and purified analyte dissolved in a small volume (typically 1-10 pi). The solution is deposited on a refractory metal filament (e.g., high-purity W or Re), where it is evaporated to dryness. The filament is then heated to temperatures of 1,000-2,500°G in the ion source by resistive heating. If the ionization potential of the analyte is low compared to the work function of the filament, some of the analyte atoms will be ionized and emitted from the filament surface. TIMS ion sources commonly incorporate a second filament arranged... 

Electrothermal atomization (ETA) for use with atomic absorption (AA) has proved to be a very sensitive technique for trace element analysis over the last three decades. However, the possibility of using the atomization/heating device for electrothermal vaporization (ETV) sample introduction into an ICP mass spectrometer was identified in the late 1980s. The ETV sampling process relies on the basic principle that a carbon furnace or metal filament can be used to thermally separate the analytes... 

Electrothermal vaporizers (ETVs), commonly used in atomic absorption spectrometry, have been successfully employed as discrete sample introduction devices for ICP-MS.The graphite furnace, carbon rod, and metallic filaments have all been used as ETV devices. In general, these devices provide... 

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