Emitter filament

The TSP interface was very popular and attractive to chromatogra-phers in the 1980s, as a result of its ease of operation and dependable performance. Commercial TSP LC/MS systems are equipped with an electron emitter filament to enhance the Cl process. 

Cl is an efficient, and relatively mild, method of ionization which takes place at a relatively high pressure, when compared to other methods of ionization used in mass spectrometry. The kinetics of the ion-molecule reactions involved would suggest that ultimate sensitivity should be obtained when ionization takes place at atmospheric pressure. It is not possible, however, to use the conventional source of electrons, a heated metallic filament, to effect the initial ionization of a reagent gas at such pressures, and an alternative, such as Ni, a emitter, or a corona discharge, must be employed. The corona discharge is used in commercially available APCI systems as it gives greater sensitivity and is less hazardous than the alternative. 

The experiment was carried out in a reaction cell shown in Fig. 3.3 with inner walls covered by a zinc oxide film having thickness 10 pm [20]. The surface area of the measuring film on the quartz plate was about 1/445 of the total film area on the wall of the vessel. The results of direct experimental measurements obtained when the adsorbent temperature was -196 C and temperature of pyrolysis filament (emitter of H-atoms) 1000°C and 1100°C, are shown on Fig. 3.4. One can see a satisfactory linear dependence between parameters A r (the change in film conductivity) and APh2 (reduction of hydrogen pressure due to adsorption of H-atoms), i.e. relations... 

It comprises an axially mounted W filament and three cylindrical collectors Ci,Ci, and Ci. All collectors are maintained at the same positive potential with respect to the W filament, but only the emission current collected from Ci is recorded. In the region of C2 the temperature and the surface concentration of adsorbate on the emitter are assumed to be uniform. 

Figure 13 shows a photocell used recently for work-function measurements (43). A metal coating inside the cell serves as the anode, and B is the cathode, which may be a metal foil or a film previously evaporated from a filament at C. Also provided at C are a W emitter for bombarding the cathode surface and a Pt electrode for thermally dissociating gas molecules. The monochromatic light which passes through the quartz window Q strikes the cathode B, and its energy can be measured by a calibrated photocell. 

In the diode the cathode is usually a W filament which can be flashed and maintained as a reference electrode at a temperature above which adsorption occurs. The anode may be of similar construction or take the form of a metal film evaporated from an adjacent filament. Experimentally, current polarization curves are obtained, first for the clean anode surface A and then for the covered anode surface A. Alternatively, resistance-voltage characteristics are measured (SO). The potential difference comprises the applied polarization and the C.P.D. between the emitter and collector. For a given anode current j,... 

In this design, a mulhple emitter assembly with heated tungsten filaments placed in parallel to the product s direction is used. The beam current is controlled by molybdenum grids held at a common potential. A planar screen... 

Thermal-ionization mass spectrometers (TIMS) combine a hot-filament source with a magnetic-sector mass spectrometer. The mass spectrometers are operated at low to moderate mass-resolving power. A large number of elements can be measured with thermal ionization mass spectrometry. Special care is taken to purify the samples using ion exchange columns. Samples are loaded onto the filaments along with an emitter, and a typical run may take several hours. Modem systems have multiple collectors so that several isotopes can be measured simultaneously. High-precision measurements are done with Faraday cup detectors, but low-abundance isotopes can be measured on electron multipliers. Modem machines are capable of precisions of 0.1 to 0.01 permit. 

In field desorption (FD), the sample is deposited onto the emitter, a high voltage is applied, and a current is passed through the emitter to heat up the filament. Mass spectra are acquired as the emitter current is gradually increased and the sample is evaporated from the emitter into the gas phase. Tlie analyte... 

The field emission microscope offers a very clear-cut and basically simple method of determining the mobility of adsorbates quantitatively. If it were possible to evaporate the gas under study from a suitable source (e.g., a heatable CuO filament for oxygen) in such a way that only a portion of the tip became contaminated, one could determine how, and at what temperatures of the tip, migration occurred. If one attempted to evaporate from a gas emitter placed on one side of the tip while the microscope tube was at room temperature, gas rebounding from the walls would instantly contaminate the whole tip and the experiment would fail. 

A needle source consists of a hairpin filament (M80 pm diameter), usuafly of a refractory metal such as tungsten, with a short length of smaller diameter (M25 pm) wire spot-welded to it, Figure lb. The tip of the latter wire, the emitter, is electrochemically etched to a point with a radius of curvature at the apex of 2-5 pm the etching technique for tungsten has been described in detail by others (7,29). As quickly as possible after the assembly is thermally cleaned under vacuum (n<10 " Pa), the emitter is dipped into a molten pool of liquid metal and then withdrawn. If done correctly, the junction formed by the bend in the filament and the emitter wire will hold a small bead of metal, and the emitter will appear shiny from the thin film of metal on its surface. 

LMI sources with needle emitters operate in essentially the same way as field ionization or field desorption sources. The filament is resistively heated to melt the metal film and/or promote its flow to the tip of the emitter. Typically, the emitter or anode is positively biased 3-5 kV with respect to its counter electrode, the cathode the actual operating voltage is determined... 

These results illustrate the importance of the chemical species of the element present in the deposit with regard to ion emission (and gives insight into the effect of the oxidizing/reducing nature of the ion emitter) but tell little about the actual mechanisms active in the ion emitting process. As an example, the ions could be emitted either from the deposit itself or from an intermediate material that formed as a consequence of the chemical properties, or it could be entirely an interface phenomenon in which the deposit only served as a repository for the uranium species and the supporting filament served as the ionization surface. 

Other than the issue of mass resolution, the major limitation of the instrument is the requirement that samples be at a constant voltage across the face of the emitter. The tube ion source gives about two orders of magnitude higher imaging resolution than the short filament source (3 pm vs. 200 pm) because of the issue... 

One advantage of a spectral radiation pyrometer is that the emissivity or emittance at only a specific wavelength (e.g. 0.653 pm) is of importance. A non-blackbody source will be less luminescent than a blackbody source at the same temperature. Thus, a falsely low temperature will be determined by sighting a calibrated disappearing filament pyrometer on the non-blackbody. This temperature has been referred to as the brightness temperature . 

The electrons needed for the primary ionization are not produced by a heated filament, as the pressure in that part of the interface is atmospheric pressure and the filament would burn, but rather using corona discharges or (3 particle emitters. These two electron sources are fairly insensitive to the presence of corrosive or oxidizing gases. 

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