Evacuated tubes

Time-of-flight (TOF) instmments utilize the times taken by ions to pass (fly) along an evacuated tube as a means of measuring m/z values and therefore of obtaining a mass spectmm. Often a reflectron is used to direct the ions back along the TOF tube. 

Vacuum Tubes. In the manufacture of vacuum tubes for use in polarized ion sources, vaporized cesium is used as a getter for residual gaseous impurities in the tube and as a coating to reduce the work function of the tungsten filaments or cathodes of the tube. The cesium vapor is generated by firing, at about 850°C within the sealed and evacuated tube, a cesium chromate pellet and zirconium (12) (see Vacuum technology). 

Astacin (P,P-carotene-3,3, 4,4 -tetraone) [514-76-1] M 592.8, m 228", 240-243"(evacuated tube), 550,000 at 498mm (pyridine). Probable impurity is astaxanthin. Purified by chromatography on alumina/fibrous clay (1 4) or sucrose, or by partition between pet ether and MeOH (alkaline). Crystd from pyridine/water. Stored in the dark under N2 at -20°. [Davis and Weedon J Chem Soc 182 I 960.]... 

Citrazinic acid (2,6-dihydroxyisonicotinic acid) [99-11-6] M 155.1, m >300°, pK] 3.0, pK2 4.76. Yellow powder with a greenish shade, but is white when ultra pure and turns blue on long standing. It is insoluble in H2O but slightly soluble in hot HCl and soluble in alkali or carbonate solutions. Purified by precipitation from alkaline solutions with dilute HCl, and dry in a vacuum over P2O5. The ethyl ester has m 232° (evacuated tube) and a pKa of 4.81 in MeOCH2CH20H [IR Pitha Coll Czech Chem Comm 28 1408 1963]. 

Nickelocene [bis-(cyclopentadienyl)nickel II] [1271-28-9] M 188.9, m 173-174 (under N2). Dissolve in Et20, filter and evaporate in a vacuum. Purify rapidly by recrystn from pet ether using a solid C02-Me2C0 bath, m 171-173°(in an evacuated tube). Also purified by vacuum sublimation. [J Am Chem Soc 76 1970 954 J Inorg Nud Chem 2 95, 110 7956.]... 

Of the many molybdenum sulfides which have been reported, only MoS, M0S2 and M02S3 are well established. A hydrated form of the trisulfide of somewhat variable composition is precipitated from aqueous molybdate solutions by H2S in classical analytical separations of molybdenum, but it is best prepared by thermal decomposition of the thiomolybdate, (NH4)2MoS4. MoS is formed by heating the calculated amounts of Mo and S in an evacuated tube. The black M0S2, however, is the most stable sulfide and, besides being the principal ore of Mo,... 

A cathode ray consisting of fast-moving electrons flows through the spark coil and into the partially evacuated tube... 

Electron beam sterilisation is a high-voltage potential established between a cathode and an anode in an evacuated tube. The cathode emits electrons, as a cathodic ray or electron beam. A high intensity of electrons is produced. These electrons are accelerated to extremely high velocities. These accelerated electron intensities have great potential as a bacteriocide. Most electron beams operate in a vacuum. As a result the unwanted organisms in the media vanish and the media is sterilised. 

Sh E-Test 4.4A A sample of neon of volume 10.0 L at 300. Torr is allowed to expand isothermally into an evacuated tube with a volume of 20.0 L. What is the final pressure of the neon in the tube ... 

Based on the photoelectric effect, electrons in evacuated tubes (photoelectrons) are released from a metal surface if it is irradiated with photons of sufficient quantum energy. These are simple photocells. Photomultipliers are more sophisticated and used in modem spectrophotometers where, via high voltage, the photoelectrons are accelerated to another electrode (dynode) where one electron releases several electrons more, and by repetition up to more than ten times a signal amplification on the order of 10 can be obtained. This means that one photon finally achieves the release of 10 electrons from the anode, which easily can be measured as an electric current. The sensitivity of such a photomultiplier resembles the sensitivity of the human eye adapted to darkness. The devices described are mainly used in laboratory-bound spectrophotometers. 

Primary X-rays are produced by the bombardment of a suitable target with a stream of accelerated electrons. A typical X-ray generator uses an evacuated tube into which the target (e.g. tungsten) projects as a cooled anode together with a tungsten filament cathode. At a potential of 20-50 keV electrons are emitted from the cathode and bombard the... 

A historical introduction into the uses of solar energy was attempted followed by a description of the various types of collectors including flat-plate, compound parabolic, evacuated tube, parabolic trough, Fresnel lens, parabolic dish and heliostat field collectors (Kalogirou, 2004a, 2004b). Solar thermal electricity systems utilize... 

The U-tube must be uniform in diameter and scrupulously clean to avoid errors due to variations in surface tension. The best final stage for the cleaning process is to heat the evacuated tube to between 280 and 300 °C. A small constriction at the base of the U does not affect the measurements and prevents sudden surges of fluid, which can be destructive. 

Evacuated tubes are used when the products or reactants are sensitive to air or water or are volatile. An example of the use of this method Is the preparation of samarium sulfide. In this case, sulfur has a low boiling temperature (717 K) and an evacuated tube Is necessary to prevent It boiling off and being lost from the reaction vessel. 

Dougherty159) reported in 1968 that the peak at m/e = 300 in the mass spectrum of hexahelicene (C26H16) is due to the ion of coronene (C24H12+), formed by an internal Diels-Alder reaction, followed by a fragmentation yielding ethylene. Indeed, traces of coronene were detected on heating hexahelicene at 485 °C for 2 h in an evacuated tube. 

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