Density gas balance

A gas density balance is shown in Fig. 71. A sealed bulb G at one end of a beam A is counterpoised by C. This is pivoted at F and enclosed in an envelope H. Gases of known (p ) and unknown (p ) densities are run separately into H until the pointer is zeroed at D in each case. Then, for pressures and p required, 

Further details are given in ref. 22c. The instrument has long been superseded for conventional analysis by the mass spectrometer. However, it remains as an absolute control for gas chromatographic analysis. 

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Material emerging from the column is detected by a thermal-conductivity cell, an ionisation method, or a gas-density balance. 

Accdg to Kirk Othmer (Ref 13, p 890), the direct detn of gas densities, by weighing a bulb before and after filling with the gas at a known pressure, is not frequently used because of the tedious operations and sources of error. Instead of this the Gas Density Balances employing the buoyancy principle are in common use. As an example of such balances is the Edwards Portable Balance developed at the National Bureau of Standards and the ArcoBalance. 

The following data provide useful guidance in the operation and optimization of procedures with the gas density balance detector in gas chromatography.1 The property values were calculated with... 

James and Martin soon developed a gas-density balance as a detector, but the sensitivity of GLC was increased enormously when the argon ionization detector was invented by James Lovelock it was he who also produced the electron capture detector in conjunction with S. R. Lipsky in 1959.162 The flame ionization detector originated in the same year.163 Another important advance in the early days of GLC was the introduction of capillary columns, which were first used by M. J. E. Golay in 1956.164165 The development of some of the first commercial instrumentation has been described.166167... 

Other Detectors. Most of the other commercially available detectors are highly selective and sensitive and find use in specialized analyses, including inorganic analyses. One exception is the gas density balance,18 which is a universal type of detector and finds use in molecular weight determinations (see Chapter 6) and for the analysis of corrosive materials. 

A balance suitable for measuring the density of carbon dioxide and other gases is described by Edwards.18 Although excellent for ordinary work it can not attain the accuracy required here. A more sensitive gas density balance has been described by Cady19 in which a balloon is suspended from an ordinary high sensitivity... 

Free-floating balloons of thin glass and rubber were first tried. The glass was too heavy the rubber too permeable. Balloons suspended from springs were too insensitive, and balloons suspended from balance beams were too mobile. A gas density balance operated by the pull of a large solenoid or an enclosed magnet was used but, although extremely sensitive, it was discarded because it involved an unnecessary and somewhat erratic variable. 

On-line analysis of the hydrocarbons in the vent gas was done on a 20-ft Porapak Q column with an internal standard. In many cases, the internal standard also allowed calculation of hydrogen by difference. A Beckman 3AM3 gas density balance provided a check on the vent gas analysis and the necessary parameter for calculating mass flow rates from orifice pressure drop data. Mass balance closure was typically better than it 2%. 

Phillips and Timms [599] described a less general method. They converted germanium and silicon in alloys into hydrides and further into chlorides by contact with gold trichloride. They performed GC on a column packed with 13% of silicone 702 on Celite with the use of a gas-density balance for detection. Juvet and Fischer [600] developed a special reactor coupled directly to the chromatographic column, in which they fluorinated metals in alloys, carbides, oxides, sulphides and salts. In these samples, they determined quantitatively uranium, sulphur, selenium, technetium, tungsten, molybdenum, rhenium, silicon, boron, osmium, vanadium, iridium and platinum as fluorides. They performed the analysis on a PTFE column packed with 15% of Kel-F oil No. 10 on Chromosorb T. Prior to analysis the column was conditioned with fluorine and chlorine trifluoride in order to remove moisture and reactive organic compounds. The thermal conductivity detector was equipped with nickel-coated filaments resistant to corrosion with metal fluorides. Fig. 5.34 illustrates the analysis of tungsten, rhenium and osmium fluorides by this method. 

Sie et al. [601—603] analysed silicon and tin in different alloys and steel samples. A sample of the material (1-50 mg) was heated at 600-900° C in a quartz tube, which was then washed with chlorine. Chlorides were trapped in a colum packed with 15% of Kel-F 40 on Haloport F and analysed on the same column at 75°C. With the use of a gas-density balance and PTFE-coated filaments a sensitivity of 50 ppm was obtained for silicon. The analysis time of 15—20 min can be reduced to 10 min. 

The Katherometer and Some of the Less Well Known Detectors The Simple Gas Density Balance The Radioactivity Detector... 

Measurements of Vm-— Direct determination of 1 using a gas density balance has in the past proved rather unsuccessful, leading to a value of B, for CHFs of the wrong sign. Recently, however, this method has been refined to the point where it may be useful in the future. 

Of the many detectors that are discussed in the literature, we shall consider three which are the most important for gas-phase kinetic studies. These are the katharometer, the hydrogen flame ionisation detector (fid), and the gas density balance. 

Fig. 55. Schematic view of the gas density balance detector. See J. H. Purnell p. 271.

An initial product identification can be carried out by comparing the retention times, under given conditions, of the unknown compound with that of suspected known compounds. To counteract minor variations in flow rate and temperature, coincidence of peaks can be checked with consecutive doping or marking of the mixture with known compounds. Coincidence should also be checked by using one or more different columns. In this context the gas density balance detector provides particular confidence in identification. 

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