Density balance detector

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... 

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. 

A gas chromatographic method using a gas density balance detector has been described407 for the determination of chlorinated phenylchlorosilanes using a column packed with Celite 545 supporting 10% of Lukopren G 1000 (a silicone elastomer), with nitrogen as carrier gas. 

The MC-2 mass chromatograph from the Spex Industries determines molecular weight of GC eluent through differential gas density measurements. A sample is split into two equal fractions they are carried by two different gases, C02 and Freon 115, through two matched GC columns into density-balance detectors. The molecular weight of the unknown is obtained from... 

Figure 5.25 Gas density balance detector, mounted as shown.

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. 

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. 

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

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. 

The purpose of the detector is to determine when and how much of a compound has emerged from the column. Although the goal of all detectors is to be as sensitive as possible, many detectors are designed to be selective for certain classes of compounds. Dozens of different types of detectors have been developed, but only a few are used routinely. Those are thermal conductivity (TC), thermionic (N/P), electron capture (ECD), flame photometric (FPD), Hall electroconductivity detector (Hall or ELCD), hydrogen flame ionization detector (FID), argon ionization (AI), photoionization (PID), gas density balance (GDB), and the mass spectrometer. Chemists usually select a detector by the following criteria, listed in priority ... 

This and the gas density balance are the only detectors that do not destroy the sample when making a measurement. In times of need, a detector of this type can be made from either the filaments of a light bulb or those from a model airplane glow plug. 

The gas density balance (GDB), one of the chromatographic detectors that first became available, has the advantage that it can be used with corrosive gases as the sample does not pass over filaments. The schematic of this detector is shown in Figure 13.9. 

The advisability of using these methods in Py-GC was pointed out in an earher book [4]. In some Py-GC work, the use of selective detectors led to the successful identification of products. For example, the molecular mass of the pyrolysis products can be determined in a much simpler way than by mass spectrometry, if two gas density balances are used [132]. The pyrolysis products were analysed on two chromatographs with density balances using two different reference gases, carbon dioxide and penta-fluorochloroethane. 

At one time GC analysis was less accurate than classical elemental analysis, but the development of simple, reliable and sensitive thermal conductivity detectors (TCDs), special electronic integrators and small computers has brought the reproducibility of chromatographic peak-area measurements to at least 0.2% (see, for example, ref. 11) the accuracy of chromatographic elemental analysis is now close to that of standard gravimetric methods, which is normally 0.3% [12]. If the GC elemental analysis is used for the determination of reactive compounds, such as halogens or hydrogen halides, use is made of gas density balances in which the sensitive elements are protected by a flow of an inert gas [13]. 

Universal detectors which can detect any kind of com x>und find a quantitative use only in GC. Examples are the thermal conductivity detector and the gas density balance (see Section 15.3.4.2). Most GC detectors are mass flow sensitive, which means that the output signal is directly proportional to the mass flow of a compound through the detector. The signal from such detectors is normally obtained from a direct effect of the detection system on the eluted compound, eg, ionization or transfer to an excited state. 

Working principle. The gas density balance uses the same measuring principle as the heat conductivity detector. However, as can be seen from Figure 15-8 two filaments are placed in... 

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