Transport detectors moving wire

Maggs [36] developed an electron-capture detector (ECD) which was based 6n the moving wire transport system. This type of detector is now available commercially (Table 3.5). Nota and Palombardi [37] described a system in which the column eluent was continuously nebulized and part of which was directed into the interior of an ECD. 

As already mentioned under transport detectors, Dugger [6] modified the moving wire detector to detect tritium and carbon. To detect carbon, the solute coated on the wire after evaporation of the solvent was oxidized to carbon dioxide and water. The radioactive carbon dioxide was passed to a Geiger counter and detected in the same manner as that described by James and Piper [7] which was discussed under GC radioactivity detectors in an earlier chapter. Tritium could be detected by passing the water vapor from the oxidation process over heated iron to reduce it to hydrogen and tritium, which was then also passed through a Geiger counter. 

A modified Pye Unicam moving-wire detector was described by Scott et al. [35] in 1974 to fit the vacuum requirements of a mass spectrometer (Figure 3.3). Part of the colunrn effluent is deposited on to a wire, which transports the liquid along a heating element to evaporate the solvents, and through a series of vacuum locks to the ion source where the analyte is thermally desorbed from the wire prior to the ionization. Ionization is independent of the LC system. Therefore, conventional El and Cl spectra can be obtained [35]. This approach was subsequently adapted in 1976 by MacFadden [36] into the moving-belt interface (Ch.4.4). 

Another instrument called the transport detector, used for detection of lipids, proteins or carbohydrates, requires the transport of the column eluent by a moving wire disc, chain or helix. The solvent is evaporated in a furnace and the nonvolatile sample passes into a flame ionization detector (FID) which is detailed later under gas chromatography (GC) wherein FID counts amongst the major detectors. 

Desolvationjtransport detectors. The principle of transport detectors, typified by the moving wire detector (Figure 6.29), was based on the concept of physically separating the solvent, which is necessarily volatile, from the involatile solute. The transport wire is passed through a coating block where eluant from the column is applied. The solvent is then evaporated, and the wire plus solute then passes to a pyrolysis or combustion... 

Mass detector. The liquid chromatographer s demand for a universal detector which overcomes some of the problems encountered with the RI detector, (such as poor sensitivity and temperature instability) led to the development about ten years ago of the mass detector described here. The transport detectors of the 1960s detected the solute by means of a flame ionization detector after removal of the solvent from the eluent stream. They were abandoned, owing to lack of sensitivity and mechanical problems associated with the moving belt or wire. The new mass detector is similar in principle, but here the eluent leaves the column and is pumped into a nebulizer, assisted by an air supply. The atomized liquid is passed into a heated evaporation column where all the solutes less volatile than the solvent are carried down the column as a cloud of fine particles. A light source and photomultiplier arranged at the bottom of the column, perpendicular to the flow, detect the cloud of particles. The output from the photomultiplier, which is proportional to the concentration, can be amplified and directed to a recorder or data system. 

The transport system for LC detection was developed to render the detector independent of the choice of mobile phase and allow any solvent to be used without compromise. The column eluent flows over the transporter, which may be a moving wire, chain or disc which takes up all, or a portion of the column eluent. The solvent is then evaporated from the transporter, usually by heating, and the solute is left as a coating on the surface. The transporter then carries the solute into a detection area, where it is sensed by suitable means, such as pyrolysis and subsequently detected by passing the pyrolysis products to a flame ionization detector. The transport detectors, by and large, are not very... 

Another group of detectors with sample transfer employs pyrolysis of macromolecules. Column effluent is deposited continuously on the appropriate moving transporter such as a wire, chain, or net. Next, the solvent is evaporated, sample is pyrolyzed and polymer concentration is assessed from the amount of caibon dioxide formed. The problems with cleaning of sample transporter to prevent base line noise and drift prevented broad use of detectors of this kind. The attempts to apply similar principle for monitoring polymer composition by engagement of complete gas chromatography of the fractions so far remained only on the level of laboratoiy experiments. Detection of such kind would provide valuable information on the sample composition, for example for statistical copolymers. There are, however, so far unsolved problems with the dependence of composition of pyrolytic products on presence of the neighboring units in copolymers. 

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