The Moving Wire Detector

The sensitivity of the wire transport detector, besides being dependent on the noise level of the system, also depends on the quantity of volatile pyrolysis products produced from the solute. Excluding synthetic polymers, which often quantitatively produce monomers on pyrolysis, many compounds only yield a few percent of their mass as volatile combustible pyrolysis products. Thus, the FID may only, in effect, detect a few percent of the solute 

The passage of hydrogen from the jet to the venturi resulted in a pressure drop around the venturi, and thus, allowed other gases to be sucked continuously into the hydrogen stream. 

Van Oijk (16) developed a spray procedure for coating the wire in an attempt to improve the sensitivity of the detector. The column effluent passed directly to an atomizer, the nozzle of 

Compton and Purdy (18) modified the FID of a Pye Unicam detector by inserting a rubidium silicate glass bead above the flame and thus changed it into a thermal ionic detector. The detector was then selectively sensitive to nitrogen and phosphorus compounds. 

Slais and Krejci (21) replaced the normal FID with the alkali FID to selectively detect chlorine compounds. These workers used a combustion technique as opposed to pyrolysis, mixed the products of combustions with hydrogen, and then passed the mixture directly to the alkali FID. At a column flow rate of 0.37 ml/min, the sensitivity of the detector was stated to be 3 x g/sec, which 

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Adsorption chromatography using small particle silica or alumina has also been employed in the separation of biologically meaningful substances. Phospholipids, for example, have been separated on silica (38). One of the big problems for such substances is detection, since many of the compounds are not U.V. active. Generally, the refractive index detector is employed for isocratic operation, and the moving wire detector for gradient operation. Formation of U.V.-active derivatives is also possible (39). 

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. 

The response peaks for the saturates and aromatics using the moving-wire detector are shown in the bottom half of Figure 2. The response peaks were retarded by 2 mL from the retention volume obtained with the RI detector. In addition, the peaks were broadened appreciably and the valley between the saturates and aromatics peaks did not return completely to the base line. Since there was a small tail on the aromatics peak as well, it was concluded that there was some hold-up in the system, most probably at the sampling point on the LCM-2 or in the 0.040-in. I.D. tubing. 

For the moving-wire detector, the precision (2o-) for the saturates response was dz5%, relative, while for the aromatics it was 8.6%, relative. The aromatic/saturate response ratio was 8%, relative. The precision for the retention volume was 1.4%, relative. 

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... 

Figure 18. Linearity curves for the moving wire detector.

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