Moving-wire detector

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). 

Silica gel G (30), and Uni si 1 (31,32). All of these LC procedures are hampered by the absence of an adequate detection system. Although the lack of on-line detection has impeded the adaptation of the LC procedures to HPLC, preparative HPLC of glycolipids has been performed on silica SI 60 with post-column, off-line TLC detection (30) and with a moving wire detector (31). The procedure described below for per-O-benzoylation of glycolipids with benzoic anhydride in pyridine and DMAP as catalyst avoids N-benzoylation problem and provides a convenient method for the detection and preparative isolation of glycolipids. The application of this procedure for the isolation of 15 mg of glycolipids in a single HPLC run is described. 

The following chromatograms were obtained with the Fractosil 200 silica gel and the Pye moving wire detector as shown in Figure 2. They were plotted by the computer. The axes are the response in millivolts versus time in seconds. When the computer reconstructed the chromatogram, the Y-axis could be magnified from 0 to 520 millivolts. 

Linearity Curve for the Modified Moving Wire Detector... 

The Separation of Blood Liquids Employing Incremental Gradient Elution and Monitored by the Modified Moving Wire Detector... 

Compton and Purdy [16] refashioned the FID of the Pye Uni cam Modified moving wire detector by inserting a rubidium silicate glass bead above the flame and thus made its response specific and changed it into a nitrogen phosphorus detector. 

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. 

Figure 4. Effect of wire speed on aromatics/saturates response ratio. Sample, 10 fiL of vacuum gas oil solvent, n-heptane (1 mL/min) column, 1-ft fi-Porakl, mode, backflush operation for aromatics detector, Pye Unicom moving-wire detector.

Table V. Comparison of Moving-Wire Detector with the Refractive Index Detector and Clay—Gel Chromatographic Separation (Vacuum Gas Oil)...

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. 

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). 

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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