Wire transport interface system

An alternative to direct liquid introduction is the moving belt, or moving-wire, transport interface. Because all l.c. solvents are evaporated before the sample is transported into the ion source, fewer restrictions are placed on solvent type, flow rates, or buffer composition. This system has been used for analysis of mixtures of pentoses, hexoses, and disaccha-rides. ... 

MS, however, also has certain advantages over other spectroscopic methods that in some ways makes it an ideal technique to combine with LC. Mass spectra can be obtained rapidly, only sub->ig amounts of material are required to provide satisfactory spectra and the data produced is highly informative with respect to molecular structure. There are two well established methods that can be used to interface a liquid chromatograph with a mass spectrometer. Firstly, the direct inlet system developed by McLafferty and co-workers (9-11) and secondly, the wire transport system developed by Scott et al. (12,13). The former takes a proportion of the column eluent and passes it directly into a conventional mass spectrometer volatilizing both solvent and solute into the ion source. The latter employs the wire transport system in the normal way and the solvent is evaporated from the wire after passage through the column eluent stream. The wire, coated with the residual solute. 

A diagram of the overall system is shown in Figure 16. The wire employed was that supplied for the wire transport detector, 0.005 in. O.D. and made of stainless steel. The wire from the drive system passes over an electrically insulated pulley, over a coating block (where the column eluent wets the wire), into the left hand interface of the mass spectrometer and thence through the ion source. It then exits through another identical interface, round another pulley and back to the drive system. In the lower portion of Figure. 16 is shown the location of the interface with respect to the ion source it is seen that the wire leaves the interface about 2 mm from the ion source and less than a centimeter from the electron beam. A potential is applied across the two pulleys causing a current of about 200 mA to pass... 

The slot in the interfaces for passage of the belt is formed by two "L"-shaped sapphire pieces which are attached to the stainless-steel flange or vacuum closure bar by epoxy cement. The belts used are either 0.05 or 0.075 mm thick and the slot tolerance is set to be 0.075 mm greater than the belt thickness (i.e., either 0.125 or 0.15 mm). The belt width is 0.317 cm and the slot width is 0.325 cm. A ribbon 0.32 cm wide travelling at a speed of 2.5 cm/sec will carry away a liquid film 0.2 mm thick from a solvent flow of 1 ml/min and if the solvent film can be evaporated without loss of solute, then the ribbon will transport virtually 100% of the solute into the mass spectrometer. Sample utilization will then depend only on the efficiency of the flash vaporization step. In practice, some sample is lost by spray processes and the flash vaporization cannot be fully efficient for all compounds. Nevertheless, yields in the range of 25-40% have been attained with an LC/MS ribbon interface system. It follows that the quantity of column eluent taken from the ribbon will be twenty times greater than that taken by the wire and provide significantly improved sensitivity. 

In principle a STM should be adequate to measure the electrical resistance of a single molecule since it suffices to measure I-V curves of the metal (tip)-molecule-metal (substrate) system. However, published results in the literature concerning this subject have to be considered cautiously because of the generally unknown nature of the molecule-metal contacts. An illustrative experiment demonstrates the relevance of the interface (Kushmerick et al, 2002). This experimental work studies charge transport using the cross-wire tunnel junction technique, where two... 

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). 

Moving Belt Interface (MBI). The concept of transport systems was first demonstrated by Scott et aL (I) who designed a system using a moving wire to carry the solvent/solute into the MS source via two vacuum locks where the vaporization of the solvent was accomplished. Then vaporization of the remaining solute was carried out by passing a current through the wire. The major drawback of this early prototype to transport systems was that the efficiency of the system was a mere %. 

There have been numerous successful approaches to LC/MS, ranging from mechanical transport of solute to the mass spectrometer after external solvent removal (belt/wire systems and parhcle-beam interfaces) to bulk soluhon introduchon (with or without splitting) involving nebulizahon and ionization direcly from the solvent stream. However, LC/MS has been surprisingly underutilized for the characterization of synthetic polymers despite its apparent advantages over the direct application of mass spectrometry. 

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