Moving belts

One of the first successful techniques for selectively removing solvent from a solution without losing the dissolved solute was to add the solution dropwise to a moving continuous belt. The drops of solution on the belt were heated sufficiently to evaporate the solvent, and the residual solute on the belt was carried into a normal El (electron ionization) or Cl (chemical ionization) ion source, where it was heated more strongly so that it in turn volatilized and could be ionized. However, the moving-belt system had some mechanical problems and could be temperamental. The more recent, less-mechanical inlets such as electrospray have displaced it. The electrospray inlet should be compared with the atmospheric-pressure chemical ionization (APCI) inlet, which is described in Chapter 9. 

In the earliest interface, a continuous moving belt (loop) was used onto which the liquid emerging from the chromatographic column was placed as a succession of drops. As the belt moved along, the drops were heated at a low temperature to evaporate the solvent and leave behind any mixture components. Finally, the dried components were carried into the ion source, where they were heated strongly to volatilize them, after which they were ionized. 

LC can be combined with all kinds of mass spectrometers, but for practical reasons only quadrapolar, magnetic/electric-sector, and TOP instruments are in wide use. A variety of interfaces are used, including thermospray, plasmaspray, electrospray, dynamic fast-atom bombardment (FAB), particle beam, and moving belt. 

Moving-belt (ribbon or wire) interface. An interface that continuously applies all, or a part of, the effluent from a liquid chromatograph to a belt (ribbon or wire) that passes through two or more orifices, with differential pumping into the mass spectrometer s vacuum system. Heat is applied to remove the solvent and to evaporate the solute into the ion source. 

Manufacturing processes have been improved by use of on-line computer control and statistical process control leading to more uniform final products. Production methods now include inverse (water-in-oil) suspension polymerization, inverse emulsion polymerization, and continuous aqueous solution polymerization on moving belts. Conventional azo, peroxy, redox, and gamma-ray initiators are used in batch and continuous processes. Recent patents describe processes for preparing transparent and stable microlatexes by inverse microemulsion polymerization. New methods have also been described for reducing residual acrylamide monomer in finished products. 

Cellulose Acetate. The extmsion process has also been used to produce ceUular ceUulose acetate (96) ia the deasity range of 96—112 kg/m (6—7 lbs /fT). A hot mixture of polymer, blowiag ageat, and nucleating agent is forced through an orifice iato the atmosphere. It expands, cools, and is carried away on a moving belt. 

Prior to deposition on a moving belt or screen, the molten polymer threads from a spinnerette must be attenuated to orient the molecular chains of the fibers in order to increase fiber strength and decrease extendibiUty. This is accompHshed by hauling the plastic fibers off immediately after they have exited the spinnerette. In practice this is done by accelerating the fibers either mechanically (18) or pneumatically (17,19,20). In most processes, the fibers are pneumatically accelerated in multiple filament bundles however, other arrangements have been described wherein a linearly aligned row(s) of individual filaments is pneumatically accelerated (21,22). 

The pneumatic deposition of the filament bundles onto the moving belt results in formation of the web. A pneumatic gun uses high pressure air to move the filaments through a constricted area of lower pressure but higher velocity, as in a venturi tube. Pneumatic jets used in spunbonded production have been described (17,24). Unfortunately, the exceUent filament uniformity coming out of the spinnerette is lost when the filaments are consoHdated going through a gun. 

Beside continuous horizontal kilns, numerous other methods for dry pyrolysis of urea have been described, eg, use of stirred batch or continuous reactors, ribbon mixers, ball mills, etc (109), heated metal surfaces such as moving belts, screws, rotating dmms, etc (110), molten tin or its alloys (111), dielectric heating (112), and fluidized beds (with performed urea cyanurate) (113). AH of these modifications yield impure CA. 

When samphng from moving belt conveyors, the cutter operates in a radial mode with the belt surrace contoured at the point of samphng by idlers, fixing radial curvature to the outer radius of the cutter. Clearance is minimized between outer edges of cutter blades and belt surface by cutter-shaft adjustment in the drive-clamping bracket. 

Cutter speed at the outer radius is recommended at twice the conveyor belt speed for through-stream extractions from moving belts. The cutter is adjusted in a lateral angle to a 30-degree position, matching the cutter extraction path through the material bed on the belt at specified speed. 

Rubber media appear as porous, flexible rubber sheets and microporous hard rubber sheets. Commercial rubber media have 1100-6400 holes/in. with pore diameters of 0.012-0.004 in. They are manufactured out of soft rubber, hard rubber, flexible hard rubber and soft neoprene. The medium is prepared on a master form, consisting of a heavy fabric belt, surfaced on one side with a layer of rubber filled with small round pits uniformly spaced. These pits are 0.020 in. deep, and the number per unit area and their surface diameter determine the porosity of the sheet. A thin layer of latex is fed to the moving belt by a spreader bar so that... 

Flat pieces of product, such as fish fillets, would suffer a change in shape in a free air blast and are better on a flat moving belt. Here, some of the heat goes direct to the cold air and some by conduction to the belt, which is usually of stainless steel. This tunnel... 

Seven different LC-MS interfaces are described in Chapter 4, with particular emphasis being placed on their advantages and disadvantages and the ways in which the interface overcomes (or fails to overcome) the incompatibilities of the two techniques. The earlier interfaces are included for historical reasons only as, for example, the moving-belt and direct-liquid-introduction interfaces, are not currently in routine use. The final chapter (Chapter 5) is devoted to a number of illustrative examples of the way in which LC-MS has been used to solve various analytical problems. 

He then joined the Central Research Establishment of the Home Office Forensic Science Service (as it then was) at Aldermaston where he developed thermogravimetry-MS, pyrolysis-MS, GC-MS and LC-MS methodologies for the identification of analytes associated with crime investigations. It was here that his interest in LC-MS began with the use of an early moving-belt interface. This interest continued during periods of employment with two manufacturers of LC-MS equipment, namely Kratos and subsequently Interion, the UK arm of the Vestec Corporation of Houston, Texas, the company set up by Marvin Vestal, the primary developer of the thermospray LC-MS interface. 

El may be used with the moving-belt and particle-beam interfaces. Cl with the moving-belt, particle-beam and direct-liquid-introduction interfaces, and FAB with the continuous-flow FAB interface. A brief description of these ionization methods will be provided here but for further details the book by Ashcroft [8] is recommended. 

In summary, it can be said that prior to the development of the thermospray interface there were an increasing nnmber of reports of the analytical application of LC-MS [3] bnt in this present anthor s opinion, based on a nnmber of years of using a moving-belt interface, the technique could not be considered to be routine . The thermospray interface changed this and with the commercial intro-dnction of the combined APCI/electrospray systems in the 1990s the technique, for it now may be considered as a true hybrid technique, has reached maturity (although this should not be taken as a suggestion that there will be no further developments). 

The first interface to be made available commercially was the moving-belt interface, shown schematically in Figure 4.1. 

---