Liquid dipping techniques

Deposition by Selective Reaction with the Surface of the Support. The carrier is left in contact with an excess of solution for a definite time, and then the excess liquid is removed, e.g. using a dipping technique. The objective is to make a strong bond with the surface. The process is little used but it has potential for grafting or anchoring active elements to a support. 

With vinyl dispersions, the processor can use convenient liquid handling techniques such as spraying, pouring, spread coating, and dipping. This system permits products to be made that otherwise would require costly and heavy melt processing equipment. 

Figure 9.1 Illustration of some liquid deposition techniques including (a) dip coating, (b) spin coating, (c) spray coating, and (d) inkjet printing.

Figure 92. Scheme of the various general steps involved in liquid deposition techniques exemplified by dip coating. 

A Suzuki, M Ikeda, H Matsunami, T Tanaka. Liquid-phase epitaxial growth of 6H-SiC by vertical dipping technique. J Electrochem Soc 122 1741, 1974. 

In another guise, the dipping technique can be used to produce flexible, thin-walled articles from natural or synthetic polymers. The various types of synthetic polymer used for dipping are discussed in the next section. In the dipping process, a suitable shape, called in the industry a mold , form or former , is dipped with an appropriate dwell time into a liquid containing the polymer. The coated former is then heated to dry and cure the polymer as necessary. Finally, the article is removed from the former, whose shape it retains. The dipping process therefore provides the means to make seamless thin-walled items with predetermined, perhaps complex, shapes. 

The color shades of the chromatogram zones and above all the pale background produced by this technique are stabilized by dipping the chromatogram in a solution of liquid paraffln — n-hexane (1 -1-2) for 2 s. The color shades produced on silica gel and RP layers are not identical. 

The concept of SPME was first introduced by Belardi and Pawliszyn in 1989. A fiber (usually fused silica) which has been coated on the outside with a suitable polymer sorbent (e.g., polydimethylsiloxane) is dipped into the headspace above the sample or directly into the liquid sample. The pesticides are partitioned from the sample into the sorbent and an equilibrium between the gas or liquid and the sorbent is established. The analytes are thermally desorbed in a GC injector or liquid desorbed in a liquid chromatography (LC) injector. The autosampler has to be specially modified for SPME but otherwise the technique is simple to use, rapid, inexpensive and solvent free. Optimization of the procedure will involve the correct choice of phase, extraction time, ionic strength of the extraction step, temperature and the time and temperature of the desorption step. According to the chemical characteristics of the pesticides determined, the extraction efficiency is often influenced by the sample matrix and pH. 

For non-volatile sample molecules, other ionisation methods must be used, namely desorption/ionisation (DI) and nebulisation ionisation methods. In DI, the unifying aspect is the rapid addition of energy into a condensed-phase sample, with subsequent generation and release of ions into the mass analyser. In El and Cl, the processes of volatilisation and ionisation are distinct and separable in DI, they are intimately associated. In nebulisation ionisation, such as ESP or TSP, an aerosol spray is used at some stage to separate sample molecules and/or ions from the solvent liquid that carries them into the source of the mass spectrometer. Less volatile but thermally stable compounds can be thermally vaporised in the direct inlet probe (DIP) situated close to the ionising molecular beam. This DIP is standard equipment on most instruments an El spectrum results. Techniques that extend the utility of mass spectrometry to the least volatile and more labile organic molecules include FD, EHD, surface ionisation (SIMS, FAB) and matrix-assisted laser desorption (MALD) as the last... 

An extended line, sometimes called a dip leg or dip pipe, reduces the electrical charge that accumulates when liquid is allowed to free fall. When using dip pipes, however, care must be taken to prevent siphoning back when the inlet flow is stopped. A commonly used method is to place a hole in the dip pipe near the top of the vessel. Another technique is to use an angle iron instead of a pipe and to let the liquid flow down the angle iron (see Figure 7-21). These methods are also used when filling drums. 

C. These freeze-dried sections were dry mounted on microscope slides which had been precoated with either Kodak NTB-3 or NTB-10 emulsion. Other techniques which thawed the frozen section, embedded the tissue in paraffin or dipped the section in liquid emulsion were demonstrated to translocate diffusible compounds. Many other similar attempts have been and are currently being made to localize diffusible compounds by autoradiography at the electron microscope level. 

Autoradiography offered an alternative technique. The earliest experiments followed the uptake of radioiodine into the thyroid by placing the tissue sections in direct contact with photographic plates (Hamilton, Soley, and Eichom, 1940 Leblond, 1943). Belanger and Leblond introduced the use of liquid emulsion in 1946. Initially this was painted onto sections mounted on microscope slides. Later, slides were dipped into liquid emulsion (Joftes and Warren, 1955) or wrapped around with stripping film (Doniach and Pelc, 1950). Semiquantitative comparisons... 

Plastisols allowing the use of particular liquid-state processing techniques casting, roto-moulding, dipping, coating... 

A needle source consists of a hairpin filament (M80 pm diameter), usuafly of a refractory metal such as tungsten, with a short length of smaller diameter (M25 pm) wire spot-welded to it, Figure lb. The tip of the latter wire, the emitter, is electrochemically etched to a point with a radius of curvature at the apex of 2-5 pm the etching technique for tungsten has been described in detail by others (7,29). As quickly as possible after the assembly is thermally cleaned under vacuum (n<10 " Pa), the emitter is dipped into a molten pool of liquid metal and then withdrawn. If done correctly, the junction formed by the bend in the filament and the emitter wire will hold a small bead of metal, and the emitter will appear shiny from the thin film of metal on its surface. 

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