Pneumatic spray application

Other temperatures and conditions not associated directly with the application of the sulfur and wax (e.g., cooling) remain the same as in the pneumatic spraying process. 

Few plant operators need to be told of the problems caused by water in compressed air. They are most apparent to those who operate pneumatic tools, rock drills, automatic pneumatic powered machinery, paint and other sprays, sandblasting equipment, and pneumatic controls. However, almost all applications, particularly of 100-psig power, could benefit from the elimination of water carryover. The principal problems might be summarized as ... 

As the vast majority of LC separations are carried out by means of gradient-elution RPLC, solvent-elimination RPLC-FUR interfaces suitable for the elimination of aqueous eluent contents are of considerable use. RPLC-FTTR systems based on TSP, PB and ultrasonic nebulisa-tion can handle relatively high flows of aqueous eluents (0.3-1 ml.min 1) and allow the use of conventional-size LC. However, due to diffuse spray characteristics and poor efficiency of analyte transfer to the substrate, their applicability is limited, with moderate (100 ng) to unfavourable (l-10pg) identification limits (mass injected). Better results (0.5-5 ng injected) are obtained with pneumatic and electrospray nebulisers, especially in combination with ZnSe substrates. Pneumatic LC-FI1R interfaces combine rapid solvent elimination with a relatively narrow spray. This allows deposition of analytes in narrow spots, so that FUR transmission microscopy achieves mass sensitivities in the low- or even sub-ng range. The flow-rates that can be handled directly by these systems are 2-50 pLmin-1, which means that micro- or narrow-bore LC (i.d. 0.2-1 mm) has to be applied. 

The ionspray (ISP, or pneumatically assisted electrospray) LC-MS interface offers all the benefits of electrospray ionisation with the additional advantages of accommodating a wide liquid flow range (up to 1 rnl.rnin ) and improved ion current stability [536]. In most LC-MS applications, one aims at introducing the highest possible flow-rate to the interface. While early ESI interfaces show best performance at 5-l() iLrnin, ion-spray interfaces are optimised for flow-rates between 50 and 200 xLmin 1. A gradient capillary HPLC system (320 xm i.d., 3-5 xLmin 1) is ideally suited for direct coupling to an electrospray mass spectrometer [537]. In sample-limited cases, nano-ISP interfaces are applied which can efficiently be operated at sub-p,Lmin 1 flow-rates [538,539]. These flow-rates are directly compatible with micro- and capillary HPLC systems, and with other separation techniques (CE, CEC). 

The first ESI design at the end of the 1980s proved to work properly as the HPLC interface with mobile phase flow rates between 1 and lOpL/min. Meanwhile, the development of the HPLC instrumentation and columns was oriented in the mL/min flow rate mode. In addition, the nebulization process based only on the application of an electrical field does not produce a stable spray from aqueous mobile phases. A modified ESI source, called ionspray, was then introduced [39], in which the nebulization of a liquid solution is pneumatically assisted by a coaxial flow of nitrogen (sheath gas) that allows the formation of a stable aerosol at mobile-phase flow rates between 10 and 500 pL/ min and the use of aqueous mobile phases. When working at higher flow rates (500-1000 pL/min), an additional nittogen flow rate can be used (auxiliary gas) to assist the desolvation of the droplets. This modified source is called turboionspray. 

Despite the distinct advantages of pneumatic nebulizers, ultrasonic nebulizers may alternatively be used, in some instances, with success. In a recent application, a variation of ultrasonic nebulizer called spray nozzle-rotating disk FTIR interface was successfully applied to confirm the presence of methyltestosterone, testosterone, fluoxymesterone, epitestosterone, and estradiol and testosterone cyp-ionate in urine, after solid-phase extraction and reversed-phase LC separation (151). Using a commercial infrared microscopy spectrometer, usable spectra from 5 ng steroid deposits could be readily obtained. To achieve success with this interface, phosphate buffers in the mobile phase were not used because these nonvolatile salts accumulate on the collection disk and their spectra tend to swamp out small mass deposits. Another limitation of the method was that only nonvolatile analytes could be analyzed because volatile compounds simply evaporated off the collection-disk surface prior to scanning. 

The main variables in the operation of atomizers are feed pressure, orifice diameter, flow rate and motive pressure for nozzles and geometry and rotation speed of wheels. Enough is known about these factors to enable prediction of size distribution and throw of droplets in specific equipment. Effects of some atomizer characteristics and other operating variables on spray dryer performance are summarized in Table 9.18. A detailed survey of theory, design and performance of atomizers is made by Masters (1976), but the conclusion is that experience and pilot plant work still are essential guides to selection of atomizers. A clear choice between nozzles and spray wheels is rarely possible and may be arbitrary. Milk dryers in the United States, for example, are equipped with nozzles, but those in Europe usually with spray wheels. Pneumatic nozzles may be favored for polymeric solutions, although data for PVC emulsions in Table 9.16(a) show that spray wheels and pressure nozzles also are used. Both pressure nozzles and spray wheels are shown to be in use for several of the applications of Table 9.16(a). 

Two-Fluid (Pneumatic) Atomizers This general category includes such diverse applications as venturi atomizers and reactor-effluent quench systems in addition to two-fluid spray nozzles. Depending on the manner in which the two fluids meet, several of the breakup mechanisms may be applicable, but the final one is high-level turbulent rupture. 

The helium interface described by Apffel et al. [15] was a eommercially available pneumatic nebulizer, spraying direetly into the redneed pressure ion source. No applications have been described other than those of the designers gronp. It has been questioned by Brains [16] whether, nnder the given experimental conditions with this interface, nebulization of the colnmn effluent aetually takes place. Considering the flow-rate (10-50 pl/min of preferentially organic solvents), the capillary diameter, and the temperature, Bruins [16] conclndes that this interfaee most likely relies on complete evaporation of the column effluent while the sample vapour is subsequently swept into the source by the helium stream. 

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