Nebulization process

The sample to be analyzed can be dissolved in an organic solvent, xylene or methylisobutyl ketone. Generally, for reasons of reproducibility and because of matrix effects (the surroundings affect the droplet size and therefore the effectiveness of the nebulization process), it is preferable to mineralize the sample in H2SO4, evaporate it and conduct the test in an aqueous environment. 

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. 

Because the plasma discharge is inefficient in dissociating droplets greater than 10p,m in diameter, the primary function of the spray chamber is to allow only the smallest droplets of about 5-10 xm to enter the plasma for dissociation, atomization, and finally ionization of the sample components. Its secondary aim is to reduce the effect of the peristaltic pump pulses on the nebulization process. Some ICP-MS spray chambers are also externally cooled (typically to 2-5 °C) to minimize the amount of solvent going into the plasma. Reduction of oxide species is the main benefit of this technical solution when volatile organic solvents are aspirated (e.g. ethanol in alcoholic beverages). 

Physical interferences can alter the aspiration, nebulization, desolvation, and volatilization processes. Substances in the sample that change the solution viseos-ity, for example, can alter the flow rate and the efficiency of the nebulization process. Combustible constituents, such as organic solvents, can change the atomizer temperature and thus affect the atomization efficiency indirectly. 

The CAN-BD process is a patented nebulization process (16,17). The experimental setup of the process has been described elsewhere (19-21). A schematic of the CAN-BD apparatus is shown in Figure 1. The unique features of this process are (a) simultaneous micronizing and drying, (b) drying at low temperatures, 10 to 65°C, (c) no organic solvent required when nebulizing an aqueous solution, (d) no high-pressure autoclave required, and (e) a continuous process, which can easily be scaled-up. 

A new nebulization process (CAN-BD) can generate fine dry particles with mean diameters of substantially less then S pm. 

The direct dilution approach works well for lubricant additives and fresh oils. However, this approach caimot handle used oils containing particulate or volatile compounds. Accurate analysis for sulfiir by ICP can be problematic if the samples contain volatile sulfur such as in sulfide form. There is a vapor enrichment phase separation that occurs during the pneumatic nebulization process in ICP. Hence, the use of closed vessel, high-pressure microwave digestion can be used to accurately prepare samples containing volatile sulfides. ICP can be used to determine the accmate sulfur result. 

Viscosity is a force that opposes the nebulization process it is the tendency for a liquid to resist flow. Thus a highly viscous liquid would be almost impossible to force through a small capillary. Viscosity is inversely related to temperature thus a highly viscous sample could be processed through a capillary if the temperature of the sample were raised. It is important that standard solutions and sample solutions have approximately the same viscosity since it is necessary that these solutions have the same sample uptake rate and produce the same particle size distribution. 

Other burners separate the nebulization process and the flame by producing small liquid droplets in a nebulizing chamber before the sample enters the flame. Figure 10-12 shows this process in the Perkin-Elmer unit. 

The ion formation in thermospray ionization, introduced in the early 1980s by Vestal and co-work-ers, was explained in terms of ion evaporation. Preformed analyte ions are evaporated from small, fast-evaporating, charged droplets generated by the thermospray nebulization process. Although the importance of ion evaporation in thermospray ionization is questioned, the emphasis put at it at the time certainly stimulated further investigation into liquid-based ionization approaches for MS. 

Other potential sources of interference are the sample matrix and the solvent used for making the sample solution. The sample matrix is anything in the sample other than the analyte. In some samples, the matrix is quite complex. Milk, for example, has a matrix that consists of an aqueous phase with suspended fat droplets and suspended micelles of milk protein, minerals, and other components of milk. The determination of calcium in milk presents matrix effects that are not found when determining calcium in drinking water. Sample solutions with high concentrations of salts other than the analyte may physically trap the analyte in particles that are slow to decompose, interfering in the vaporization step and causing interference. Differences in viscosity or surface tension between the standard solutions and the samples, or between different samples, will result in interference. Interference due to viscosity or surface tension occurs in the nebulization process for FAAS... 

The spray chamber restricts the movement of large droplets ( 10 pm in diameter or more) toward the plasma allowing only fine droplets ( 5-10 pm diameter) to enter the plasma. The reason is that the plasma is not efficient enough to dissociate large droplets. The spray chamber also smoothes out the pulses that form during the nebulization process due to the pumping. 

These nebulizers and their components are typically constructed from polymer materials, such as polytetrafluoroethylene (PTFE), perfluoroalkoxy (PFA), or poly-vinylfluoride (PVF), although some designs are available in borosilicate glass or quartz. The excellent corrosion resistance of the polymer nebulizers means they have naturally low blank levels. This characteristic, together with their ability to handle small sample volumes found in applications such as vapor phase decomposition (VPD), makes them an ideal choice for semiconductor laboratories that are carrying out ultratrace element analysis. - A microflow concentric nebulizer made from PFA is shown in Figure 3.8, and a typical spray pattern of the nebulization process is shown in Figure 3.9. 

A schematic representation of the thermospray nebulization process is shown in Figure 2. Initially, in the first part of the vaporizer tube, the liquid is heated until, at a certain stage, the onset of vaporization takes place. The vaporization process will start at the heated capillary walls and results in tearing of the liquid bubbles are formed within the liquid. 

Upon continuing vaporization, the stage of bubbles in the liquid transforms to liquid droplets in a vapour. The temperature measured at the vaporizer tube wall over the length of the capillary is also shown in Figure 2. When complete solvent evaporation inside the tube would be achieved, a sharp increase of the capillary wall temperature would be observed, where the vapour is heated. However, optimum ionization conditions are achieved at nearly complete inside-tube vaporization. From this description of the nebulization process, it may be concluded that the contact time between the liquid and the analyte molecules dissolved in the liquid and the hot surface of the capillary is relatively short. This limits the extent of thermal decomposition of labile analytes. 

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