Aerosol desolvation

Ultrasonic nebulization This has been applied since the early work on ICP-AES [151], Both nebulizer types where the sample liquid flows over the nebulizer transducer crystal and types where the ultrasonic radiation (at 1 MHz frequency) is focussed through a membrane on the standing sample solution have been used. When applying aerosol desolvation the power of detection of ICP-AES can be improved by a factor of 10 by using ultrasonic nebulization. This specifically applies to elements such as As, Cd and Pb, which are of environmental interest. However, because of the limitations discussed in Section 3.2, the approach is of particular use in the case of dilute analytes such as in water analysis [150]. Additional fine detailed development, however, is regularly carried out, as with ICP-AES the process is crucial for elements such as Cd, As and Pb for which threshold values in fresh water samples can just still be measured reliably with this type of sample introduction. Such a development is the microultrasonic nebulizer (pUSN) operated with argon carrier gas, as described by Tarr et al. [410]. 

It should be kept in mind that both thermospray nebulization and high-pressure nebulization [143] successfully allow the analyte introduction efficiency to be increased and thus also the power of detection, however, again only when aerosol desolvation is applied. They are especially interesting for speciation by on-line coupling of ICP-AES and HPLC, as shown later. 

Tab. 14. Detection limits (both on a 3er basis) obatined with OES using a toroidal MIP and pneumatic nebulization of solutions without desolvation as compared with those using a filament-type MIP and aerosol desolvation obtained by Beenakker et al. (Reprinted with permission from Ref. [450].)...

Zr 0+ with Pd+, i Hf 0+ with " Pt+), different approaches have been reported (e.g., Krachler etal. 1998 Lustig etal. 1997), including mathematical corrections, matrix-matched standards, use of various different sample introduction devices (including aerosol desolvation), and the use of so-called high-resolution mass spectrometers (Kollensperger etal. 2000). 

Appropriate sample digestion, element-matrix separation techniques, aerosol desolvation [67, 68], and use of an alternative sample introduction system [66, 69, 70] are means to avoid (some cases of) spectral overlap. In present-day ICP-MS instrumentation, some more general strategies to tackle spectral overlap can also be deployed. In general, spectral interferences are an even greater nuisance in isotopic than in trace element analysis. A first reason for this lies in the fact that in isotopic analysis, at least two nuclides should display a signal free from spectral overlap. Moreover, whereas a limited extent of spectral overlap (e.g., 0.1-1%) may be negligible in trace element determination, this is definitely not the case in isotope ratio determination. 

An aerosol produced instrumentally has similar properties, except that the aerosol is usually produced from solutions and not from pure liquids. For solutions of analytes, the droplets consist of solute and solvent, from which the latter can evaporate to give smaller droplets of increasingly concentrated solution (Figure 19.1). If the solvent evaporates entirely from a droplet, the desolvated dry solute appears as small solid particles, often simply called particulate matter. 

In a concentric-tube nebulizer, the sample solution is drawn through the inner capillary by the vacuum created when the argon gas stream flows over the end (nozzle) at high linear velocity. As the solution is drawn out, the edges of the liquid forming a film over the end of the inner capillary are blown away as a spray of droplets and solvent vapor. This aerosol may pass through spray and desolvation chambers before reaching the plasma flame. 

To assist evaporation of solvent, the argon stream carrying the aerosol can be passed through a heated tube called a desolvation chamber, operated at temperatures up to about 150°C. 

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. 

The flow rate accepted by the interface ranges between 100 and lOOOnL/min for most applications. The interfacing mechanism is based on the formation of the aerosol in high-vacuum conditions, followed by a quick droplet desolvation and the hnal vaporization of the solute on a target surface prior to ionization. The process is fast and requires less than 8 mm of path length. At the core of the interface there are a nano-nebulizer and a treated surface. The nebulizer tip... 

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