Nebulizers design

The term nebulizer is used generally as a description for any spraying device, such as the hair spray mentioned above. It is normally applied to any means of forming an aerosol spray in which a volume of liquid is broken into a mist of vapor and small droplets and possibly even solid matter. There is a variety of nebulizer designs for transporting a solution of analyte in droplet form to a plasma torch in ICP/MS and to the inlet/ionization sources used in electrospray and mass spectrometry (ES/MS) and atmospheric-pressure chemical ionization and mass spectrometry (APCI/MS). 

The transfer efficiencies of analyte solution from the nebulizer to the plasma flame depend on nebulizer design and vary widely from about 5-20% up to nearly 100%. 

The following discussion will focus upon three areas of importance to successful utilization of an ICP. The three specific areas discussed are nebulizer design, spectral rejection and computer processing capability and have significant influence upon the quality and quantity of data obtained from an ICP-AES. These three features will significantly contribute to the analytical sample rate and the accuracy of the data. 

Five basic nebulizer designs, known to be used on ICP systems are cross flow, concentric, flow shear, ultrasonic, and FAA. The cross flow and concentric designs are most commonly supplied with commercial systems. FAA nebulizers were supplied with early multi-element systems and there are some commercial suppliers who provide ultrasonic nebulizers. 

Rau, J. L., Ari, A., and Restrepo, R. D. (2004), Performance comparison of nebulizer designs Constant-output, breath-enhanced, and dosimetric, Respir. Care, 49, 174-179. 

Although there has been limited use with CE interfaces, the direct injection nebulizer (DIN) was first described by Shum et al. - and later used by Liu et al. for CE (Fig. 2E). In this design, the nebulizer introduces the sample very near the plasma inside the ICP torch and eliminates the spray chamber assembly. Close to 100% analyte transport efficiency can theoretically be obtained with the DIN, but the nebulizer is restricted to very low liquid flow rate and thus is well matched to CE interfacing. This design does induce local plasma cooling due the lack of desolvation and detection limits are only slightly improved over other nebulizer designs. 

Kirlew, P.W. Caruso, J.A. Investigation of a modified oscillating capillary nebulizer design as an interface for CE-ICP-MS. Appl. Spectrosc. 1998, 52, 770-772. 

When coupling a low-pressure detector such as the ELSD with SFC, detection takes place at atmospheric pressure, usually downstream of the back-pressure regulator [2]. Figure la shows a common SFC-ELSD interface with downstream pressure control. Factors affecting ELSD response in this configuration include nebulizer design, evaporation conditions, carrier gas flow rate, and the use of makeup fluid. 

The column is designed to provide a moderate plate count (approximately 500 plates) at a low flow rate (typically 75 pL/min) and a low backpressure (typically 50 psi). After the column, the flow is directed to a Cetac MCN-100 nebulizer, designed to operate at low flow conditions, and then to an inductively coupled plasma and finally to a mass spectrometer. 

The nebulizer described is a self-aspirating, pneumatic nebulizer and is the one shown schematically on the lower left of Fig. 6.8(b). The nebulizer capillary is usually made of stainless steel but other materials such as Pt, Ta, and polymers may be used for corrosive solvents when contamination from the elements in steel must be avoided. A variety of other nebulizer designs have been developed for specific applications but are not commonly used in AAS. These other nebulizers are often used in atomic emission spectrometry and will be described in Chapter 7. 

New nebulizer designs that are only just emerging whose nebulized aerosol applications have not yet been fully defined—e.g., Touch-Spray, Microjet, and Ganan-Calvo technology. 

The European Standard presents a set of in vitro methods that are expected to reflect in vivo deposition and have been shown to provide repeatable and consistent results. It is clear that some nebulizer designs cannot easily be adapted into this or any other standard and that some flexibility is required in interpreting and applying such to these systems. In particular, obtaining a realistic profile of aerosol size distribution from the small aerosol boluses mixed with entrained ambient air from the Halolite, Circulaire, and AERx is particularly problanatic. However, such technical difficulties can be overcome and realistic measures of... 

Dennis JH. Drug nebulizer design and performance breath-enhanced jet vs constant output jet vs ultrasonic. J Aerosol Med 1995 8(3) 277-280. 

Figure 6.8 (a) Premix burner system. [ 1993-2014 PerkinElmer, Inc. All rights reserved. Printed with permission, (www.perkinelmer.com).] (b) Schematic nebulizer designs. (Top), modified Babington type (left), concentric (the most common in FAAS) (right) cross-flow type. (From Parsons, M.L., Atomic absorption and flame emission, in Ewing, G.A., ed.. Analytical Instrumentation Handbook, 2nd edn., Marcel Dekker, Inc., New York, 1997. Used with permission.)... 

---