Sample introduction by pneumatic nebulization

In the analysis of solutions, pneumatic nebulization and then the introduction of the aerosol into the source are already well known from the early work on flame emission spectrometry. Pneumatic nebulizers must enable a fine aerosol to be produced, which leads to a high efficiency, and the gas flow used must be low enough to transport the droplets of the aerosol through the source with a low velocity. The production of fine droplets and the use of low gas flows are essential to obtaining complete atomization in the source. 

Pneumatic nebulization of liquids is based on the viscous drag forces of a gas flow passing over a liquid surface and entraining parts of the liquid, by which small independent droplets are produced. This may occur when the liquid is forced through a capillary tube and at the exit the gas then flows concentrically around the tube or perpendicularly with respect to the liquid stream. A fiit can also be used, which is continuously wetted and through which the gas passes. 

The spray chamber needs some time to fill up with the aerosol produced and some tailing of signals is seen, resultng from removal of the sample aerosol by the new incoming gas and aerosol. These build-up and wash-out times limit the speed of analysis and lead to a flattening of transient signals, when a vapor cloud passes through the spray chamber, as is the case in flow injection analysis. In order to 

20 ng/mL of each element mentioned. (Reprinted with permission from Ref [41].) 

RSD (relative standard deviations) in % resulting from 10 replicate 

In the pneumatic nebulizers used in atomic spectrometry, the liquid flow is usually of the order of 1 to a few mL/min and the full efficiency of the nebulizer (a few %) can actually be realized at gas flows of 2 L/min. However, even with gas flows below 2 L/min, droplet diameters as low as about 10 pm and injection velocities below about 10 m/ s are obtained. Pneumatic nebulization can be realized with a number of types of nebulizers. For flame emission and atomic absorption as well as for plasma spectrometry, they include concentric nebulizers, so-called cross-flow nebulizers, Babington nebulizers and fritted-disk nebulizers (Fig. 40) [95]. 

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LC is typically operated at room temperature. The eluent flow rate in LC corresponds well with the sample uptake rate of the traditional introduction system (pneumatic nebulizer) used for MC-ICP-MS [26]. As a consequence, LC-MC-ICP-MS coupling can be simply accomplished by connecting the column outlet of the chromatographic system with the nebulizer of the MC-ICP-MS unit (Figure 17.2). 

The introduction of samples via nebulizers requires that they are either pneumatically or peristaltically pumped into the nebulizer for aerosol formation. This restricts the range of viscosities that can be easily handled by the nebulizer. For example highly saline or oil samples may well have to be diluted by an order of magnitude or greater. This dilution can be carried out either in a batch mode or continuously. Batch systems are quite complex in design but the rate of analysis is high. It is often the case that where dilution is required, in addition, a fast rate of analysis is also desirable. Some batch systems have been introduced commercially, notably to monitor wear metals in the oil industry. 

The commonest form of sample introduction is by means of an aerosol generated using a pneumatic nebulizer. Several types of nebulizer can be used. All-glass concentric nebulizers (Fig. 4.7a) operate in a similar manner to those used for FAAS. Cross-flow nebulizers (Fig. 4.7b) operate by directing a high-velocity stream of gas across the mouth of a capillary... 

From the sample solution to be analyzed, small droplets are formed by the nebulization of the solution using an appropriate concentric or cross-flow pneumatic nebulizer/spray chamber system. Quite different solution introduction systems have been created for the appropriate generation of an aerosol from a liquid sample and for separation of large size droplets. Such an arrangement provides an efficiency of the analyte introduction in the plasma of 1-3 % only.6 The rest (97 % to 99%) goes down in the drain.7 Beside the conventional Meinhard nebulizer, together with cooled or non-cooled Scott spray chamber or conical spray chamber, several types of micronebulizers together with cyclonic spray chambers are employed for routine measurements in ICP-MS laboratories. The solvent evaporated from each droplet forms a particle which is vaporized into atoms and molecules... 

Most flame spectrometers use a premix burner, such as that in Figure 21-5, in which fuel, oxidant, and sample are mixed before introduction into the flame. Sample solution is drawn into the pneumatic nebulizer by the rapid flow of oxidant (usually air) past the tip of the sample capillary. Liquid breaks into a fine mist as it leaves the capillary. The spray is directed against a glass bead, upon which the droplets break into smaller particles. The formation of small droplets is termed nebulization. A fine suspension of liquid (or solid) particles in a gas is called an aerosol. The nebulizer creates an aerosol from the liquid sample. The mist, oxi-... 

The most common introduction of the samples in this source consists of a pneumatic nebulizer which is driven by the same flow of argon which carries the resulting droplets in the plasma. An ultrasonic nebulizer and heated desolvation tube are also used because they allow a better droplet size distribution which increases the load of sample into the plasma. Generally, the sample solutions are continuously introduced in the nebulizer at the rate of about 1 ml min-1 with the help of a peristaltic pump. However, this is not acceptable with small-sample solutions. Therefore an alternative method using the flow injection technique is employed to introduce a small sample of about 100 pi. The sample solution is injected into a reference blank flow so that the sample is transported in the nebulizer and a transitory signal is observed. 

Matrix interferences are often associated with the sample introduction process. For example, pneumatic nebulization can be affected by the dissolved-solids content of the aqueous sample, which affects the uptake rate of the nebulizer and hence the sensitivity of the assay. Matrix effects in the plasma source typically involve the presence of easily ionizable elements (EIEs), e.g. alkali metals, within the plasma source. 

For environmental, biological, or health-related investigations, the primary limitation of ICAP analysis is related to the totality of the analysis. The concentrations of specific element forms are often more important than total element content in these disciplines. ICAP analysis by direct pneumatic nebulization to date must be considered to provide only the total content of each element analyzed, independent of the forms. If a specific element form is to be determined, appropriate separation techniques must be used prior to analysis by conventional sample introduction to an ICAP. 

Interfacing micro-LC and MS via a capillary inlet interface coimected to a GC-MS jet separator was described in 1978 by Takeuchi et al. [55]. In the period until 1982, this system was subsequently developed towards a vacuum nebuhzer, in which the column effluent is pneumatically nebulized into a modified jet-separator type of device [56-57]. The instrumental developments of the vacuum nebulizer interfaces are discussed in Ch. 4.3. Pneumatic nebulization of column effluents directly into the Cl somce was described by a number of groups [58-61]. A so-called helium interface for the introduction of organic solvents was described by Apffel et al. [59]. It was primarily applied to the analysis of pesticides in aqueous samples. Although the system was commercially available, it did not find wide application. 

OCN), which is a variation of the pneumatic concentric nebulizer built from flexible capillary mbes, was used in an interface. The OCN has had little application in CE interfaces, owing to its generally lower sensitivity performance when compared to other pneumatic nebulizers used with ICP-MS detection.The direct injection nebulizer (DIN), previously described in The Nebulizer, was used by Liu et al. in a CE interface. The electrophoretic capillary was directly inserted through the central sample introduction capillary of the DIN. A platinum grounding electrode was positioned into a three-port connector. This connector contained the DIN sample introduction capillary as well as a make-up buffer flow. These alternative nebulizers have been successfully used in CE interfaces, but the pneumatic designs dominate the interface systems reported in the literature. 

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