Sample introduction pneumatic nebulizers

Bjorn E, Freeh W, Hoffmann E, Liidke C (1998) Investigation and quantification of spectroscopic interferences from polyatomic species in inductively coupled plasma mass spectrometry using electrothermal vaporization or pneumatic nebulization for sample introduction. Spectrochim Acta 53B 1766... 

Montaser A, Tan H, lishi II, Nam SFI, CaiM (1991) Argon inductively coupled plasma mass spectrometry with thermospray, ultrasonic, and pneumatic nebulization. Anal Chem 63 2660-2665 Montaser A, Minnich MG, Liu FI, Gustavsson AGT, Browner RF (1998) Fundamental aspects of sample introduction in ICP spectrometry. In Inductively Coupled Plasma Mass Spectrometry. Montaser A (ed), Wiley-VCH, New York, p 335-420... 

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-... 

There are several drawbacks to ultrasonic nebulizer/desolvation systems. Precision is typically somewhat poorer (1% to 3% relative standard deviation) than for pneumatic nebulizers (0.5% to 1.0% relative standard deviation) and washout times are often longer (60 to 90 sec compared to 20 to 30 sec for a pneumatic nebulizer/spray chamber without desolvation). Furthermore, chemical matrix effects are dependent on the amount of concomitant species that enter the ICP per second. Therefore, use of any sample introduction device that increases the amount of sample entering the plasma per second also naturally leads to more severe matrix effects when the sample contains high concentrations of concomitant species. 

When using a pneumatic nebulizer, an unheated spray chamber, and a quadrupole mass spectrometer, ICP-MS detection limits are 1 part per trillion or less for 40 to 60 elements (Table 3.4) in clean solutions. Detection limits in the parts per quadrillion range can be obtained for many elements with higher-efficiency sample introduction systems and/or a magnetic sector mass spectrometer used in low-resolution mode. Blank levels, spectral overlaps, and control of sample contamination during preparation, storage, and analysis often prohibit attainment of the ultimate detection limits. 

Liquid chromatography (LC) is the most commonly used technique for trace element speciation with ICP-MS detection. The mobile phase flow rates used with most LC techniques (0.5-2.0 mL min-1) are compatible for ICP-MS introduction using conventional sample introduction systems (pneumatic nebulization with cross flow and concentric nebulizers and double-pass spray chambers). An interface, known as a transfer line, must be constructed to allow connection between the outlet of the LC column and the nebulizer of the ICP-MS. Inert plastic tubing is commonly used for this purpose with the inner diameter and length kept to 20-50 cm in order to minimize peak broadening. 

Conventional ICP-MS sample introduction systems with pneumatic nebulization are inefficient as the amount of sample reaching the plasma is generally less than 2% of the amount of sample actually entering the nebulizer. To obtain im-... 

From the late 1960s onwards, a number of research groups around the world began to investigate alternatives to pneumatic nebulization for sample introduction, in an attempt to overcome transport efficiency limitations. The most successful approaches were those which involved heating small, discrete liquid samples, and sometimes even solid samples, directly on a metal filament, boat, or cup which could be positioned reproducibly into a flame. However, since the temperature of the metal would be lower than that of the flame itself, the techniques were confined to the determination of relatively easily atomized elements such as arsenic, bismuth, cadmium, copper, mercury, lead, selenium, silver, tellurium, thallium, and zinc. 

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. 

ICP elevated cost, low efficiency of sample introduction via pneumatic nebulization, poor tolerance of plasma to organic solvents, and high salt contents ... 

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. 

The specific design of the various sample introduction devices or spray probes depends to a large extent on the technique applied, i.e., ESI, APCI, or other. With respect to ESI, systems have been described for conventional pure ESI, pneumatically-assisted ESI or ionspray, ultrasonically-assisted ESI, thermally-assisted ESI, and micro- and nano-ESI (Ch. 5.5). The heated-nebulizer system (Ch. 5.6.2) is used in APCI and atmospheric-pressure photoionization (APPI). 

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. 

The liquid sample introduction system most commonly used on an ICP-MS is very similar to that used on a flame Atomic Absorption Spectrometer or an ICP-OES. Liquid samples can be directly injected using a pneumatic nebulizer and a spray chamber. 

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