Direct insertion nebulizers

Microconcentric nebulizers have been developed that put solution directly in the ICP torch sample capillary, thereby eliminating the need for a spray chamber. These nebulizers, also called direct insertion nebulizers (DIN), can be used for very small (microliter) sample volumes and for direct coupling of hquid chromatographs to the ICP as an element-specific detector. 

For solids, there is now a very wide range of inlet and ionization opportunities, so most types of solids can be examined, either neat or in solution. However, the inlet/ionization methods are often not simply interchangeable, even if they use the same mass analyzer. Thus a direct-insertion probe will normally be used with El or Cl (and desorption chemical ionization, DCl) methods of ionization. An LC is used with ES or APCI for solutions, and nebulizers can be used with plasma torches for other solutions. MALDI or laser ablation are used for direct analysis of solids. 

The second type of direct injection nebulizer, called the direct injection high-efficiency nebulizer (DIHEN), is a specific type of the Meinhard HEN [38] that is inserted into the ICP torch in place of the center, injector tube. The main advantage of the DIHEN compared to the Cetac DEN is that a high-pressure pump is not needed to deliver sample to the nebulizer. An unusually low nebulizer gas flow rate (0.25 L/min) and high ICP power (1.5 kW) were found to provide optimal ICP-MS sensitivity when DIHEN is used [38]. 

A direct injection nebulizer has been used for CE-ICP-MS [101] with the capillary actually inserted through the central sample introduction capillary of the DEN (Fig. 10.17). This nebulizer is very well suited to the very low flow rates associated with CE and also offers approximately 100% sample transport efficiency. A platinum grounding electrode was used, situated in a three-port connector, which also accommodated the DEN capillary and a makeup buffer flow. The makeup buffer was used so that the DIN operation was independent of the EOF, and the two flows were combined at the capillary exit. The system was used for feasibility studies for As and Se speciation. 

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. 

Limits of detection for an ETV source are in the picogram to nanogram range and are often better than for direct solution introduction via a nebulizer. As with direct-insertion methods (DSl, see below), graphite supports for solid samples often lead to carbide formation with elements that resist volatilization. In these cases, a thermal reagent is commonly added to convert the sample elements into a more volatile form. For example, chlorine- or fluorine-containing compounds, such... 

Recently, Houk s laboratory [18,19] described a direct injection nebulizer for use in ICP-MS consisting of a stainless steel tube with an inner diameter of 250 p.m held in a ceramic support tube that is inserted directly into the quartz injector tube of the ICP torch. A positive displacement gas pump is used to achieve a sample flow rate of 120 p,L/min. The low flow rates decrease sample consumption and the essentially 100% transfer efficiency improves detection limits by an order of magnitude. However, solvent loading presents a difficulty and the plasma content of solvent-derived polyatomics such as metal oxides was increased up to threefold [18]. 

Hydride generation Cross-flow pneumatic nebulizer Direct insertion... 

Liquid nebulization as a means of obtaining aerosols is commonly used for activities such as drug administration, hair spraying or perfume application [1,2]. Direct nebulization of a liquid phase containing the target analytes has been widely used in analytical chemistry for sample insertion into some detection systems (particularly atomic spectrometers). The main purpose of analytical nebulizers is to insert the maximum possible amount of sample, and hence of analyte, in the form of aerosol consisting of very small droplets, into the detection system. 

The pneumatic nebulizer has for many years been the most universal sample insertion device for plasma-based spectrometry. The inherent lack of transport efficiency, coupled with the continuing need for increased sensitivity, has promoted research into the use of ultrasonic nebulizers to boost detection capabilities. Such research has focused on various aspects including fundamental aerosol properties [86-88], instrument development [89], nebulizer comparisons [90,91], desolvation effects [92,93], direct nebulization applications [94,95] and speciation [96]. 

Different auxiliary methods of administration can be used in conjunction with nebulizers to deliver aerosol to the patient [144]. A mouthpiece may be inserted in the mouth or a face mask may be attached tightly to the face. A large-bore inlet adapter attaches tubing from the nebulizer outlet to the mouthpiece or mask. It is possible to compensate for exhaled aerosol without increasing resistance to prevent condensation. A face tent fits more loosely around the patient s mouth, allowing speech. The latter arrangement is frequently used with ultrasonic nebulizers. A tracheostomy mask may be fitted to the patient s tracheostomy tube directly and requires a T-shaped adapter. Environmental chambers are used to enhance therapy and include incubators, pediatric croup tents, and hoods. 

Many of the operating parameters normally optimized for ICP-AES with solution nebulization, are also important in direct sample analyses. These include observation height, RF power, gas flow rates, integration time, and wavelength. In direct sample insertion, sample insertion heights for drying, ashing, and vaporization, rate and duration of insertion, and sample size must also be optimized. 

Sample solutions should be measured immediately following calibration. If the instrument is shut down for any reason (the gas tank runs out, the power fails in a thunderstorm, the lamp burns out, the nebulizer clogs up and needs to be cleaned, the graphite tube cracks, the analyst goes to lunch, etc.), the calibration must be repeated when the instrument is turned back on to be sure that items 3-6 are exactly the same for samples and standards. For GFAAS, the platform and tube must be the same for the calibration curve and the samples. If a tube cracks during a run, a new tube and platform are inserted, conditioned per the manufacturer s directions and the calibration standards rerun before samples are analyzed. For extremely complex sample matrices, it may not be possible to make external standards with a similar matrix. In this case, the MSA should be used for quantitative analysis. 

Contrary to V-EASI, in the case of polarization induced (PI)-ESI [e.g., contactless atmospheric pressure ionization (C-API) and ultrasonication-assisted spray ionization (UASl)] [39-41], one does not need to supply nebulizing gas, which makes the monitoring system even more simplistic. In C-API, a glass capillary ( 20 cm, inner diameter 50 pm) was inserted to the reaction vial to direct the reaction solution toward the mass spectrometer. The tapered end of this capillary was placed in proximity ( 1 mm) to the inlet of the mass spectrometer (Figure 11.2a). The flow of sample was mainly due to capillary action and the influence of the electric field near the mass spectrometer, which may induce some electroosmotic flow (EOF) (see Chapter 6). In this approach, the sample flow rate is low ( 100 nl min" ). The sampling time can be reduced by using a shorter capillary as the sampling tube. Similarly to V-EASI, the capillary combines... 

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