Ultrasonic devices nebulizers

Thermospray nebulizers are somewhat expensive but can be used on-line to a liquid chromatographic column. About 10% of sample solution is transferred to the plasma flame. The overall performance of the thermospray device compares well with pneumatic and ultrasonic sprays. When used with microbore liquid chromatographic columns, which produce only about 100 pl/min of eluant, the need for spray and desolvation chambers is reduced, and detection sensitivities similar to those of the ultrasonic devices can be attained both are some 20 times better than the sensitivities routinely found in pneumatic nebulizers. 

Nebulizers are used to introduce analyte solutions as an aerosol spray into a mass spectrometer. For use with plasma torches, it is necessary to produce a fine spray and to remove as much solvent as possible before the aerosol reaches the flame of the torch. Various designs of nebulizer are available, but most work on the principle of interacting gas and liquid streams or the use of ultrasonic devices to cause droplet formation. For nebulization applications in thermospray, APCI, and electrospray, see Chapters 8 and 11. 

Except for a few laboratory-made units and some devices marketed by other firms, most applications of ultrasonic nebulization (USNn) in atomic spectrometry have been developed with two commeroial devioes from CETAC Technologies viz. the U-5000AT+ and U-6000AT+). It should be noted that these two commercial ultrasonic devices were originally designed for ICP instruments but have been used with other types of detectors [20]. 

This may be either a continuous process, used when the sample size is relatively large (1 ml or more), or a discrete process, used with samples of less than 20 /il. Continuous-flow systems are simpler to use and more precise, but they are less sensitive. They employ a nebulizer in association with a flame or gas plasma, and either a rotating electrode (Rotrode) or drip-feed to the electrode with the arc or spark. The pneumatic nebulizer has an efficiency of 5-10% and generates an inhomogeneous aerosol. Efiiciency can be improved by proper design of the nebulizer and spray chamber (N4), by use of heated nebulizer gas (R6) or ultrasonic devices (S23). The maximum improvement is a 5- to 10-fold increase in sensitivity. There is also an increase in the complexity and cost of the instrument which usually offsets these benefits. The effect... 

Numerous investigators have compared the aerosol droplet size of nebulized aerosols from ultrasonic and air-jet devices. " Because droplet size is inversely proportional to the acoustic frequency, smaller droplets are generated from ultrasonic devices with higher frequencies. Ultrasonic nebulizers with high operating frequencies (2-3 MHz) are capable of... 

Warming may have a beneficial effect. For instance, the temperature of fluids atomized in air-jet nebulizers decreases by approximately 10-15°C during use, resulting in bronchoconstriction in some asthma suffers.Bronchoconstriction, which is most marked at 5°C, disappears at 37°C and thus may be minimized by using an ultrasonic device. Furthermore, when solutions of drugs with low solubility are to be nebulized, ultrasonic nebulizers, which warm the solutions, may be preferable to air-jet devices, which cool them and may cause precipitation. However, the heat generated may harm heat-labile materials such as diethylenetriaminepentoacetic acid ( mTc-/DTPA), " proteins, and some antibiotic solutions. Thus, ultrasonic nebulizers are specifically prohibited for aero-solization of recombinant human deoxyribonuclease (rhDNase). ... 

The British Thoracic Society Nebulizer Project Group published a set of guidelines for nebulizer treatment. Their findings relating to ultrasonic devices are summarized as follows ... 

Jet and ultrasonic nebulizers have been used to aerosolize antimicrobial therapy. Jet nebulizers used compressed air or oxygen passed through a liquid to produce an aerosol gas. With ultrasonic devices, a piezoelectric element vibrates to generate small aerosol particles. Both systems have advantages and limitations related to their components and technique of use. These are discussed in more detail elsewhere in this text (see Chs. 8, 12). 

Ultrasonic Nebulizer. The ultrasonic devices are more efficient nebuliza-tion systems than the pneumatic nebulizers, but they possess a lower nebulization rate. By the use of these systems, it is possible to convert about 40% of small sample volumes to a mist of useful droplet size, but sensitivity is generally not increased because of the slowness of the nebulization. 

Figure 24 Diagram of neonatal ventilator circuit incorporating a nebulizer. This example illustrates an ultrasonic device. Cascade impaction can be performed at the distal tip of the endotracheal tube. (From Ref. 36.)...

A second form of desolvation chamber relies on diffusion of small vapor molecules through pores in a Teflon membrane in preference to the much larger droplets (molecular agglomerations), which are held back. These devices have proved popular with thermospray and ultrasonic nebulizers, both of which produce large quantities of solvent and droplets in a short space of time. Bundles of heated hollow polyimide or Naflon fibers have been introduced as short, high-surface-area membranes for efficient desolvation. 

As described in Section 3.3 in more detah, particles in the aerosol cloud should preferably have an aerodynamic diameter between 0.5 and 7.5 pm. Currently three different types of devices are used to generate aerosol clouds for inhalation nebulizers (jet or ultrasonic), (pressurized) metered dose inhalers (pMDIs) and dry powder inhalers (DPIs). The basic function of these three completely different devices is to generate a drug-containing aerosol cloud that contains the highest possible fraction of particles in the desired size range. 

Wu-Pong and Byron [15] have contributed a review of the issues associated with pulmonary delivery of ASOs. Since ASOs are freely soluble and highly hygroscopic, it would be reasonable to assume that initial dosage forms will rely upon the aerosolization of simple aqueous solutions. Our data indicate that commercially available nebulization devices will generate suitable aerosolizations of ASO solutions at concentrations up to 180 mg/mL [3]. Ultrasonic and jet nebulizations were found to have essentially no effect on the phosphorothioate stability of the ASO over 40 min, which is longer than typical treatment times. 

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. 

Nebulizer jet nebulization ultrasonic nebulization. Generates small particles with higher delivery capacities than pMDIs and DPIs no coordination required. Inconvenient long inhalation times poor dose control lack of portability expensive. More compact and portable devices breath enhanced nebulizers dosimetric nebulizers. 

Several cfPcicnt sample introduction systems have been conceived to achieve higher precision, accuracy, and sensitivity, such as thermospray nebulization (TN) [95], ETV [88, 99], and hydraulic high pressure nebulization. The use of desolva-tation devices, for example, ultrasonic nebulization (UN), in combination with... 

Nebulization is a physical process widely used in analytical chemistry for introducing samples into atomic spectrometers [66], Ultrasonic nebulizers are the most effective devices for this operation. Rather than a step preceding sample preparation, nebulization is a sample preparation operation and so close to detection that the nebulizer is a component of flame and plasma spectrometers that influences their efficiency. This warrants separate discussion on ultrasonic nebulizers in Chapter 8. 

Ultrasonic nebulizers have also been employed in continuous flow systems as interfaces between sample preparation steps in the analytical process and detection by virtue of their suitability for operating in a continuous mode. Thus, preconcentration devices have commonly been coupled to atomic spectrometers in order to increase the sensitivity of some analytical methods. An enhancement factor of 100 (10 due to USNn and 10 due to preconcentration) was obtained in the determination of platinum in water using a column packed with polyurethane foam loaded with thiocyanate to form a platinum-thiocyanate complex [51]. An enhancement factor of 216 (12 with USNn and 18 with preconcentration) was obtained in the determination of low cadmium concentrations in wine by sorption of metallic complexes with pyridylazo reagents on the inner walls of a PTFE knotted reactor [52]. One special example is the sequential determination of As(lll) and As(V) in water by coupling a preconcentration system to an ICP-AES instrument equipped with a USN. For this purpose, two columns packed with two different resins selective for each arsenic species were connected via a 16-port valve in order to concentrate them for their subsequent sequential elution to the spectrometer [53]. 

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