Heated spray chamber

A frequently used micronebulizer with heated spray chamber and membrane desolvator is the Aridus from CETAC Technologies, Ohama, NE. The experimental setup of the Aridus II microconcentric nebulizer is shown in Figure 5.16. 

Most ultrasonic nebulizers use a somewhat larger sample uptake rate (2-3 mL/min) than pneumatic nebulizers. Typically the spray chamber and/or a tube following the spray chamber is heated to evaporate water partially from the aerosol. Because the aerosol transport efficiency is higher when an ultrasonic nebulizer is used, particularly with a heated spray chamber, a system to remove solvent (typically a condenser and/or membrane separator) is essential to prevent deleterious cooling of the ICP by excess water. 

The combination of the ultrasonic nebulizer, heated spray chamber and condenser/desolvator leads to improvements in detection limits by a factor of about 10 compared to that of a pneumatic nebulizer without a desolvation system. This is the main reason ultrasonic nebulizers are used despite their higher cost (approximately U.S. 15,000 in 1998). 

Desolvation systems can provide three potential advantages for ICP-MS higher analyte transport efficiencies, reduced molecular oxide ion signals, and reduced solvent loading of the plasma. Two different approaches have been used for desolvation in ICP-MS. The heated spray chamber/condenser combination has been discussed it is the most commonly used system. The extent of evaporation of the solvent from the aerosol and cooling to reduce vapor loading varies from system to system. The second approach is the use of a membrane separator to remove solvent vapor before it enters the ICP. 

Heated Spray Chambers. The use of a heated spray chamber to evaporate the aerosol partially leads to reduction in drop size and therefore higher analyte transport efficiencies. Often the drying of the aerosol droplets is incomplete. 

Using a microcentric nebulizer at low flow rates (typically about 50 pJL/min), a heated spray chamber, and a heated microporous membrane desolva-tor, the Cetac MCN-6000 system can provide analyte transport efficiencies of 50% to 90%. This system is made completely of HF resistance materials. 

In this device the liquid sample is sprayed into a heated spray chamber, where the nebulizer gas transfers the aerosol through the membrane desolvator. An argon flow removes the solvent vapour from the exterior of the membrane. If compared to conventional pneumatic nebulizers, this system enhances analyte transport efficiency and limits solvent loading to the plasma. Oxide and hydride polyatomic ion interferences are significantly reduced, improving the detection limits by an order of magnitude. 

The ions are transferred into the mass analyzer by use of the same atmosphere-to-vacuum interface as employed in ESI. Therefore, ESI sources can easily be switched to APCI operation. To do so, the ESI spray head is exchanged for a unit comprising a pneumatic nebulizer and a heated spray chamber with a needle electrode mounted in front of the sampling orifice, while the atmospheie-to-vacuum interface stays in place (Fig. 12.40) [51,173]. Different from ESI, APCI requires high liquid flow (200-1000 pi min" ) for effective vaporization. 

Such is the added capability and widespread use of these nebulizers across all application areas that manufacturers are developing application-specific integrated systems that include the spray chamber and a choice of different desolvation techniques to reduce the amount of solvent aerosol entering the plasma. Depending on the types of samples being analyzed, some of these systems include a low-flow nebulizer, Peltier-cooled spray chambers, heated spray chambers, Peltier-cooled condensers, and membrane desolvation technology. Some of the commercially available equipment include the following ... 

Microflow nebulizer with heated spray chamber and Peltier-cooled condenser An example of this design is the Apex inlet system from ESI. This unit includes a microflow nebulizer, heated cyclonic spray chamber (up to 140°C), and a Peltier multipass condenser/cooler (down to -5°C). A number of different spray chamber and nebulizer options and materials are available, depending on the application requirements. Also, the system is available with Teflon or Nafion microporous membrane desolvation, depending on the types of samples being analyzed. Figure 17.12 shows a schematic of the Apex sample inlet system with the ctoss-Aow nebulizer. 

Microflow nebulizer with heated spray chamber and Peltier-cooled condenser An example of this design is the Apex inlet system from ESI. This unit includes a microflow nebulizer, heated cyclonic spray chamber (up to 140°C), and a Peltier multipass condenser/cooler (down to -5°C). 

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