5. Nebulizers, Spray Chambers, and Torches
There has been a tremendous activity in the area of sample introduction over the past 30 years since ICP has been commercially available. The objective of this section is to acquaint the reader with the basic options available to the ICP operator for the introduction of 'liquid' samples.
Some of the considerations in selecting an introduction system include dissolved solids content, suspended solids presence, presence of HF or caustic, detection limit requirements, precision requirements, sample load requirements, sample size limitations, and operating budget. In the last section, the concentric nebulizer and all glass introduction systems were given top billing but they may not work at all for your application. The analyst is left with the task of choosing the best introduction components after taking into account the appropriate considerations.
Nebulizers
Pneumatic Nebulizers:
The term "pneumatic" is defined as 'of or relating to or using air or a similar gas'. The word "nebulizer" is derived from the Latin "nebula" meaning mist and is defined as 'an instrument for converting a liquid into a fine spray'. Therefore, a pneumatic nebulizer is literally an instrument for converting a liquid into a fine spray that uses a gas as the driving force.
Some of the most popular ICP pneumatic nebulizers are:
• Concentric glass
• Concentric PFA
• Fixed Cross-Flow
• Lichte (modified)
• Micro-concentric glass
• Adjustable Cross-Flow
• High-Pressure Fixed Cross-Flow (MAK)
• Babington V-Groove (high solids)
• GMK Babington (high solids)
• Hildebrand dual grid (high solids)
• Ebdon slurry (high solids)
• Cone Spray (high solids)
The concentric and fixed cross-flow are still the most common designs. The construction of both types is described in the following article by ICP expert Robert Thomas (see Figures 4 & 5):
» A Beginner's Guide to
ICP-MS - Part II: The Sample-Introduction System
ICP manufacturers will give you an option as to the type of nebulizer to use depending upon your analytical requirements and the instrumental design. For more information on choosing the right nebulizer, the following link may be useful:
» Precision Glassblowing: Guide to Nebulizers
Ultrasonic Nebulizers:
Sound can be used instead of a gas as the energy source for converting a liquid to a mist. These nebulizers use an ultrasonic generator at a frequency of between 200 kHz and 10 MHz to drive a piezoelectric crystal. A pressure is produced that breaks the surface of the liquid - air interface. Ultrasonic nebulizers are more expensive and difficult to use but they will improve (lower) detection limits by about a factor of 10. For more information on ultrasonic nebulizers, visit the following link:
» CETAC U-5000+ Ultrasonic Nebulizer with Axial ICP-OES
Spray Chambers
Assorted spray chambers
The basic designs that have remained over the years are the Scott double-pass and the Cyclonic. To review the designs of these two components, see Figures 8 & 9 in Robert Thomas' article:
» A Beginner's Guide to
ICP-MS - Part II: The Sample-Introduction System
The Cyclonic design is relatively new but is very popular. The purpose of the spray chamber is to remove droplets produced by the nebulizer that are > 8µm in diameter. Considerations include the wash-in-time, washout time, stability, and sensitivity. The drainage characteristics are important in part due to pressure changes that may occur during drainage. It is important that the drainage process be smooth and continuous. The analyst may observe faster washout times with the Cyclonic design. The chamber material of construction as well as the sample matrix and the chemistry of the element will influence the washout time. In addition, the analyst may observe faster washout times with glass construction than with polymers. This is due in part to better wet ability of the glass (lack of beading). Both designs are excellent and the analysts may wish to experiment with each to determine which yields the best performance for their specific analyses.