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.
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):
ICP manufacturers will give you an option as to the type of nebulizer to use depending upon your analytical requirements and the instrumental design.
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:
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:
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.
The two basic torch designs are the Greenfield and Fassel torches. The Greenfield torch requires higher gas flows and RF powers. The Greenfield torch is more rugged (less likely to extinguish due to misalignment and introduction of air) whereas the Fassel torch requires less Ar and power. Both designs produce similar detection limits.
Some nebulizer designs work better with one torch design over another. Before experimenting with torches, it is best to contact your instrument manufacturer to determine the torch design recommended for your instrument as well as any design specifications, operating conditions, and dimensions that must be observed.
The following are some questions you may want to consider, whether you are looking to purchase a new ICP or already have one or more existing units:
- What torch design is used and what are the power and Ar gas flow requirements? (It may be helpful to calculate/determine your annual Ar expense).
- What nebulizer and spray chamber designs are available and can they be obtained from alternate suppliers?
- Are there specific nebulizer designs that cannot be used with the recommended torch/spray chamber?
- What are the costs of the individual introduction system components and what are the upkeep costs over a year of operation?
- What is the lifetime of the torch and what is the most common reason for failure?
- How tolerant is the system to slight changes in torch alignment?
- How tolerant is the system to air coming from the nebulizer and will it extinguish after a few seconds?
- How tolerant is the system to the introduction of organic solvents?
- What is the lowest boiling point solvent that can be introduced?
- How tolerant is the system to the torch building up coke for aromatic and aliphatic non-polar solvents?
- What is the most precise nebulizer that can be used with your ICP and what precision should be obtained?
- What are the detection limits for your analytes of interest? Are you achieving the detection limits required for your application?
- When looking for lower detection limits have you considered axial view ICP? Ultrasonic nebulizers? Both?
- Do your analytical solution samples contain high levels of dissolved solids? Do they contain any suspended solids?
- Do you experience nebulizer salting out or plugging?
- Which high solids nebulizer is recommended for your current or potential ICP and what is the precision to be expected? How rugged is it and what does it cost?
- How difficult is it to connect an ultrasonic nebulizer?
- Can either the Scott or Cyclonic spray chamber designs be used?
- What are the washout times for Hg, B, Y, and Cu in nitric acid? In HCl?
- Are corrosion resistant/HF resistant introduction systems available? What do they cost? How easy are they to switch in and out?
- When analyzing for Si in trace HF, how much of a Si, B, Na, and, Al background signal do you get?
- If the introduction system contains glass, how much HF can be tolerated before signals from Si are observed? What about before damage occurs?
In the next section sample introduction compatibility and precision issues will be addressed.