Sample Introduction Systems
Overview
The most common form of ICP sample introduction is liquid. The purpose of this section is to introduce the beginner to the most popular components of liquid sample introduction systems used for the introduction of samples to ICP-OES and ICP-MS instrumentation (hereafter referred to as ICP) and to alert the reader to some common problems.
System Components
Before continuing any further, I strongly encourage you to read the following:
In the above article, the author gives an excellent overview of the most popular commercially available nebulizers and spray chambers. He also provides guidance and basic theory behind the available designs, as well as an overall understanding of ICP introduction systems.
The key elements of a sample introduction system start with the sipper tube and end with the torch. They are listed as follows:
- Sipper (typically plastic)
- Teflon tubing going from the sipper to the peristaltic pump tubing
- Peristaltic pump tubing
- Teflon tubing going from the peristaltic pump tubing to the nebulizer
- Spray chamber
- Torch
Troubleshooting
Connection Checks
The main difficulty I have experienced with introduction system failure is that of connections between components. The connections are listed as follows:
- Sipper to Teflon tubing
- Teflon tubing to peristaltic tubing (both into and out of)
- Teflon tubing from peristaltic pump to nebulizer
- Nebulizer to spray chamber
- Spray chamber to waste drain tube
- Spray chamber to torch
If any one of these connections is not airtight, the operator will experience anything from poor precision to an inability to light the plasma. One of the many reasons I prefer concentric glass nebulizers is that they are 'free flow' (i.e., the liquid will flow from the sample container to the nebulizer without assistance from the peristaltic pump). A simple check is to determine if you obtain a fine steady mist (using water as the sample) without the peristaltic pump (pressure lever released) so that free flow can occur. This can be done with the nebulizer disconnected from the spray chamber (plasma has not yet been lit) so that the mist can be easily visualized. You can also check for the appearance of any small air bubbles in the Teflon tubing, which should never be present and indicate a poor connection somewhere between and/or including the sipper and the nebulizer.
Another connection that is often taken for granted is the spray chamber drain/waste tube connection. This connection is absolutely critical. One way to test this connection is to put some water in the spray chamber using a wash bottle and determine if it drains smoothly and without leaks. Poor precision or the inability to light the plasma is a common symptom of a poor drain tube connection. During this test you should also observe the absence of water droplets in the spray chamber (assuming glass construction). A dirty spray chamber will leave water droplets and cause poor precision and carryover problems. Make sure the plasma is not lit whenever you perform this test.
Spray Chambers
Spray chambers can be made of all glass, all plastic, and glass with plastic end caps. If you do not use HF (all plastic systems must be used with HF) and therefore have the luxury of using glass components, attempt to use a spray chamber without the plastic end cap (i.e., all glass). They are typically used with glass concentric nebulizers and use only two 'O rings' to connect the nebulizer to the spray chamber. I have found that the plastic end cap may cause longer washout times, carry over problems, and is a very large connection surface where connection problems can occur. Using a glass concentric nebulizer and all glass spray chamber a precision of between 0.2 and 0.5% RSD should be observed. If an all glass system gives a precision of 1% RSD or greater, then there is most likely a connection problem or the nebulizer gas flow rate is too high (look for spitting when checking the nebulizer free flow and do not be afraid to lower the gas pressure {argon sample flow} to the nebulizer).
Peristalic Pump Tubing
Another weak link in the introduction system is the peristaltic pump tubing. When you start the day the tubing is fresh and the pressure can be set to give a steady mist when the pump in running. The problem is that the pump tubing stretches and either the pressure is not enough to drive the solution through the tubing or you over tighten and get a pulsating mist spray. This is a problem that each analyst has to be aware of and solve through experimentation. This problem is particularly troublesome for ICP-MS users because the argon flow changes as the tubing stretches. This causes a relative increase in the sensitivity of the higher atomic number elements.
Maintenance
I prefer glass components because of their ease of operation and cleaning. It is always best to start the day with a clean nebulizer, spray chamber, and torch. Cleaning the torch daily will also extend its life. There are many cleaning solutions that can be used. Some of our analysts prefer 1:1 Nitric acid/water and others prefer sulfuric acid and hydrogen peroxide. Another common cleaning solution is 1:1 HCl/nitric. All of these solutions will work depending upon the nature of the contaminants. The sulfuric/peroxide is generally a severe approach and needed only if organics such as grease, etc, are suspected.
Be advised that ultrasonic baths are great for cleaning. However, NEVER use them to clean a glass concentric nebulizer. Glass concentric nebulizers are cleaned by leaching and occasionally by applying a backpressure with water to remove lodged particles. The use of a cleaning wire or ultrasonic bath is a sure way to destroy the nebulizer.
In summary, when it comes to ICP introduction systems there is no substitute for experience. Relatively speaking, introduction systems are simple but they are not easy to maintain and they are challenging to operate to their maximum potential.