Nebulizers for Inductively Coupled Plasma Spectroscopy

When contemplating how to perform atomic spectrochemical analysis for trace metals, the following key components immediately come to mind:

1. sample

2. excitation (or atomization) source

3. spectrometer

4. detector

5. data readout

These five components represent assemblies made up of a complex array of mechanical, optical, and electronic parts that serve to make up a complete instrument. A very important subset between sample and excitation source (components 1 and 2 above) is the general category of sample introduction — two otherwise very simple words that represent research and development rivaling that devoted to the other four components combined. Proper sample introduction may mean the difference between a correct or incorrect answer when it comes to reporting analytical results, and the right kind of sample introduction, in this case, is some kind of nebulizer.

To keep things a bit more focused, this article devotes itself to methods using an inductively coupled plasma (ICP) with an emphasis toward atomic emission spectroscopy, although the atomic absorption technique also uses nebulizers. References that describe nebulizer designs and their application to chemical analysis are available in the usual sources of information (1–4).

Why Atomic Spectroscopy Works

To perform atomic spectroscopy, free atoms must be produced that can be excited to be able to spectrally observe their signature radiation profile. Producing free atoms requires a fair amount of energy. First, energy is needed to break up molecules in the sample matrix into free atoms. Second, enough energy needs to be left over to excite the free atoms sufficiently to the point where they will emit radiation, specifically radiation that is in the target wavelength region. The process of sample nebulization can significantly affect the efficiency whereby free analyte atoms are produced.

Before any further discussion about nebulization, an understanding of energy is necessary, or more to the point, energy management. Unfortunately, energy is not a limitless commodity. To the spectroscopist, energy is to a large degree measured as temperature. Temperature is associated with everything from dissociation of molecules into free atoms, to the degree of excitation of free atoms to excited states. Harnessing energy to produce a high enough temperature is the purpose of the excitation source (component 2 above). A flame (that is, the chemical combustion resulting in the breaking of chemical bonds) was the historical excitation source for producing high temperatures. Now, much higher temperatures are provided by a general category of atmospheric pressure discharges termed plasmas or ionized gases. These sources, which supply seemingly limitless temperatures, are in fact by their nature, limited. It is this notion of energy limits that presents a boundary for the spectroscopist to consider when choosing a method of sample introduction. The argument is not complete without at least mentioning that standing between the seemingly endless supply of energy and production of free atoms is the concept of energy transfer. The efficiency of energy transfer during sample introduction is a huge part of what turns an average analytical method into an exceptional one. The nebulizer is once again the largest factor in maximizing that energy transfer efficiency.

How an ICP Is Formed

An excitation source by nature has to be hot. Just how hot is defined by how it is produced. Historically, the flame was the simplest source available to produce elevated temperatures. Besides ease of control, the flame quickly became a useful analytical tool because it was easy to introduce a liquid sample. Flame temperatures could be changed by using different gases for combustion, but eventually temperatures reached a plateau. The process of breaking chemical bonds through burning had reached its limit. Soon thereafter, researchers began using electricity to produce an excitation source with much higher temperatures. The only limit was how much electric current could be generated. Controlling the current and voltage across a gap, the spectroscopist could produce either a direct current arc or a momentary but repeated spark to heat the sample. Energy transfer occurred by directly coupling the electrical energy to the sample and creating very high temperatures that produced both free atoms and excited atoms nearly simultaneously. The two biggest problems with these types of sources were contamination from the electrodes and inconvenience of handling liquid samples. Electrodeless discharges, or plasmas, emerged as an alternative. With the concept of electromagnetic induction, electrical energy could be coupled to an inert gas to create a plasma by way of an induction coil carrying electrical current at radio frequencies (rf). As a result, ICP emerged as a commercial product in the early 1970s (5,6).

An argon ICP is an ionized gas that contains argon ions at a concentration of about 1%, and about an equal number of free electrons. Argon turns out to be an economical gas extremely well suited for producing hot plasmas at atmospheric pressure with rf current. Additionally, this type of plasma brought back the advantage of handling liquid sample introduction much like the flame. Along with argon ions and electrons, argon ICPs also contain another valuable species that gives the argon ICP its amazing analytical sensitivity — argon metastables. These ions are a critical part of the energy transfer balance in the plasma, and their concentration is directly tied to the temperature in the plasma. Perturbing that temperature by, for example, introducing sample aerosol into the plasma affects the equilibrium concentration of the argon metastable ions, which thereby affects analytical performance. For this reason, it is vital that nebulizers produce the right properties of aerosol to minimize changes to the temperature in the plasma and, therefore, the argon metastable concentration.

What a Nebulizer Does

Figure 1 shows what happens to an aerosol particle as it passes through a plasma.

The challenge with every analysis presented to an analytical spectroscopist starts with the sample. It can be a solid or a liquid, and sometimes a vapor or a gas. If the sample is a solid, conventional wisdom says to somehow render that solid into a liquid. If it is a liquid, it might be clear or have suspended or dissolved solids. If the sample is a vapor or a gas, the concern is how to maintain the integrity of that vapor or gas to preserve it as much as possible and prevent loss of the analyte. Remember, the final goal with any method is to produce free atoms in the plasma. Keeping in mind the motto "free atoms lead to good excitation," the charge of the analytical chemist is to prepare that sample into a form that makes it easiest to introduce into the plasma. With any of these aforementioned sample matrices, the atomic spectroscopist knows that the farther he or she can refine the sample, the more the excitation source will achieve efficient production of free atoms. Said another way, the more that the method helps increase the transfer of energy from the source to the sample, the more excited analyte atoms are produced, and consequently, the better the method. On a more practical note, every sample preparation method should yield as simple a matrix as possible before introduction to the plasma. Most often this matrix is defined as either aqueous or organic, with the common characteristic of being homogeneous.

Figure 1: Diagram showing a single ærosol particle as it progresses from liquid droplet to free atoms. Adapted from reference 7.

To perform metals analysis by ICP spectroscopy, free atoms need to be produced that in turn can become excited. The energy available from the plasma needs to be transferred to the sample as efficiently as possible. To make that happen, we must have a sample that is in a form that readily allows efficient transfer of that energy. That form is primarily produced with a nebulizer.

This all sounds very straightforward, but in reality, nothing could be further from the truth. If it really were all that straightforward, we wouldn't have created such an extensive literature base describing everything from directly inserting the sample into the plasma on the tip of a graphite rod (8), to electrothermal vaporization of difficult-to-digest sample matrices (9). The difficult nature of samples submitted for metals analysis has led to the development of a wide variety of nebulizer types. Furthermore, nebulizers have to satisfy stringent analytical figures of merit including stability, piece-to-piece reproducibility, ruggedness, ease of operation and cleaning, and continued market availability. Nebulizers often define one manufacturer's performance edge over the competition, and today, where this performance edge is razor thin, nebulizer design is an aggressively protected art.

Types of Nebulizers

Nebulizer types generally fall into either pneumatic or nonpneumatic categories (Table I). These are all available either through major instrument manufacturers or through independent distributors of analytical accessories.

Table I: General categories of nebulizers used in inductively coupled plasma spectroscopy

A new instrument user is usually presented with the nebulizer that comes with their instrument. The manufacturer typically provides one or two options for other difficult matrices. If the instrument was purchased for a specific purpose, one or two nebulizer types can be sufficient. Still, it is a good idea to select the best nebulizer for the matrix and analytical method at hand. Any discussion of which nebulizer is the best one for an application usually involves knowing the properties of the sample. The following are some criteria that become important when deciding on a particular nebulizer.

Construction material: Nebulizers used to be made of borosilicate glass or quartz. Now nebulizers are available made from a variety of materials, including polytetrafluoroethylene (PTFE), perfluoroalkoxy (PFA), or polyetheretherketone (PEEK), for better compatibility with the matrix chemistry. For example, matrices rich in hydrofluoric acid should be run with non-glass nebulizer bodies. Applications in the precious metals industry might benefit from the tight manufacturing specifications and reproducibility gained with a glass concentric design.

Handling of dissolved solids: The discussion of percent dissolved total solids (%TS) in the sample can apply as much to describing sea water as a solution of monomer or polymer in an organic solvent. Both liquids are clear, but contain dissolved solids that can interfere with the nebulization process. Any of the high solids nebulizers available today are robust and maintain their performance despite being exposed to harsh chemicals. They do not readily plug, and are not easily damaged if cleaning becomes necessary. Users typically select one of the high solids nebulizers for samples with a %TS above 2%, and for levels below 1 %TS one of the low flow or high efficiency nebulizers is considered safe to use. Some concentric nebulizers are designed for moderate dissolved solids levels but care must be taken when there is particulate matter present that can clog the inner capillary.

Primary aerosol production efficiency: Nebulizers produce an aerosol that contains a wide range of particle sizes. The more a nebulizer produces an aerosol rich in particles approaching 1 µm or less, the better. Smaller aerosol particles are more easily transported and dissociate more quickly in the plasma, producing free analyte atoms more efficiently. Chromatographic methods interfaced with ICPs typically deliver only microliter amounts of sample. Selecting a high efficiency or low-flow nebulizer in this instance, with the added benefit that these nebulizers also produce a very fine mist, helps get the most performance from the small amount of sample available for analysis.

Self priming: Concentric nebulizers have a natural tendency to draw liquid sample in the process of producing the aerosol. Negative pressure at the point of aspiration causes fluid to flow through the sample line without any external pumping action, albeit under a limited set of circumstances. These are termed self priming. Other nebulizers, though they are considered to be pneumatic, fail to produce that zone of negative pressure, and thereby require an external pump to deliver sample to the nebulizer to produce an aerosol. A self-priming nebulizer might become necessary when a sample matrix is incompatible with the pump tubing used for delivery, as is the case with a harsh organic solvent. In another situation where the pump tubing produces high levels of contamination the only choice may be for the nebulizer to directly sip sample solution from its base to avoid high background signals.

Sample consumption: Nebulizers have to, by nature, consume sample volume. When there is ample sample volume available for analysis, the amount of sample a nebulizer consumes is of little concern. However, in instances where sample volume is at a premium (for example, with precious metal digests or biological materials), sample consumption is of greater concern. (See the discussion above on primary aerosol production.) For these applications there are a variety of high efficiency and low-flow nebulizers available. Unfortunately, these types of nebulizers usually cannot tolerate high levels of dissolved solids before plugging or salting up. Babington and frit nebulizers use higher volumes of sample while being able to tolerate percent levels of dissolved solids. The reverse is true with low-flow nebulizers (low volume with low levels of dissolved solids). In addition, analysis time is affected by the lower sample consumption because of the time required for the sample to be transported between the sample vessel and the nebulizer. This can be enhanced by choosing capillary tubing, but has the added risk that the line can plug.

Gas flow versus back pressure: One direct consequence of pneumatics when it comes to producing an aerosol is the relationship between gas flow, back pressure, and sample flow on the quality of aerosol production. Generally speaking, the higher the back pressure the better the properties of the primary aerosol. While most ICP instrument manufacturers specify an inlet pressure for argon of as high as 100 psig, many high efficiency nebulizers require back pressures approaching 150 psig or higher. This level of pressure can easily be delivered from a compressed gas cylinder but not from the liquid argon Dewar containers typically used in ICP laboratories. For this reason, nebulizer designers targeted their performance metrics for the lower pressures even though better performance can be realized at the higher pressures.

Pneumatic versus nonpneumatic: In the past, nebulization was produced by passing a gas under high pressure through an orifice and relying on the rapid expansion of the gas at that orifice to break down the liquid into minute aerosol droplets. This created an effect whereby gas flow rate directly influenced the properties of the aerosol produced; that is, increasing the gas flow raised the pressure and consequently produced a better aerosol. This also meant that changing gas flows to optimize the ICP performance also changed the performance of the nebulizer posing difficulty to researchers. This spawned research into methods that could produce an aerosol independently of gas flow. Devices were developed with which a vibrating quartz membrane excited at microwave frequencies produced a primary aerosol rich in small particle sizes. With this method, a sample aerosol could be produced independently of gas flow without requiring elevated gas pressures. This turned out to be so efficient that the aerosol itself would overwhelm the energy balance in an ICP and necessitated desolvating that aerosol before introducing it to the plasma. Desolvating the aerosol ended up improving analytical performance by removing vapor from the aerosol and reducing the degree of sample loading in the plasma, which consequently prevented the temperature in the plasma from becoming cooler. Though nonpneumatic nebulizers solved many of the problems created with pneumatic nebulizers, they were not as compact as the typical glass or plastic nebulizer and required an extra layer of equipment maintenance.

Future Trends

There is a slow, but deliberate progression toward introducing samples at 100% efficiency. The earliest nebulizers were 1–2% efficient. That meant that 98–99% of the sample was wasted while introducing only microliter volumes of sample into the ICP. Recent experiments have shown that by introducing sample into the plasma at extremely low flow rates (less than 50 µL/min), sample can be introduced at essentially 100% with little sacrifice to the overall detection limit. Amazingly, this 100% efficiency is achieved without overloading the plasma and avoids many of the problems associated with matrix effects in the plasma due to sample loading (10,11). This resulted in optimum energy transfer efficiency leading to maximum generation of free analyte atoms, with minimal reduction in overall plasma temperature — all factors discussed above as being important to good method development. The sample best suited to this nebulizer must be very clean and contain no suspended particles, and possibly also have a relatively low percentage of dissolved solids.

Have we seen the last of new nebulizers for ICP? Perhaps not. Nebulization is not only for us to own. The world is full of nebulizers, whether they are used for fuel injection in cars, air brush painting of model airplanes, or controlling humidity in hospitals and laboratories. It remains to be seen whether a development in these other fields matches the needs of atomic spectrometric chemical analysis.

Conclusion

There will continue to be those who seek the perfect nebulizer for each special application. Others are content with one or two types and have used those same nebulizers for many years. However, it's never good to remain too comfortable in a routine. New matrices and detection limit pressures will always challenge conventional wisdom and skill. While not touched on here, new designs in spray chambers (often referred to as aerosol chambers or filters) are showing properties of aerosols previously hidden that are challenging the theory of the purpose of the chamber in the first place (12,13). We still need nebulizers (or something else) that get us from the world of samples to the world of free analyte atoms with as much efficiency as scientifically possible. That time may be at our doorstep.

References

(1) J.L. Todoli and J.M. Mermet, Liquid Sample Introduction in ICP Spectrometry, A Practical Guide, First Edition (Elsevier, 2008).

(2) A. Montaser and D.W. Golightly, Eds., Inductively Coupled Plasmas in Analytical Atomic Spectrometry (VCH Publishers, Inc., 1992).

(3) B.L. Sharp, J. Anal. At. Spectrom. 3, 613–652 (1988).

(4) R.F. Browner and A.W. Boorn, Anal. Chem. 56, 786A–798A (1984).

(5) S. Greenfield, I.L. Jones, and C.T. Berry, Analyst (London) 89, 713–720 (1964).

(6) R.H. Wendt and V.A. Fassel, Anal. Chem. 37, 920–922 (1965).

(7) Meinhard An Elemental Scientific Company, used with permission.

(8) V. Karanassios and G. Horlick, Spectrochim. Acta 45B, 85–104 (1990).

(9) M.W. Tikkanen and T.M. Niemczyk, Anal. Chem. 58, 366–370 (1986).

(10) J.L Todoli, Anal. Bioanal. Chem. 378(1), 57–59 (2004).

(11) J.L Todoli, R. Sanchez, A. Francisco, and M. Grotti, "Poster TP05" presented at the 2012 Winter Conference on Plasma Spectrochemistry, Tuscon, Arizona, 2012.

(12) J.L. Todoli, S. Maestre, J. Mora, A. Canals, and V. Hernandis, Fresenius' J. Anal. Chem. 368(8), 773–779 (2000).

(13) G. Meyer, "Paper M05" presented at the 2010 Winter Conference on Plasma Spectrochemistry, Fort Myers, Florida, 2010.

Gerhard Meyer, PhD, is the lead analyst for analytical quality and control at Promerus LLC in Brecksville, Ohio. Direct correspondence to: Gary.Meyer@promerus.com.