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