Plasma torches

The term plasma refers to a partially ionized gas, composed essentially of photons, ions, and free electrons as well as atoms and molecules in their fundamental or excited states. Because of the presence of free charges, plasma is electrically conductive and follows the law of electromagnetism. Thus, a static electric field, for example, will tend to separate and accelerate plasma charges. Overall, however, plasma is neutral since the number of negative-charge carriers is equal to the number of the positive ones. 

Because of its unique properties, plasma is often called the fourth state of matter according to the following scheme expressing an increase in energy level  

Plasmas can exist within solids as well as liquids, but the most common are gas plasmas. In view of thermal conditions, the following basic types of plasma are distinguished  

Cold plasmas tens of degrees (near ambient) 

Electrodeless devices are used to generate cold plasma, typically within a quartz tube by a high-frequency electromagnetic energy. A surface-wave applicator with longitudinal propagation of microwave energy is especially 

---

The sample should be liquid or in solution. It is pumped and nebulized in an argon atmosphere, then sent through a plasma torch that is, in an environment where the material is strongly ionized resulting from the electromagnetic radiation produced by an induction coil. Refer to the schematic diagram in Figure 2.8. 

Much of the energy deposited in a sample by a laser pulse or beam ablates as neutral material and not ions. Ordinarily, the neutral substances are simply pumped away, and the ions are analyzed by the mass spectrometer. To increase the number of ions formed, there is often a second ion source to produce ions from the neutral materials, thereby enhancing the total ion yield. This secondary or additional mode of ionization can be effected by electrons (electron ionization, El), reagent gases (chemical ionization. Cl), a plasma torch, or even a second laser pulse. The additional ionization is often organized as a pulse (electrons, reagent gas, or laser) that follows very shortly after the... 

Lasers can be used in either pulsed or continuous mode to desorb material from a sample, which can then be examined as such or mixed or dissolved in a matrix. The desorbed (ablated) material contains few or sometimes even no ions, and a second ionization step is frequently needed to improve the yield of ions. The most common methods of providing the second ionization use MALDI to give protonated molecular ions or a plasma torch to give atomic ions for isotope ratio measurement. By adjusting the laser s focus and power, laser desorption can be used for either depth or surface profiling. 

By use of a piezoelectric device, as in a gas lighter, a small spark containing electrons and ions can be produced. If the spark is introduced into the gas in a discharge tube, it will provide the extra initial electrons and ions needed to start a continuous discharge. A plasma torch is frequently lit (started) in this fashion. 

This chapter should be read in conjunction with Chapter 6, Coronas, Plasmas, and Arcs. A plasma is defined as a gaseous phase containing neutral molecules, ions, and electrons. The numbers of ions and electrons are usually almost equal. In a plasma torch, the plasma is normally formed in a monatomic gas such as argon flowing between two concentric quartz tubes (Figure 14.1). 

If a sample solution is introduced into the center of the plasma, the constituent molecules are bombarded by the energetic atoms, ions, electrons, and even photons from the plasma itself. Under these vigorous conditions, sample molecules are both ionized and fragmented repeatedly until only their constituent elemental atoms or ions survive. The ions are drawn off into a mass analyzer for measurement of abundances and mJz values. Plasma torches provide a powerful method for introducing and ionizing a wide range of sample types into a mass spectrometer (inductively coupled plasma mass spectrometry, ICP/MS). 

Because light emitted from inductively coupled plasma torches is characteristic of the elements present, the torches were originally introduced for instruments that optically measured the frequencies and intensities of the emitted light and used them, rather than ions, to estimate the amounts and types of elements present (inductively coupled plasma atomic emission spectroscopy. 

The degree of ionization increases with temperature, and at 6000-8000 K, where ionization efficiencies are 90 to 95%, nearly all atoms exist as ions in the plasma. Operation of the plasma torch under conditions that produce a cooler flame (cold plasma) has important advantages for some applications, which are discussed after the next section. 

Sample Inlets for Plasma Torches, Part A Gases... 

Fundamentally, introduction of a gaseous sample is the easiest option for ICP/MS because all of the sample can be passed efficiently along the inlet tube and into the center of the flame. Unfortunately, gases are mainly confined to low-molecular-mass compounds, and many of the samples that need to be examined cannot be vaporized easily. Nevertheless, there are some key analyses that are carried out in this fashion the major one i.s the generation of volatile hydrides. Other methods for volatiles are discussed below. An important method of analysis uses lasers to vaporize nonvolatile samples such as bone or ceramics. With a laser, ablated (vaporized) sample material is swept into the plasma flame before it can condense out again. Similarly, electrically heated filaments or ovens are also used to volatilize solids, the vapor of which is then swept by argon makeup gas into the plasma torch. However, for convenience, the methods of introducing solid samples are discussed fully in Part C (Chapter 17). 

The volatile hydride (arsine in Equation 15.1) is swept by a. stream of argon gas into the inlet of the plasma torch. The plasma flame decomposes the hydride to give elemental ions. For example, arsine gives arsenic ions at m/z 75. The other elements listed in Figure 15.2 also yield volatile hydrides, except for mercury salts which are reduced to the element (Fig), which is volatile. In the plasma flame, the arsine of Equation 15.1 is transformed into As ions. The other elements of Figure 15.2 are converted similarly into their elemental ions. 

To examine a sample by inductively coupled plasma mass spectrometry (ICP/MS) or inductively coupled plasma atomic-emission spectroscopy (ICP/AES) the sample must be transported into the flame of a plasma torch. Once in the flame, sample molecules are literally ripped apart to form ions of their constituent elements. These fragmentation and ionization processes are described in Chapters 6 and 14. To introduce samples into the center of the (plasma) flame, they must be transported there as gases, as finely dispersed droplets of a solution, or as fine particulate matter. The various methods of sample introduction are described here in three parts — A, B, and C Chapters 15, 16, and 17 — to cover gases, solutions (liquids), and solids. Some types of sample inlets are multipurpose and can be used with gases and liquids or with liquids and solids, but others have been designed specifically for only one kind of analysis. However, the principles governing the operation of inlet systems fall into a small number of categories. This chapter discusses specifically substances that are normally liquids at ambient temperatures. This sort of inlet is the commonest in analytical work. 

With a typical ablated particle size of about 1 -pm diameter, the efficiency of transport of the ablated material is normally about 50% most of the lost material is deposited on contact with cold surfaces or by gravitational deposition. From a practical viewpoint, this deposition may require frequent cleaning of the ablation cell, transfer lines, and plasma torch. 

In principle, DSI is the simplest method for sample introduction into a plasma torch since the sample is placed into the base of the flame, which then heats, evaporates, and ionizes the sample, all in one small region. Inherent sensitivity is high because the sample components are already in the flame. A diagrammatic representation of a DSI assembly is shown in Figure 17.4. 

These direct-insertion devices are often incorporated within an autosampling device that not only loads sample consecutively but also places the sample carefully into the flame. Usually, the sample on its electrode is first placed just below the load coil of the plasma torch, where it remains for a short time to allow conditions in the plasma to restabilize. The sample is then moved into the base of the flame. Either this last movement can be made quickly so sample evaporation occurs rapidly, or it can be made slowly to allow differential evaporation of components of a sample over a longer period of time. The positioning of the sample in the flame, its rate of introduction, and the length of time in the flame are all important criteria for obtaining reproducible results. 

Solid samples can be analyzed using a plasma torch by first ablating the solid to form an aerosol, which is swept into the plasma flame. The major ablation devices are lasers, arcs and sparks, electrothermal heating, and direct insertion into the flame. 

The term nebulizer is used generally as a description for any spraying device, such as the hair spray mentioned above. It is normally applied to any means of forming an aerosol spray in which a volume of liquid is broken into a mist of vapor and small droplets and possibly even solid matter. There is a variety of nebulizer designs for transporting a solution of analyte in droplet form to a plasma torch in ICP/MS and to the inlet/ionization sources used in electrospray and mass spectrometry (ES/MS) and atmospheric-pressure chemical ionization and mass spectrometry (APCI/MS). 

Nebulizers are used to introduce analyte solutions as an aerosol spray into a mass spectrometer. For use with plasma torches, it is necessary to produce a fine spray and to remove as much solvent as possible before the aerosol reaches the flame of the torch. Various designs of nebulizer are available, but most work on the principle of interacting gas and liquid streams or the use of ultrasonic devices to cause droplet formation. For nebulization applications in thermospray, APCI, and electrospray, see Chapters 8 and 11. 

Solutions of solids may need to be converted into aerosols by pneumatic or sonic-spraying techniques. After solvent has evaporated from the aerosol droplets, the residual particulate solid matter can be ionized by a plasma torch. 

---